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The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far northwest Tasmania A technical report to Cradle Coast Natural Resource Management September 2012

Paul Donaldson, Chris Sharples & Robert J Anders Blue Wren Group School of Geography and Environmental Studies University of Tasmania

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Blue Wren Group, University of Tasmania The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far northwest Tasmania

Acknowledgements This project was supported by Cradle Coast NRM through funding from the Australian Government’s Caring for our Country. The authors of this report would like to especially thank: Chris Watson, UTAS, for assistance with tide data processing and analysis; Dave Shaw, for designing, constructing and operating the sediment corer; Matthew and Niko Campbell-Ellis, for helping to deploy, download and retrieve the remote water level loggers, as well as providing general field support and warm hospitality; and Richard Mount, BOM, for initiating the research project, providing advice on seagrass and blue carbon and reviewing the draft report. Thanks also go to: John Gibson and Jeff Ross, IMAS, for practical advice on the operation of the water level loggers; Mike Johnson, for assistance with the engineering design for the sediment sampling platform; Mark Underwood and David Hughes, CSIRO, for advice regarding stilling well design requirements; Brett Greene and Shane Underwood, Black Reef Fisheries, for boating assistance during our sediment sampling program; Cradle Coast NRM staff, including James Shaddock for his enthusiasm towards this project and early assistance in with the contract management and Sue Botting for her critical comments on the draft report; and ESRI Australia, for assistance with ArcGIS.

Photo credits Cover photos: Western Robbins Passage tidal channel and Boullanger Bay (top, Vishnu Prahalad), Posidonia australis meadows, Boullanger Bay (lower-middle, Richard Mount), fibrous rich seagrass sediment core from Boullanger Bay (bottom, Paul Donaldson). All other photos by Paul Donaldson and Chris Sharples, with the exception of Figure 8c (p 24) by Matthew Campbell-Ellis and Figure 20 (p 48) by David Shaw.

Cover design Paul Donaldson

Citation DONALDON, P., SHARPLES, C., ANDERS, R.J., 2012: The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far northwest Tasmania. A technical report to Cradle Coast Natural Resource Management. Blue Wren Group, School of Geography and Environmental Studies, University of Tasmania, Hobart.

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The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far northwest Tasmania A technical report to Cradle Coast Natural Resource Management

Paul Donaldson, Chris Sharples & Robert J Anders September 2012

Blue Wren Group School of Geography and Environmental Studies University of Tasmania

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

SUMMARY This natural resource management research project was initiated by the Cradle Coast NRM, in response to the knowledge gaps identified by the Blue Wren Group in understanding elements of Robbins Passage-Boullanger Bay (RP-BB) coastal processes. The purpose of this study was to:  

Improve the understanding of RP-BB tides, based on observational data, and Investigate the carbon sequestration potential and palaeo-environmental evolution of RP-BB shallow seagrass beds, based on a set of shallow marine sediment cores.

The key findings of this report are:  

 

 

RP-BB receives strongly semi-diurnal meso-tides which vary in their range and time of arrival Predicted mean spring tide ranges and total tide ranges were found to be 2.80 m and 3.15 m at Howie Island, 2.20 m and 2.63 m at Kangaroo Island, and 2.01 m and 2.42 at Welcome Inlet The National Tide Centre’s modelled tide range was found to underestimate the tide range for eastern Boullanger Bay by approximately 30% Three unique sedimentary deposits (i.e. facies) were identified in the sediment cores, interpreted as a Late Pleistocene alluvial/lacustrine deposit (SF1), Mid-Holocene intertidal or shallow subtidal sand flats (SF2), and Mid-Late Holocene seagrass associated deposits (SF3) Large carbon rich sediment deposits exist beneath the subtidal seagrass meadows at RP-BB RP-BB Posidonia australis dominated subtidal seagrass meadows are highly effective at sequestering carbon.

ROBBINS PASSAGE-BOULLANGER BAY TIDES The tides in far northwest Tasmania are highly variable and poorly documented. This study recorded about 5 months of sea level observation from three locations across RPBB, and analysed this data with T-TIDE in MATLAB, to better define the range and timing and astronomical components of the regions tides. A comprehensive literature review was initially conducted to provide background information on the science of tides, the available methods for collecting and analysing tide data, and the current understanding of the tidal characteristics of the far northwest Tasmania.

Tide data collection Sea level observations from three locations were recorded using pressure gauges deployed in ‘remote stilling wells’ purposefully constructed for this project. These were designed for deployment in meso-tidal conditions and constructed from relatively cheap and readily available materials. The stilling wells functioned to protect the loggers and improve the data accuracy by mechanically filtering out the effect of waves on the observed water level data.

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Summary Data was collected at 5 minute intervals with the HOBO U20 water level loggers from November/December 2010 to May 2012. The loggers were surveyed to the Australian Height Datum (AHD) with differential GPS.

Tide data processing and analysis The total pressure data collected from the three sea level observation sites were converted to water level using the Barometric compensation assistant in HOBOware Pro and the regional air pressure observations collected in this study. These water levels were subsequently corrected to AHD. A tidal analysis was conducted on the corrected data using T-TIDE in MATLAB to define the astronomical constituents for the three sites. This data was then used to predict the astronomical tides (i.e. those driven by the gravitational interactions between sun, moon and earth) and define the environmental effects on the observed tides (such as weather and shallow water). Various tidal planes were defined for each observation site, using standard observationand harmonic- based definitions. Prediction-based definitions (which apply observationbased definitions to the predicted data) were also applied, to avoid the issues associated with applying observation- and harmonic - based definitions on our dataset.

Tide characteristics and behaviour RP-BB receives strongly semi-diurnal meso-tides which vary geographically in its height and time of arrival.

Tide range RP-BB receives strongly semi-diurnal (F ≤ 0.2) meso-tides which vary geographically in their height. Tide range increases from west to east – a trend which is consistent with the regional gradient experienced across the greater northwest coast. Table S1: Key tide ranges for Robbins Passage – Boullanger Bay, derived from harmonic-based2 and prediction-based3 tide planes. Tidal Plane Total tide range3 Indian Spring Water tide range Mean Spring tide range Mean Neap tide range

3

3

2

Howie Island (m)

Kangaroo Island (m)

Welcome Inlet (m)

3.151

2.626

2.422

2.866

2.410

2.122

2.796

2.200

2.094

1.839

1.312

1.311

The tide planes defined for Kangaroo Island and Welcome Inlet show that teh tide range in eastern Boullanger Bay is greater than previously realised (e.g. NTC standard tide range model).

Tide timing Howie Island is the first observational location to receive high tide by 20 minutes, occurring approximately 55 minutes after Stanley. Welcome Island is the last site to experience ebb tide, some 40 minutes after Howie Island and Kangaroo Island. All three measured sites experience asymmetrical tides with lagged ebb times due to shallow water conditions.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania Table S2: Relative timing of the predicted tidal wave at Robbins Passage-Boullanger Bay. Kangaroo Island

Arrival time after Howie Island

Welcome Inlet

High Tide

Low Tide

High Tide

Low Tide

Maximum (minutes)

30

30

35

60

Minimum (minutes)

10

-20

05

10

Average (minutes ± 1sd)

20 (±05)

00 (±10)

20 (±05)

40 (±10)

Astronomical tides Tidal harmonic analysis solved between 21- 23 harmonic constituents for all observational sites, with the Principal Lunar semidiurnal constituent accounting for ~50 % of the astronomical tidal variation. Higher order - shallow water harmonics (such as M4) were also found to be significant for all three sites. Predicted astronomical tides based on the summation of the solved tidal harmonics account for ~97% of the observed data.

Tide behaviour The tides timing indicate that two separate tide waves enter Robbins Passage, from east and west. The decrease in tide range experienced from west to east suggest a net hydrodynamic flow occurs in the same direction. This observation supports Mount et al.’s (2010a) interpretation of the long term sediment transport dynamics through Robbins Passage. The timing of tides from the two eastern Boullanger Bay sites suggests that northwest propagating flood tides are experienced here. This may indicate that two separate tide waves enter the bay from the north and west, but more tide observations are needed.

Management applications The improved tide range data defined in this study shows that the National Tide Centre’s (NTC) tide range model underestimates eastern Boullanger Bay tides by approximately 30%. Additionally, the second stage of the Coastal Inundation Mapping for Tasmania (Lacey et al., 2012) which maps the onshore sea level rise hazard based in part on a derivative of the NTC’s tide range data has subsequently underestimated the flood hazard for this region. Both the NTC’s tide range model and future inundation mapping should incorporate our improved tide range data for this region. Table S3: Indian Spring High Water level comparisons between various sources of tide data. Tide level source

Welcome Inlet (B. Bay – centre)

Kangaroo Island (B. Bay – east / R. Passage – west)

Howie Island (R Passage – east)

This study

1.06

1.19

1.44

0.76

0.76

1.48

0.74

0.81

1.30

1

NTC

Lacey et al. (2012)

2

1

The closest National Tide Centre data points to the observational sites in this study are ~7km NE of Welcome Inlet, ~4.5km NNW of Kangaroo Island and ~9km E of Howie Island. 2 Tide data used for the second stage of the Coastal Inundation Mapping for Tasmanian (Lacey et al., 2012).

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Summary

RP-BB SEAGRASS SEDIMENTOLOGY Six sediment cores were extracted from RP-BB shallow marine environment to investigate the carbon sequestration potential of the regions extensive seagrass meadows, based on recent literature which highlights the significant role which seagrasses play within the global carbon cycle.

Sediment coring Continuous shallow marine sediments cores penetrating depths of up to 3 m were sampled from the RP-BB seagrass beds using a purpose built double tube percussion corer. The coring program focussed on sampling sediments from the extensive subtidal Posidonia australis dominated meadows, which were hypothesised to form valuable carbon sinks. Four cores (BB1- BB4) were extracted from Boullanger Bay and two (RP1RP2) from eastern Robbins Passage. The stratigraphy and relative carbon content of the six cores were visually ‘logged’ (i.e. examined and described) and interpreted.

Core stratigraphy and interpretation Stratigraphic analysis identified three unique sedimentary deposits (i.e. sedimentary facies) within the sampled sedimentary record, including a:   

Clay rich terrestrial facies (SF1: Late Pleistocene alluvial/lacustrine deposit), Well sorted sandy marine facies (SF2: Mid-Holocene intertidal or shallow subtidal sand flats), and Silty sand to sandy marine facies variably rich in organic fibres (SF3: Mid-Late Holocene seagrass associated deposits).

The seagrass associated facies (SF3) was further divided into four sub-facies comprising:    

Organic silty sands rich in cellulose fibres (SF3a: subtidal P. australis seagrass platform), Organic silty sands with silt rich laminae (SF3b: palaeo-tidal channel infill deposit), Organic sands rich cellulose fibres (SF3c: subtidal P. seagrass flats), and Organic sands with variable organic fibres (SF3d: intertidal mixed seagrass flats).

Table 1: Sedimentary facies summary description. Facies & subfacies

Sedimentary facies & sub-facies summary description

Present in cores:

Overlies facies:

Underlies facies:

Relative ‘blue carbon’ abundance:

SF1

Mottled silty clayey sand to clay, cohesive, massive, no organic fibres of shell material present.

RB1, RB2

?

SF3c

None

SF2

Well sorted grey quartzcarbonate sands. Sands fine to medium.

BB1, BB2, BB3

?

SF3a

Low

SF3a

Olive grey organic silty quartzcarbonate sand, variable rich in cellulose fibres. Moderately sorted. Sands fine. Some broken and whole shells.

BB1, BB2, BB3, BB4

SF2, SF3b

-

High

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

SF3b

Dark grey organic rich silty sand with regular silt rich laminae, minor cellulose fibres. Moderately sorted. Sands fine. Broken and whole shells present.

BB4

?

SF3a

Moderate (to high?)

SF3c

Olive grey organic quartzcarbonate sand, variable rich in cellulose fibres. Moderately sorted. Sands fine. Some broken and whole shells.

RP1, RP2

SF1

SF3d

High

SF3d

Olive grey quartz-carbonate sand, with dark organic fibres. Moderate-well sorted. Sands fine. Some broken and whole shells.

RP2

SF3c

Moderate

Carbon sequestration potential of RP-BB seagrasses All seagrass associated sedimentary deposits were found to have significant carbon content in their sediments, with the subtidal P. australis seagrass platform and the subtidal P. seagrass flats notably rich in organic carbon. Thus the two key finding were:  

RP-BB Posidonia australis dominated subtidal seagrass meadows are highly efficient at sequestering carbon, and Large carbon stocks exist beneath the subtidal seagrass meadows at RP-BB.

Coastal habitat mapping indicates that RP-BB subtidal seagrass cover an area of 61km2, and the cores found the subtidal P. australis seagrasses to form carbon rich deposits approximately 1.25 m thick, thus the total volume of carbon rich sediments is likely at least 76.25 km3. However the cores did not sample towards the outer edge of the seagrass meadows where deeper carbon rich profiles have likely formed. Further analysis is needed to estimate the total amount of carbon storage in these sediments, and the rate at which it is sequestered.

Management implications RP-BB seagrasses have the capacity to continue sequestering significant amounts of carbon with rising sea levels if properly managed, but like all global seagrasses they are highly susceptible to degradation from human disturbances. Management effort should focus on reducing nutrient loads in the coastal waters, and preserving water clarity through conserving, managing and improving the regions coastal and riparian vegetation, and minimising physical disturbance of coastal sediments. Mismanagement of the regions wetlands could result in seagrass health degradation and erosion of carbon rich sediments which would have long term global implications for atmospheric carbon concentrations.

RECOMMENDATIONS Recommendation are made for both studies, which advise to provide the datasets created in this project to appropriate authorities, and detail the need for, and steps required to, undertake follow up detailed investigations for these two preliminary studies.

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Summary

RP-BB tides 

Tide data should be provided to the National Tide Centre for archiving

This report should be provided to the National Tide Centre to highlight the errors associated with their tide range model for eastern Boullanger Bay

Additional sea level observations should be made throughout the greater (northern and western) Boullanger Bay to better define the tidal characteristics of this poorly understood and documented region

Redeploy a tide logger at one existing RP-BB survey location to increase the observational period, as this will allow longer period tidal constituents to be resolved

RP-BB seagrasses 

Manage seagrass health, to ensure carbon sequestration continues by: o

limiting human practices which increase nutrient and sediment input into the passage/bay, including those which physically disturb the nutrient rich RP-BB coastal sediments

o

conserving and improving RP-BB coastal and riparian vegetation to improve coastal waters nutrient levels and clarity

Disseminate our ‘blue carbon’ findings to the greater scientific community by providing this report to appropriate research bodies (e.g. UNESCO Blue Carbon International Scientific Working Group)

Undertake additional research to better define RP-BB seagrass carbon stocks by:

o

Measuring carbon content of the existing core samples

o

Undertake additional coring programs and/or seismic survey to better resolve the 3D geometry of the blue carbon stocks

Radiocarbon date the existing seagrass sediments to define the long term rate of carbon sequestration

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

TABLE of CONTENTS SUMMARY ........................................................................................................... 4 1

2

INTRODUCTION .......................................................................................... 12 1.1

Project aims ........................................................................................... 12

1.2

Study area ............................................................................................. 13

1.3

Work undertaken and report structure ....................................................... 13

1.4

Scope .................................................................................................... 13

TIDAL OBSERVATIONS, ANALYSIS AND CHARACTERISTICS ....................... 14 2.1

Introduction ........................................................................................... 14

2.2

Background ............................................................................................ 14

2.2.1

Causes of sea level variation .............................................................. 14

2.2.2

Ocean tides ...................................................................................... 15

2.2.3

Tidal analysis and prediction............................................................... 16

2.2.4

Tidal variability in far northern Tasmania ............................................. 17

2.2.5

Tidal measurements .......................................................................... 18

2.3

Methods ................................................................................................. 20

2.3.1

Data collection.................................................................................. 20

2.3.2

Data analysis ................................................................................... 28

2.4

Results .................................................................................................. 32

2.4.1

Observed tides ................................................................................. 32

2.4.2

Tide analysis and prediction ............................................................... 32

2.4.3

Tide characteristics ........................................................................... 36

2.5

Discussion .............................................................................................. 38

2.6

Management applications ......................................................................... 39

2.6.1

Improved tide range ......................................................................... 39

2.6.2

Management recommendations .......................................................... 41

2.7

Data outputs .......................................................................................... 41

2.8

Future work ........................................................................................... 41

3 STRATIGRAPHIC AND BLUE CARBON INVESTIGATION OF THE SEAGRASS BEDS ................................................................................................................ 42 3.1

Introduction ........................................................................................... 42

3.2

Background ............................................................................................ 42

3.2.1

Blue Carbon: natural coastal carbon sinks ............................................ 43

3.2.2

Blue carbon in seagrasses meadows .................................................... 44

3.2.3

Palaeo-environmental investigations using soft sediment cores ............... 45

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Table of contents 3.3

Methods ................................................................................................. 46

3.3.1

Site selection.................................................................................... 46

3.3.2

Soft sediment coring ......................................................................... 47

3.3.3

Visual logging of sediment core .......................................................... 50

3.4

Results .................................................................................................. 51

3.4.1

Site location and description............................................................... 51

3.4.2

Core stratigraphy .............................................................................. 53

3.4.3

Sedimentary facies ........................................................................... 66

3.4.4

Sedimentary facies interpretation ....................................................... 68

3.5 Carbon sequestration potential of Robbins Passage – Boullanger Bay seagrass meadows ......................................................................................................... 70

4

3.6

Management implications ......................................................................... 71

3.7

Management recommendations ................................................................ 71

3.8

Future work ........................................................................................... 72

References .................................................................................................. 73

Appendix .......................................................................................................... 76

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

1 INTRODUCTION This technical research project investigates aspects of Robbins Passage – Boullanger Bay wetlands environmental functioning. This study was initiated by the Cradle Coast NRM in response to the lack of research into, and understanding of, two specific coastal processes in the region. These knowledge gaps were identified by the Blue Wren Group, University of Tasmania, following our earlier study into Circular Head’s coastal habitat (Mount et al., 2010a).

1.1 Project aims The aim of this study was to investigate two elements of the Robbins Passage Boullanger Bay (RP-BB) environment and produce new datasets that both strengthen the current understanding of the regions coastal processes and contribute to ongoing environmental monitoring and management of the region. The specific objectives were: 

To collect 5 months of sea level observational data from RP-BB (Figure 1) for the purpose of investigating the tide range and characteristics of the region, and;

To obtain a set of shallow marine sediment cores (Figure 1) for the purpose of: o Investigating the carbon sequestration potential of the RP-BB seagrass meadows, and o Improving the understanding of the modern sedimentary processes and palaeo-environmental evolution of the RP-BB region.

Figure 1: The Robbins Passage – Boullanger Bay study areas, far northwest Tasmania.

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Introduction

1.2 Study area The Robbins Passage and Boullanger Bay area is located off the far western end of Tasmania’s north coast (Figure 1). This region’s coastline is complex, comprising many inlets and offshore islands which are inter-dispersed with vast sandy and dense seagrass inhabited intertidal and shallow subtidal flats. These areas are dissected by an extensive network of tidal channels. The region is hydrodynamically complex, experiencing mesotides which vary geographically in their range and time of arrival. The RP-BB coastal setting is sheltered from ocean swell, but experiences strong tidal currents and regular wind derived waves.

1.3 Work undertaken and report structure This present study was undertaken by the Blue Wren Group, School of Geography and Environmental Studies, University of Tasmania, in response to the knowledge gaps we indentified in the understanding of the Circular Heads coastal processes during an earlier study of the coastal region (see Mount et al., 2010a). Within this study we completed two independent technical research projects, each addressing specific research aims. These two projects were: 

Improving the tide range observations for RP-BB (see Section ‎2). This was achieved by measuring the regions tides and analysing the observational data.

A sediment coring investigation into the shallow marine coastal stratigraphy and carbon sequestration potential of RP-BB (see Section 3 ‎ ). This was achieved by collect a set of shallow marine sedimentary cores form the extensive seagrass beds.

This report is split into two stand alone sections, each detailing these two independent studies.

1.4 Scope We should highlight that both bodies of research undertaken on the RP-BB environmental functioning are preliminary in nature: the tides have been investigated from a relatively short period of sea level observations from three locations; and the seagrass bed sedimentology was analysed for the region from six sediment cores by simply visually analysing and documenting their sedimentary record. Despite the preliminary nature of this project, our study has produced new insight into the environmental processes at RP-BB, which have important implications for the natural resource management of this region. Additionally, the datasets compiled from our research can be further studied or built on, and we outline the required steps to do so.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

2 TIDAL OBSERVATIONS, ANALYSIS AND CHARACTERISTICS 2.1 Introduction Far northwest Tasmania experiences diurnal tides which vary substantially in their range and time of the arrival. Measured observational tide data from this region is sparse and details of the sharp gradient and behaviour of the tides experienced between Tasmania’s northern western tip at Cape Woolnorth and Stanley have not been previously measured to our knowledge. The Robbins Passage – Boullanger Bay (RP-BB) region falls within this poorly documented region in an area where the tide range gradient is likely the greatest. This knowledge gap was highlighted by the Blue Wren Group following on from our detailed study into the vulnerability of the Circular Heads coastal foreshore habitats to sea level rise (Mount et al., 2010a). We also identified that future coastal management for the far northwest would benefit from improving the understanding of the regions tidal characteristics by: 

Allowing more accurate projections of sea level rise to be made and mapped onto the regions shores (i.e. more precisely defining future sea level rise coastal footprints);

Having the tide data available for future process studies (such as hydrodynamic modelling which would provide valuable information about ecological, physiochemical and geomorphological processes in the area).

Our study described here measured the local tides at 3 locations in RP-BB over a 5 month period. The datasets created from these sea level observations has improved the understanding of the areas coastal processes and will benefit the natural resource management of the region.

2.2 Background The process of measuring tides is complex, requiring not only technical equipment and knowledge of the methodology required to collect accurate sea level data, but also a sound knowledge of the physical sources which contribute to sea level variation. Knowledge of these sources is necessary to enable meaningful tidal data to be extracted from the sea level observations. The following sub-sections briefly review some important background information which needs to be considered when conducting a tidal survey. Further information on this topic can be gained from technical manuals such as Forrester (1983) and IOC (2006).

2.2.1

Causes of sea level variation

Sea level variation occurs due to the combination of a number of different physical sources, ranging from short term (1 - 20 second) variations in ocean water levels caused by wind generated swell and waves, to long term (10’s of thousands of years) variation in sea level primarily driven by changes in global temperatures. However, the most predominant and regular variations observed are caused by (1/2 – 1 day) periodic tidal waves which are driven by gravitational interactions between the earth, sun and moon (IOC, 2006). A summary of the various components that contribute to the sea level variation, which include both tidal and non tidal sources, are provided in Table 2. To

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Tide observations and analysis understand the true contribution (and characteristics) of the astronomical tides to a sea level observational dataset, other contributing sources of change (listed below) must be accounted for by undertaking tidal analysis on the data. Table 2: Causes of changes in sea levels, modified from IOC (2006). Causes of sea level variation

Period

Description

Surface waves (wind or swell waves)

1-20 seconds.

Surface waves created from winds, including wind waves and swell which can vary in amplitude from centimetres to metres.

Seiches

Minutes to hours.

Local periodic variation in sea level due to causes such as strong wind or current, or a sudden change in atmospheric pressure.

Tsunami

Minutes to hours.

A series of long wavelength waves created from the vertical displacement of the water column, nominally due to tectonic faulting of the ocean floor, but also from other disturbances such as landslides and meteorite impacts.

Astronomical tides

½ - 1 day.

Predominant changes in sea level due to oceans response to gravitational attraction of predominantly the moon and sun, and also solar radiation.

Meteorological effects (e.g. storm surges)

Several days to inter-annual and decadal.

Changes in sea level due to transfer of energy between ocean and atmosphere, due to the changes in air pressure (where a drop in barometric pressure slightly increases sea levels, and vice versa) and the stress of wind on the sea surface (where onshore wind drag sets up sea level, and vice versa).

Long term trends in sea level (including eustatic and isostatic changes in SL)

Long term changes observed over years to 10’s of thousands of years.

Long term changes in mean sea level in relation to a fixed point, or datum, due to changes in the ocean water volume (eustatic changes), and/or changes in the shape of the earth surface (isostatic changes).

2.2.2

Ocean tides

Ocean tides are the periodic rise and fall in sea levels, produced by the gravitational attraction of the sun and moon. Tide levels are also influenced by non astronomical forces collectively known as ‘environmental effects’, which include the influence of meteorological (i.e. weather) conditions on sea levels, as well as factors such as seabed bathymetry, water depth and coastal topography (UCAR, 2006; ICSM, 2011). The combination of astronomical forced tides and environmental effects on sea level produce tides that vary significantly in behaviour (i.e. range, timing and periodicity) throughout the world’s oceans and estuaries.

Astronomical tides The primary daily variations in ocean water levels are caused by the gravitational interactions between the sun, earth and moon, which create very long wave length waves that traverse the global oceans. These tidal waves have wave lengths of 100s to 1000s km’s in the open ocean, where the wave crest forms the high tide and the trough form the low tide. Tides periodicity can vary between diurnal tides: one high and one low tide per day; to semi diurnal tides: two high and two low tides per day. Astronomical tides are driven by the combined result of a number of physical astronomical forcing mechanisms, also known as tidal constituents. Each tidal

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania constituent has an individual periodicity that is constant across the globe, but varies in strength and timing depending on the geographic location. For example, the “principal lunar semi-diurnal� gravitational force (known as M2 tidal constituent) forms the most important astronomical force on the ocean tides which is commonly expressed as two high and two low tides each day. However differences in the timing and amplitude of the high and low tides produced by the principal lunar semi-diurnal constituent varies across the globe. Furthermore, differences experienced at any one location between consecutive high and low tides water levels occur due to the varying influences of the additional astronomically forced mechanisms. For example the regular larger tidal ranges (spring tides) occur once every two weeks on the full or new moon, when then Earth, Moon and Sun become aligned. Conversely, the regular smaller tide range of the same period (neap tides) occurs when the Earth, Moon and Sun are at positioned at 90 degrees (i.e. at half moon). A complete cycle of all the tide-producing forces, known as a tidal epoch, has a ~19 year return period (NOAA, 2012). Tide characteristics also vary with time and space due to a number of non astronomical effects, such as meteorological effects and sea floor bathymetry.

Meteorological (weather) tides Sea level elevations vary due to the meteorological effects of barometric pressure and wind, which can superimpose their effect on astronomical tide to increase or diminish the water levels (NOAA, 2012). Air pressure variations produce what is known as the reverse barometric effect, where increases in air pressure depress the ocean surface thus lowering water levels, and vice versa. Additionally, the frictional force of winds can increase water levels by literally pilling up the ocean in the down wind direction, most notably occurring when onshore winds push water into enclosed embayments. The most dramatic variations in meteorological tides occur during storm surges, where deep low pressure systems and prolonged onshore winds combine to significantly increase water level heights (Pugh, 1987).

Bathymetry and shallow water effects Tide behaviour is complex and affected by geographic factors such as seabed bathymetry, water depth and coastal topography. Tide form varies when traversing from the deep to shallow water; tide wave propagation slows, wave lengths shorten and amplitude increases (UCAR 2006). Conversely, tide waves become distorted when moving into shallower waters through restricted embayment openings with ranges diminishing and waters levels rising more rapidly rate than they fall (ICSM, 2011). Additionally, shallow water conditions also affecT-TIDE propagation as it becomes lagged due to friction with the seafloor (ICSM, 2011).

2.2.3

Tidal analysis and prediction

Sea level observations can be analysed in a way which enables future astronomical tides to be predicted. The most common method of tide analysis is called the harmonic method. This method analyses the tidal constituents that comprise the total astronomical tide for a particular location, based on simultaneous linear regression which solves the amplitudes and phases of the astronomical constituents for their set of known harmonic frequencies. This harmonic data (i.e. the frequencies, phases and amplitudes of the tidal constituents) can then be combined to simulate the astronomical tide (ICSM, 2011; Table 3, Figure 2). Longer observed tidal records allow higher numbers of tidal constituents to be solved, and more accurate predictions to be made. All tidal

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Tide observations and analysis constituents can only be solved when analysing observation records equal to, or greater than a full tidal epoch of approximately 19 years length. (NOAA, 2012). However, appropriate tidal analysis and predictions can still be made from analysing shorter observational datasets (C. Watson pers. comm., 2012), with a minimum of one month’s data required to permit proper tidal analysis (Forrester, 1983). Table 3: Major tidal constituents (modified from ICSM, 2011 and NOAA, 2000).

Harmonic constituent

Definition

Major semi-diurnal constituents M2

Principal lunar semidiurnal constituent Principal solar semidiurnal constituent Larger lunar elliptic semi diurnal constituent

S2 N2

Major diurnal constituents K1 O1 P1

Principle Lunisolar diurnal constituent Principle Lunar diurnal constituent Solar diurnal constituent

Figure 2: Schematic example of how a number of defined harmonic tidal constituents can be summed to produce a predicted astronomical tidal curve (modified from DONPS, 2012).

The harmonic analysis method also defines the contribution to the total tide record from environmental (i.e. non-astronomical) sources which equals the non linear component of the observed data. These sources mostly comprise weather effects (i.e. meteorological tides), but can also include other site specific environmental influences such as the time lag caused by friction due to shallow water conditions.

2.2.4

Tidal variability in far northern Tasmania

Far northwest Tasmania experiences diurnal tides which varies significantly in their range and time of arrival (Short, 2006; Figure 3). The northern-west coast receives microtides, with Granville Harbour experiencing a mean spring tide range of 0.9 m. This is in stark contrast to the western-north coast which receives meso-tides, with Stanley experiencing a mean spring tide range of 2.6 m from tides that arrives ~1 ¼ hrs after Granville Harbour (Short, 2006; tide timing obtained from the Bureau of Meteorology, Figure 3). The strong variation in Tasmania’s far northwesT-TIDEs results from the interactions between the offshore approaching tidal wave and the regional complex bathymetry. Tasmania’s tides initially arrive from the east and meet towards the western end of the north coast some 5 hours later. The tides first enter Bass Strait from the east, and subsequently refract around Tasmania’s south and west coast to finally enter the strait from the west (Short, 2006; Figure 3). The funnel like nature of Bass Strait’s shallow seafloor and its surrounding coastline geography leads to a significant increase in the tidal range toward the middle of the north coast. The strongest gradient in tides is experienced along the Tasmania’s far northwest shores where the tidal wave becomes substantially amplified over relatively short distances (Figure 3).

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Figure 3: The behaviour of Tasmania’s tides, showing co-range (left) and co-tidal (right) lines, where: co-range lines indicates the variation in spring tide range in metres; and co-tidal lines indicate the relative time of arrival. Note how: the tidal range is significantly amplified in Bass Strait with the gradient greatest in the far northwest; and the tide moves clockwise around Tasmania to enter Bass Straight in the west 5 hours later than in the east.

Sea level observation data defining the far northwest coasts tides is limited. Observations are routinely measured at Burnie and Devonport only and historically from Stanley. Prior to our research the actual range and timing of tides west of Stanley (where the gradient is likely greatest) was not studied.

2.2.5

Tidal measurements

Measuring sea level variation is a technical process, requiring specialist instrumentation and equipment. Of the four main types of technical equipment used for measuring sea levels (Table 4), pressure sensing systems are arguably the most affordable and versatile method for recording suitably accurate observational data. Pressure sensors can be used for sea level studies in a variety of ways. One such method includes deploying an absolute pressure sensor offshore in a stilling well. The sensor records absolute pressure variation through time, which is then converted to sea level based on the knowledge of the barometric pressure and seawater density. The stilling well provides the important function of increasing the accuracy of sea level observations by mechanically dampening out high frequency water level variations due to waves. We employed this method to monitor the tides at RP-BB, and it is reviewed in detail below. Table 4: The four fundamental types of sea level measuring technology, as described in IOC (2006). Methods for directly measuring sea level variation:  

Stilling well and float

Acoustic systems

Pressure systems

Radar Systems

Stilling wells Stilling wells are a technical requirement for a number of the sea level monitoring methods. Their function is ‘to still’ (i.e. mechanically filter out) the high frequency fluctuations in sea level (e.g. waves) to improve the accuracy of measuring the low frequency tidal signal (i.e. astronomical plus weather tides; IOC, 2006). Their use is important when measuring tides in open, to semi open waters which are commonly exposed to waves. Figure 4 below shows the damping effect of stilling wells on sea level processes of varying frequencies (i.e. astronomical tides, harbour seiches, and wave swell; modified from Forrester, 1983). Page 18 of 88


Tide observations and analysis

Figure 4: Damping effect of stilling well for sea level oscillations with a period of 12 hours (top), 6 minutes (middle) and 6 seconds (bottom), which are representative of varying sources of sea level variation including (diurnal) astronomical tides (top), harbour seiches (middle), and wave swell (bottom). Modified from Forrester (1983).

The stilling wells reduce wave activity by housing the water level logger in a vertical enclosure (i.e. the well), which is long enough to adequately cover the tidal range of the site, and has only limited access to the outside sea at its base (Forrester, 1983; IOC, 1985; IOC, 2006). This configuration acts as a ‘low pass’ filter by forcing the sea to slowly infiltrate into, and out of, the well, mechanically damping out the effect of high frequency processes such as waves. Detailed design requirements of stilling well are described in various technical manuals (e.g. Forrester, 1983) and are summarised in Table 5 and Figure 5.

Pressure sensor water level loggers Pressure gauges are commonly used for measuring sea level variation. This is achieved by deploying the pressure sensor in the sea, measuring the variation in total subsurface pressure (due to the combination of sea level and atmosphere) through time, and subsequently converted to the total pressure time series dataset to sea level. Converting the absolute pressure to sea level requires knowledge of barometric pressure, seawater density and gravitational acceleration, according to the law:

h=(p-pa)/(ρg) where

h = height of sea level above the pressure sensor p = measured pressure pa = atmospheric (barometric) pressure ρ = seawater density g = gravitational acceleration (IOC, 2006)

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania Table 5: Generic design requirements for pressure gauge stilling wells to mechanically dampen out wave action (modified from Forrester, 1983; IOC, 2006). Stilling well requirements: 

The well should stand vertical and extend below the lowest water level to above the highest water level (including wave action outside of well).

The well should be water tight over the portion that is submerged, except for the intake hole(s) at, or near the base of the well.

The intake holes should not be too close to the bottom that they may become blocked by sediments.

The well should be sturdily constructed and secured to minimise motion of the logger from wave action.

The top of the well should be vented, to equalise pressure differences between the inside and outside of the well.

The pressure transducer sensor must be secured in position, relative to a predefined stilling well datum over duration of survey.

Figure 5: Schematic description of a stilling well design for the measurement of sea levels with pressure transducer data logger, such as that required for the HOBO U20 water level loggers used in this study.

Atmospheric pressure and seawater density must also be measured and so an identical pressure sensor must be deployed in a position exposed to the open atmosphere and measure barometric pressure (IOC, 2006). Additionally, the elevation of the pressure sensor deployed in the sea must also be surveyed, to relate the measured water level data (which is relative to the pressure sensor) to a specific benchmark, or datum (e.g. the Australian Height Datum, or AHD).

2.3 Methods 2.3.1

Data collection

A number of stages were involved in collecting tide data from RP-BB. These included: selecting 3 key (and practical) survey locations; designing and constructing appropriate stilling wells to house the water level loggers; programming the data loggers; deploying the barometric and water level loggers (and their stilling wells); surveying the elevation of the water level logger pressure sensors; and periodically downloading the data loggers. These stages are detailed below.

Site selection We selected three key tidal survey sites and one barometric survey site from our target RP-BB region, based on their geographic location, accessibility and local infrastructure availability for stilling well attachment. Figure 6 shows the location of the data logger sites, including the three tide survey sites at ‘Howie Island’, ‘Kangaroo Island’ and ‘Welcome Inlet’, and the barometric survey site at ‘Stony Point’ (see Table 6).

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Tide observations and analysis Howie Island and Kangaroo Island water level sites for Robbins Passage were determined by the availability of navigation posts within the passage which were used to secure both the loggers and stilling wells to. These sites were accessed by boat. The Welcome Inlet site was located at the head of Boullanger Bay and was accessed on foot. This site had no available infrastructure for stilling well attachment. The onshore barometric site was placed at ‘Stony Point’ (Montagu) for accessibility reasons.

Figure 6: Logger location map, Robbins Passage-Boullanger Bay, Tasmania (image modified from Google Earth, accessed May 2012). Table 6: Tide survey site location details. Site name

Coordinates (GDA94)

Description

Howie Island (Robbins Passageeast)

328, 821 mE 5, 488, 136 mN

Attached to channel marker, immediately adjacent (north) Robbins Passage main easterly channel on shallow subtidal sandy seagrass flats. Approximately 700 m southeast of Howie Island.

Kangaroo Island (Robbins Passagewest)

318, 580 mE 5, 492, 736 mN

Attached to channel marker, immediately adjacent (east) Robbins Passage main north-westerly channel on shallow subtidal sand flats. Approximately 1 km east of Kangaroo Island.

Welcome Inlet (Boullanger Bay)

321, 250 mE 5, 490, 914 mN

Secured by a concrete anchor adjacent the Welcome River channel, on the eastern shallow sandy-muddy sub tidal seagrass flats. Located towards the mouth of the inlet, bordering Boullanger Bay.

Stony Point (Montagu)

328, 696 mE 5, 487, 140 mN

Housed in dry observation well secured below the ground surface, near sea level (<10 m), on the foreshore slopes between Stony Point and eastern Montagu Beach.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Water level loggers We measured tidal water levels with HOBO U20 Water Level Loggers (i.e. pressure gauges). Three tidal loggers were deployed offshore to measure total subsurface pressure and one barometric logger was positioned onshore to measure air pressure at ~sea level. All loggers were programmed to record pressure and temperature at 5 minute intervals, beginning at 09:00:00 Eastern Standard Time (EST) on 27/11/2011 (i.e. 23:00:00 Coordinated Universal Time, or UTC, on 26/11/2011), however the deployment of the Welcome Inlet logger failed, and was relaunched at 15:25:00 EST on 23/12/2011 (i.e. 05:25:00 UTC on the same date). Data was periodically retrieved onsite using a HOBO U-DTW-1 Waterproof Shuttle and 2B coupler. On downloading the data the logger’s memories were automatically cleared and relaunched using the existing survey parameters. On each data download the logger’s internal clocks were reset to match the shuttles clock, which was in turn synchronised with the host computer. Preceding the final logger launch, the shuttles clock was mistakenly set to local daylight saving time (i.e. EST + 1 hr), which resulted in the last data files for all the loggers being offset from their preceding files by + 1 hour. This error was corrected in the data processing stages; however the original HOBO data files have not, and cannot be corrected.

Stilling wells The three water level loggers deployed offshore were individually housed in stilling wells purposefully built for the local conditions. Each stilling well was designed and constructed to emulate the ‘high-tech’ stilling wells commonly used for long term monitoring of sea level (e.g. GLOSS network), whilst complying with the criteria in Table 7.

Table 7: Robbins Passage-Boullanger Bay remote stilling well design criteria. Robbins Passage-Boullanger Bay remote stilling well design criteria: 

Effectively filters high frequency variations in sea level (i.e. wave action), by including the stilling well requirements outlined in Figure 5 where practically possible.

Can be constructed from relatively cheap, and readily available materials

Is mobile (i.e. can be easily transported to remote location by small boat1 or foot2 and removed at completion of the survey)

Can be secured to navigation aid1 or sandy subtidal seafloor2

Is able to withstand local environmental conditions (i.e. semi-open coastal environment, strong tidal currents, meso-tidal sea level fluctuations)

Provides easy access to data logger for periodic retrieval, data downloads and redeployment

Has a small to no environmental footprint

1

Refers to Howie Island and Kangaroo Island locations; Table 6 for location details)

2

refers to Welcome Inlet locations (see Figure 6 and

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Figure 7: Remote navigation post stilling well at Howie Island and Kangaroo Island; (a) schematic design of a stilling well that extends above the highest water level and beneath the mean low water level; (b) a saddle clamp securing the well to the navigation post; (c) details of the low tide logger access with a â&#x20AC;&#x2DC;Y connectorâ&#x20AC;&#x2122; which allows the logger to be retrieved from part way up the 4.5 m stilling well; (d) the wellâ&#x20AC;&#x2122;s base, with 5 inlet holes and two protruding threaded rods which form a stable platform for the logger (and logger housing) to sit on when deployed. These rods also form the external datum for surveying the logger in with the DGPS.

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

Figure 8: Remote anchored stilling well at Welcome Inlet; (a) schematic design of a stilling well with self supporting concrete anchor secured to the seafloor with star pickets; (b) details of well cap, wire line and logger housing; (c) details of concrete anchor; (d) the Welcome Inlet survey site (looking north) located on the outer margins of the intertidal seagrass flats.

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Tide observations and analysis

Two different well designs were required, due to the variable local conditions, and infrastructure availability (or their lack of), at each survey site. The Howie Island and Kangaroo Island survey sites are relatively remote, located ~1 km and ~16 km by boat from Stony Point located on marginal inter-subtidal sand flats, adjacent the eastern and western Robbins Passage channel, respectively. The stilling wells for both these sites were designed identically, to be affixed to their available navigation posts (Figure 7). The remote Welcome Inlet survey site is located 100 m from the adjacent shoreline, on marginal inter-subtidal sandy seagrass flats. No infrastructure was available in this region to secure the stilling well to, thus we designed the well to be self supporting (Figure 8). Building a self supporting stilling well which extended the full range of the tide would have required a significant investment of time, money and infrastructure that was outside the scope of the project. As such, a relatively cheap, self supporting, low environmental impact well was alternately designed that extended to mid tide height only. The two stilling well designs are described below.

Remote navigation post-stilling well design: Howie and Kangaroo Island The Howie Island and Kangaroo Island stilling wells (Figure 7) are constructed from 2 inch diameter ABS pipe and ABS pipe fittings and joined with ABS cement. They consist of an outer, vertical 4.5 m stilling tube with an adjoining pipe and screw cap located part way up the well for low tide access of the logger. The wells are mostly water/air tight, with the exception of five 6 mm Ø basal inlet holes, and an air pressure vent on the access cap and at top of the well. The data logger is housed in an inner 178 mm section of 1 inch Ø ABS pipe which is in turn deployed within the stilling well. The logger sits in the housing on a bottom screw and is secured by a zip tie. The housing is attached to the outer wells low tide access cap via a wire line, connected to an upper screw in the housing and an eye bolt on the access cap, allowing for easy periodic retrieval, download and redeployment of the logger. The housing sits near the bottom of the well on two 200 mm lengths of 6 mm Ø threaded rods which protrude perpendicularly from the well to one side, serving as the stilling wells fixed datum, which can be easily surveyed at low tide. All the hardware components of the stilling well and logger housing are marine grade stainless steel (SS-316). The well is attached to the navigation pole with a series of galvanised steel saddle clips for 2 inch Ø pipe.

Remote anchored-stilling well design: Welcome Inlet The Welcome Inlet stilling well (Figure 8) is identical to that described above, with the exception of the vertical outer well being 1.6 m in height only due to practical limitation of the site; the logger and housing are accessed by the vented screw cap at the top of the well and the well is secured to a ~25 kg concrete anchor at its base. The anchor is a rectangular prism, with two bent protruding reinforced steel bars handles and the wells basal cap concreted into place. Note that this well does not extend above the highest water levels, and thus its ability to dampen out the effect of wave action is reduced.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Logger elevation survey GPS equipment and software The water level logger’s elevation was surveyed using differential geodetic grade Leica Viva GPS receivers and AS10 antennas (with quoted accuracy of 10 mm + 2 ppm). Measured GPS data were processed with Leica GeoOffice version 7 software (LGO), using AUSGeoid09 to calculate Australian Height Datum (AHD) heights from the GPS derived ellipsoidal heights (http://www.ga.gov.au/ausgeoid/nvalcomp.jsp). This data was additionally processed by AUSPOS (http://www.ga.gov.au/earthmonitoring/geodesy/auspos-online-gps-processing-service.html) for a less accurate but independent verification of the LGO processing.

Differential GPS survey Standard differential GPS (DGPS) methods were applied to measure the tide loggers elevation, with a reference base station GPS setup measuring an onshore benchmark (Control Point 730/70 at Stony Point Caravan Park, Montagu), whilst a rover GPS was simultaneously measuring the offshore tide loggers. GPS data was recorded at 2-second interval in standard RINEX format. The rover GPS antenna was mounted on a pole and secured to the stilling well datum using a purpose built mounting bracket. This pole was rigidly fixed to the navigation post to eliminate movement of the GPS antenna (Figure 9).

Figure 9: Differential GPS (DGPS) elevation survey of water level loggers at (a) Kangaroo Island and (b) Welcome Inlet. The DGPS pole mounting bracket (c) secured the roving GPS antenna to the stilling wells (and data loggers) external survey datum.

Tide loggers at Kangaroo Island and Welcome Inlet were surveyed for 1 hour over a baseline of 12 and 17 km respectively, and the Howie Island tide logger was measured over a 1 km base line for 16 minutes1 only due to unavoidable time and weather limitations experienced. All ambiguities in processing the GPS data in LGO were resolved for each survey. Position of the onshore reference benchmark 730/70 was subsequently determined by differential connection to a GPS Primary Control Point at the Smithton Airport (ST1087). Over a 13 km baseline, this connection was measured for 4.5-hours and processed in LGO using AUSGeoid09 for a AHD height determination.

1

The 16 minute measuring period for Howie Island didn’t allow for a comparison to an AUSPOS solution.

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Tide observations and analysis

Potential sources of error Excluding gross errors, potential sources of error in surveying of the logger heights are attributed to systematic and random errors associated with height datum’s and the GPS equipment. We estimate an error margin of ±0.138 m for the surveyed water level loggers AHD heights. These errors are discussed below. All heights were surveyed in the Australian Height Datum (AHD). The AHD is Australia’s official national height datum and is classed as a 3rd Order Network only, as it has inherent known systematic errors (Featherstone et. al., 2010; Featherstone and Kuhn, 2006). The AHD was defined in 1971 by the simultaneous adjustment of 97,230km of two-way levelling and setting mean sea-level height of 30 tide gauges Australia wide to a height of 0.000m (ICSM, 2009). Known errors in the AHD include gross errors with the two-way levelling, using limited tidal data (2-3 years), not accounting for ocean temperature differences between northern and southern Australia (introduces a ~1 m tilt in the north-south direction), uncertainties of measuring gravity, and associated issues with developing a reliable geoid model (Featherstone et. al., 2010; Featherstone and Kuhn, 2006). Brown (2010) estimates an accuracy of ±0.050m across most of Australian using the AUSGeoid09 model to convert ellipsoidal heights derived from GPS measurements to AHD heights. This newly available geoid model (released in April 2011) provides greater accuracy than preceding models for converting GPS measurements to AHD and combines the Earth Geopotential Model 2008 (EGM2008; Pavlis, 2012) with Australian land gravity data and a ‘correction grid’ between AHD and the gravimetric geoid (Figure 10).

Figure 10: The various height datums used to compute AUSGeoid09, using the following equation: O = h – Nag – Hahd, where O = the difference between the AHD and gravimetric geoid; h = ellipsoidal height from GPS observations; Nag = gravimetric geoid-ellipsoid separation; Hahd = AHD height (modified from Brown, 2010).

Instrumental errors were minimised by using high accuracy equipment (10 mm + 2 ppm). Issues associated with excessive baseline distances were minimised by using the 730/70 as a reference GPS base station, approximately mid-way between the primary GPS control point at Smithton Airport and the far field sites at Welcome Inlet and Kangaroo Island. Additionally, GPS data was measured at a high frequency and sufficiently long periods to ensure satisfactory processing of the data in LGO. Over the maximum baseline length of 17 km for Welcome Inlet, a horizontal and vertical accuracy of the order of ±0.044 m and ±0.088 m is expected. Combined with a ±0.050m error associated in converting ellipsoidal heights derived from GPS to AHD heights using AUSGeoid09, the accuracy of AHD heights of the water level loggers is ±0.138 m.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

2.3.2

Data analysis

Data processing All individual data logger time series datasets were converted to Coordinated Universal Time (UTC) to correct for the time offset error (UTC+10 to UTC+11) which occurred between the second and third download. The Howie Island, Kangaroo Island and Welcome Inlet total pressure data was converted to water levels relative to their pressure sensor using HOBOware Pro Barometric Compensation Assistant and the Montagu barometer dataset. The default salt water fluid density input of 1025 kg/m3 was used for this conversation. In reviewing the literature for available data on the locally measured salinity levels we found that this default fluid density was sufficient2, with minor variations between the local salinity levels and the default salinity density found to have an insignificant influence on the calculated water levels (i.e. < 0.1 cm). Corrected water level data files were merged and exported for subsequent tidal harmonic analysis.

Tidal analysis – Harmonic constituents and tidal predictions Dr Chris Watson (UTAS) conducted a tidal analysis on the corrected observational water level data, using T-TIDE, a MATLAB based harmonic analysis and prediction program which uses a least squared approach to solve the amplitude and phase (and compute error estimates) for the known tidal constituent frequencies that could be determined for each data set . Signal to noise ratios (e.g. amplitude/amplitude error) were determined for each solved tidal constituent, and those with values less than 1 were discarded. The astronomical tide was subsequently modelled for the duration of the observed data in T-TIDE by summing the solved tidal constituents (see Figure 2 for schematic example of this process). The predicted astronomical tide was then subtracted from the observed total tide to compute the non-tidal residual (i.e. the non-linear environmental effects on the observed total tide, including meteorological tides and local physical effects such as bathymetry). Additional 10 year tide predictions (until June 1, 2022) for each site were computed using their determined tidal constituents.

Tidal classification – harmonic based classification The common harmonic based method for classifying tides, known as the ‘form factor’, was used to characterise the tides for each site (Table 8). Table 8: Harmonic based Form factor scheme for used to classify Robbins Passage – Boullanger Bay tides (as outlined in the Australian Tide Manual; ICSM, 2011). Form factor (“F”) – harmonic tide classification scheme F = (K1 + O1) / (M2 + S2) where: F = form factor, and K1, O1, M2 and S2 = amplitudes of the 4 main tidal constituents, as derived from the tidal analysis. when: F < 0.5, the tides are considered semi-diurnal, and F > 0.5 the tides are considered diurnal note: Semi-diurnal tides = 2 high and low tides/day, and Diurnal tides =1 high and low tide/day. 2

Edgar et al (1999) measured a single surface salinity at Welcome Inlet in February, 1997 at 34.8‰, equalling a fluid density between ~1027 to ~1025 kg/m3 at 10 to 20 °C, and sixteen salinity samples from 4 sites in Robbins Passage at Montagu measured by DHHS (2011) through 2010 averaged 33.6‰, equalling a fluid density of ~1026 and ~1024 kg/m3 at 10 to 20 °C.

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Tide observations and analysis

Tidal planes The various tide levels which can be derived from sea level datasets (e.g. high water, mean sea level) are collectively known as tidal planes. Tidal planes were defined for each site, based on the observation-based, harmonic-based and prediction-based definitions as outlined in, or modified from, the Australian Tide Manual (ICSM, 2011). Observation-based definitions are the only universally accepted method of defining tidal planes, however they require “sufficiently long” periods (preferably 19 years) of sea level observations to adequately define each level (ICSM, 2011). The use of short records (such as our RP-BB dataset) may result in an over or underestimation of the tidal planes due to the variable influence of seasonal, annual or inter-annual environmental effects on short term sea level records. Harmonic-based tide plane definitions are considered “convenient simplifications” of these levels and are based on tidal harmonics (such as those we derived in our RP-BB tidal analysis). These definitions are commonly used in Australia, however they assume that M2, S2, O1 and K1 are the dominant four astronomical components of the local tide, which is not necessarily true for all locations (ICSM, 2011), and was also not found to be the case at RP-BB. Due to issues associated with applying the above methods to our RP-BB tide data, we developed alternate prediction-based tidal plane definitions, which include applying the observation-based definitions to sufficiently long periods of predicted sea levels (≥5 years). Details of the observation-based, harmonic-based and prediction-based definitions we applied to the RP-BB data are outlined in Table 9.

Table 9: Tidal planes. Tidal Plane

Definition 1 observation-based 2 harmonic-based 3 prediction-based

Method applied in this study

Notes

HAT3

Highest Astronomical Tide3: The highest level of water which can be predicted to occur under any combination of astronomical conditions.

Highest astronomical water level predicted from 20 year period of predicted astronomical tides modelled from the harmonic constituents defined in this study (only).

Our results are indicative only and underestimate the true HAT levels, as not all astronomical constituents were solved in our tidal analysis, hence our tide predictions do not model ‘any (possible) combination of astronomical conditions’.

ISHW2

Indian Spring High Water2: ISHW = Z0 + (M2 + S2 + K1 + 01); where Z0 = mean sea level and M2, S2, K1, 01 = their usual harmonic constituents.

Same as the definition

This is a tidal plane designed for regions with mixed tides; ISPW * 2 is the standard NTC Australian tide range model, and is used to map SL inundation footprints in Tasmania (e.g. Mount et al., 2010b, Lacey et. al. 2012).

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania MHWS1

Mean High Water Springs1: The average of all high water observations at the time of spring tide over a sufficiently long period of time.

The average of all spring high water observations over our ~5 month period of data collection.

Our results are indicative only, due to the relatively short time period of observations used in this analysis.

MHWS2

Mean High Water Springs2: MHWS = Z0 + (M2 + S2); where Z0 = mean sea level and M2 & S2 = their usual harmonic constituents.

Same as the definition.

Our results are indicative only, as the harmonic based definitions assume that M2, S2, O1, and K1 are the dominant four components, which is not the case at RP-BB.

MHWS3

Mean High Water Springs3: The average of all predicted high waters at the time of spring tide over a sufficiently long period of time.

The average of all the high waters predicted at the time of spring tide over a 5 year period.

Our results are indicative only, as not all astronomical constituents were solved in our tidal analysis, hence our tide predictions do not model every combination of astronomical conditions.

MHW1

Mean High Water1: The average of all high waters observed over a sufficiently long period.

The average of all high water observations over our ~5 month period of data collection.

Our results are indicative only, due to the relatively short time period of observations used in this analysis.

MHW3

Mean High Water3: The average of all high waters predicted over a sufficiently long period.

The average of all the predicted high water over a 5 year period.

Our results are indicative only, due to issues previously noted in MHWS3 regarding tide predictions derived from limited harmonics.

MHWN2

Mean High Water Neap2: MHWS = Z0 + (M2 - S2); where Z0 = mean sea level and M2 & S2 = their usual harmonic constituents.

Same as the definition.

Our results are indicative only, due to the issues regarding the harmonic definitions previously noted under MHWS2.

MHWN3

Mean High Water Neap3: The average of all predicted high waters at the time of neap tide over a sufficiently long period.

The average of all the predicted high waters at the time of neap tide over a 5 year period.

Our results are indicative only, due to issues previously noted in MHWS3 regarding tide predictions derived from limited harmonics.

MSL1

Mean Sea Level1: The arithmetic mean of hourly heights of the sea at the tidal station observed over a period of time (preferably 19 years).

The average of all sea level observations over our ~5 month period of data collection.

Our results are indicative only, due to the relatively short time period of observations used in this analysis.

MLWN2

Mean Low Water Neap2: MLWN = Z0 - (M2 - S2); where Z0 = mean sea level and M2 & S2 = their usual harmonic constituents.

Same as the definition.

Our results are indicative only, due to the issues regarding the harmonic definitions previously noted under MHWS2.

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Tide observations and analysis MLWN3

Mean Low Water Neap3: The average of all predicted low waters at the time of neap tide over a sufficiently long period.

The average of all the predicted low waters at the time of neap tide over a 5 year period.

Our results are indicative only, due to issues previously noted in MHWS3 regarding tide predictions derived from limited harmonics.

MLW1

Mean Low Water1: The average of all low waters observed over a sufficiently long period.

NA

This level cannot be extracted from our data, as our tide loggers were deployed in shallow conditions above the lowest water levels and thus these levels were not observed.

MLW3

Mean Low Water3: The average of all low waters predicted over a sufficiently long period.

The average of all predicted low water observations over a 5 year period.

Our results are indicative only, due to issues previously noted in MHWS3 regarding tide predictions derived from limited harmonics.

MLWS1

Mean Low Water Springs1: The average of all low water observations at the time of spring tide over a sufficiently long period.

NA

This level cannot be extracted from our data, as our tide loggers were deployed in shallow conditions above the lowest water levels and thus these levels were not observed.

MLWS2

Mean Low Water Springs2: MLWS = Z0 - (M2 + S2); where Z0 = mean sea level and M2 & S2 = their usual harmonic constituents.

Same as the definition.

Our results are indicative only, due to the issues regarding the harmonic definitions previously noted under MHWS2.

MLWS3

Mean Low Water Spring3: The average of all predicted low waters at the time of spring tide over a period of time (preferably 19 years).

The average of all the low waters predicted at the time of spring tide over a 5 year period.

Our results are indicative only, due to issues previously noted in MHWS3 regarding tide predictions derived from limited harmonics.

ISLW2

Indian Spring Low Water2: ISPW = Z0 â&#x20AC;&#x201C; (M2 + S2 + K1 + 01); where Z0 = mean sea level and M2, S2, K1, 01 = their usual harmonic constituents.

Same as the definition

See ISHW notes.

LAT3

Lowest Astronomical Tide3: The lowest level of water which can be predicted to occur under any combination of astronomical conditions.

Lowest astronomical water level predicted from 20 year period of predicted astronomical tides modelled from the harmonic constituents defined in this study (only).

Page 31 of 88

Our results are indicative only and overestimate the true LAT level, (thus underestimating the true tide range) due to the tide prediction issues notes in HAT3.


Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

2.4 Results 2.4.1

Observed tides

Selected site location and tide characteristic data for Howie Island, Kangaroo Island and Welcome Inlet are given in Table 10, and the observed total tides for these sites are presented in Figure 11 â&#x20AC;&#x201C; 13. Table 10: Selected data associated with water level observations. Howie Island

Kangaroo Island

Welcome Inlet

Easting (MGA)

328,821 mE

318,580 mE

312,250 mE

Northing (MGA)

5,488,136 mN

5,492,736 mN

5,490,914 mN

Start date + time (UTC + 0hrs)

26/11/2011 23:00:00

26/11/2011 23:00:00

23/12/2011 05:30:00

No. obs. (5 mins)

47016

46755

39282

Period obs. (days)

163.25

162.34

136.4

Max obs. height (m > AHD)

1.792

1.451

1.318

Mean obs. height (m > AHD)

0.009

0.018

0.001

Min obs. Height1 (m > AHD)

-1.409

-1.261

-0.947

Pressure sensor height1 (m > AHD)

-1.390

-1.226

-0.920

1

Note the minimum observed water levels pressure sensor height, as these loggers were not deployed below the lowest water levels. Minor discrepancies (up to ~3.5 cm) are likely due to the artefacts associated with using regional barometric pressure to correct local total pressure to water levels.

2.4.2

Tide analysis and prediction

Harmonic analysis conducted on the observed sea level datasets solved 21 tidal constituents for both Howie and Kangaroo Island (Table 11 and Table 12), and 23 tidal constituents for Welcome Inlet (Table 13). The Principal Lunar semidiurnal constituent, M2, was found to have the greatest amplitude for each site, accounting for ~50 % of the astronomical tidal variation. Higher order-shallow water harmonics (such as M4) were also found to be significant for all three sites and accounted for at least 14 % of the astronomical tides at Welcome Inlet. The predicted tides, formed by combining the solved tidal constituents, accounted for ~97 % of the total observed tidal variability at RP-BB (Figure 11, Figure 12 and Figure 13). The remaining 3 % of observed total tide variability not accounted for is largely due to the environmental effects on sea levels and is defined by the non-tidal residuals computed for each site (see below). The standard deviation of the non-tidal residual is ~12 cm for all three sites and the maximum variation in observed sea levels from the predicted tide was -67.7 cm at Welcome Inlet.

Page 32 of 88


Tide observations and analysis

Figure 11: Observed sea levels and predicted astronomical tides (top), and environmental effects (i.e. non tidal residual; bottom) for Howie Island, Robbins Passage-east.

Figure 12: Observed sea levels and predicted astronomical tides (top), and environmental effects (i.e. non tidal residual; bottom) for Kangaroo Island, Robbins Passage-west.

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

Figure 13: Observed sea levels and predicted astronomical tides (top), and environmental effects (i.e. non tidal residual; bottom) for Welcome Inlet, Boullanger Bay. Table 11: Physical attributes for the 21 tidal constituents solved for Howie Island, derived from the harmonic analysis of 163 continuous days of observed tide data. Frequency

Amplitude

Amplitude error

M2

0.0805114

1.0871

0.012

73.2

0.6

8.00E+03

N2

0.0789992

0.2115

0.013

48.57

3.67

2.80E+02

S2

0.0833333

0.1399

0.012

218.26

5.4

1.30E+02

Tidal constituent

Phase

Phase error

Signal to noise ratio

K1

0.0417807

0.1316

0.013

314.8

6.54

1.00E+02

O1

0.0387307

0.0836

0.014

299.67

10.18

34

M4

0.1610228

0.083

0.005

81.72

3.79

3.00E+02

L2

0.0820236

0.0718

0.012

82.97

8.33

38

MU2

0.0776895

0.0596

0.011

44.49

10.11

31

MN4

0.1595106

0.0365

0.005

59.86

7.82

52

MK3

0.1222921

0.0229

0.004

336.89

11.49

28

M6

0.2415342

0.0219

0.002

268.38

6.33

1.00E+02

MS4

0.1638447

0.02

0.005

223.03

15.34

18

EPS2

0.0761773

0.0184

0.013

20.04

32.97

2.1

Q1

0.0372185

0.0167

0.013

293.36

46.6

1.6

MO3

0.1192421

0.0149

0.004

338.47

16.33

12

2MN6

0.2400221

0.0135

0.002

227.13

10.89

31

2MS6

0.2443561

0.0097

0.003

54.33

12.62

14

M3

0.1207671

0.0081

0.005

209.55

30.91

2.9

2MK5

0.2028035

0.0073

0.003

37.17

20.66

7.2

M8

0.3220456

0.0048

0.001

269.21

13.01

24

3MK7

0.2833149

0.0021

0.001

121.32

30.81

4.5

Page 34 of 88


Tide observations and analysis Table 12: Physical attributes for the 21 tidal constituents solved for Kangaroo Island. derived from the harmonic analysis of 162 continuous days of observed tide data. Tidal constituent

Frequency

Amplitude

Amplitude error

Phase

Phase error

Signal to noise ratio

M2

0.0805114

0.8753

0.01

75.24

0.71

7.40E+03

N2

0.0789992

0.1693

0.01

49.7

4.05

2.80E+02

K1

0.0417807

0.1154

0.013

295.91

6.52

84

S2

0.0833333

0.1139

0.012

195.52

5.92

93

O1

0.0387307

0.0824

0.013

284.48

8.78

39

L2

0.0820236

0.0486

0.011

89.4

14.95

19

M4

0.1610228

0.039

0.004

107.92

5.71

1.00E+02

MU2

0.0776895

0.0385

0.01

40.99

16.42

15

M6

0.2415342

0.0348

0.003

294.44

5.1

1.30E+02

Q1

0.0372185

0.0204

0.013

265.1

32.3

2.5

2MN6

0.2400221

0.0199

0.003

252.1

9.56

41

MN4

0.1595106

0.0178

0.004

87.17

13.38

16

2MS6

0.2443561

0.0139

0.003

88.6

12.87

21

EPS2

0.0761773

0.013

0.012

10.47

49.95

1.2

MK3

0.1222921

0.0111

0.002

31.71

11.34

25

MO3

0.1192421

0.0065

0.002

50.45

20.1

7.4

MS4

0.1638447

0.0061

0.004

276.96

34.88

2.1

2MK5

0.2028035

0.0046

0.002

19.58

31.93

3.5

M3

0.1207671

0.004

0.002

233.79

31.75

2.5

SK3

0.1251141

0.0025

0.002

228.1

60.6

1.2

3MK7

0.2833149

0.0024

0.001

178.36

39.45

2.7

Table 13: Physical attributes for the 23 tidal constituents solved for Welcome Inlet. derived from the harmonic analysis of 136 continuous days of observed tide data. Tidal constituent

Frequency

Amplitude

Amplitude error

Phase

Phase error

Signal to noise ratio

M2

0.0805114

0.797

0.01

81.26

0.77

6.00E+03

N2

0.0789992

0.1517

0.01

59.26

3.58

2.40E+02

S2

0.0833333

0.1041

0.011

202.9

5.19

93

K1

0.0417807

0.0946

0.012

299.85

7.51

62

M4

0.1610228

0.0945

0.006

103.05

3.66

2.30E+02

O1

0.0387307

0.0664

0.012

286.05

10.3

29

L2

0.0820236

0.0527

0.009

84.03

12.24

37

MN4

0.1595106

0.0423

0.007

82.11

8.84

42

M6

0.2415342

0.0356

0.004

345.21

6.9

76

MU2

0.0776895

0.0344

0.009

46.85

18.74

13

MK3

0.1222921

0.0216

0.004

357.29

13.42

26

MS4

0.1638447

0.0191

0.007

230.54

19.68

7

2MS6

0.2443561

0.0189

0.005

125.54

13.17

16

2MN6

0.2400221

0.0188

0.004

313.31

13.09

20

Q1

0.0372185

0.0171

0.013

268.2

45.41

1.8

MO3

0.1192421

0.0158

0.005

352.08

15.5

11

M8

0.3220456

0.0153

0.002

341.84

9.76

46

EPS2

0.0761773

0.0116

0.009

27.17

50

1.6

M3

0.1207671

0.0065

0.004

199.88

40.8

2.5

2MK5

0.2028035

0.0058

0.002

301.97

23.95

5.5

3MK7

0.2833149

0.0057

0.002

211.05

21

6.3

2SM6

0.2471781

0.0042

0.004

267.56

57.21

1.1

2SK5

0.2084474

0.0021

0.002

118.33

62.68

1

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

2.4.3

Tide characteristics

Tidal Planes The RP-BB region receives strongly semi-diurnal (F ≤ 0.2) meso-tides, with Howie Island (Robbins Passage-east) receiving the greatest predicted maximum tide range of 3.15 m, compared with 2.63 m at Kangaroo Island (Robbins Passage-west) and 2.42 m at Welcome Inlet (bay head of Boullanger Bay). Observed3 and predicted mean spring tide ranges were found to be 2.97 and 2.80 m at Howie Island, 2.37 and 2.20 m at Kangaroo Island and 2.22 and 2.09 m at Welcome Inlet. A greater number of tidal planes derived for these three sites by observation-based, harmonic-based and prediction-based definitions are detailed in Table 14, with key tide ranges summarised in Table 15. Table 14: Tidal planes of Robbins Passage – Boullanger Bay, derived from observation-based1, harmonic-based2 and prediction-based3 definitions. Tidal Plane HAT3

Howie Island

Kangaroo Island

Welcome Inlet

(m > MSL)

(m > AHD)

(m > MSL)

(m > AHD)

(m > MSL)

(m > AHD)

1.512

1.521

1.296

1.278

1.186

1.187

2

1.442

1.451

1.187

1.169

1.062

1.063

1

1.484

1.493

1.187

1.169

1.110

1.111

2

1.227

1.236

0.989

0.971

0.901

0.902

3

MHWS

1.375

1.384

1.169

1.151

1.047

1.048

1

1.200

1.209

0.958

0.940

0.910

0.911

MHW3

1.180

1.189

0.944

0.926

0.873

0.874

MHWN

2

0.947

0.956

0.761

0.743

0.693

0.694

MHWN

3

1.006

1.015

0.734

0.716

0.714

0.715

ISHW

MHWS MHWS MHW

MSL

1

0.000

0.009

0.000

-0.018

0.000

0.001

MLWN

2

-0.947

-0.938

-0.761

-0.779

-0.693

-0.692

MLWN

3

-0.833

-0.824

-0.578

-0.596

-0.597

-0.596

MLW

1

NA

NA

NA

NA

NA

NA

MLW

3

-1.110

-1.101

-0.891

-0.909

-0.808

-0.807

1

NA

NA

NA

NA

NA

NA

2

-1.227

-1.218

-0.989

-1.007

-0.901

-0.900

3

-1.421

-1.412

-1.031

-1.049

-1.047

-1.046

2

- 1.442

-1.433

-1.187

-1.205

-1.062

-1.061

-1.639

-1.630

-1.330

-1.348

-1.236

-1.235

MLWS MLWS MLWS ISLW

3

LAT

Table 15: Key tide ranges for Robbins Passage – Boullanger Bay, derived from harmonic-based2 and prediction-based3 tide planes. Tidal Plane Total tide range3 Indian Spring Water tide range Mean Spring tide range Mean Neap tide range

3

3

2

Howie Island (m)

Kangaroo Island (m)

Welcome Inlet (m)

3.151

2.626

2.422

2.866

2.410

2.122

2.796

2.200

2.094

1.839

1.312

1.311

3

The observed mean spring range equals double the observed MSHW, as the observed MSLW could not be defined from the observed data.

Page 36 of 88


Tide observations and analysis

Timing of the tides Analysis of the predicted high and low tides show the timing of the RP-BB tides to be variable (Table 16). Howie Island is first to receive high tide, followed by Kangaroo Island and Welcome Inlet some 20 minutes later. Low tide is generally experienced concurrently at Howie Island and Kangaroo Island, and is followed 40 minutes later at Welcome Inlet. Howie tides occur approximately 55 (±5) minutes after Stanley. Table 16: Relative timing of the tidal wave at Robbins Passage-Boullanger Bay. Arrival time after Howie Island

Kangaroo Island

Welcome Inlet

High Tide

Low Tide

High Tide

Low Tide

Maximum (minutes)

30

30

35

60

Minimum (minutes)

10

-20

05

10

Average (minutes ± 1sd)

20 (±05)

00 (±10)

20 (±05)

40 (±10)

All three measured locations experience shorter flood times than ebb times (Figure 14) due to their shallow water conditions slowing the speed of the receding tidal wave trough. This behaviour is most pronounced at Welcome River where the rate of water level change during the ebbing tide abruptly slows at -0.5 m AHD (in water depths of around 40 cm).

Figure 14: One and a half days of selected observational sea level data characteristic of average tidal conditions (top), detailing the variation in timing, range and influence of shallow water (and other environmental effects) on tides at Howie Island (red), Kangaroo Island (blue) and Welcome Inlet (green). Time lines of the semi-diurnal flood-ebb tidal period (~12.4 hours) spanning 28-29 march, 2012 are shown in the time line (bottom). Note the asymmetrical nature of all three tidal curves, notably Welcome Inlets, where the frictional force of shallow water conditions on the receding tide wave lags the timing of ebb tide (see arrow).

Page 37 of 88


Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

2.5 Discussion The ~5 months of sea level observations collected from Robbins Passage-Boullanger Bay in this study provides the first detailed measurement of the regions tidal characteristics and behaviour. These tide observations show that the tide range, timing and tidal constituents at RP-BB are geographically variable. In defining the tidal planes for RP-BB, we applied a variety of methods as neither the commonly used observation-based nor harmonic-based definitions were absolutely suited to our dataset. The universally excepted observation-based method requires long periods of data, and thus our short period of observations may have under- or overestimated the true tidal plane heights due to short term environmental effects skewing the results. Also, the harmonic-based definition assumes that the tidal constituents K1, O1, M2 and S2 are the four primary tidal constituents, which we found to not be the case for RP-BB. Due to these potential issues we also applied prediction-based methods to define the RP-BB tide plane heights. Furthermore, we suggest that these heights are likely the best approximation of the true tidal planes, as they are not affected by the issues associated with the observation- and harmonic-based definitions. Additionally, we found the prediction-based levels to form the median values for those tidal planes where all three definitions were applied4. The average difference between coincidenT-TIDE planes heights produced by varying definitions is 10 cm, ranging from a minimum difference of 1 cm to a maximum of difference 26 cm 5. The tide planes we produced for RP-BB show the regions tide range to increase from west to east, with a predicted mean spring tide range of 2.09 at the head of Boullanger Bay (Welcome Inlet) increasing to 2.80 m in the eastern channel of Robbins Passage (Howie Island). This increase from west to east is consistent with the regional gradient experienced across the greater northwest coast which sees the Indian Spring Water tide range becoming amplified from micro-tides on the west coast (‘standard’ NTC tide range < 1.5 m) to meso-tides across the mid north coast (‘standard’ NTC tide range > 3.0). In comparing our RP-BB tide ranges with the NTC tide range models we found their modelled data for the eastern region of Boullanger Bay to underestimate the tide ranges by ~30% when compared to our adjacent observational sites at Welcome Inlet and Kangaroo Island. Here our ‘standard’ and total tide range is 2.12 and 2.42 m for Welcome Inlet and 2.41 and 2.62 m for Kangaroo Island, compared with the closest NTC modelled range of 1.53 and 1.72 m. The smaller tide range for this modelled point of concern was likely skewed by the historical observational data from the relatively nearby Stack Island which likely experiences micro-tidal conditions similar to the adjacent west coast. We however have found eastern Boullanger Bay to experience meso-tides, which are characteristic of Tasmanian north coast. The management implications arising from the differences between these two datasets are discussed below (see Section ‎2.6: Management applications). Inferences can be made on the hydrodynamics of the RP-BB region based on the timing of our observed tides. As expected, the comparable timing of high tides in the eastern and western channels of Robbins Passage (at Howie Island and Kangaroo Island) 4

The range of mean spring water levels for derived Kangaroo Island demonstrate how the prediction-based tide planes forms the median value, where the observation-, prediction- and harmonic-based levels are 1.17, 1.15 and 0.97 m, respectively. 5 The minimum difference between coincidenT-TIDE planes heights was between the observationbased and prediction-based MHW level at Kangaroo Island; whereas the maximum difference was between the observation-based and harmonic-based MHWS level at Howie Island.

Page 38 of 88


Tide observations and analysis indicates that two separate tidal waves enter the passage from opposite ends and meet in the middle of the passage. The coincident arrival times of the tidal wave at Welcome inlet at the head of the bay, and Kangaroo Island in the far east of the bay broadly suggests that Boullanger Bay experiences a broadly SSE propagating tidal wave front. This may indicate that the tides here are sourced from a combination of a southerly propagating wave sourced from Bass Strait between Hunter Island and Walker Island and an easterly propagating wave, sourced from west coast between Woolnorth point and Hunter Island, however more tide observations are required to constrain this hypothesis (see Section 2 ‎ .8: Future work). The distorted-asymmetrical RP-BB tide curves shows that the regions tides experience more rapid rise times than fall times. This is due RP-BB’s coastal setting and bathymetry which produces stronger flood currents and lagged ebbing tides due to the frictional forces of the shallow water conditions (Pugh, 2004). The Welcome InleT-TIDEs experience the most asymmetrical tidal signal of all of our three observational sites, with the timing of ebb tide lagging 40 minutes behind both Howie Island and Kangaroo Island. Here the falling tide is notably distorted due to shallow water conditions at -0.4 m AHD where the rate of change in water level abruptly deceases in water depths of ~0.75 m (Figure 14).

2.6 Management applications The tide data produced from our observations has a number of management applications. These are summarised below: 

Tide planes – the improved understanding of the regions tide range will allow more accurate projections of sea level rise to be made and mapped onto the adjacent foreshore of the area. This will assist in more precisely defining sea level rise inundation footprints for the region (see below).

Tide observations and tidal constituents – can be used in future tide studies, including as input into 2D (and 3D) hydrodynamic modelling. Such modelling would provide valuable information about a large number of physical and ecological processes, including: o Nutrient and toxicant concentrations (dilution rates, flushing time)

o

Water quality modelling (algal bloom modelling)

o

Geomorphological studies (sediment transport e.g. through Robbins Passage)

o

Habitat responses (seagrass depth ranges, wetting and drying regimes)

Tide predictions – can be used for a multitude of purposes, including for persons and authorities engaged in recreation, tourism and marine resource related industries to name a few.

2.6.1

Improved tide range

The tide ranges we defined for RP-BB show that adjustments need to be made to the NTC modelled tide range grid for Tasmania in eastern Boullanger Bay. The single NTC data point for this region models the ‘standard’ tide range and total tide range heights ~30% less than our more rigorously defined tide range data at Kangaroo Island and Welcome Inlet (Figure 15).

Page 39 of 88


Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Figure 15: Comparisons between our tide range data and existing modelled ranges for the Robbins Passage-Boullanger Bay region. Note the ‘standard tide range’ shown equals the water level height variation between Indian Spring Low Water and Indian Spring High Water.

The NTC’s models underestimation of the true tide range for Boullanger Bay has implications for the natural resource management of adjacent coastal regions. These underestimated data have been incorporated into the coastal flooding hazard identification methodology being applied by Tasmanian Department of Premier and Cabinet (DPAC) to map the future sea level rise inundation footprints onshore for a variety of different sea level scenarios (Mount et al., 2010b; Lacey et al., 2012). As such the most recently mapped flood hazard area underestimates the true flood hazard. Comparisons between NTC modelled tide ranges, the tide ranges used in the latest Tasmanian inundation mapping and our calculated tide ranges are provided below (Table 17). We recommend that future updates to the Tasmanian inundation mapping should incorporate our Indian Spring High Water (‘half tide range’) levels for the RP-BB region. Table 17: Comparison between our harmonically defined Indian Spring High Water level, the National Tide Centre modelled heights and those being used for the updated Tasmanian DPAC Coastal Inundation Mapping (Lacey et al., 2012). Welcome Inlet / Boullanger Bay Tide level source

Kangaroo Island / Robbins Passage west

Howie Island / Robbins Passage east

ISHW (½ tide) range (m)

HAT (m)

ISHW (½ tide) range (m)

HAT (m)

ISHW (½ tide) range (m)

HAT (m)

1.06

1.19

1.19

1.28

1.44

1.52

NTC

0.76

0.93

0.76

0.93

1.48

1.75

Tas Inundation2

0.74

0.91

0.81

NA

1.30

NA

This study 1

1

The closest NTC data point is compared with our observed data, which is ~7km NE of Welcome Inlet, ~4.5km NNW of Kangaroo Island and ~9km E of Howie Island (Figure 15). 2 Data to be used in updated Tasmanian Inundation Mapping (Mount et al., 2010b; Lacey et al., 2012).

Page 40 of 88


Tide observations and analysis

2.6.2

Management recommendations

We recommend that both the raw sea level observations and predicted astronomical tides produced for our three RP-BB sites be supplied to, and archived at, the National Tide Centre.

2.7 Data outputs Data outputs, included raw sea level observations, tide analysis results and predicted tides are provided in digital appendices. These include:     

Raw HOBO water level logger (total and barometric pressure) observations Water level logger observations and corrected sea levels Observed sea levels, predicted astronomical tides and non tidal residuals Predicted astronomical tide cycle Predicted astronomical high and low tides

The meta data for these data files are provided in the Appendix: Metadata for sea level observations and tide predictions data files, Robbins Passage and Boullanger Bay, Tasmania which follows this report.

2.8 Future work Given the significant discrepancies we have highlighted between the NTC modelled tide ranges and our observational based data for eastern Boullanger Bay, we recommend that additional sea level observations be made for this region. Future observational points should include towards the western end of the bay (e.g. Murkay Islets), and the north-eastern end of the bay (e.g. Petrel Islands). This greater spread of data would allow the spatial variation of Boullanger Bay’s tide range to be more accurately defined. Also, these additional data points would provide information on how the tide wave propagates through this tidally complex area. Additionally, it would be useful to redeploy a water level logger at one of the existing sites (e.g. Howie Island) for the purpose of extending the period of observation. Such data would allow an additional tide analysis to define the longer period tidal constituents which could not be resolved from our ~5 month dataset, and subsequently allow more precise tide predictions to be made from the existing and future tide data in the region. We also recommend that future data be collected at 15 minute intervals only (in comparison with the 5 minute intervals used in this study) to reduce the time and resources required to periodically download the loggers.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

3 STRATIGRAPHIC AND BLUE CARBON INVESTIGATION OF THE SEAGRASS BEDS 3.1 Introduction The Robbins Passage – Boullanger Bay (RP-BB) coastal marine environment forms a unique and complex wetland system, comprising a diversity of habitat types ranging from beaches and sand flats to tidal channels and seagrass beds (Mount et al., 2010a). Seagrass meadows dominate the RP-BB intertidal flats and shallow subtidal areas, covering intertidal and subtidal areas of ~50 km2and ~60 km2, respectively6. These two habitat types were identified by the Blue Wren Group (Mount et al., 2010) as highly significant for their carbon sequestration potential - based on the recent global studies into the capacity for seagrasses to deposit and accrete significant amounts of biogenic carbon (e.g. Kennedy and Björk, 2009). This study was undertaken as a follow up to the hypothesis of Mount et al. (2010a) that the RP-BB seagrass beds house a significant carbon sink. The main aim of this study therefore was to investigate the sedimentary record of the hypothesised carbon rich seagrass sediments and collect preliminary data on the nature of these sediments. This was achieved by extracting a set of six shallow marine cores from the RP-BB seagrass meadows and visually analysing their sedimentology and relative abundance of carbon. It is envisaged that this study will have management outcomes for the RP-BB wetlands, by: 

Improving the knowledge of the regions environmental history and informing current understanding of ongoing, present day geomorphic and ecological processes.

Confirming the carbon sequestration value of these RP-BB seagrasses and providing a stronger argument for their future protection and management.

The preliminary results obtained from this research will also contribute to the understanding of the role which cool temperate seagrass meadows play within the global carbon cycle. Additional objectives of this study were to develop a relatively cheap and functional shallow marine coring rig which could be used for similar studies and identify follow-up investigations needed to further improve the scientific understanding of northwest Tasmania’s ‘blue carbon’ sink.

3.2 Background The value of managing and conserving ecosystems with high carbon sequestration capabilities (in addition to reducing anthropogenic CO2 emission) is becoming globally recognised as a practical and cost effective strategy for mitigating climate change (Canadell and Raupach, 2008). Recent research has highlighted the significant role which coastal ecosystems play in sequestering carbon dioxide, which subsequently prompted the undertaking of this study. The carbon stored within coastal ecosystems has been termed blue carbon (Mcleod et. al., 2011). 6

Aerial extent for Robbins Passage – Boullanger Bay region only, based on GIS habitat mapping by Mount et al., (2010a) of the Circular Head coastal foreshore region.

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Seagrass bed sedimentology The capacity of seagrasses to accumulate and store carbon over the long term is high, exceeding that of many terrestrial ecosystems (Kennedy and Björk, 2009). Investigations into the carbon stocks of a seagrass ecosystem require their subsurface sediments to be extracted with a sediment corer and their carbon content analysed (e.g. Fourqurean et al., 2011). In the following section we provide background information for our study into the RP-BB seagrass sedimentology. We initially overview the importance of natural coastal ‘blue carbon’ sinks and then focus on the carbon sequestration capabilities of seagrass meadows. We finish this section with a brief review of principles behind soft sediment coring as a method for investigating palaeo-environments and processes.

3.2.1

Blue Carbon: natural coastal carbon sinks

Vegetated coastal habitats have relatively recently become recognised as being superior at fixating carbon in comparison with land based (terrestrial) vegetated ecosystems (Laffoley and Grimsditch, 2009). Three coastal ecosystems found to be particularly good as sequestering carbon include mangrove forests, tidal salt marshes and seagrass meadows. These coastal vegetation communities are known as ‘blue carbon’ ecosystems, and their role in the global carbon cycle is summarised in Table 18.

Table 18: Overview of the carbon sequestration potential of coastal vegetation communities (Modified from Laffoley and Grimsditch, 20091 and Mcleod et al., 20112). Fast facts: Blue carbon 

Coastal ecosystems have recently become well recognised as being highly important global carbon sinks1.

Notable carbon sequestering coastal vegetated communities include mangrove forests, seagrass meadows and tidal salt marshes, and are termed ‘blue carbon’ ecosystems2.

Hectare for hectare, the long term carbon sequestration contribution of coastal blue carbon ecosystems is greater than terrestrial vegetated ecosystems, however their global areal extent is significantly less1, 2.

Blue carbon ecosystems sequester carbon in their: o

living biomass (leaves, stems and roots),

o

non-living biomass (e.g. litter, dead wood)2, and

o

underlying sediments.

Vegetated coastal ecosystems can sequester carbon from internal (organic) and external (inorganic, i.e. calcium carbonate sediments) sources1, 2.

The C-rich biomass produced by blue carbon ecosystems is sequestered over the short term (decennial), whereas the C-rich sediments they fixate are sequestered over the long term (millennial)2. The long term rate and size of blue carbon sinks continue to increase over time, because unlike terrestrial vegetation their soils/sediments have the capacity to accrete vertically in response to sea level rise (see Figure 16 below for comparison of coastal and terrestrial long term C sequestration rates)2.

Blue carbon sinks are globally valuable for their capacity to store carbon, as well as the local ecosystem services which they provide, however these ecosystems are under threat worldwide due to human pressures with 1/3 of such ecosystems estimated to have been lost over the past few decades.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Figure 16: Average long term rates of carbon sequestration in soils of terrestrial and coastal (blue carbon) ecosystems, modified from Mcleod et al. (2011). The significantly increased ability of coastal ecosystems to sequester carbon over the long term (i.e. millennia) is due to their ability to vertically accrete sediment (i.e. build soil profiles upwards) in line with rising sea levels, as well as having an abundant external supply of carbon rich sediment in the form of calcium carbonate skeletal sediments (e.g. broken shells).

3.2.2

Blue carbon in seagrasses meadows

Seagrasses are globally distributed flowering submarine plants that can form extensive meadows in the intertidal and shallow subtidal coastal zone. Seagrasses are a blue carbon ecosystem of high significance, being globally important for their ability to sequester carbon, and locally important for the range of ‘good and services’ they provide to the surrounding marine environment. Examples of the local goods and services include stabilising sediments, fixating nutrients and provide the basis for their surrounding coastal food web (Mount et al., 2010a); Globally, seagrasses are responsible for storing 15% of the oceans total carbon storage (Kennedy and Björk, 2009). The seagrass Posidonia oceanica is endemic to the Mediterranean and is currently recognised as the most efficient seagrass at fixating carbon. Posidonia australis has a similar structure to its relative Mediterranean species, with strap like leaves and dense rhizome mat and lives in temperate southern Australian sub-tidal waters, including in far northwest Tasmania. Like P. oceanica, P. australis develops high below ground biomass in the form of a dense matte of roots and rhizomes which stores large amounts of carbon which can persist for millennia (Kennedy and Björk, 2009). An overview of seagrasses, including Posidonia sp, and their carbon fixating potential, is summarised in Table 19.

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Seagrass bed sedimentology Table 19: Blue carbon and seagrass overview, modified from FSC (2009)1, Kennedy and Björk, (2009)2 and Mcleod et al. (2011)3. Fast facts: Seagrass meadows and blue carbon 

Seagrasses are flowering submarine plants that can form extensive meadows in the intertidal and shallow subtidal coastal waters.

They are globally distributed, covering approximately 1% of the oceans, and have important environmental functions, including providing ecosystem services and playing a significant role in the global carbon cycle2.

Seagrass meadows store ~15% of the global oceans total carbon storage 2, 3.

Posidonia oceanica is a Mediterranean endemic seagrass which is thought to be the most effective carbon sequestering seagrass2.

Similar in structure to P. oceanica, Posidonia australis lives in southern Australia’s shallow subtidal temperate waters, and hence our regional Posidonia seagrass meadows may potentially house huge carbon stocks.

P. australis form a dense rhizome mat buried in sand and/or mud which produce vertical shoots that emerge through the sediment with 2-4 strap-like leaves; plants are slow to develop (+ 10 years) and their meadows are slow to expand1.

Large seagrass species like P. australis encourage sediment deposition, hence long term carbon deposition can occur through seagrass sediments accreting (building) vertically and/or prograding (building) seawards.

Globally seagrass ecosystems are in decline, due to human related activities leading to nutrient runoff (and coastal eutrophication), increased sedimentation and physical disturbances2.

Management aimed at preserving the health of seagrass ecosystems, such as those of the Circular Head coastal region, is necessary to maintain their important carbon and ecosystem services2.

3.2.3

Palaeo-environmental investigations using soft

sediment cores Coring is a common geological technique that is used to investigate past softunconsolidated sedimentary environments. In its most simple form this method includes a three stage process: (a) pushing a length of tube into the ground; (b) retrieving the tube and the subsurface sediments contained within it (i.e. the sediment core); and (c) examining the cored sediments for past (palaeo-) environmental information. The sedimentary record (i.e. the stratigraphy) of soft sediment cores provide geological information on how the local-regional environmental conditions have evolved through time, where each distinctive sedimentary unit (or suite of related units known as a sedimentary facies) intersected in the core is representative of a past environment that was experienced regionally during its time of deposition. Sedimentary units increase in age down the core. Past environmental conditions are determined through the sedimentary characteristics contained within each unit, or facies (including grain size, sorting, rounding, mineralogy, structures and fossils). Periods of past erosion are recorded as breaks in the sedimentary record, often represented by sharpunconformable boundaries between adjacent sedimentary units. Selecting optimised sample sites is an important consideration to be made for any sampling program. The ideal coring sites for a particular coastal investigation may vary depending on the research objective. For example, programs aimed at reconstructing past sedimentary environments usually sample from a transect perpendicular to the shore located in an area representative of the greater region. This methodology has the advantage of minimising the land based edge effects on the sedimentary record (e.g. Page 45 of 88


Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania mobile tidal channels) and optimises the record of regional influences on their depositional environment (e.g. sea level change; Ellison, 2008). However coastal coring programs aimed specifically at investigating the local variation of a particular sedimentary environment across a region, such as documenting the variability in blue carbon stored beneath a seagrass meadow, may design a sampling program which targets the influences of a number of local variables on the sediments of interest (e.g. geographic location, habitat, surface depth). This latter approach has been applied to seagrass blue carbon studies at Shark Bay, WA and Florida Bay, Florida (see Fourqurean et al., 2011). The aim of our study was to both investigate the extent of sequestered blue carbon beneath the RP-BB seagrass meadows as well as the palaeo-environmental evolution of the region, and hence we applied a combination of these two approaches.

3.3 Methods 3.3.1

Site selection

The Boullanger Bay coring program focussed on investigating the sedimentology and depositional history of the Posidonia australis dominated subtidal seagrass beds. This target habitat was chosen as Mount et al. (2010a) hypothesised that northwest Tasmaniaâ&#x20AC;&#x2122;s Posidonia seagrass meadows are highly effectively at accumulating carbon rich deposits within their sub surface sediments. Six sites were initially chosen across the dense subtidal seagrass meadows at Boullanger Bay, based on detailed regional coastal habitat mapping. These sites included a three core north-south transect aimed at investigate the depositional evolution of the Boullanger Bay seagrass beds, and three additional cores spread east to west across the bay to investigate the regional variability in sequestered carbon. However time restrictions and weather conditions limited our Boullanger Bay sampling program to four cores only. An additional two cores were subsequently sampled from the more protected and accessible seagrass meadows in the far east of Robbins Passage (Figure 17).

Figure 17: Seagrass sediment core location from the Robbins Passage - Boullanger Bay.

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Seagrass bed sedimentology

3.3.2

Soft sediment coring

The coring equipment, sampling platform and personal were transported to the remote RP-BB sampling sites by a chartered fishing vessel (Figure 18). Cores were collected using a custom made semi-automated double tube percussion corer. The corer was operated from a purpose designed shallow marine sampling platform which included a twin hull dingy with a central well installed for water based coring purposes. Continuous cores were collected, penetrating depths of approximately 1.8 - 3 m. Extracted cored sediments were spit lengthways, with half being archived and the other half incrementally sampled and frozen for future analysis.

Figure 18: We used a 24 foot Shark Cat (a) to tow the sampling platform and equipment to the remote sites in Boullanger Bay (b) and eastern Robbins Passage.

Design and operation of the corer Our semi-automated double tube percussion corer was constructed by David Shaw. This sediment coring rig was initially adapted from Tratt and Burne (1980) for a palaeotsunami project (Cochran and Wilson, 2007). Additional design modifications were made during this project to enable its use in shallow marine conditions to increase its penetration depth and improve its efficiency of operation. The primary advantage of the double tube coring method is that the outer tube, or sleeve, reduces the friction on the sampled tube (i.e. the coring tube) during the withdrawal process, and hence facilitates the successful recovery of moderate length (>1 m) continuous cores. All of our coring was undertaken from a small twin hull aluminium dinghy through a central well in the boat (Figure 19). The boat was secured with three anchors while sampling. The components of our corer are shown in Figure 20 and listed in Table 20. The corer consists of two 3 m lengths of PVC tube; an inner 50 mm PVC sampling tube and an outer 65 mm PVC sleeve. The inner tube sits snugly within the outer while the corer is driven into the sediment. Steel leads with bevelled edges are attached to the leading end of both tubes for reinforcement and improve the cores ability to penetrate hard substrates (e.g. shell rich layers). An internal core catcher is attached to the inside of the sampling tube to maximise core recovery. A pipe adapter is temporarily secured to the inner sampling tube with a removable pin prior to sampling. The drive anvil is placed over the top of the corer during the sampling process, and the corer is driven into the sediments with an air compressor powered

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

Figure 19: Operation of the semi-automated double tube percussion corer, showing the sampling (insert a, b, c) and withdrawal (insert d) process.

pneumatic post driver. Once the top of the corer approaches the water level, extension â&#x20AC;&#x2DC;pushâ&#x20AC;&#x2122; rods are attached to the sampling tube with the pipe adapter to allow the entire sampling tube to be driven into the submarine sediments. A tripod mounted winch is used to first withdraw the inner sampling tube, by hooking onto the lift plug chain. The lift plug is attached to the top of push tube, which is in turn attached to the inner tube (and then to the pipe adapter once the inner core is lifted above the water level). Following the withdrawal of the sampled inner tube, the outer tube is then retrieved (where possible) with the winch, or else cut at ground level.

Figure 20: Semi-automated double tube percussion corer hardware (see Table 20 for description of individual components)

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Seagrass bed sedimentology Table 20: Individual components of the semi-automated double tube percussion corer. Figure 15 label #

1

Core component

Details

1

Lift plug (with chain)

Steel lift plug with machined thread; screws onto pipe adapter post sampling to withdrawal the inner tube with a tripod and winch; the winch hooks onto the chain.

2

Drive anvil

Rubber anvil; sits on top of tubes for protection when corer is being driven into the sediment by the post driver.

3

Pipe adapter

Steel machined adapter; attaches inner tube with attachment pin to the lift plug and chain for core withdrawal.

4

Attachment pin

10 mm steel rod; fixes the inner tube to the pipe adapter during withdrawal by slotting through appropriate holes drilled into each.

5

Core catcher (flata and rolledb)

Thin brass core catcher, cut with tin snips; flat (5a) and rolled (5b); joined to inner tube and steel inner lead with araldite; teeth face up the tube to allows sampled sediment to move past with ease and inhibits sediment loss during core withdrawal.

6

Inner lead

50 mm steel bevelled leading edge; joined to inside of inner tube with araldite.

7

Outer lead

65 mm steel bevelled leading edge; joined to outside of outer tube with araldite.

8

Attachment pin hole template

Steel template for drilling pin holes in inner tube.

9

Inner tuber

1.5 m length of 50 mm PVC pipe (Class 12)1; inner leading edge of tube widened to allow flush attachment of inner lead and core catcher; attachment pin holes drills near top.

10

Outer tube (sleeve)

1.5 m length of 65 mm PVC pipe (DWC)1

NA

Post driver (and air compressor)

Pneumatic post driver; drives the corer into the sediments; powered by an air compressor.

NA

Push tubes

1.5 m lengths of steel pipe; attaches to inner and outer tube for to push whole length of tubes beneath the water level

NA

Tripod and winch

Tripod with winch used for extracting cores; winch wire hooks onto lift plug.

Three metre length tubes were used for this study, but 1.5 m lengths are shown in Figure 20.

Core extraction, handling, splitting and sampling The cores were extracted from the subsurface with a tripod and winch mounted on the sampling platform. Once extracted, each core was sealed at both ends with sample bags and tape to contain the saturated sediments within the tube. The tubes were stored horizontally on the sampling platform (Figure 19). Some mixing of the upper (~10-20 cm) saturated sediments occurred. The cores were cut into 1 m lengths and then split lengthwise with an angle grinder. Once the cores were visually logged and photographed, one half of the core was sub sampled at 10 cm increments and stored in a freezer to stop oxidisation of the sediments and allow future chemical analysis to be conducted on the samples. The other half was wrapped in plastic and archived.

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

Figure 21: The cores were split lengthways in the field with an angle grinder, prior to being visually logged and photographed and systematically sampled.

3.3.3

Visual logging of sediment core

Sediment cores were visually logged and photographed back at the field camp documenting their sedimentological characteristics, including: unit (and sub-unit) thickness and their boundary relationships, lithology, texture (e.g. grainsize), sedimentary structures (where apparent), colour, and blue carbon content including the presence of organic and/or inorganic carbon (e.g. seagrass material and/or shelly material). Lithology, texture and sedimentary structures were visually recorded following standard sedimentological classification schemes (e.g. see Tucker, 2003). Wet sediment colour was recorded using a Munsell Colour Chart (e.g. see Munsell, 2009). Presence and relative abundance of blue carbon were recorded from visual observation of organic and inorganic carbon contents greater than silt in size. Potential blue carbon content in the silt to clay fraction could not be determined from visual analysis of the cores. Unit thickness was measured and recorded from the extracted cores, however these thicknesses are less than the true thicknesses of the in situ sediments due to core shortening7 (also known as core compaction) occurring in all cores. On average, each core was shortened by 24.4%, however the degree of shortening which occurred within the core was likely non linear and thus no thickness corrections were estimated for each unit8.

7

Core shortening is the sampling artefact of the coring processes resulting in the length of the extracted sediments being less than the depth of tube penetration. This occurs due to the progressively increasing pressure which forms ahead of the sampling tube which laterally displaces the underlying in situ sediments prior to them being sampled (Glew et al., 2001). 8 The degree of the shortening which occurred within the cores is not likely linear due to the progressive thinning which takes place, and thus no thickness corrections were made. However, total core lengths were significantly shortened for all cores, where: core shortening (%) = 100 [core penetration depth (m) / core recovery] x 100. Core 1 experienced the least shortening of 17.6% and Core 3 experienced the greatest shortening of 29.6%. Core penetration depth was not recorded in the field for Core 5.

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Seagrass bed sedimentology

3.4 Results 3.4.1

Site location and description

Boullanger Bay Boullanger Bay is a large swell sheltered, crescent shaped embayment located off the far western end of the Tasmaniaâ&#x20AC;&#x2122;s north coast, bound by Robbins Passage, Robbins Island and Walker Island in the east; a string of islands connecting Woolnorth Point to Hunter Island in the west; and Bass Strait in the north (Figure 17). The bay comprises an area of approximately 160 km2, including vast intertidal sand and seagrass flats and extensive subtidal seagrass inhabited platform, both of which are both dissected by a complex network of mobile tidal channels. The surrounding shorelines are dominated by beaches and saltmarsh, and include estuarine inlets (Shoal Inlet, Welcome Inlet and Swan Bay) and sections of rocky coasts. Sandy and rocky islands, and off shore rocky reefs are also located throughout the bay. Our initial plan was to extract a total of 6 cores spread throughout the subtidal P. australis beds of Boullanger Bay; three from a south to north transect line perpendicular to the shore in the middle of the bay (to investigate the depositional evolution of the seagrass beds); and three cores spread across the bay from west to east targeting the outer deep edge of the beds where the carbon accumulation is thought to be the greatest (to investigate the regional variability in sequestered carbon across the seagrass beds). Time and weather restrictions limited our Boullanger Bay field sampling program to retrieving four (BB1 â&#x20AC;&#x201C; BB4) out of the six planned cores, Of the four cores sampled, one was located from the western beds and three from a middle transect in the bay (Figure 22). The western core (BB1) was sampled from marginal subtidal seagrass beds, which formed dense P. australis dominated meadows approximately 1.5 km of the east of the rocky Murkay Islets and 0.5 km northeast of the wide sandy tidal channel. Here the surface sediments felt muddy underfoot. Bare patches and mixed seagrass beds were observed in the greater area. The three cores collected from the south to north transect (BB4, BB2, BB3) sampled the shallower subtidal beds across the middle of the bay. The cores were spaced at approximately 650 m intervals and sampled single species P. australis subtidal meadows, including: the shallowest, landwards core (BB4) which sampled dense marginal subtidal Posidonia beds, located approximately 100 m north from a tidal channel; the middle core (BB2) which sampled dense Posidonia beds; and the outer core (BB3) that was extracted from a region of locally dense Posidonia beds which was within an area mapped as patchy seagrass (Figure 22).

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Figure 22: Boullanger Bay core location map, with subtidal seagrass habitat shown (adapted from Mount et al., 2010a).

Robbins Passage - east Robbins Passage forms an extensive network of tidal channel and flats which separate Robbins Island from the mainland. The passage spans an area of approximately 60 km2, which can be broadly separated into two geographic regions, Robbins Passage – east, and Robbins Passage – west. Each region is characterised by a two main tidal channels that are separated by a low tide - exposed drainage divide at Robbins Crossing. Robbins Passage – east forms a shallow marine environment comprising easterly draining network of tidal channels, rocky reefs, sand flats and seagrass meadows. Two Robbins Passage cores were extracted from seagrass meadows near the far southeast coast or Robbins Island, a few hundred metres off shore from the Robbins Creek Inlet (Figure 23). The aim of coring these alternate locations was to investigate the sedimentology of various seagrass habitats. The first core (RP1) was sampled from dense P. australis dominated subtidal meadows, and the second core (RP2) was sampled from marginal mixed intertidal seagrass meadows located adjacent sand flats that extended to the shore.

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Seagrass bed sedimentology

Figure 23: Robbins Passage – east core location map, with subtidal seagrass habitat shown (adapted from Mount et al., 2010a).

3.4.2

Core stratigraphy

Boullanger Bay: Core BB1 – BB4 Similarities exist between the sedimentary record of all four Boullanger Bay cores, with the lower sediments from three of the four cores comprising well sorted clean quartzcarbonate sands, and the upper surficial sediments comprising moderately sorted, fibrous silty quartz-carbonate sands. A third silt rich fibrous layer was also found at the base of one of the cores. The Boullanger Bay stratigraphy is summarised in Table 21 – 24 and Figure 24 – 27).

Robbins Passage: Core RP1 – RP2 Similarities exist within the sampled sedimentary record from Robbins Passage, and between the Robbins Passage and Boullanger Bay cores. The lower sediments extracted from eastern Robbins Passage comprise a cohesive clay rich layer, which is unconformably overlain by moderate to well sorted fibrous quartz carbonate sands. The Robbins Passage stratigraphy is summarised in Table 25 – 26 and Figure 28 – 29.

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

Boullanger Bay: Core BB1 (Core 1) Table 21: Sedimentology of Core BB1, western Boullanger Bay. BB1 (Core 1) â&#x20AC;&#x201C; Western Bay, Boullanger Bay Coords (GDA94)

0310575 mE

Site Description

East of Murkay Islands, in patchy-mixed Posidonia dominated meadows, near inter- to sub-tidal boundary, 500 m northeast of the primary tidal channel which joins to the Welcome Inlet.

Unit

5495920 mN

From (cm)

To (cm)

Munsell colour (wet)

Description

Sedimentary

1.1

000

005

5Y 4/1 Olive grey

Olive grey organic rich muddy quartz-carbonate fine sands, with abundant fibrous cellulose material and leaf sheaths; minor shell grit; loosely packed - very high water content; gradational boundary.

1.2

005

027

As above

Olive grey fine quartz-carbonate sands in a silty organic rich matrix; shell grit common throughout, with minor whole shells; minor cellulose fibrous material; moderately sorted; high water content; gradational boundary.

1.3

027

126

As above

Olive grey silty fine quartz-carbonate sand with abundant fibrous cellulose material, locally enriched; shell grit common with minor whole shells; moderately sorted; sharp boundary.

1.4

126

169

5Y 6/1 Light olive grey

Light olive grey silty fine quartz-carbonate sand; fibrous seagrass material common; abundant shell grit throughout with minor whole shells; moderately sorted; silty fraction decreases with depth; gradational boundary.

1.5

169

180

As above

Light grey fine quartz-carbonate sands; shell grit abundant throughout with minor broken shells; minor silt enriched laminae; no seagrass material present; moderately-well sorted; 180 cm end of hole.

Facies

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Palaeoenvironment

SF3a

Subtidal Posidonia australis seagrass platform.

SF2

Intertidal or subtidal sand flats


Seagrass bed sedimentology

Figure 24: Stratigraphy of Core BB1, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note the 1 cm scale bar).

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Boullanger Bay: Core BB2 (Core 2) Table 22: Sedimentology of Core BB2, (middle) central Boullanger Bay. BB2 (Core 2) – Centre bay (middle transect), Boullanger Bay Coords (GDA94)

0312906 mE

Site description

Subtidal seagrass meadow in middle of the bay, with abundant P. australis.

Unit

5494432 mN

From (cm)

To (cm)

Munsell colour (wet)

Description

Sedimentary

2.1

000

005

5Y 4/1 Olive grey

Olive grey organic rich silty fine quartz-carbonate sands; abundant fibrous cellulose material and leaf sheaths; minor shell grit and broken shell pieces; moderately sorted; loosely packed - very high water content; gradational boundary.

2.2

005

036

As above

Olive grey fibrous-rich silty very fine-carbonate quartz sands; fibrous cellulose material abundant throughout; common shell grit and irregular whole and broken shells; moderate sorting; gradational boundary.

2.3

036

069

As above

Olive grey organic silty fine quartz-carbonate sands; moderate fibrous cellulose material dispersed throughout; moderate shell grit and irregular whole and broken shells; moderate sorting; gradational boundary.

2.4

069

168

As above

Olive grey fibrous rich silty fine quartz-carbonate sands, with common shell grit; abundant fibrous cellulose material, locally enriched (154 – 168 cm); broken and whole shells dispersed throughout; moderate sorting; gradational boundary with whole shells at base.

2.5

168

199

5B 7/1 Light bluish grey

Clean light bluish grey fine quartz-carbonate sand; minor seagrass fibres and silt rich laminae at upper boundary only (170 -175 cm), no fibrous material in remainder of unit; burrows infilled with silty sand present; well to very well sorted; 199 cm end of hole.

Facies

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Palaeoenvironment

SF3a

Subtidal Posidonia australis seagrass platform.

SF2

Intertidal or subtidal sand flats


Seagrass bed sedimentology

Figure 25: Stratigraphy of Core BB2, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note the 1 cm scale bar).

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Boullanger Bay: Core BB3 (Core 3) Table 23: Sedimentology of Core BB3, (outer) central Boullanger Bay. BB3 (Core 3) – Centre bay (outer transect), Boullanger Bay Coords (GDA94)

0312971 mE

5495114 mN

Site description

Dense Posidonia meadow in the middle of the bay; on outer shallow subtidal flats.

Unit

From (cm)

To (cm)

Munsell colour (wet)

Description

Sedimentary Facies

Palaeoenvironment

3.1

000

005

5Y 4/1 Olive grey

Olive grey organic rich silty fine quartz sands, with abundant shell grit; abundant fibrous seagrass material and leaf sheaths; moderately sorted; loosely packed - very high water content; gradational boundary.

SF3a

3.2

005

036

As above

Olive grey fibrous rich, silty fine quartz sands, with abundant shell grit; abundant fibrous seagrass material throughout; broken shells sporadic; moderately sorted; gradational boundary

Subtidal Posidonia australis seagrass platform.

3.3

036

045

As above

Olive grey organic, silty fine quartz sand, with abundant shell grit; moderate fibrous seagrass material throughout; broken shells sporadic; moderately sorted; gradational boundary

3.4

045

132

As above

Olive grey fibrous rich, silty fine quartz sands, with abundant shell grit; abundant- highly abundant organic fibrous material, increasing with depth (112 – 132); broken shells sporadic and minor whole shells; moderately sorted; peat clast present near base; gradational boundary.

3.5

132

198

5B 6/1 Lightmedium bluish grey

Clean light-medium bluish grey medium quartz sand; massive; whole shells and silty sands present in upper gradational boundary only (132 135 cm); no fibrous material present; burrows infilled with clean quartz sand present; well to very well sorted; 198 cm end of hole.

SF2

Intertidal or subtidal sand flats

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Seagrass bed sedimentology

Figure 26: Stratigraphy of Core BB3, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note the 1 cm scale bar).

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Boullanger Bay: Core BB4 (Core 4) Table 24: Sedimentology of Core BB4, (inner) central Boullanger Bay. BB4 (Core 4) – Centre bay (inner transect), Boullanger Bay Coords (GDA94)

0312921 mE

5493806 mN

Site description

Dense Posidonia beds in middle of the bay, close to tidal channel.

Unit

From (cm)

To (cm)

Munsell colour (wet)

Description

Sedimentary Facies

Palaeoenvironment

4.1

000

010

5Y 4/1 Olive grey

Olive grey organic rich silty fine quartz-carbonate sand; common leaf sheaths and fibrous cellulose material; shell grit and small shells present; moderately sorted; loosely packed (sloped around in core) very high water content; gradational boundary.

SF3a

4.2

010

056

As above

Olive grey fibrous rich, silty quartz fine sands, with minor shell grit; moderately abundant to abundant fibrous cellulose material; some whole shell present; moderately-poorly sorted; gradational boundary.

Subtidal Posidonia australis seagrass platform.

4.3

056

178

Mottled N2 Dark grey

Mottled dark grey silty fine quartz-carbonate sands, with regular dark grey organic rich silty laminae; minor fibrous cellulose material, increasing in abundance at base (from 174 -178 cm – fibrous plug/coring artefact?); regular broken shells and minor whole shells; moderately sorted; 178 end of hole.

SF3b

Palaeo-tidal channel infill.

Page 60 of 88


Seagrass bed sedimentology

Figure 27: Stratigraphy of Core BB4, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note the 1 cm scale bar).

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Robbins Passage: Core RP1 (Core 5) Table 25: Sedimentology of Core RP1, (inner) far eastern Robbins Passage RP1 (Core 5) – SE Robbins Island, Robbins Passage – east Coords (GDA94)

0334278 mE

5490375 mN

Site description

Shallow subtidal Posidonia beds adjacent Robbins Island, in Robbins Passage (east), in area of most locally continuous Posidonia beds.

Unit

From (cm)

To (cm)

Munsell colour (wet)

Description

5.1

000

008

5Y 4/1 Olive grey

Olive grey organic rich quartz fine sand, with minor shell grit; abundant seagrass fibres and leaf sheaths; whole shell present; moderately sorted; gradational boundary

5.2

008

108

5Y 6/1 Light olive grey

Light olive grey fibrous rich fine quartz sand, with moderate shell grit; seagrass fibres abundant, locally enriched; whole and broken shells infrequently dispersed throughout; massive; moderately sorted; sharp basal boundary.

5.3

108

134

Mottled

Mottled greenish black and brownish black clayey very fine quartz sand; massive; cohesive; no fibrous seagrass or shelly material present; 134 cm end of hole.

5GY 2/1 Greenish black & 5YR 2/1 Brownish black

Page 62 of 88

Sedimentary Facies

Palaeoenvironment

SF3c

Subtidal Posidonia australis seagrass sand flats.

SF1

Terrestrial alluvial/swamp deposits.


Seagrass bed sedimentology

Figure 28: Stratigraphy of Core RP1, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-c; note the 1 cm scale bar).

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

Robbins Passage: Core RP2 (Core 6) Table 26: Sedimentology of Core RP2, (outer) far eastern Robbins Passage. RP2 (Core 6) â&#x20AC;&#x201C; SE Robbins Island, Robbins Passage - east Coords (GDA94)

0335199 mE

Site description

5490490 mN

Boundary of mixed seagrass meadows and sand flats, in marginal sub-intertidal zone; ~200 m off Robbins Island shoreline - Robbins Passage (east). Multiple seagrasses present.

Unit

From (cm)

To (cm)

Munsell colour (wet)

Description

Sedimentary Facies

Palaeoenvironment

6.1

000

016

N6 Light grey

Light grey massive quartz fine sands, with subordinate dark organic fibres and minor shell grit; moderately well sorted; gradational boundary.

SF3d

Intertidal mixed species seagrass flats.

6.2

016

100

Mottled 5Y 6/1 Light olive grey & 5Y 4/1 Olive grey

Mottled light olive grey and olive grey clean fine quartz sands with some shell grit and whole shells; minor to moderate dark organic fibres; moderately well sorted; gradational boundary.

6.2

100

126

As above

Mottled light olive grey and olive grey clean fine quartz sands with shell grit and whole shells; moderate cellulose fibres, locally abundant massive; moderately well sorted; sharp boundary.

SF3c

Subtidal Posidonia australis seagrass sand flats.

6.3

126

154

5GY 2/1 Greenish black

Greenish black silty-clay with very fine sand; massive; cohesive; no seagrass fibres or shelly material present; gradational boundary.

SF1

Terrestrial alluvial/swamp deposits.

6.4

154

164

Mottled As above & 10YR 4/2 Dark yellow brown

Mottled greenish grey and dark yellow brown clay; cohesive; no seagrass fibres or shell material present; 164 cm end of hole.

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Seagrass bed sedimentology

Figure 29: Stratigraphy of Core RP2, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note the 1 cm scale bar).

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Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

3.4.3

Sedimentary facies

To assist with interpreting the depositional environments and associated sub environments of the cored sediments, we have differentiated comparable sediment bodies by their geological characteristics such as litho logy (grain size, sorting, and mineralogy), presence/absence of fossil biota (seagrass fibres, shells) and sedimentary structures. These differentiated sediment bodies are referred to as sedimentary facies. We identified three unique sedimentary facies from the six RP-BB cores, with each facies containing an important suite of features indicative of its depositional environment. These three facies include: a clay rich facies, comprising terrestrial sediments only; a silty sand to sandy facies represented by the presence of organic fibres; and a well sorted sandy facies largely devoid of organic fibres. We have further divided the seagrass associated sedimentary facies (i.e. the facies containing organic fibres) into four sub-facies, based on subtle, yet important sediment logical variations which are indicative their unique depositional sub environments. These are summarised in Table 27, and detailed below. Table 27: Sedimentary facies summary description. Sedimentary facies & sub-facies

Sedimentary facies & sub-facies summary description

Present in cores:

Overlies facies:

Underlies facies:

Relative â&#x20AC;&#x2DC;blue carbonâ&#x20AC;&#x2122; abundance:

SF1

Mottled silty clayey sand to clay, cohesive, massive, no organic fibres of shell material present.

RB1, RB2

?

SF3c

None

SF2

Well sorted grey quartzcarbonate sands. Sands fine to medium.

BB1, BB2, BB3

?

SF3a

Low

SF3a

Olive grey organic silty quartz-carbonate sand, variably rich in cellulose fibres. Moderately sorted. Sands fine. Some broken and whole shells.

BB1, BB2, BB3, BB4

SF2, SF3b

-

High

SF3b

Dark grey organic rich silty sand with regular silt rich laminae, minor cellulose fibres. Moderately sorted. Sands fine. Broken and whole shells present.

BB4

?

SF3a

Moderate (to high?)

SF3c

Olive grey organic quartzcarbonate sand, variably rich in cellulose fibres. Moderately sorted. Sands fine. Some broken and whole shells.

RP1, RP2

SF1

SF3d

High

SF3d

Olive grey quartzcarbonate sand, with dark organic fibres. Moderatewell sorted. Sands fine. Some broken and whole shells.

RP2

SF3c

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Moderate


Seagrass bed sedimentology The basal sediments from the Robbins Passage cores RP1 and RP2 forms the first sedimentary facies (SF1), represented by cohesive and structure less clay rich sediments which vary from clayey sand to clay. Where present, the sands are very fine and terrigenous in nature, with no marine-sourced calcium carbonate sediments and seagrass sourced organic fibres present. Its colouring is variable, but often mottled and dark. The second sedimentary facies (SF2) found at the base of Boullanger Bay cores BB1, BB2 and BB3 consists of well to very well sorted quartz-carbonate sands. Sands are quartz dominated, range from fine to medium and are greyish in colour. Infilled burrows and comminuted marine shells are present. This facies is found underlying the seagrassassociated sub-facies (SFa) where present, with a gradational boundary (2-5 cm) often consisting of silty sand laminae and whole and broken shells, and minor seagrass fibres. With the exception of its gradational upper boundary, no seagrass material is present in this facies. The third sedimentary facies is associated with the colonisation of seagrass, as indicated through the presence of organic cellulose fibres. This facies has been split into four subfacies based on discernible changes in type and abundance of organic fibres, sediment sorting, the presence/absence of silt and sedimentary structures. The most common seagrass sediments found in Boullanger Bay is sub-facies SF3a, which consists of moderately sorted olive grey silty quartz-carbonate sands which are variably rich in pale cellulose fibres. The cellulose fibres bind the SFa sediments and are often matted together in dense clumps. The sands are dominated by quartz and mostly fine, with comminuted shell common and whole shells also present. Silt content is variable. This surficial facies is present in the upper section of all four Boullanger Bay cores which were universally extracted from subtidal P. australis meadows. Shortened thicknessâ&#x20AC;&#x2122; of the sub-facies SF3a ranges from 0.56 â&#x20AC;&#x201C; 1.68 m. The sub-facies SF3b forms a unique sedimentary layer found at the base of Boullanger Bay core BB4 only, consisting of moderately sorted organic rich silty quartz-carbonate sands with dispersed cellulose fibres and regular silt rich laminae. This sub-facies is distinguished from SF3a by its higher silt content which is increasingly expressed as silt rich laminae towards the base; relatively minor abundance and dispersed nature of cellulose fibres; and mottled dark grey colouring. Sands are mostly fine, dominated by quartz and include comminuted shells. Broken shells are common, with minor whole shells present. This sub-facies is overlain by sub-facies SF3a, with a gradational boundary. Sub-facies SF3c was sampled in both Robbins Passage cores, consisting of olive grey quartz-carbonate sands, with variably abundant and locally enriched cellulose fibres and organics. This sub-facies is comparable with the Boullanger Bay sub-facies SF3c, but silt is absent. Sediments are moderately-moderately well sorted; sands are fine and dominated by quartz. Broken and whole shells are common. This sub-facies comprises the upper-most sediments in core RP1, which was extracted from a subtidal environment, and forms the middle sub-facies in Core RP2 which was extracted from the intertidal zone. Finally, sub-facies SF3d is represented by olive grey quartz-carbonate sands with dispersed dark organic fibres. Sands are mostly fine, dominated by quartz and moderately well sorted. Articulated carbonate sands, and broken and whole shells are present. The pale, cellulose fibres common in all of the other seagrass associated subfacies is absent. This sub-facies is found in core RP2 only which was extracted from the Page 67 of 88


Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania intertidal zone, in a marginal seagrass sand flat habitat, and forms the surficial sedimentary layer in this core. Its lower boundary with sub-facies SF3d is gradational. It has a gradational lower boundary with sub-facies SF3c.

3.4.4

Sedimentary facies interpretation

Terrestrial alluvial/lacustrine deposit (Robbins Passage) The sedimentary facies, SF1, found in the basal sediments of both Robbins Passage cores are represented by cohesive and structureless clay-rich sediments devoid of any seagrass fibres, marine shells or skeletal sediments. As such, facies SF1 is interpreted to be terrestrial in origin and therefore Pleistocene in age, deposited under lower sea levels and colder climatic conditions. The fine nature of facies SF1â&#x20AC;&#x2122;s sediments indicates deposition occurred in a low to very low energy environment, by alluvial or lacustrine processes in a freshwater swamp or lake environment. Such palaeo-environments have been previously interpreted to exist in this region. For example, Sharples (in Mount et al., 2010a) hypothesised that under the last glacial climatic phase (when sea levels were below present, and much of the Bass Strait was sub-aerially exposed) the Circular Head coastal landscape primarily formed an extensive dunefield comprising mobile linear sand dunes with interspersed widespread swamps and lakes forming in dune swales and in deflation hollows. Alternatively, this basal clayey sedimentary facies may have formed a distal floodplain deposit of the palaeo-Montagu River which likely crossed modern eastern Robbins Passage, following a similar path to the present main tidal channel.

Intertidal or subtidal sand flats (Boullanger Bay) The well sorted quartz-carbonate sands of the lower Boullanger Bay facies SF2 is marine in origin, as indicated by the presence of comminuted marine shell. The well sorted nature of this deposit, and absence of fines indicates that deposition occurred under relatively high hydrodynamic energy indicating deposition in a marginal marine environment where wave energy and/or tidal currents favoured the winnowing of silts and muds, and concentration of sands. Based on this evidence, facies SF2 is interpreted to form either an intertidal sand flat/sandy beach, or else a shallow subtidal sand flat/sand wave. The subtidal bathymetric range of this facies would indicate deposition to have occurred in either the mid Holocene (circa 6-8 ka BP) under rising sea levels to form an intertidal deposit, or else during the Mid-Late Holocene (<6 ka BP) to form a subtidal deposit under stable sea levels comparable to modern day conditions. Infilled burrows also occur in the basal sands of cores BB2 and BB3, indicating the presence of burrowing fauna during and/or after their deposition. Such trace fossils are also common in marginal marine environments. This sandy marginal marine deposit grades up into the seagrass sub-facies SF3a indicating that the conditions which promoted deposition persisted during the time when seagrasses colonised the bay.

Subtidal Posidonia australis seagrass platform (Boullanger Bay) The modern depositional environment of the surficial seagrass sub-facies SF3a at Boullanger Bay comprises a shallow subtidal seagrass platform dominated by a dense P. australis dominated meadows. The sediments of this sub-facies are characterised by moderately sorted, fibrous rich silty quartz-carbonate sands with common broken shells, indicating that deposition occurred in an environment analogous to the modern subtidal setting. Pale cellulose fibres analogous to those within this facies have been previously documented in P. australis sediments from the Spencer Gulf in South Australia and were identified as forming the decay resistant residue of P. australis leaf sheath fibres (Kuo, 1978 in Belpario et al., 1984). The moderately sorted nature of the sub-facies SF3a sediments are representative of the sedimentary processes associated with P. australis Page 68 of 88


Seagrass bed sedimentology habitat; the high density foliage of these meadows increases the seafloor roughness and locally reduces the hydrodynamic (wave/tide current) energy near which encourages sediments to become trapped and accumulate. Additionally, direct deposition of epiphytic derived carbonate sediments associated with P. australis meadows also occurs, forming the broken shelly fraction that is associated with this facies. The source of the silt is unknown, however it may originate from either decaying seagrass plant material, fine alluvial sediments sourced from terrestrial inputs (e.g. Welcome Inlet), or else a combination of these sources. If so, it is possibly rich in organic carbon.

Palaeo-tidal channel infill (Boullanger Bay) Tidal channels are dynamic features with lateral migration, cutting and abandonment of channels occurring episodically (Bridge and Demicco, 2008). The basal sedimentary subfacies SF3b of Boullanger Bay Core BB4 is interpreted to form a palaeo-tidal channel infill deposit based on its geographic location, the presence of regular silt rich laminae and the comparably lower abundance and more dispersed nature of cellulose fibres of this seagrass sub-facies. Core BB4 was extracted from the margins of a subtidal seagrass meadow some 50-100 m south of a modern tidal channel. This location falls within the area which may have been previously traversed by the adjacent modern channel. The silt rich laminae of this sub-facies indicate that periodic deposition occurred in a low energy environment, like that which would occur in a tidal channel cut-off. The cyclic nature of these laminae forms a sedimentary record of a relict depositional process which was cyclic in nature, possibly including those produced by tides9 or climatic events (e.g. periodic high rainfall)10. Finally, the dispersed nature of the seagrass sediments of this sub-facies indicates that seagrasses did not densely colonise the seafloor during its deposition, as the sedimentary signature of in situ seagrass debris deposition includes densely matted fibres like that described in the seagrass sub-facies SF3a and SF3c. Rather, we suggest that these dispersed fibres represent the periodically accumulation of sheaths which were transported into the infilling cut-off channel. Possibly the reasons that seagrasses did not grow here is because once the cut-off was formed, sediments infilled this palaeo-channel depression at a rate faster than the seagrasses could establish. This sub-facies grades into a subtidal seagrass platform sediments (sub-facies SF3a), indicating that P. australis habitat eventually colonised this location. This probably occurred once the palaeo-channel became infilled.

Subtidal Posidonia australis seagrass sand flats (Robbins Passage) Robbins Passage seagrass sub-facies are characterised by moderately sorted fibrous quartz-carbonate shelly sands which are largely devoid of fines. The sub-facies SF3c is interpreted as a subtidal Posidonia australis seagrass flat deposit, based on the abundant presence of cellulose fibres, and environmental location with which both the Robbins Passage cores were extracted. This sedimentary facies is comparable to the Boullanger Bay subtidal P. australis seagrass platform sediments (see Subtidal Posidonia australis seagrass platform (Boullanger Bay)), however an absence of silt indicates deposition occurred under conditions with higher hydrodynamic energy which allowed these fines to be winnowed. In Core RP2, this subtidal sub-facies grades up to an intertidal sub-facies at depths of about 1 m below the cores surface. This change in depositional environment is consistent with the bathymetric range from which those sediments were extracted.

9

The cyclic silt rich laminae may have been a tidal deposit, where the silty layer were laid down during slack water conditions at high and low tide, or else during ~fortnightly occurrence of neap tides when the bays tidal energy is at its lowest. 10 High rainfall events associated with cyclic low pressure systems may also be responsible for the silty laminae, with increased terrestrial runoff increasing the influx of alluvial silts entering the bay.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Intertidal mixed species seagrass flats (Boullanger Bay) The upper sediments of Core RP2 form a seagrass sub-facies comprising moderately-well sorted olive grey quartz-carbonate sands with dispersed dark organic fibres. These sediments are interpreted to have deposited in intertidal (mixed species) seagrass flats, like those of the modern seafloor environment where the core was extracted from. The organic fibres in this sub-facies differ from the cellulose fibres common to the other seagrass associated sub-facies, being dark in colour, less abundant and more dispersed. The sandy nature and moderately-well sorted nature of the sandy sediments are indicative of the relatively higher hydrodynamic energy of an intertidal flats environment.

3.5 Carbon sequestration potential of Robbins Passage – Boullanger Bay seagrass meadows Visual analysis of the cored RP-BB sedimentology has shown the seagrass-associated facies (SF3a-d) to have significant blue carbon content in their sediments, with the subfacies SF3a and SF3c being notably rich in organic carbon. This indicates that the subtidal Posidonia seagrass platform in Boullanger Bay and the subtidal Posidonia seagrass flats in Robbins Passage, have a high capacity to preserve blue carbon. As such, two key findings can be deduced from these observations:  

The Posidonia australis dominated subtidal seagrass meadows at RP-BB are highly effective at sequestering carbon, and Large carbon stocks exist beneath the subtidal seagrass meadows of RP-BB.

These finding provide additional evidence to show that Australian P. australis meadows form valuable coastal carbon sinks, supporting previous studies from Spencer Gulf, South Australia (Belperio et al., 1984) and Shark Bay, Western Australia (Forqurean et al., 2011). Additionally, our research demonstrates the carbon sequestration capacity of Tasmanian P. australis meadows, which unlike the seagrass habitats of the Spencer Gulf and Shark Bay experience a cool-temperate climate. The volume of sequestered carbon at RP-BB is likely significant. Habitat mapping of RPBB shows subtidal seagrass meadows to cover an area of approximately 61 km2, with P. australis and/or Amphibolis antarctica dominated meadows occupying much of this area (Mount et al., 2010a). Thus the subsurface extent of the carbon rich sediments associated with the P. australis subfacies (SF3a and SF3c) is likely to be comparable to, or even greater than this area11. Assuming this is the case and based on the depths of the carbon rich deposits found in our 6 cores (which were commonly between 1 – 1.5, see Table 28), a crude estimation of the volume of carbon rich sediments (61 km 2 x 1.25 m) is approximately 76,250 m3 (note: this should be considered as an indicative minimum volume only, as the carbon rich seagrass deposits likely deepen seawards towards Walker Channel in Boullanger Bay). Further investigations are required to calculate the total carbon pool stored beneath the RP-BB meadows and/or infer the rate of seagrass sediment deposition/carbon sequestrations (see Section 3 ‎ .8: Future work).

11

As shown with the Core RP2, subtidal seagrass sub-facies stratigraphically underlie the intertidal seagrass sub-facies, and therefore the older subtidal seagrass sediments likely comprise a carbon rich wedge that extends and thins shoreward’s beneath the more modern intertidal deposits. As such, the aerial extent of carbon rich sediments is most likely greater than the mapped 61km 2 of modern seagrass sediments.

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Seagrass bed sedimentology

Table 28: Depth of carbon rich sediments. Core #

1

Site (and geographic location; relative to shoreline and site)

Carbon rich (P. australis) sub-facies

Thickness1 (cm)

BB1

B. Bay (mid-west)

SF3a

169

BB4

B. Bay (inner-centre)

SF3a

56

BB2

B. Bay (inner mid-centre)

SF3a

168

BB3

B. Bay (mid-centre)

SF3a

132

RP1

R. Passage (inner mid-east)

SF3c

108

RP2

R. Passage (inner-east)

SF3c

26

Compressed thickness (cores were shortened on average by 25%).

3.6 Management implications Seagrass meadows have the capacity to accrete vertically in response to sea level rise and thus the seagrass habitat of RP-BB will continue to perform as a carbon sink into the future as long as their health is maintained. However, seagrass ecosystems are inherently susceptible to degradation by human disturbances, including eutrophication and siltation of coastal waters. Increasing human pressures on the coastal environment is leading to continued significant decline in the global distribution of seagrasses (Kennedy and Björk, 2009). To ensure that RP-BB seagrass beds effectively sequester carbon into the future, the terrestrial wetland habitats (importantly riparian and shoreline vegetation), their associated waterways and the physical foreshore environment of this region must be properly managed. Environmental risks associated with the mismanagement of the RP-BB wetlands are high. Large scale loss of seagrass habitat would not only lead to the loss of their important carbon sink service but would also expose their underlying carbon-rich sediments to erosion. Such an occurrence would have long term consequences for global atmospheric carbon concentrations (Mount et al., 2010a). Management recommendation for the RP-BB seagrass meadows are provided below.

3.7 Management recommendations Management efforts should aim to preserve the general health of RP-BB seagrass meadows through: 

Reducing nutrient loads in the coastal waters (Kennedy and Björk, 2009).

Preserving water clarity through conserving, managing and improving the regions coastal and riparian vegetation (Kennedy and Björk, 2009).

Avoid physical disturbance of the RP-BB coastal sediments (e.g. from vehicular driving and foreshore engineering), as these sediments were found to have high nutrient levels (Mount et al., 2010a).

Page 71 of 88


Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania We also recommend that this report be provided to appropriate research bodies actively working in the area blue carbon (e.g. UNESCO Blue Carbon International Scientific Working Group), to: 

Assist in the dissemination of information attained through this study. This will increase the likelihood of further research being undertaken on RP-BB’s significant blue carbon stock

Contribute to the development global blue carbon inventories (e.g. Forqurean et al., 2012)

3.8 Future work The results obtained in this research provide a very preliminary assessment of the RP-BB capacity to sequester and store carbon. Future work is required to adequately estimate the total carbon pool beneath the RP-BB seagrass beds, as well as to calculate the rate at which these meadows are sequestering carbon. Thus to fully assess the carbon sequestration potential of the RP-BB seagrass beds, we recommend that the following work be done: Measure the carbon stocks in RP-BB sediments. 

Measure carbon content of the seagrass sediments by undertaking geochemical analysis of their sediments. This analysis could be completed on our stored core samples. Measure the dry bulk density of the RP-BB seagrass sediments. This requires additional cores to be sampled using a piston corer to collect uncompressed cores. The mass of the RP-BB seagrass sediments per volume can then be accurately measured from these uncompressed cores. These data (i.e. carbon content and dry bulk density), along with our data on the depth of seagrass sediments could be used to produce a crude approximation of the total carbon pool. However, additional data on the three dimensional nature of the carbon rich sediments is required to conduct a more rigorous assessment of the total RP-BB carbon stocks (see below).

Define the three dimensional body of RP-BB’s carbon rich sediments. 

Undertake a comprehensive coring program to determine the varying depth of seagrass sediments across the region, notably towards the outer limits of the seagrass meadows. A vibracore rig may be suitable to extract long deep cores for this purpose. Alternatively, undertake high resolution seismic survey, coupled with a small number of additional cores to determine the three dimensional body of the carbon rich sediments (see Lo Iacono et al., 2008, for methods regarding this approach).

Calculate the carbon sequestration rate at RP-BB 

Measure the rate of sediment accumulation on the RP-BB seagrass sediments by undertaking multiple radio carbon dating of the buried seagrass fibres. Dating various depths within the cored sedimentary record will allow the annual sequestration rate to be calculated, as well as improving understanding of the Mid- to Late Holocene geomorphic history of the region (which will additionally contribute to understanding the carbon sequestration processes).

Page 72 of 88


References

4 References BELPERIO, A.P., HAILS, J.R., GOSTIN & V.A., POLACH, H.A., 1984: The stratigraphy of coastal carbonate banks and Holocene sea levels of northern Spencer Gulf, South Australia. Marine Geology. 61 (2-4), 297-313. BRIDGE, J & DEMICCO, R., 2008: Earth surface processes, landforms and sediment deposits. Cambridge University Press, Cambridge, pp. 815. BROWN, N., 2010. AUSGeoid09: Converting GPS heights to AHD heights. AUSGEO News, 97 (March). Downloaded June 2012, from: <http://www.ga.gov.au/image_cache/GA16650.pdf> CANADELL, J.G. & RAUPACH M.R., 2008: Managing forests for climate change mitigation. Science. 320, 1456-1457. COCHRAN, U. & WILSON, K., 2007: Tasmanian palaeotsunami project. GNS Science Consultancy Report 2007/222. DHHS, 2011: Montagu triennial data review 2010. The Tasmanian shellfish quality assurance program. Public and Environmental Health Service, Department of Health and Human Services, Tasmania. Downloaded June 2012, <http://www.dhhs.tas.gov.au/__data/assets/pdf_file/0020/85430/Montagu_Triennial_2 010.pdf> DONPS, 2012: Basic concepts in physical oceanography: Tides. Navy Operational Ocean Circulation and Tide Models, Department of Oceanography, Naval Postgraduate School. Viewed 03 July, 2012, <http://www.oc.nps.edu/nom/day1/partc.html> EDGAR, G.J., BARRETT, N.S. & GRADDON, D.J., 1999: A classification of Tasmanian estuaries and assessment of their conservation significance using ecological and physical attributes, population and land use. Technical report series no 2. Marine Research Laboratories – Tasmanian Aquaculture and Fisheries Institute, University of Tasmania. ELLISON, J., 2008: Long-term retrospection on mangrove development using sediment cores and pollen analysis: A review. Aquatic Botany. 89, 93 – 104. FEATHERSTONE, W.E., KIRBY, J.F., HIRT, C., FILMER, M.S., CLAESSENS, S.J., BROWN, N.J., HU, G. & JOHNSTON, G.M., 2010: The AUSGeoid09 model of the Australian Height Datum, Journal of Geodesy 85 (3). FEATHERSTONE, W.E. & KUHN, M., 2006: Height Systems and Vertical Datums: a Review in the Australian Context. Spatial Science 51 (1). FORRESTER, W.D., 1983: Canadian Tidal Manual. Department of Fisheries and Oceans, Canadian Hydrographic Service, Ottawa, pp. 75-79. FOURQUREAN, J., MARBA, N., KENNEDY, H., MATEO, M.A., DUARTE, C., HOLMER, M., APOSTOLAKI, E., MCGLATHERY, K. & KENDRICK, G., 2011: Blue Carbon in seagrass ecosystems: how much is there and what’s it worth? Visual presentation, The Seagrass Blue Carbon Task Force, Blue Carbon Science Working Group meeting, Bali, Indonesia, 26 July, 2011.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania FOURQUREAN, J W., DUARTE, C.M., KENNEDY, H., MARBA., HOLMER, M., MATEO, M.A., APOSTOLAKI, E.T., KENDRICK, G.A., KRAUSE-JENSEN, D., K.J., & SERRANO, O., 2012: Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience. 5, 505509. FSC, 2009: Proposed determination: The seagrass ‘Posidonia australis’ as Endangered Population in Port Hacking, Botany Bay, Sydney Harbour, Pittwater, Brisbane Water and Lake Macquarie (NSW), Ref. No. PD44. Fisheries Scientific Committee, NSW Department of Primary Industries. GLEW, J.R., SMOL, J.P. & LAST, W.M., 2001: ‘Sediment core collection and extrusion’ in LAST, W.M. & SMOL, J.P. (eds) Tracking environmental change using lake sediments. Volume 1: Basin analysis, coring, and chronological techniques. Kluwer Academic Publishers, Dordrecht, pp. 73-105. ICSM, 2009. Geocentric Datum of Australia Technical Manual Version 2.3 (1). Intergovernmental Committee on Surveying and Mapping. Downloaded June 2012, <http://www.icsm.gov.au/icsm/gda/gdatm/gdav2.3.pdf>. ICSM, 2011. Australian Tides Manual. Permanent Committee for Tides and Mean Sea Level Special Publication No.9, Version 4.1. Permanent Committee on Tides and Mean Sea Level (PCTMSL) and Intergovernmental Committee on Surveying and Mapping. Downloaded June 2012, <http://www.icsm.gov.au/tides/SP9_Australian_Tides_Manual_V4.1.pdf> IOC, 1985: Manual on Sea level Measurement and Interpretation. Volume 1 - Basic procedures. Intergovernmental Oceanographic Commission of UNESCO, Manuals and Guides No. 14, Paris, 83 pp. IOC, 2006: Manual on Sea level Measurements and Interpretation, Volume IV: An update to 2006. Intergovernmental Oceanographic Commission of UNESCO, Manual and Guides No. 14, Paris, 78 pp. KUO, J., 1978: Morphology, anatomy and histochemistry of the Australian seagrasses of the genus Posidonia König (Posidoniaceae). I. Leaf blade and leaf sheath of Posidonia australis Hook.f. Aquatic Botany. 5, 171–190, in BELPERIO, A.P., HAILS, J.R., GOSTIN & V.A., POLACH, H.A., 1984: The stratigraphy of coastal carbonate banks and Holocene sea levels of northern Spencer Gulf, South Australia. Marine Geology. 61 (2-4), 297-313. KENNEDY, H. & BJÖRK, 2009: ‘Seagrass Meadows’ in LAFFOLEY, D & GRIMSDITCH, G., (eds) The management of natural carbon sinks. International Union for Conservation for Nature, Gland, Switzerland. 53pp. LACEY, M.J., HUNTER, J.R. & MOUNT, R.E., 2012: Coastal Inundation Mapping for Tasmania - Stage 2. Report to the Department of Premier and Cabinet by the Blue Wren Group, School of Geography and Environmental Studies, University of Tasmania and the Antarctic Climate and Ecosystems Cooperative Research Centre. LAFFOLEY, D & GRIMSDITCH, G., (eds) 2009: The management of natural carbon sinks. International Union for Conservation for Nature, Gland, Switzerland. 53pp.

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References LO IACONO, C., MATEO, M. A., GRÀCIA, E., GUASCH, L., CARBONELL, R., SERRANO, L., SERRANO, O. & DAÑOBEITIA, J., 2008: Very high-resolution seismo-acoustic imaging of seagrass meadows (Mediterranean Sea): Implications for carbon sink estimates, Geophysical Research Letters. 35, L18601. MCLEOD, E., CHMURA, G.L., BOUILLON, S., SALM, R., BJÖRK, M., DUARTE, C.M., LOVELOCK, C.E., SCHLESINGER, W.H. & SILLIMAN, B.R. 2011. A blueprint for blue carbon: toward an improved understanding of the role of vegetated coastal habitats in sequestering CO2. Frontiers in Ecology and the Environment. 9, 552–560. MOUNT, R.E., PREHALAD, V., SHARPLES, C., TILDEN, J., MORRISON, B., LACEY, M., ELLISON, J., HELMAN, M. & NEWTON J., 2010a Circular Head Coastal Foreshore Habitats: Sea Level Rise Vulnerability Assessment: Final Project Report to Cradle Coast NRM. School of Geography and Environmental Studies, University of Tasmania, Hobart. MOUNT, R.E., LACEY, M. & HUNTER J.R., 2010b: Tasmanian coastal inundation mapping project report. A report to the Tasmanian Planning Commission. School of Geography and Environmental Studies, University of Tasmania. MUNSELL COLOUR, 2009: Munsell soil colour charts. Grand Rapids, MI. NOAA, 2000: Tide and current glossary. National Oceanic and Atmospheric Administration, US Department of Commerce. Downloaded June 2012, <http://www.co-ops.nos.noaa.gov/pub.html> NOAA, 2012: Our restless tides. National Ocean and Atmospheric Administration Centre for Operational Oceanographic Products and Services. Viewed 29 June, 2012, <http://www.co-ops.nos.noaa.gov/restles1.html#Intro> PAVLIS, N.K., HOLMES, S.A., KENYON, S.C. & FACTOR, J.K., 2012: The development and evaluation of the Earth Gravitational Model 2008 (EGM2008), Journal of Geophysical Research, 117, B04406. PUGH, D.T. 1987: Tides, Surges, and Mean Sea level, John Wiley and Sons, Chichester, 472 pp. PUGH, D.T., 2004: Changing Sea Levels: Effects of Tides, Weather and Climate, Cambridge University Press, Cambridge, 265 pp. SHORT, A.D., 2006: Beaches of the Tasmanian Coast and Islands: A guide to their nature, characteristics, surf and safety. Sydney University Press, Sydney, 353 pp. TRATT, M.H. & BURNE, R.V., 1980: An inexpensive and efficient double-tube, handcoring device. BMR Journal of Australian Geology and Geophysics. 5, 156-158. TUCKER, M., 2003: Sedimentary rocks in the field. The geological field guide series. Wiley, West Sussex. UCAR 2006: Introduction to ocean tides. MetEd, Oceanography/Marine Meteorology module. University Corporation for Atmospheric Research. Viewed 29 June, 2012, <https://www.meted.ucar.edu/training_module.php?id=223>

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

Appendix: Metadata for sea level observations and tide predictions data files, Robbins Passage and Boullanger Bay, Tasmania A.1 Introduction These data were collected and produced by the Blue Wren Group, School of Geography and Environmental Studies, University of Tasmania for the Cradle Coast Natural Research Management initiated 2011-2012 research project to investigate the tides characteristics in far northwest Tasmania. The sea level observations were made from November/December 2011 to May 2012 using four HOBO U20 water level loggers (pressure transducers); three deployed in near shore waters across Robbins Passage-Boullanger Bay to measure total (submarine) pressure; and one positioned onshore to measure regional barometric (subaerial) pressure, near sea level. The water level loggers were deployed in remote stilling wells to mechanically dampen the influence of high frequency fluctuations in water level (i.e. waves) on the observational data. The Howie Island and Kangaroo Island stilling wells extended above the highest water mark and were most effective at filtering out sea level fluctuations due to waves, the Welcome Inlet stilling well extended to approximate half tide level and the some noise from waves can be seen in this data. The total (submarine) pressure data were converted to water levels (i.e. water depth with respect to the loggers pressure sensor) with the HOBOware Pro Barometric Compensation Assistant, using the regional barometric data and applying the softwares default fluid density input for salt water (1,025 kg/m3). Water levels were corrected to the Australian Height Datum based on a differential GPS levelling of the loggers’ sensors. Tidal analysis was conducted on the sea level observations using T-TIDE (version 1.3) in MATLAB, to solve 21-23 tidal constituents for each site. These data were subsequently used to compute the astronomical tidal predictions. The final Robbins Passage-Boullanger Bay datasets detailed here include: 

Raw HOBO water level logger (total and barometric pressure) observations, for the period of November/December 2011 – May 2012 (see Section A.2).

Water level logger observations and corrected sea levels, for the period of November/December 2011 – May 2012 (see Section A.3).

Observed sea levels, predicted astronomical tides and non tidal residuals, for the period of November/December 2011 – May 2012 (see Section A.4).

Predicted astronomical tidal cycle, for the period of June 2012 – June 2022 (see Section A.5).

Predicted astronomical high and low tides, for the period of June 2012 – June 2022 (see Section A.6).

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Appendix

A.1.1

Period of sea level observation

The period of data collection is detailed in Table 29, below. Table 29: Period of water level logger data collection. Survey Id

Logger type

Howie Island

UTC (UTC + 0 hrs)

EST (UTC + 10 hrs)

From (Date, Time)

To (Date, Time)

From (Date, Time)

To (Date, Time)

Water level logger

26/11/2011 23:00:00

8/05/2012 04:55:00

27/11/2011 09:00:00

8/05/2012 14:55:00

Kangaroo Island

Water level logger

26/11/2011 23:00:00

07/05/2012 07:10:00

27/11/2011 09:00:00

7/05/2012 17:10:00

Welcome Inlet

Water level logger

23/12/2011 05:25:00

07/05/2012 14:50:00

23/12/2011 15:25:00

8/05/2012 00:50:00

Stony Point

Barometric logger

26/11/2011 23:00:00

08/05/2012 06:05:00

27/11/2011 09:00:00

08/05/2012 16:05:00

A.1.2

Sea level logger location and heights

The water level logger’s elevation and geographic location is detailed below. The loggers were surveyed using differential geodetic grade Leica Viva GPS receivers and Leica AS10 antennas. The measured elevation data were processed with Leica GeoOffice version 7 (LGO) using the AUSGeoid09 to calculate Australian Height Datum (AHD) heights. Table 30: Logger survey details. Survey Id

Logger type

Easting (AHD)

Northing (AHD)

Height (m, AHD)

Howie Island

Water level logger

328821.1012

5488136.0736

-1.377

Kangaroo Island

Water level logger

318,580.208

5,492,735.900

-1.213

Welcome Inlet

Water level logger

312,250.475

5,490,913.792

-0.907

Stony Point

Barometric logger

328,696.421

5,487,139.538

NA

Potential sources of error in surveying the logger heights are attributed to systematic and random errors associated with AHD height datum’s and the GPS equipment. A error margin of ±0.138 m for is estimated

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

A.2 Raw HOBO water level logger (total and barometric pressure) observations (Nov/Dec 2011 – May 2012). A.2.1 Files

Data model Howie Island HOBO U20 water level logger observations 26 Nov 2011 – 08 May 2012 (howie_water_level_logger_1_1_GMT+10.hobo; howie_water_level_logger_1_2_GMT+10.hobo; howie_water_level_logger_1_3_GMT+11.hobo) Kangaroo Island HOBO U20 water level logger observations 26 Nov 2011 – 07 May 2012 (kangaroo _water_level_logger_2_1_GMT+10.hobo; kangaroo_water_level_logger_2_2_GMT+10.hobo; kangaroo _water_level_logger_2_3_GMT+11.hobo) Welcome Inlet HOBO U20 water level logger observations 23 Dec 2011 – 07 May 2012 (welcome_water_level_logger_3_1_GMT+10.hobo; welcome_water_level_logger_3_2_GMT+10.hobo; welcome _water_level_logger_3_3_GMT+11.hobo) Stony Point HOBO U20 barometric logger observations 26 Nov 2011 – 07 May 2012 (stonypoint_barometric_logger_3_1_GMT+10.hobo; stonypoint_barometric_logger_3_2_GMT+10.hobo; stonypoint_barometric_logger_3_3_GMT+11.hobo)

Type

Onset HOBO U20 (.hobo) data file

Description Raw observed barometric and total pressure downloaded from data loggers. Datasets span time period of 26 Nov 2011 to 08 May 2012 for Howie Island and Kangaroo Island, and 23 Dec 2011 to 07 May 2012 and are divided into separate blocks according to periodic field download. All data is recorded at 5 minute intervals. Notes

There are errors with the timing within the HOBO dataset. The times in the first and second (only) data downloads for all loggers are in UTC + 10 hours (EST), and thus their header files stating “Launch GMT offset: 11 Hr 00 Min” is incorrect. To plot data in EST (UTC + 10hrs) in HOBOware Pro, set the “offset from GMT” to "+11" hours in the plot setup. The times in the third data download for all loggers in UTC + 11. To plot data in EST (UTC + 10hrs) in HOBOware Pro, set the “offset from GMT” to "+10" hours in the plot setup.

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Appendix Table 31: Howie Island HOBO data files summary information.

File info

Howie Island HOBO data files

File name

howie_water_level_logger_1_1_G MT+10.hobo

howie_water_level_logger_1_2_G MT+10.hobo

howie_water_level_logger_1_2_G MT+11.hobo

UTC time offset

UTC + 10 hours

UTC + 10 hours

UTC + 11 hours

File start

27/11/2011 9:00:00 AM

21/12/2011 2:45:00 PM

23/02/2012 8:20:00 AM

File end

21/12/2011 2:40:00 PM

Data series

 

23/02/2012 7:15:00 AM  Date time (UTC + 10)  Absolute (water) pressure (kPa)  Temperate (°C)

8/05/2012 3:55:00 PM  Date time (UTC + 11)  Absolute (water) pressure (kPa)  Temperate (°C)

10011600

10011600

Logger serial number

Date & time (UTC + 10) Absolute (water) pressure (kPa)  Temperate (°C) 10011600

Launch description

water level logger_1

water level logger_1

water level logger_1

Deployment number

11

12

13

Notes

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+10" hrs in plot setup; Deployment info in file header is correct: “Launch GMT offset” IS “11 Hr 00 Min”.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania Table 32: Kangaroo Island HOBO data file summary information.

File info

Kangaroo Island HOBO data files

File name

kangaroo_water_level_logger_2_ 1_GMT+10.hobo

kangaroo_water_level_logger_2_ 2_GMT+10.hobo

kangaroo_water_level_logger_2_ 2_GMT+11.hobo

UTC time offset

UTC + 10 hours

UTC + 10 hours

UTC + 11 hours

File start

27/11/2011 9:00:00 AM

21/12/2011 5:50:00 PM

22/02/2012 10:35:00 AM

File end

21/12/2011 5:45:00 PM

Data series

 

22/02/2012 9:30:00 AM  Date time (UTC + 10)  Absolute (water) pressure (kPa)  Temperate (°C)

7/05/2012 6:10:00 PM  Date time (UTC + 11)  Absolute (water) pressure (kPa)  Temperate (°C)

10011601

10011601

Logger serial number

Date & time (UTC + 10) Absolute (water) pressure (kPa)  Temperate (°C) 10011601

Launch description

water level logger_2

water level logger_2

water level logger_2

Deployment number

8

9

10

Notes

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+10" hrs in plot setup; Deployment info in file header is correct: “Launch GMT offset” IS “11 Hr 00 Min”.

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Appendix Table 33: Welcome Inlet HOBO data files summary information.

File info

Welcome Island HOBO data files

File name

welcome_water_level_logger_1_3 _GMT+10.hobo

welcome_water_level_logger_3_2 _GMT+10.hobo

welcome_water_level_logger_3_2 _GMT+11.hobo

UTC time offset

UTC + 10 hours

UTC + 10 hours

UTC + 11 hours

File start

NA

23/12/2011 3:25:00 PM

22/02/2012 6:15:00 PM

File end

NA

Data series

 

22/02/2012 5:10:00 PM  Date time (UTC + 10)  Absolute (barometric) pressure (kPa)  Temperate (°C)

8/05/2012 1:50:00 AM  Date time (UTC + 11)  Absolute (barometric) pressure (kPa)  Temperate (°C)

10011602

10011602

Logger serial number

Date & time (UTC + 10) Absolute (barometric) pressure (kPa)  Temperate (°C) 10011602

Launch description

water level logger_2

water level logger_2

water level logger_2

Deployment number

11

12

13

Notes

Launch failed, data series blank.

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+10" hrs in plot setup; Deployment info in file header is correct: “Launch GMT offset” IS “11 Hr 00 Min”.

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania Table 34: Stony Point HOBO data files summary information.

File info

Stony Point HOBO data files

Data file

stonypoint_barometric_logger_1_ 1_UTC+10.hobo

stonypoint_barometric_logger_1_ 2_ UTC +10.hobo

stonypoint_barometric_logger_1_ 2_ UTC +11.hobo

UTC time offset

UTC + 10 hours

UTC + 10 hours

UTC + 11 hours

File start

27/11/2011 9:00:00 AM

21/12/2011 1:35:00 PM

23/02/2012 9:25:00 AM

File end

21/12/2011 1:30:00 PM

Data series

8/05/2012 5:05:00 PM  Date time (UTC + 11)  Barometric pressure (kPa)  Temperate (°C)

Logger serial number

 Date & time (UTC + 10)  Barometric pressure (kPa)  Temperate (°C) 9991888

23/02/2012 8:20:00 AM  Date & time (UTC + 10)  Barometric pressure (kPa)  Temperate (°C) 9991888

9991888

Launch description

barometric_logger_1

barometric_logger_1

barometric_logger_1

Deployment number

8

9

10

Notes

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+11" hrs in plot setup; Deployment info in file header is incorrect!: “Launch GMT offset” IS “10 Hr 00 Min” (and NOT “11 Hr 00 Min” as recorded in file).

To plot data in EST (UTC + 10hrs) in HOBOware Pro, set “offset from GMT” to "+10" hrs in plot setup; Deployment info in file header is correct: “Launch GMT offset” IS “11 Hr 00 Min”.

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Appendix

A.3 Water level logger observations and corrected sea levels (Nov/Dec 2011 – May 2012). A.3.1 Files

Data model Howie Island sea level observations 26 Nov 2011 – 08 May 2012 (howie_sea-level_observations_nov2011_may2012.csv) Kangaroo Island sea level observational data 26 Nov 2011 - 07 May 2012 (kangaroo_ sea-level_observations_nov2011_may2012.csv) Welcome Inlet sea level observational data 23 Dec 2011 – 07 May 2012 (welcome_sea-level_observations_dec2012_may2012.csv)

Type

Comma-separated value (CSV) files

Description Observed barometric and total pressure, corrected water depths and sea levels. Datasets span time period of 26 Nov 2011 to 08 May 2012 for Howie Island and Kangaroo Island, and 23 Dec 2011 to 07 May 2012. All data is recorded at 5 minute intervals. Table 35: Data model for the water level logger observations and corrected sea levels data files.

Colum header

Data attributes

Comments

Date Time UTC (UTC + 0)

Date and time of data in Coordinated Universal Time (UTC).

Refers to date and time of observed data. This date coincides with the time data in UTC.

Stated in 24 hour format as: DD/MM/YYYY HH:MM Date Time EST (UTC+10)

SP Pressure, Barometric (kPa)

Date and time of data in Australian Eastern Standard Time (UTC + 10).

Refers to date and time of observed data in EST (UTC + 10 hours; above).

Stated in 24 hour format as: DD/MM/YYYY HH:MM

Note that day light saving time is disregarded.

Barometric pressure in kilopascals (kPa)

Barometric pressure observations was observed from Stony Point (Montagu) only, from near sea level. This site was located some 1 km from Howie Island, 11.5 km from Kangaroo Island and 17 km from Welcome Inlet Island sites.

This data shows the variation in barometric pressure observed over time at Stony Point

This regional barometric pressure data was used to correct all three absolute (water) pressure observational datasets to water level

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Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

HI/KI/WI Pressure, Total (kPa)

Total (water) pressure in kilopascals (kPa) This data shows the variation in total (water + air) submerged pressure observed over time at Howie Island (HI), Kangaroo Island (KI) and Welcome Inlet (WI).

Total pressure observations were observed from the near shore offshore remote pressure sensors, deployed in remote stilling wells at Howie Island (HI), Kangaroo Island (KI) and Welcome Inlet (WI). Each site specific total pressure dataset was corrected to water level with the regional barometric pressure data from to Stony Point. Note that the total pressure equals the barometric pressure when the loggers’ sensor periodically became exposed to the air during notably low tides.

HI/KI/WI Sensor Depth (m)

Depth of the water level logger pressure sensor in metres (m) This data shows the variation in water level above the submerged pressure sensor over time at Howie Island (HI), Kangaroo Island (KI) and Welcome Inlet (WI).

HI/KI/WI Observed Sea Level (m, AHD)

Elevation of the sea level in relation to the Australian Height Datum (AHD). This data shows the variation in sea level over time at Howie Island (HI), Kangaroo Island (KI) and Welcome Inlet (WI).

Page 84 of 88

Depth of the sensor has been converted from each sites total pressure data and the regional barometric pressure data. Note that the water level equals 0 m (±0.005 m) equals when the loggers’ sensor periodically became exposed to the air during notably low tides. Sea level has been calculated from the surveyed loggers’ sensor elevation, derived from a Differential GPS survey. Note that a null value (“NaN”) has been given to those times when the loggers’ pressure sensor became exposed to the air. As sea level were not recorded for these times (as they had receded beneath the loggers’ sensor).


Appendix

A.4 Observed sea levels, predicted astronomical tides and nontidal residuals (Nov/Dec 2011 – May 2012). A.4.1 Files

Data model Howie Island observed and predicted sea levels, and non-tidal residual 26 Nov 2011 – 08 May 2012 (howie_observed_predicted_sea-levels_nov2011_may2012.csv) Kangaroo Island sea level observational data 26 Nov 2011 – 07 May 2012 (kangaroo_observed_predicted_sea-levels_nov2011_may2012.csv) Welcome Inlet sea level observational data 23 Dec 2011 - 07 May 2012 (welcome_observed_predicted_sea-levels_dec2011_may2012.csv)

Type

Comma-separated value (CSV) files

Description Sea level observations, astronomical tide predictions and non tidal residual computed for the three survey locations. Datasets span time period of 26 Nov 2011 to 08 May 2012 for Howie Island and Kangaroo Island, and 23 Dec 2011 to 07 May 2012. All data is recorded at 5 minute intervals. Tide predictions produced from tidal analysis of observed data using T-TIDE V1.3. Table 33: Data model for the observed sea levels, predicted astronomical tides and non-tidal residuals data files.

Colum header

Data attributes

Comments

Date Time UTC (UTC + 0)

Date and time of data in Coordinated Universal Time (UTC).

Refers to date and time of observed data in UTC.

Stated in 24 hour format as: DD/MM/YYYY HH:MM Date Time EST (UTC + 10)

HI/KI/WI Observed Sea Level (m, AHD)

Date and time of data in Australian Eastern Standard Time (UTC + 10).

Refers to date and time of observed data in EST (UTC + 10 hours; above).

Stated in 24 hour format as: DD/MM/YYYY HH:MM

Note that day light saving time is disregarded.

Observed elevation of the sea level in relation to the Australian Height Datum (AHD).

Sea level has been calculated from the site specific total (subaqueous) pressure and regional barometric pressure observations.

This data shows the variation in sea level over time at Howie Island (HI), Kangaroo Island (KI) or Welcome Inlet (WI).

Page 85 of 88

Note that a null value (“NaN”) has been given to those times when the loggers’ pressure sensor became exposed to the air during periods of notably low tides.


Tide and seagrass sedimentology report, Robbins Passageâ&#x20AC;&#x201C;Boullanger Bay, Tasmania

HI/KI/WI Predicted Sea Level (m, AHD)

Predicted elevation of the astronomical tides in metres above the Australian Height Datum (AHD). This data shows the predicted variation in tides over time at Howie Island (HI), Kangaroo Island (KI) or Welcome Inlet (WI).

The astronomical tides have been predicted in MATLAB based T-TIDE (version 1.3), from summing the 21-23 tidal constituents which were solved for each site from undertaking a tidal analysis on their corresponding observational data. The predicted tides account for 97% of the observed data.

HI/KI/WI Non-Tidal Residual (m)

Non-tidal residual of the predicted tides in metres. This data shows the variation in water level heights between the observed and predicted sea level data for Howie Island (HI), Kangaroo Island (KI) or Welcome Inlet (WI).

Page 86 of 88

Non-tidal residual is calculated by subtracting the predicted tides from the observed tides. This difference between the observed (total) and predicted (astronomical) sea levels are largely due to environmental effects e.g. weather tides, shallow water conditions).


Appendix

A.5 Predicted astronomical tide cycle (June 2012– June 2022). A.5.1 Files

Data model Howie Island predicted astronomical tides 01 Jun 2012 – 01 Jun 2022 (howie_predicted_sea-levels_jun2012_jun2022.csv) Kangaroo Island predicted astronomical tides 01 Jun 2012 – 01 Jun 2022 (kangaroo_predicted_sea-levels_jun2012_jun2022.csv) Welcome Inlet predicted astronomical tides 01 Jun 2012 - 01 Jun 2022 (welcome_predicted_sea-levels_jun2012_jun2022.csv)

Type

Comma-separated value (CSV) files

Description Astronomical tide predictions computed for the three survey locations. All tide predictions are computed from 01 June, 2012 to 01 June, 2022 at 30 minute intervals, produced from tidal analysis of observed data using TTIDE V1.3. Table 34: Data model for the predicted astronomical tide cycle data files.

Colum header

Data attributes

Comments

Date Time UTC (UTC + 0)

Date and time of data in Coordinated Universal Time (UTC).

Refers to date and time of observed data in UTC.

Stated in 24 hour format as: DD/MM/YYYY HH:MM Date Time EST (UTC + 10)

HI/KI/WI Predicted Sea Level (m, AHD)

Date and time of data in Australian Eastern Standard Time (UTC + 10).

Refers to date and time of observed data in EST (UTC + 10 hours; above).

Stated in 24 hour format as: DD/MM/YYYY HH:MM

Note that day light saving times are disregarded.

Predicted elevation of the astronomical tides in metres above the Australian Height Datum (AHD).

The astronomical tides have been predicted in MATLAB based T-TIDE (version 1.3) from summing the 21-23 tidal constituents which were solved for each site from undertaking a tidal analysis on their corresponding observational data.

This data shows the predicted variation in tides over time at Howie Island (HI), Kangaroo Island (KI) or Welcome Inlet (WI).

The predicted tides account for 97% of the observed data.

Page 87 of 88


Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania

A.6 Predicted astronomical high and low tides (June 2012– June 2022). A.6.1 Files

Data model Howie Island predicted high and low astronomical tides 01 June 2012 – 01 June 2022 (howie_predicted_high-low_tides_jun2012_jun2022.csv) Kangaroo Island predicted high and low astronomical tides 01 June 2012 - 01 June 2022 (kangaroo_predicted_high-low_tides_jun2012_jun2022.csv) Welcome Inlet predicted high and low astronomical tides 01 June2012 – 01 June 2022 (welcome_predicted_high-low_tides_jun2012_jun2022.csv)

Type

Comma-separated value (CSV) files

Description High and low tide predictions for the three survey locations. All tide predictions are computed from 01June, 2012 to 01June, 2022 to the closest 5 minute interval, produced from tidal analysis of observed data using T-TIDE V1.3. Table 35: Data model for the predicted astronomical high and low tides data files.

Colum header

Data attributes

Comments

Date Time UTC (UTC + 0)

Date and time of data in Coordinated Universal Time (UTC).

Refers to date and time of observed data in UTC.

Stated in 24 hour format as: DD/MM/YYYY HH:MM Date Time EST (UTC + 10)

HI/KI/WI Predicted Sea Level (m, AHD)

Date and time of data in Australian Eastern Standard Time (UTC + 10).

Refers to date and time of observed data in EST (UTC + 10 hours; above).

Stated in 24 hour format as: DD/MM/YYYY HH:MM

Note that day light saving times are disregarded.

Predicted elevation of the astronomical tides in metres above the Australian Height Datum (AHD).

The astronomical tides have been predicted in MATLAB based T-TIDE (version 1.3), from summing the 21-23 tidal constituents which were solved for each site from undertaking a tidal analysis on their corresponding observational data.

This data shows the predicted variation in tides over time at Howie Island (HI), Kangaroo Island (KI) or Welcome Inlet (WI).

The predicted tides account for 97% of the observed data. Page 88 of 88

Profile for Cradle Coast Tasmania

The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage  

Technical Report: The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far northwest T...

The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage  

Technical Report: The tidal characteristics and shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far northwest T...