River forth model aberfoyle recalibration final report

Stirling Council Aberfoyle Flood Study

River Forth Hydrodynamic Model Update and Recalibration Report

June 2010

Notice This document and its contents have been prepared and are intended solely for Stirling Council’s information and use in relation to the River Forth Model Recalibration.

Atkins assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents.

Document History JOB NUMBER: 5025898

DOCUMENT REF: 5025898/60/DG/002

1

Final Issue

RH

RS

TR

SE

June 2010

0.1

Draft for Client Comment

RS

EM

TR

SE

March 2010

EJA Revision

Purpose Description

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Originated

Checked

Reviewed

Authorised

Date

River Forth Hydrodynamic Model Update and Recalibration

Contents Section

Page

1.

Introduction

5

2.

Review and Update of Hydrological Analysis

6

2.1 2.2 2.3 2.4 2.5

Schematisation of the Model FEH Statistical Method FEH Rainfall Runoff Method Comparison of Hydrological Methods Conclusions and Final Design Estimations

6 6 7 7 9

3.

Review of the River Forth Hydraulic Model

11

3.1

Refining Out-of-Bank Conveyance

11

4.

Calibration of the Updated Hydraulic Model

15

4.1 4.2 4.3 4.4

Introduction Updated Model Calibration Model Verification Sensitivity to Hydraulic Model Revisions

15 16 24 29

5.

Flood Extents

30

6.

Conclusions and Recommendations

31

6.1 6.2

Conclusions Recommendations

31 31

List of Tables Table 3.1 – Log of Changes to the Revised Hydraulic Model 12 Table 4.1 – Rainfall Gauge Details 17 Table 4.2 – Comparison of Observed and Final Model Flows and Levels for Milton Gauge, September 2009 event 19 Table 4.3 – Final Calibration Parameter Factors for Milton Gauge 19 Table 4.4 – Comparison of Observed and Final Model Levels at Aberfoyle, 8th September 2009 25 th Table 4.5 – Final Parameter Adjustments to Reproduce Observed Water Levels at Aberfoyle, 8 September 2009 25

List of Figures Figure 3.1 – Original Model Flood Plain Storage Reservoirs Figure 3.2 – Updated Model Extended Sections Figure 4.1 – Location of Milton Gauge.

Figure 4.2 – Location of River Level Monitoring Station, Aberfoyle Figure 4.3 – Rainfall Data and Gauged Flows June-September 2009 Figure 4.4 – Modelled and Gauged Flows at Milton July – September 2009 Figure 4.5 – Modelled and Gauged River Stage at Milton July – September 2009 Figure 4.6 – Modelled and Gauged Flows at Milton 5th – 20thSeptember 2009 Figure 4.7 – Modelled and Gauged River Stage at Milton 5th – 20thSeptember 2009 Figure 4.8 – 8th September 2009 Event: Comparison of Observed and Modelled River Stage at Aberfoyle River Forth Model Recalibration V 0.4.doc

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River Forth Hydrodynamic Model Update and Recalibration

Appendices Appendix A – FEH Calculation Record Appendix B – Flood Maps for Updated Model Appendix C – Maximum Modelled River Stage

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Glossary of Terms Term

Meaning/Definition

AEP

Annual Exceedance Probability

DTM

Digital Terrain Model

SEPA

Scottish Environment Protection Agency

FEH

Flood Estimated Handbook

m AOD

Meter Above Ordinance Datum

Event Severity The severity of the events discussed in this document are defined as Annual Exceedance Probabilities (AEP), the table below provides a summary of AEP and corresponding Return Periods.

The AEP is the probability that there will be an event exceeding a particular severity in any one year. The Return Period is the average duration (in years) between events of a particular severity.

Annual Exceedance Probability

Return Period

50%

1 in 2 years

10%

1 in 10 years

5%

1 in 20 years

2%

1 in 50 years

1%

1 in 100 years

0.5%

1 in 200 years

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1.

Introduction Atkins previously completed the Aberfoyle River Forth Flood Study Hydraulic Modelling Report (2009). This study involved building a hydraulic model of the River Forth at Aberfoyle which extended to include the tributary, the Avon Dhu, and the whole of Loch Ard. The model was calibrated using gauged event data provided by SEPA, with the primary objective of assessing flood risk to Aberfoyle, Kinlochard and to the B829. For details on hydraulic model build, sensitivity testing and assessment of flooding mechanisms reference should be made to the 2009 modelling report.

Since the 2009 study was completed there was a significant flood event on 8th September 2009.

This event resulted in flooding along the Main Street in Aberfoyle. On the basis of the hydrological analysis already undertaken and the flood maps produced by the Aberfoyle Flood Study the frequency of the event that would be required to generate this extent of fluvial flooding would be estimated at approximately 0.5% AEP. In conjunction with other recent flooding from the Forth at Aberfoyle in 2006 and also historic flooding in 1950, 1979, 1986, 2001 and 2005, it was considered that the existing model was under-predicting flood extents and/or frequency.

Therefore, Atkins was commissioned to undertake an additional calibration exercise on the model using observed flow and level data from the 8th September 2009 event. This also provided the opportunity to review and if necessary to revise the design hydrology if it was considered that this was required to generate a more realistic flood-frequency relationship within the study area.

Since the original study was completed additional data has become available in the form of recorded river levels at the Manse Road bridge in Aberfoyle. This was recorded by a flood warning level gauge that was installed by Stirling Council in 2008. This additional data allowed for a more comprehensive validation than was possible in the original modelling study by providing flood levels for the 8th September 2009 event at Aberfoyle as well as gauge data further upstream on the Avon Dhu at Milton. As a result of validating the model at two locations this increases the level of confidence in the model’s performance.

The objectives of this study were: 1. Obtain recent flooding information from SEPA and Stirling council including rain gauge data for the Duchray Castle, Loch Katrine and High Corrie, flow gauging data from Milton and observed flood levels for Manse Road Bridge, Aberfoyle and the Aberfoyle Main Street area for the 8th September 2009 event , along with anecdotal evidence of flooding and soil moisture deficit data; 2. Review the design flows using the FEH Version 2.0 (2006) in accordance with the latest guidelines and if appropriate adopt a revised approach to provide a better agreement with observed flood extents and flood frequency; 3. Review and, if appropriate to, update the existing hydraulic model to improve the modelling of out-of-bank flows and the mapping of flood extents to provide a better agreement with observed flood extents and flood frequency; 4. Calibrate the hydraulic model to the Milton flow gauge data for 8th September 2009 event; and, 5. Update the flood risk maps using flood outlines and levels from the design events routed through the hydraulic model.

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2.

Review and Update of Hydrological Analysis Following a significant flood event in Aberfoyle in September 2009 a review of the hydrological model has been undertaken taking into consideration recent developments in the Flood Estimation Handbook (FEH) and additional available hydrometric data.

2.1 Schematisation of the Model In order to produce a detailed hydrological model of the catchment, the catchment was subdivided using information from the FEH CD-ROM, with sub-catchments based on direct tributary inflows and lateral flow routes to the main channel.

In addition to the direct inflows and lateral inflows, Points of Interest (POIs) were identified, and catchments delineated, at strategic locations along the model reaches. There are POIs located at the Milton gauge, the Duchray direct inflow and the downstream extent of the model. The POIs along the main channel were used to identify critical storm durations, which change along the length of the watercourse.

FEH catchment descriptors were obtained from the FEH CD-ROM Version 2.0 for each direct inflow and POI. There were no changes to the schematisation of the model or the catchment descriptors as part of the review and update of the hydrological analysis.

2.2 FEH Statistical Method At the POIs, peak flows for the required return periods were estimated using the FEH statistical method. The methodology is described in detail in the previous report (Aberfoyle River Forth Flood Study Hydraulic Modelling Report, Atkins 2009). The updated hydrological analysis for Aberfoyle was based on the most up to date HiFlows data which superseded the previous work undertaken. As part of the update to HiFlows a review of the suitability of the gauge for QMED and pooling analysis is undertaken and additional AMAX and POT data are included.

The updates to HiFlows has resulted in different stations being included in the calculation of QMED and the derivation of the pooling group to derive a growth curve for each of the POIs. For each subject site, the product of QMED and the growth curve was used to obtain a peak flow for each return period. Table 2.1 shows the resulting statistical estimates of peak flows.

Appendix A presents the calculation records for these analyses.

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Table 2.1: Peak flows estimated using the FEH statistical method Name

Flood peak (m3/s) for the following percentage chance of an event occurring in any one year (with return periods in years in brackets).

50% (2)

Duchray Milton Gauge d/s model extent

20% (5)

10% (10)

5% (20)

2% (50)

1% (100)

0.5% (200)

83.65

104.50

118.93

133.92

155.59

173.86

194.07

10.30

13.01

14.90

16.88

19.76

22.20

24.92

74.99

94.23

107.64

121.64

141.99

159.23

178.38

2.3 FEH Rainfall Runoff Method The rainfall-runoff method estimates flows by examining the relationship between rainfall and the hydrological response of a catchment to a storm event. Catchment descriptors for each direct inflow, lateral inflow and POI were entered into ISIS FEH boundary units (FEHBDY) and flow hydrographs were estimated for each return period required. The inflows were routed through the hydraulic model, and as expected significant attenuation of the hydrographs was observed, due primarily to Loch Ard. Peak flow estimates for each of the direct inflows and POIs are given in Table 2.2. It should be noted that these values are pre-calibration flows.

Table 2.2: Rainfall Runoff peak flow estimates – Precalibration flows

Name

Duchray Milton Gauge d/s model extent

Critical storm duration (hours)

Flood peak (m3/s) for the following percentage chance of an event occurring in any one year (with return periods in years in brackets).

50% (2)

20% (5)

10% (10)

5% (20)

2% (50)

1% (100)

0.5% (200)

11.4

63.0

87.0

104.1

123.3

150.0

170.7

195.2

8.3

35.3

48.8

58.8

70.0

85.6

97.8

112.2

11.0

105.8

147.1

176.0

208.5

253.9

288.9

330.6

Calibration of the rainfall runoff models was undertaken for four events and the calibrated model was then verified for the September 2009 event as discussed in Section 4.

2.4 Comparison of Hydrological Methods The peak flows calculated using the FEH rainfall-rainfall and statistical methods are summarised in Figure 2.1.

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Figure 2.1: FEH peak flows

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The peak flow estimates generated by the different methods have increasing disparity for the higher return periods for the Milton Gauge and d/s model extent as the rainfall runoff model does not take into consideration the hydraulic attenuation from Loch Ard. Further development of the rainfall runoff method was undertaken including calibration and routing of the rainfall runoff flows through the hydraulic model.

2.5 Conclusions and Final Design Estimations The standard approach taken in a hydrological analysis for flood risk mapping purposes is to use a combination of FEH statistical and rainfall runoff techniques to generate a design flood hydrograph at each of the points of interest.

The rainfall runoff method is used to provide a series of sub-catchment inflows which are routed through the catchment by means of a hydraulic model. In this study, the rainfall runoff model has been calibrated, and as a result decreases in SPR have been applied across the catchment (see Section 4).

An analysis of critical storms at various locations around the catchment has been undertaken to determine the design rainfall parameters for the catchment. The design rainfall was used to generate flows within the hydraulic model for the 50%, 20%, 10%, 5%, 2% 1% and 0.5% AEP events (the 2, 5, 10, 20, 50, 100 and 200 year return periods respectively). Comparison of the statistical and rainfall runoff flows indicated that while rainfall runoff flows were generally higher than the statistical flows, the effects of hydraulic attenuation have a significant effect on the hydrological analysis for this catchment.

Design flows were derived from FEH rainfall-runoff estimates. The critical storm area, duration and profile applied were that for the d/s model extent catchment.

The FEH rainfall-runoff design storm models were then scaled to fit the statistical peak flow at the Milton Gauge and Duchray POIs. Table 2.3 outlines the peak flow of the sub-catchment inflows for each of the return periods.

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Table 2.3 – Final Flood Estimations Name

50%

20%

10%

5%

2%

1%

0.5%

Loch Ard

54.49

70.35

81.62

92.21

108.06

118.11

128.62

Duchray

87.31

115.01

133.78

159.21

191.26

221.21

265.16

Lower Ard

6.28

8.11

9.47

10.76

12.68

13.91

15.19

Lateral

4.46

5.85

6.82

8.11

9.74

11.26

13.48

Trib 1

8.13

10.64

12.38

14.73

17.68

20.43

24.47

Trib 2

8.53

11.19

13.05

15.55

18.70

21.64

25.95

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3.

Review of the River Forth Hydraulic Model The InfoWorks RS hydraulic model was reviewed to consider all possible options for improving the way in which the model represents and maps flood extents, so as to replicate as well as possible the September 2009 event.

3.1 Refining Out-of-Bank Conveyance Following the flooding of the Aberfoyle Main Street and Kinlochard Road areas on 8th/9th September 2009 it was considered prudent to review the implementation of extended river crosssections and flood plain storage areas in the model. This was done in order to assess if any improvements in the representation of conveyance and storage of flows on the floodplain could be identified, with a view to improving the final mapped flood extents.

The area of flood plain along the left over-bank of the River Forth at the B829 Kinlochard Road between Milton and Aberfoyle was previously modelled using three flood storage reservoirs and associated lateral spill units to represent bank tops. Whilst this approach is standard in 1D modelling where flood plain areas lie partially below bank top level it does not model conveyance within or between storage reservoirs and storage areas do not contain any data on upstream to downstream length. As a result, the storage reservoirs act as standing bodies of water with instantaneous flow occurring across the spill units once they are overtopped by water levels.

Lowering spill co-efficients values to 0.8 to 1.0 does act to counter this effect to a degree but flood waters may still be modelled as being conveyed from upstream to downstream with reduced travel time and therefore potentially under-predict stage. This often may not be a significant factor in estimating flood extents, but where properties are within the flood plain storage reservoirs this could be worth investigating if it is felt that the model is under-predicting flood extents or depths.

Therefore, it was considered appropriate to modify the model between Milton and Aberfoyle to quantify any effect this may have had on modelled peak stage. The changes made to the model are given in Table 3.1. Whilst the changes in peak water levels are minimal (no more than 0.001m difference at any model node), the revised model arrangement with extended river sections was taken forward for the final recalibration and flood mapping exercise as it was considered to be technically more representative for this particular location. Sensitivity of the model to these changes is further discussed in section 4.4.

The updated extended sections compared to the original model arrangement are displayed in the Figures 3.1 and 3.2.

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Table 3.1 – Log of Changes to the Revised Hydraulic Model

River Cross-Sections Extended

Spill Units Removed

Flood Storage Reservoirs Removed

855

FP Res 05 Spill A

FP Res 05

0973 D

FP Res 09

FP Res 05 Spill B 0973 E

FP Res 09

FP Res 05 Spill D 1150 FP Res 05 Spill E 1338 FP Res 05 Spill F 1473 FP Res 10 Spill A 1683 FP Res 10 Spill B 1873 FP Res 10 Spill C 2098 FP Res 9 Spill A 2452 FP Res 9 Spill B 2792 2817 A 2817 B 2856 2868 2990 3065 int 3090 int

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Figure 3.1 – Original Model Flood Plain Storage Reservoirs

Reproduced by permission of Ordnance Survey on behalf of HMSO. Š Crown copyright and database right. All rights reserved. Ordnance Survey Licence number 0100020780 River Forth Model Recalibration V 0.4.doc

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River Forth Hydrodynamic Model Update and Recalibration

Figure 3.2 – Updated Model Extended Sections

Reproduced by permission of Ordnance Survey on behalf of HMSO. Š Crown copyright and database right. All rights reserved. Ordnance Survey Licence number 0100020780 River Forth Model Recalibration V 0.4.doc

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River Forth Hydrodynamic Model Update and Recalibration

4.

Calibration of the Updated Hydraulic Model 4.1 Introduction The updated hydraulic model was calibrated to the flood event of 8th September 2009. The observed flow and river stage were recorded at the SEPA gauge at Milton on the Avon Dhu. This gauge is located on the downstream face of a masonry arch road bridge approximately 300m upstream of the confluence of the Avon Dhu and the Duchray Water. The SEPA gauge is located at cross-section 3127 E in the hydraulic model (see Figure 4.1). No additional review of the Milton gauge rating was undertaken on top of that described in the Aberfoyle River Forth Flood Study Hydraulic Modelling Report (2009).

Figure 4.1 – Location of Milton Gauge.

Reproduced by permission of Ordnance Survey on behalf of HMSO. © Crown copyright and database right. All rights reserved. Ordnance Survey Licence number 0100020780

The results of the calibration are outlined in section 4.2. A further model validation exercise was undertaken using recorded peak river levels in Aberfoyle at the Manse Road bridge (model section 0973 D – see figure 4.2) and a post-event trash line survey carried also in Aberfoyle, both provided by Stirling Council. This is described in section 4.3. As some parts of the model had River Forth Model Recalibration V 0.4.doc

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been updated, as described in Section 3.1, the flow and stage output for each version of the model were compared to check the sensitivity of the model to these changes. This has been outlined in section 4.4.

Figure 4.2 – Location of River Level Monitoring Station, Aberfoyle

Reproduced by permission of Ordnance Survey on behalf of HMSO. Š Crown copyright and database right. All rights reserved. Ordnance Survey Licence number 0100020780

4.2 Updated Model Calibration The observed event used for the calibration was the 8th September 2009 events. Observed rainfall from this event was used to generate hydrographs at the inflow boundaries of the model.

Rainfall data was available for three sites in the Callander area; Loch Katrine, Duchray Castle and High Corrie. Only the Duchray Castle gauge is in the Aberfoyle catchment with the Loch Katrine raingauge located to the north of Aberfoyle in the Eas Gobhain catchment and the High Corrie raingauge south of the Aberfoyle catchment within the Kelty Water catchment. A summary of gauge characteristics and data availability is presented in Table 4.1 below. These are the same rain gauge stations that were used in the original model. The period of record requested was in order to provide data collected after 1st April 2008, which was the cut-off date of the data used previously and to specifically cover the period up to and including the 8th September 2009 event.

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Table 4.1 – Rainfall Gauge Details

Station name

Catchment

Observation

Grid ref

Period of record provided for calibration update

01/04/08 – 15/11/08, Duchray Castle

Loch Ard Burn

Tipping Bucket

NS467987

30/01/09 – 11/06/09, and 28/08/09 30/09/09

01/04/08 – 11/04/08, and Loch Katrine

Eas Gobhain

Tipping Bucket

NS491065 28/05/08 – 30/09/09

01/04/08 – 01/05/08, 21/06/08 – 09/08/08, High Corrie (Gartmore)

Kelty Water

Tipping Bucket

NS479953

09/10/08 – 14/11/08, 16/01/09 – 20/07/09, and 27/08/09 – 25/09/09

The observed flow and stage from the SEPA Milton river gauge on the Avon Dhu were available for the period from 1st July 2008 to 30 September 2009. The gauged flow data from the Milton gauge was plotted along with the rainfall data for the three rain gauges (Figure 4.3). Visual inspection of the plotted data shows that the 8th September event was the last of a number of hydrograph peaks that can be considered as together comprising a single large event beginning on or around the 2nd July. This period of protracted wet weather would have resulted in increased soil moisture levels across the catchment and high antecedent water levels in Loch Ard prior to the 8th September event. As a result the catchment would have shown an increased response to rainfall by the time of the flood event compared to more typical summer conditions.

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Figure 4.3 – Rainfall Data and Gauged Flows June-September 2009

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The initial approach adopted to calibrate the model was to use the Loch Katrine rainfall to simulate the event starting on 2nd July 2009 and running through to 30th September 2009. Due to the percentage runoff being fixed throughout the duration of a FEH rainfall-runoff model simulation, in order to match the hydrograph peak of interest the model therefore tends to over-predict flow during the earlier parts of the event. This illustrates the limitations of the FEH rainfall-runoff model for simulating long multi-peaked events but this does not have any implications for the modelling of the main flood peak that this report is concerned with. In order to speed up model run times the event was truncated to the period 5th September to 20th September to model the peak of interest.

Initial conditions in Loch Ard for 5th September were taken from an initial run of the full event using the final calibration parameters from the original 2009 calibration and then finalised using the adopted adjustment parameters once the main peak had been calibrated. Catchment Wetness Index was calculated separately for 2nd July and 5th September depending on the date when the modelled event was started. Output for model runs starting on 2nd July are shown in Figures 4.4 and 4.5. The final calibration plots for stage and flow at the Milton gauge (5th to 20th September 2009) are compared with the model output taken from river section 3127 E for the event in Figures 4.6 and 4.7 and results are presented in Table 4.2.

In order to achieve the final calibration at the Milton gauge, rainfall-runoff boundary parameters Tp (Time to Peak) and SPR (Standard Percentage Runoff) were adjusted for the inflows upstream of the Milton gauge in Loch Ard. The final parameter adjustments are summarised in Table 4.3.

SPR was adjusted by a factor of 0.7 (-30% adjustment to the default value) compared to an adjustment by a factor of 0.8 in the original model calibration. The model generates slightly lower flow and higher stage as a result of differences in the model and gauge rating curves, as discussed in the Aberfoyle River Forth Flood Study Hydraulic Modelling Report (2009). It should be noted though that the largest check gauging flow and stage available was 5.2m3/s and 21.148m AOD, which is below the flow and stage of the September 2009 event and as such it is surmised that the theoretical rating relationship provided by SEPA may be inaccurate at higher flows at or exceeding bank top. As this study achieved the set calibration standard of within 150mm to the observed stage values, no further adjustment was made to the model.

The main observation is that the timing and magnitude of the peak give a reasonable fit to the recorded data and, as a result, the model is considered to be replicating the hydrology and hydraulics of the catchment upstream of the Milton gauge sufficiently accurately for the purposes of this flood study.

Table 4.2 – Comparison of Observed and Final Model Flows and Levels for Milton Gauge, September 2009 event

Period

Observed Peak Flow (m3/s)

Modelled Peak Flow (m3/s)

Observed Peak Stage (m AOD)

Modelled Peak Stage (m AOD)

Observed Peak Stage Time

Modelled Peak Stage Time

05/09/0920/09/09

7.98

7.32

21.24

21.28

12:45

12:00

08/09/09

08/09/09

Table 4.3 – Final Calibration Parameter Factors for Milton Gauge

Rainfall-Runoff Inflow Boundary

SPR adjustment factor

Tp adjustment factor

Loch Ard

0.7

0.97

Lower Ard

0.7

0.97

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Figure 4.4 – Modelled and Gauged Flows at Milton July – September 2009

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Figure 4.5 – Modelled and Gauged River Stage at Milton July – September 2009

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th th Figure 4.6 – Modelled and Gauged Flows at Milton 5 – 20 September 2009

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Figure 4.7 – Modelled and Gauged River Stage at Milton 5th – 20thSeptember 2009

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4.3 Model Verification Since the original River Forth model was built and calibrated, Stirling Council has installed a water level monitoring station located on the upstream face of the Manse Road bridge in Aberfoyle. This location is represented by river section 0973 D in the InfoWorks RS model. This level monitoring station has been installed to provide a telemetered warning when water levels at Aberfoyle rise above a predetermined threshold. The availability of this data allowed a further check on the models performance to be carried out in the lower half of the modelled reach and at the key point of interest in Aberfoyle itself. Whilst the quality and nature of this data does not lend itself to use in formal calibration it was used to ensure that the flood event in Aberfoyle could be broadly reproduced using the model.

However, Stirling Council have observed during check levelling that the recorded level at this station shows a considerable amount of drift from its calibration. A check level was taken by Stirling Council on 8th September 2009 and the monitoring equipment was found to be 211mm below the actual level taken to Ordnance Datum.

The recorded data from the level monitoring was useful in determining both the timing and level of the flood peak water. Figure 4.6 shows the modelled stage plots for Manse Bridge, Aberfoyle compared to the recorded stage (adjusted by +211mm to correspond to the check levelling of 08/09/09). The first model run plotted was obtained using the SPR and Tp adjustment factors derived from the calibration to the Milton gauge, discussed in section 4.2 above. This shows that applying the same parameter adjustment to all inflows in the model results in an under-prediction of water levels for this event. Therefore, SPR and Tp were adjusted for the inflows in the model downstream of the Milton gauge to improve the modelled flood levels in Aberfoyle when compared to the observed. The channel roughness co-efficient, Mannings ‘n’, was also increased from 0.03 to 0.04 for all sections downstream of the confluence of the Duchray Water and Avon Dhu to assist in raising water levels; this was considered a realistic and justifiable adjustment given the meandering planform geometry of the River Forth upstream of Aberfoyle.

It was also decided to use a different rainfall record, taken from the Duchray Castle rain gauge, for all the inflows downstream of the Milton gauge. It was necessary to use the rainfall from this gauge as it recorded a greater total rainfall during this event than the Loch Katrine gauge used in section 4.2. This was necessary in order to generate sufficient flows at the Duchray Water inflow boundary to reproduce the flood levels observed in Aberfoyle. Whilst flows at the Milton gauge are heavily affected by the attenuation provided by Loch Ard, the Duchray Water catchment appears to be more sensitive to short-term rainfall in terms of flood generation.

Whilst the final model shows an earlier rise and descent in levels, the timing of the peak is in good agreement with the observation, as is the maximum water level itself. A comparison of modelled and observed water levels at Aberfoyle is presented in Table 4.4 and the final parameter adjustments adopted to achieve this water level and timing are given in Table 4.5.

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Table 4.4 – Comparison of Observed and Final Model Levels at Aberfoyle, 8th September 2009

Period

Observed Peak Stage (m AOD)

Modelled Peak Stage (m AOD)

Observed Peak Stage Time

Modelled Peak Stage Time

08/09/0909/09/09

20.58

20.56

12:35

12:15

08/09/09

08/09/09

Table 4.5 – Final Parameter Adjustments to Reproduce Observed Water Levels at Aberfoyle, 8th September 2009

Rainfall-Runoff Inflow Boundary

SPR adjustment factor

Tp adjustment factor

Loch Ard & Lower Ard

0.7

0.97

Duchray

1.8

1.2

Lateral, Trib 1 & Trib 2

2.2

1.2

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Figure 4.8 – 8th September 2009 Event: Comparison of Observed and Modelled River Stage at Aberfoyle

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River Forth Hydrodynamic Model Update and Recalibration

Comparisons of modelled maximum water level and observed levels and flood extents were also carried out using a trash line level survey and photographs of the 8th September 2009 event, which were provided by Stirling Council. The trash line survey indicates that water levels reached 20.659 mAOD at the upstream face of the Manse Road bridge and 20.573 mAOD at the junction of the A81 and B829.

The range of surveyed levels is generally in good agreement with the model output. The higher level surveyed at the Manse Road bridge when compared to the model and the adjusted level monitoring may be the result of either local variation in water level due to wave action and flow constriction at the bridge opening or possibly due to a loss of accuracy when identifying trash lines some time after the flood event. It should also be noted that observed flood extents and levels may have been the result of multiple factors. For example, surface runoff may have contributed in part to the flooding in addition to the flooding from the Forth. The InfoWorks model simulates flooding from the Forth only and this needs to be appreciated when comparing the model results with observations.

Modelled and observed flood extents in the area around the Manse Road bridge are provided in Figure 4.7 and show that the model has been used to successfully reproduce the general flood outline observed on 8th September 2009. The model shows a slightly larger flood extent and it is thought that this is most likely due to local inaccuracies inherent in the Lidar ground model used to generate the flood outline.

The modelled peak flow at Aberfoyle required to generate this flood outline was 191m3/s at Manse Road bridge. This is broadly equal to the 2% AEP design event (194m3/s at Manse Road bridge).

This analysis suggests that the flood experienced on 8th September 2009 was a relatively infrequent event, although as discussed in section 5, more frequent flood events do still pose a flood risk to Aberfoyle Main Street and surrounding areas.

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River Forth Hydrodynamic Model Update and Recalibration

Figure 4.7 – Modelled and Observed Flood Extents at Aberfoyle 8thSeptember 2009. Blue shading indicates modelled flood extents and red hashing represents Stirling Council post-event survey.

Reproduced by permission of Ordnance Survey on behalf of HMSO. Š Crown copyright and database right. All rights reserved. Ordnance Survey Licence number 0100020780 River Forth Model Recalibration V 0.4.doc

28

River Forth Hydrodynamic Model Update and Recalibration

4.4 Sensitivity to Hydraulic Model Revisions As discussed in section 3, some minor modifications were made to the approach to modelling the flows out over the left bank between Milton and Aberfoyle. Also, as outlined in section 4.3 Mannings ‘n’ was adjusted to help reproduce the flood levels and extents observed in Aberfoyle on 8th September 2009. In order to quantify any effects that this may have had on the model results, a check was carried out comparing the modelled water levels for the original and revised models. The original 0.5% AEP design event used in the 2009 modelling study was routed through both versions of the model. Comparison of the results demonstrated that the changes made have a negligible effect on model output with a maximum change in water level of 1mm.

This is not surprising as, with reference to section 8.3 of the Aberfoyle River Forth Flood Study Hydraulic Modelling Report (2009), the model does not demonstrate sensitivity to changes in Mannings ‘n’.

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River Forth Hydrodynamic Model Update and Recalibration

5.

Flood Extents Revised flood maps of the Aberfoyle and Kinlochard area were constructed for a range of design events. This covered the 50%, 20%, 10%, 5%, 2%, 1% and 0.5% AEP events. The 0.5% AEP design event was also run with a 20% increase in flows applied to represent an allowance for future climate change as per the current SEPA guidance. Flood maps are provided in Appendix B and peak modelled river stage for all model nodes is given Appendix C. Following the recalibration and revisions to the design hydrology the flood-frequency relationship shown by the modelled flood extents provides an improved correlation with the observed history of flooding in the Aberfoyle area.

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River Forth Hydrodynamic Model Update and Recalibration

6.

Conclusions and Recommendations

6.1 Conclusions 1. The River Forth hydraulic model was revised from the previous 2009 hydraulic model to refine the representation of out-of-bank flows.

2. The updated model was calibrated to the September 2009 event.

3. The updated model provided a good comparison to the September 2009 flood event.

4. The design hydrology was updated from the 2009 report. The final design flows estimated from this study are greater than those in the previous study.

5. The flood outlines produced from the updated model cover a larger area than those produced by the previous 2009 hydraulic modelling.

6. The principle conclusions regarding flood risk in the area remain unchanged from the 2009 study. These are as follows: 

There are a number of areas at risk of flooding in the upper Forth catchment. The areas experiencing flooding at the 0.5% AEP event are Main Street, Aberfoyle; Lochard Road, Aberfoyle, and Kinlochard Road between Milton and Kinlochard.  The major hydraulic influences in the study area are the Duchray Water for flooding in Aberfoyle and the antecedent water levels in Loch Ard prior to the onset of a storm event.  The bed level in The Narrows has a very minor effect on maximum stage in either Loch Ard or at Aberfoyle.

7. The updated model indicates that flooding in the Tourist Information Centre Car Park area of Aberfoyle has a 10% annual probability of flooding from the River Forth with more extensive flooding in the Main Street area at 5% annual probability.

8. The updated model indicates that flooding to the B829 on the north shore of Loch Ard has a 10% annual probability of occurrence with extensive flooding to this road at 5% annual probability.

9. The updated model suggests that the conclusions of the 2009 report regarding the proposed flood defences at Aberfoyle (Stirling Council Stage 2 Flood Prevention Study, Bullen and Partners, 2000) should be revised. The design level of the proposed flood embankment would be insufficient to provide protection against the 0.5% AEP event.

6.2 Recommendations If the model is to be used to evaluate and develop flood mitigation options in detail, Atkins recommends that the hydraulic component of the model is revised and updated to include a greater level of detail. This may include additional river channel surveys to increase model extents, and surveying of defacto defences such as road levels, walls, field boundaries along with modelling of floodplain conveyance in 2D to refine out-of-bank flow paths. This will help reduce localised errors in flood mapping that arise from using Lidar ground model data, especially in urban environments.

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River Forth Hydrodynamic Model Update and Recalibration

Appendix A – FEH Calculation Record

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Flood estimation calculation record

Introduction This document is a supporting document to the Environment Agency’s flood estimation guidelines. It provides a record of the calculations and decisions made during flood estimation. It will often be complemented by more general hydrological information given in a project report. The information given here should enable the work to be reproduced in the future. This version of the record is for studies where flood estimates are needed at a multiple locations.

Note for analysts: This document contains guidance notes shown in hidden text. If they are not visible, they can be revealed by clicking the Show ¶ button on the toolbar.

Contents

Page 1

Method statement -------------------------------------------------------------------------------------------------- 3

2

Locations where flood estimates required ----------------------------------------------------------------- 6

3

Statistical method -------------------------------------------------------------------------------------------------- 8

4

Revitalised flood hydrograph (refh) method ------------- ERROR! BOOKMARK NOT DEFINED.

5

FEH rainfall-runoff method ------------------------------------------------------------------------------------ 11

6

Small catchment methods ------------------------------------- ERROR! BOOKMARK NOT DEFINED.

7

Discussion and summary of results------------------------------------------------------------------------ 13

8

Annex - supporting information----------------------------------------------------------------------------- 15

Approval

Signature

Name and qualifications

For Environment Agency staff: Competence level (see below)

Calculations prepared by:

Esther Ainsworth BSC, MRes, MCIWEM, CSci, CEnv CWEM

Level 2

Calculations checked by:

Tracey Ashworth BSC, MSc, MCIWEM, CSci, CEnv, CWEM

Level 2

Calculations approved by: Environment Agency competence levels are covered in Section 2.1 of the flood estimation guidelines: • Level 1 – Hydrologist with minimum approved experience in flood estimation • Level 2 – Senior Hydrologist • Level 3 – Senior Hydrologist with extensive experience of flood estimation

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ABBREVIATIONS

AM AREA BFI BFIHOST CFMP CPRE FARL FEH FSR HOST NRFA POT QMED ReFH SAAR SPR SPRHOST Tp(0) URBAN URBEXT WINFAP-FEH

Annual Maximum 2 Catchment area (km ) Base Flow Index Base Flow Index derived using the HOST soil classification Catchment Flood Management Plan Council for the Protection of Rural England FEH index of flood attenuation due to reservoirs and lakes Flood Estimation Handbook Flood Studies Report Hydrology of Soil Types National River Flow Archive Peaks Over a Threshold Median Annual Flood (with return period 2 years) Revitalised Flood Hydrograph method Standard Average Annual Rainfall (mm) Standard percentage runoff Standard percentage runoff derived using the HOST soil classification Time to peak of the instantaneous unit hydrograph Flood Studies Report index of fractional urban extent FEH index of fractional urban extent Windows Frequency Analysis Package – used for FEH statistical method

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1

Method statement

This section should be completed and if possible agreed with the project manager at the inception of the project.

1.1

Overview of requirements for flood estimates

Item Purpose of study Approx. no. of flood estimates required Peak flows or hydrographs?

Range of return periods and locations.

Approx. time available

1.2

Overview of catchment

Item Brief description of catchment, or reference to section in accompanying report

1.3

Comments The study catchment is located in the headwaters of the River Forth, 30 km 2 west-north-west of Stirling, and has an area of 129 km . The National Grid Reference of the point of interest is NN 521 009. Aberfoyle is located downstream of the confluence of two tributary branches, the Avon Dhu and Loch Ard Burn (also known as the Duchray Water). Two lochs are located on the Avon Dhu branch: Loch Ard and Loch Chon. This is an important feature of the catchment for this study, as storage capacity in the lochs may have an attenuating effect on flood peaks passed downstream.

Source of flood peak data

Was the HiFlows UK dataset used? If so, which version? If not, why not? Record any changes made

1.4

Comments To derive inflow hydrographs for a number of return periods (2, 5, 10, 20, 50, 100 and 200) for a Flood Risk Assessment for Aberfoyle.

Yes – Version 2.2.1, December 2008.

No changes have been made to this data.

Gauging stations (flow or level)

(at the sites of flood estimates or nearby at potential donor sites) WaterStation Gauging NRFA Grid course name authority number reference number (used in FEH) Avon Dhu Milton 18022 NN503 013

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Catchment area (km²) 44.5

Type (rated / ultrasonic / level…)

Velocity Area

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Period of record

1989present

1.5

Data available at each flow gauging station

Station name

Milton

Period of data in HiFlowsUK None

Update for this study?

None

Suitable for QMED?

Suitable for pooling?

Yes

No

Data quality check needed?

No

Other comments on station and flow data quality

Give link/reference to any further data quality checks carried out Any flood peak data not in HiFlows-UK (e.g. extra stations, recent years or altered flows) should be given here or in the annex.

1.6

Rating equations Station name

Milton

Type of rating e.g. theoretical, empirical

empirical

Rating review needed?

No

Reasons

Give link/reference to any rating reviews carried out 1.7

Other data available and how it has been obtained

Flow gaugings (if planned to review ratings) Historic flood data

Anecdotal evidence of historical flooding events

Flow data for events Rainfall data for events Results from previous studies (e.g. CFMPs, Strategies) Other data or information (e.g. groundwater, tides) 1.8

Initial choice of approach

Is FEH appropriate? (it may not be for very small, heavily urbanised or complex catchments) If not, describe other methods to be used.

Yes

Outline the conceptual model.

Where are the main sites of interest?

What is likely to cause flooding at those

The points of interest are located at the Milton gauge, the Duchray direct inflow and the downstream extent of the model.

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locations? (peak flows, flood volumes, combinations of peaks, groundwater, snowmelt, tides…)

Might those locations flood from runoff generated on part of the catchment only, e.g. downstream of a reservoir?

Is there a need to consider temporary debris dams that could collapse?

Any unusual catchment features to take into account? e.g.

• highly permeable (SPRHOST<20%) – avoid ReFH, use permeable catchment adjustment for statistical method • highly urbanised – prefer FEH statistical, or consider alternative methods • pumped watercourse – consider lowland catchment version of rainfall-runoff method • major reservoir influence (FARL<0.90) – consider flood routing • extensive floodplain storage – consider choice of method carefully

Initial choice of method(s) and reasons Will the catchment be split into subcatchments? If so, how?

Two lochs are located on the Avon Dhu branch: Loch Ard and Loch Chon. This is an important feature of the catchment for this study, as storage capacity in the lochs may have an attenuating effect on flood peaks passed downstream

Hybrid – rainfall runoff scaled to statistical.

The catchment was divided into six subcatchments

Software to be used (with version numbers)

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2

Locations where flood estimates required

The table below lists the locations of subject sites. The site codes listed below are used in all subsequent tables to save space.

2.1

Summary of subject sites Site code

Watercourse

Site

Easting

Northing

1

Loch Ard

Direct Inflow to Upper Loch Ard

248650

701350

2

Duchray

250600

701300

3

Lower Ard

249100

701450

4

Lateral

Lateral area downstream of Milton gauge

251550

700850

5

Trib 1

Lateral area representing tributary of the River Forth

252200

700500

6

Trib 2

Lateral area representing tributary of the River Forth

252600

700650

7

Duchray

Duchray Water at confluence with Avon Dhu

250600

701300

8

Milton Gauge

Milton Gauge on Avon Dhu

250350

701350

44.5

9

d/s model extent

D/s model extent at Aberfoyle

252650

700550

128.95

Direct Inflow of Duchray Water to confluence with Avon Dhu Lateral area between Lower Loch Ard at the Narrows and Milton gauge

AREA on FEH CDROM 2 (km )

Revised AREA if altered

37.9 70.5

4.4 2.8 6.7 6.7 70.5

Reasons for choosing above locations 2.2

Important catchment descriptors at each subject site (incorporating any changes made)

Site code

2.3

FARL

PROPWET

BFIHOST

1

0.655

0.74

0.438

DPLBA R (km) 7.53

DPSBAR (m/km) 218.3

SAAR (mm) 2106

SPRHOST

URBEXT

42.4

0

2

0.983

0.74

0.373

13.62

207.5

2280

48.52

0

3

1

0.74

0.487

1.61

297.7

1857

39.15

0

4

0.818

0.74

0.573

1.26

142.6

1654

39.25

0

5

0.999

0.74

0.581

4.03

125.5

1725

37.17

0

6

0.995

0.74

0.54

3.47

205.1

1793

34.78

0

7

0.983

0.74

0.373

13.62

207.5

2280

48.52

0

8

0.653

0.74

0.446

8.68

225.8

2076

41.84

0

9

0.845

0.74

0.403

13.5

214.3

2191

45.92

0

Checking catchment descriptors

Record how catchment boundary was checked and describe any changes (refer to maps if needed)

Catchment boundaries checked with OS Maps

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Record how other catchment descriptors (especially soils) were checked and describe any changes. Include before/after table if necessary.

Catchment descriptors for the laterals were obtained using one of two methods; either from a carefully selected donor catchment with the correct AREA and URBEXT for the lateral, or from a spreadsheet (FEHCAL.xls).

This spreadsheet automatically calculates FEH catchment descriptors for a lateral area using weighted averages of the descriptors for the catchments at the upstream and downstream extents of the lateral area.

Source of URBEXT

URBEXT2000

Method for updating of URBEXT

Method in R&D Technical report FD1919/TR Eq. 7.2

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3

Statistical method

3.1

Overview of estimation of QMED at each subject site (1)

Site code

Method

Initial estimate of QMED 3 (m /s)

AM or POT Adjust-ment for climatic (2) variation?

7

CD

83.65

-

8

CD

10.65

-

8

AM

10.30

9

CD

74.99

Data transfer NRFA numbers for donor/analogue sites used (see 3.4) and reasons for (3) choice 86002 – highest geographically close gauge

Final estimate of QMED 3 (m /s)

QMED adjustment factor 1

83.65

86001– highest geographically close gauge

1

10.65

-

86001– highest geographically close gauge

1

10.3

-

86002– highest geographically close gauge

1

74.99

Are the values of QMED consistent, for example at confluences?

Notes 1. Methods: AM – Annual maxima; POT – Peaks over threshold; DT – Data transfer; CD – Catchment descriptors alone.

2. Give details of any adjustment for climatic variation below.

3. If more than one donor or analogue has been used, give the weights used in the averaging.

3.2

Search for donor or analogue sites for QMED

Comment on potential donor sites

None within HiFlows

Method for seeking out analogue sites

Pooling Group with geographical proximity

3.3

Characteristics of potential donor and analogue sites

Subject sites needing donors or analogues Site code

AREA

7

70.27

8 9

FARL

BFIHOST

SAAR

SPRHOST

URBEXT

0.983

0.373

2280

48.52

0

44.50

0.654

0.447

2074

41.80

0

129

0.845

0.845

2191

45.92

0

BFIHOST

SAAR

SPRHOST

URBEXT

Potential donors and analogues NRFA no.

Watercourse

Station

AREA

FARL

86002

Eachaig

Eckford

138.63

0.836

0.379

2470

48.90

0

86001

Little Eachaig

Dalinlongart

31.84

1

0.393

2340

47.17

0

85003

Falloch

Glen Falloch

79.62

0.988

0.379

2848

49.31

0

16003

Ruchill Water

Cultybraggan

98.48

1

0.428

1900

44.46

0

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3.4

Decision on choosing or rejecting donor and analogue sites

NRFA no.

86002

Reasons for choosing or rejecting (mention location, catchment properties, data quality…)

Geographically close

86001

Geographically close

3.5

Method (AM or POT) AM

Adjustment for climatic variation?

-

AM

-

QMED from flow data (A) 80.98

QMED from catchment descriptors (B) 89.67

Adjustment ratio (A/B) 0.98

43.18

44.08

1

Derivation of pooling groups

The composition of the pooling groups is given in the Annex. Several subject sites may use the same pooling group.

Target return period (years) for all pooling groups Name of group

Site code for which group derived

Changes made to default pooling group, with reasons Note also any sites that were investigated but retained in the group.

100 Distribution and reason for choice

Parameters (before urban adjustment) Note any permeable catchment adjustments Location – 1.000 Scale – 0.161 Shape – -0.154 Bound – -0.044

Duchray

7

Remove 76001, 94001, 95001, 73010, 73006, 73002, 76015, 21034, 84017, 75003, 4006, 75004, 86002, 75005, 57015, 93001, 15016, 21020, 76003, 74005 – Low FARL Remove 4005, 47011, 96004 – seasonality outlier Remove 79003 – steep growth curve Remove 85003, 89804 skewness/kurtosis outlier

Generalised Logistic

Milton

8

Remove 95001, 73002, 76015, 84017, 75004, 57015, 74005, 76001, 73006, 76001, 73010, 4006, 86002, 75005, 93001 – Low FARL Remove 21030, 48009 skewness/kurtosis outlier Remove 4005 – seasonality outlier

Generalised Logistic

Location – 1.000 Scale – 0.169 Shape – -0.162 Bound – -0.041

d/s extent

9

Remove 76001, 94001, 95001, 85001, 73010, 73002, 76015, 15016, 21034, 84017, 75003, 4006, 21020, 75004, 86002, 75002, 75005, 57015, 74005, 76003, 93001, 15007, 8008 – Low FARL Remove 71011, 85003, 21030, 72006, 89804, 76014, 78005, 4005 - skewness/ kurtosis outlier Remove 96004, 47011 – seasonality outlier Remove 79003, 48001 – steep growth curve

Generalised Logistic

Location – 1.000 Scale – 0.1654 Shape – -0.161 Bound – -0.029

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3.6 Site code

7 8 9

Derivation of flood growth curves at subject sites Method:

P

If P or J, name of pooling group (3.5) Duchray

-

-

Growth factor for 200-year return period 2.109

P

Milton

-

-

2.379

P

d/s extent

-

-

2.150

SS – Single site P – Pooled J – Joint analysis

If SS, distribution used and reason for choice If J, details of averaging

If SS, parameters of distribution (location, scale and shape)

Any relevant frequency plots from WINFAP-FEH, particularly showing any comparisons between single-site and pooled growth curves, should be shown here or in a project report. Also record here any unusual choices such as not applying the urban adjustment to pooled growth curves.

3.7 Site code 7

Flood estimates from the statistical method 3

2

Flood peak (m /s) for the following return periods (in years) 5 10 20 50 100 200

83.65

104.50

118.93

133.92

155.59

173.86

194.07

8

10.3

13.01

14.90

16.88

19.76

22.20

24.92

9

74.99

94.23

107.64

121.64

141.99

159.23

178.38

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4

FEH rainfall-runoff method

This method has been superseded by the ReFH method for most applications This section can be left blank or removed if not needed.

4.1

Parameters for FEH rainfall-runoff model

Methods:

FEA : Flood event analysis LAG : Catchment lag DT : Catchment descriptors with data transfer from donor catchment CD : Catchment descriptors alone BFI : SPR derived from baseflow index calculated from flow data

Site code

Rural (R) or urban (U)

Tp(0): method

Tp(0): value (hours)

SPR: method

1

CD

2.5

CD

2

R R

CD

3.4

3

R

CD

4

R

CD

5

R

6

R

7

R

8

R

9

R

4.2 N o.

SPR: value (%)

BF: method

BF: value 3 (m /s)

42.4

CD

2.4

CD

48.5

CD

4.9

1.0

CD

39.2

CD

0.2

1.1

CD

39.3

CD

0.1

CD

2.1

CD

37.2

CD

0.3

CD

1.7

CD

34.8

CD

0.4

CD

3.4

CD

48.5

CD

4.9

CD

2.7

CD

41.8

CD

2.7

CD

3.4

CD

45.9

CD

8.6

If DT, numbers of donor sites used (see Section 5.2) and reasons

Donor sites for FEH rainfall-runoff parameters Watercourse

Station

Tp(0) from data (A)

Tp(0) from CDs (B)

Adjustment ratio for Tp(0) (A/B)

SPR from data (C)

SPR from CDs (D)

Adjustment ratio for SPR (C/D)

1 A description of the flood event analysis or lag analysis used to derive the rainfall-runoff parameters should be given here or in a project report.

4.3 Site code

Inputs to and outputs from FEH rainfall-runoff model Storm duration (hours)

Storm area for ARF (if not catchment area)

3

2

3

Flood peaks (m /s) or volumes (m ) for the following return periods (in years) 20 50 100 200 5 10

1

7.8

33.6

46.4

56.0

66.7

81.6

93.2

107.1

2

11.4

63.0

87.0

104.1

123.3

150.0

170.7

195.2

3

2.9

5.2

7.2

8.5

10.1

12.7

14.7

17.1

4

3.0

3.0

4.0

4.8

5.7

7.1

8.2

9.6

5

5.9

4.9

6.6

8.0

9.7

11.9

13.7

15.8

6

4.5

5.4

7.3

8.7

10.6

13.2

15.2

17.6

7

11.4

63.0

87.0

104.1

123.3

150.0

170.7

195.2

8

8.3

35.3

48.8

58.8

70.0

85.6

97.8

112.2

9

11

105.8

147.1

176.0

208.5

253.9

288.9

330.6

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Site code

Storm duration (hours)

3

3

Storm area Flood peaks (m /s) or volumes (m ) for the following return for ARF (if periods (in years) 20 50 100 200 not 2 5 10 catchment area) Yes Are the storm durations likely to be changed in the next stage of the study, e.g. by optimisation within a hydraulic model?

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5

Discussion and summary of results

5.1

Comparison of results from different methods

This table compares peak flows from various methods with those from the FEH Statistical method at example sites for two key return periods. Blank cells indicate that results for a particular site were not calculated using that method.

Ratio of peak flow to FEH Statistical peak Return period 200 years Return period 2 years Site code 7

FEH rainfallrunoff 0.75

8

3.43

3.81

9

1.41

1.85

5.2

Final choice of method

ReFH

Other method

ReFH

FEH rainfallrunoff 1.01

Other method

Remember to refer to the type of study, type of data and type of catchment to justify choice of method.

Choice of method and reasons

5.3

Rainfall runoff routed through the hydraulic model and scaled to statistical

Assumptions, limitations and uncertainty

List the main assumptions made (specific to this study)

Discuss any particular limitations, e.g. applying methods outside the range of catchment types or return periods for which they were developed Give what information you can on uncertainty in the results, for example in the QMED estimates using FEH 3 12.5 or 13.8 Comment on the suitability of the results for future studies, e.g. at nearby locations or for different purposes.

Give any other comments on the study, for example suggestions for additional work.

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5.4

Checks

Are the results consistent, for example at confluences?

What do the results imply regarding the return periods of floods during the period of record?

What is the range of 100-year growth factors? Is this realistic?

(The guidance suggests a typical range of 2.1 to 4.0) If 1000-year flows have been derived, what is the range of ratios for 1000-year flow over 100-year flow?

What range of specific runoffs (l/s/ha) do the results equate to?

Are there any inconsistencies?

How do the results compare with those of other studies?

Are the results compatible with the longer-term flood history?

Describe any other checks on the results

5.5

Site code

Final results â&#x20AC;&#x201C; See Report for Design Flows following Calibration Show the final results here for all flow estimation points and return periods, and give any other data or results needed for the next stage of the study.

3 3 Flood peak (m /s) or volume (m ) for the following return periods (in years) 2

If flood hydrographs are needed for the next stage of the study, where are they provided? (e.g. give filename of spreadsheet, name of ISIS model, or reference to table below)

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6

Annex - supporting information

6.1

Pooling group composition

Site Code 7 – Duchray Water

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Site Code 8 – Milton Gauge

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Site Code 9 – D/s model extent

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List the gauging stations included in each pooling group, and their periods of record.

6.2

Additional supporting information

List any annual maximum series which have been altered from the HiFlows-UK data.

Give details of any flow data quality checks or rating reviews here or in a separate report.

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River Forth Hydrodynamic Model Update and Recalibration

Appendix B â&#x20AC;&#x201C; Flood Maps for Updated Model

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River Forth Hydrodynamic Model Update and Recalibration

Appendix C â&#x20AC;&#x201C; Maximum Modelled River Stage

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River Forth Hydrodynamic Model Update and Recalibration

Model node label

50%AEP max stage (m AOD)

20% AEP max stage (m AOD)

10% AEP max stage (m AOD)

5% AEP max stage (m AOD)

2% AEP max stage (m AOD)

1%AEP max stage (m AOD)

0.5% AEP max stage(m AOD)

0.5%AEP+20%CC max stage (m AOD)

0 217 430 530 607 855 0973 E 0973 D 1150 1338 1473 1683 1873 2098 2452 2792 2817 A 2817 B 2856 2868 2990 3065 int 3090 int 3127 E 3127 D 3183 3267 3283 3294 3387 3419 duchray_01 duchray_02 duchray_03 Lower_Ard 01 Lower_Ard 01_1 Lower_Ard 01_2 Lower_Ard 01_3 Lower_Ard 01_4 Lower_Ard 01_5 Lower_Ard 02 Lower_Ard 03 Lower_Ard 04 4000 4301 4646 Helen's Rock Rob Roy cave Upper Ard 05 Upper_Ard 01 Upper_Ard 02 Upper_Ard 03 Upper_ard_04 Lower_Ard 05 loch ard us

18.479 18.922 18.926 18.929 18.930 19.138 19.296 19.425 19.623 19.659 19.700 19.782 19.787 19.932 20.441 20.581 20.634 20.645 20.645 20.645 20.646 20.657 20.673 21.388 21.900 23.313 25.233 25.472 31.290 31.813 32.091 20.645 20.765 20.944 32.364 32.404 32.425 32.429 32.433 32.438 32.449 32.491 32.540 32.540 32.557 32.605 32.692 32.744 32.744 32.744 32.744 32.744 32.744 32.744 32.744

18.878 19.319 19.308 19.302 19.302 19.436 19.601 19.799 20.034 20.034 20.061 20.120 20.125 20.154 20.517 20.738 20.753 20.753 20.754 20.755 20.768 20.785 20.751 21.519 22.150 23.416 25.318 25.622 31.395 31.922 32.239 20.753 20.903 20.903 32.520 32.432 32.475 32.502 32.509 32.514 32.535 32.579 32.631 32.647 32.702 32.811 32.877 32.877 32.877 32.877 32.877 32.877 32.877 32.631 32.877

19.115 19.511 19.511 19.522 19.530 19.616 19.778 20.038 20.284 20.295 20.306 20.356 20.360 20.374 20.547 20.798 20.817 20.817 20.818 20.820 20.839 20.844 20.858 21.600 22.311 23.490 25.358 25.686 31.459 31.988 32.311 20.817 20.979 21.215 32.493 32.548 32.576 32.584 32.590 32.597 32.613 32.655 32.705 32.705 32.719 32.776 32.895 32.968 32.968 32.969 32.969 32.969 32.969 32.969 32.969

19.384 19.751 19.751 19.766 19.772 19.838 19.983 20.302 20.566 20.584 20.585 20.630 20.632 20.638 20.697 20.865 20.887 20.887 20.888 20.893 20.919 20.936 21.060 21.685 22.486 23.562 25.392 25.727 31.516 32.048 32.374 20.887 21.063 21.320 32.558 32.613 32.642 32.652 32.657 32.665 32.683 32.723 32.770 32.770 32.784 32.843 32.971 33.051 33.051 33.051 33.051 33.051 33.051 33.051 33.051

19.699 20.038 20.039 20.058 20.062 20.119 20.225 20.583 20.874 20.891 20.901 20.935 20.936 20.942 20.972 20.990 21.012 21.012 21.015 21.021 21.053 21.074 21.200 21.805 22.739 23.679 25.455 25.798 31.614 32.147 32.493 21.012 21.153 21.412 32.638 32.699 32.732 32.744 32.750 32.759 32.781 32.817 32.861 32.861 32.876 32.939 33.082 33.173 33.173 33.173 33.173 33.173 33.173 33.173 33.173

19.978 20.298 20.299 20.317 20.323 20.369 20.433 20.809 21.118 21.134 21.150 21.178 21.179 21.186 21.206 21.209 21.225 21.225 21.228 21.234 21.259 21.273 21.305 21.893 22.937 23.774 25.501 25.859 31.675 32.209 32.556 21.225 21.303 21.483 32.702 32.763 32.798 32.811 32.818 32.827 32.851 32.883 32.923 32.923 32.937 33.002 33.152 33.248 33.248 33.248 33.248 33.248 33.248 33.248 33.248

20.356 20.658 20.665 20.665 20.689 20.724 20.734 21.096 21.428 21.443 21.467 21.482 21.488 21.490 21.498 21.507 21.516 21.526 21.526 21.529 21.533 21.548 21.556 21.987 23.151 23.909 25.564 25.930 31.742 32.271 32.621 21.526 21.566 21.579 32.769 32.829 32.867 32.881 32.888 32.897 32.924 32.953 32.990 32.991 33.004 33.070 33.228 33.330 33.330 33.330 33.330 33.330 33.330 33.330 33.330

20.900 21.212 21.161 21.181 21.165 21.230 21.191 21.518 21.907 21.861 21.876 21.923 21.925 21.936 21.948 21.941 21.963 21.963 21.963 21.964 21.972 21.977 21.941 22.286 23.753 24.153 25.649 26.058 31.869 32.389 32.732 21.963 21.983 21.981 33.040 32.908 32.966 33.008 33.024 33.031 33.070 33.096 33.129 33.142 33.209 33.379 33.490 33.490 33.490 33.490 33.490 33.490 33.490 33.130 33.490

Table C-1. Maximum modelled river stage for the full range of design events for all river sections

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