Key Biscayne Beach Management Feasibility Study

Key Biscayne Beach Management Feasibility Study Village of Key Biscayne Miami-Dade County, Florida February, 2018

Prepared by:

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Table of Contents 1.

2.

3.

4.

5.

6.

7.

8.

9.

Introduction ..................................................................................................................................... 1 Engineering Study Approach .......................................................................................................... 3 2.1.

Existing Data and Literature Review ...................................................................................... 3

2.2.

Beach Nourishment Conceptual Design ................................................................................. 3

2.3.

Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis ..................... 4

2.4.

Preliminary Submerged Breakwater Design ........................................................................... 4

Existing Data and Literature Review .............................................................................................. 5 3.1.

Tide and Storm Surge .............................................................................................................. 7

3.2.

Resiliency ................................................................................................................................ 7

Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis ............................. 8 4.1.

WIS Hindcast Wave Data........................................................................................................ 8

4.2.

Nearshore Wave Transformation .......................................................................................... 11

4.3.

Wave Dissipation on the Submerged Breakwaters ............................................................... 22

Beach Nourishment Conceptual Design ....................................................................................... 24 5.1.

Beach Nourishment Development ........................................................................................ 24

5.2.

Potential Impacts on the Nearshore Seagrasses .................................................................... 27

Preliminary Submerged Breakwater Design ................................................................................. 32 6.1.

Design Criteria ...................................................................................................................... 32

6.2.

Submerged Breakwater Design ............................................................................................. 32

Opinion of Probable Construction Cost ........................................................................................ 34 Conclusions and Recommendations ............................................................................................. 36 8.1.

Conclusions ........................................................................................................................... 37

8.2.

Recommendations ................................................................................................................. 38

References ..................................................................................................................................... 39

Appendix A: Wind and Wave Roses at WIS Station 63417 9699-04

Moffatt & Nichol

ii

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Appendix B: Dean’s Method

List of Figures Figure 1: Location Map .......................................................................................................................... 2 Figure 2: Historic Beach Renourishment ............................................................................................... 6 Figure 3: Location of WIS Station 63471 .............................................................................................. 8 Figure 4: Annual Wave Rose at WIS Station 63471 .............................................................................. 9 Figure 5: Exponential Relationship between Wave Heights and Wave Periods at WIS Station 63471 .............................................................................................................................................................. 10 Figure 6: Storm Event Return Period of 35-yr (1980-2014) at WIS Station 63471 ............................ 10 Figure 7: Wave Model Mesh and Bathymetry ..................................................................................... 12 Figure 8: Bathymetry along Key Biscayne .......................................................................................... 13 Figure 9: Wave Roses along -3.28 feet (-1 m), MLLW contour (1980-2014) ..................................... 15 Figure 10: Wave Roses along -6.56 feet (-2 m), MLLW contour (1980-2014) ................................... 16 Figure 11: Wave Roses along -9.84 feet (-3 m), MLLW contour (1980-2014) ................................... 17 Figure 12: Typical Annual Net Longshore Sediment Transport Pattern ............................................. 18 Figure 13: Wave Height Distributions (Annual 90-percentile non-exceedance) at Project Site.......... 19 Figure 14: Wave Height Distributions when Incident Waves from Northeast (25-year Return Period) .............................................................................................................................................................. 19 Figure 15: Wave Height Distributions when Incident Waves from East-northeast (25-year Return Period) .................................................................................................................................................. 20 Figure 16: Wave Height Distributions when Incident Waves from East (25-year Return Period) ...... 20 Figure 17: Wave Height Distributions when Incident Waves from East-southeast (25-year Return Period) .................................................................................................................................................. 21 Figure 18: Wave Height Distributions when Incident Waves from Southeast (25-year Return Period) .............................................................................................................................................................. 21 Figure 19: Wave Transmission Coefficient, Kt vs. Breakwater Crest Widths .................................... 23 9699-04

Moffatt & Nichol

iii

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 20: Beach Fill Design at R-102 ................................................................................................. 25 Figure 21: Beach Fill Design at R-103 ................................................................................................. 25 Figure 22: Beach Fill Design at R-104 ................................................................................................. 26 Figure 23: Beach Fill Design at R-105 ................................................................................................. 26 Figure 24: Beach Fill Design at R-106 ................................................................................................. 27 Figure 25: Beach Fill Design at R-107 ................................................................................................. 27 Figure 26: Predicted Equilibrium Profile at R-103 .............................................................................. 29 Figure 27: Predicted Equilibrium Profile at R-104 .............................................................................. 29 Figure 28: Predicted Equilibrium Profile at R-105 .............................................................................. 30 Figure 29: Predicted Equilibrium Profile at R-106 .............................................................................. 30 Figure 30: Predicted Equilibrium Profile at R-107 .............................................................................. 31 Figure 31: Initial Submerged Breakwater Locations ........................................................................... 32

9699-04

Moffatt & Nichol

iv

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

List of Tables Table 1:Key Biscayne History ............................................................................................................... 5 Table 2: Tidal Elevations at Station 8723214 ........................................................................................ 7 Table 3: Storm Surges ............................................................................................................................ 7 Table 4: Design Wave Conditions at WIS Station 63417 .................................................................... 11 Table 5: Design Wind Conditions at WIS Station 63417 .................................................................... 11 Table 6: Design Wave Conditions along Key Biscayne ...................................................................... 22 Table 7: Predicted Wave Transmission Coefficients ........................................................................... 23 Table 8: Dimensions of the Proposed Beach Nourishment Project ..................................................... 24 Table 9: Impacted Areas on the Nearshore Seagrasses ........................................................................ 29 Table 10: Armor Rock Design Parameters........................................................................................... 33 Table 11: Design Armor Rock Sizes .................................................................................................... 33 Table 12: Underlayer Stone Sizes ........................................................................................................ 33 Table 13: Material Quantities for Beach Management Project ............................................................ 33

9699-04

Moffatt & Nichol

v

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

1. Introduction Moffatt & Nichol (M&N) and EAC Consulting, Inc. were retained to provide a study along with conceptual engineering designs of beach nourishment and submerged breakwaters for the beach along Key Biscayne. After completion of the 2017 truck haul beach nourishment, the Village of Key Biscayne (Village) has retained a team of professionals to assist the Village in planning a long term and largescale beach nourishment. This beach nourishment would substantially increase the beach fill volume as compared to the currently permitted truck haul maintenance project that was completed in 2012 along with the recently completed maintenance nourishment constructed in May, 2017. This larger beach nourishment plan was proposed to the Village in 2011, and due to schedule and environmental permitting challenges, the 2012 truck haul maintenance project was constructed. The longer-term project proposed included a 100,000+ cubic yard nourishment. For added shore protection, the Village and their team of professionals have requested an evaluation of submerged breakwaters (coastal structures).

Key Biscayne is the southernmost and largest of the natural sandy barrier islands lying south of Miami Beach and Government Cut (see Figure 1). These series of barrier islands including neighboring Virginia Key and Fisher Island are separated by tidal inlets Bear Cut, Norris Cut, and Government Cut connecting north Biscayne Bay to the Atlantic Ocean. The next series barrier islands, Sands Key and Elliott Key, are located approximately 10 miles south of Key Biscayne. The Village of Key Biscayne lies within the central and most developed part of Key Biscayne comprising approximately 1.2 miles of the total Atlantic shoreline. Miami-Dade County’s Crandon Park is located to the north with nearly 2 miles of Atlantic shoreline and Bill Baggs Cape Florida State Recreation Area is to the south comprising approximately 1.2 miles of Atlantic shoreline.

9699-04

Moffatt & Nichol | Introduction

1

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 1: Location Map

9699-04

Moffatt & Nichol | Introduction

2

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

2. Engineering Study Approach 2.1.

Existing Data and Literature Review

M&N reviewed and compiled existing available data sets regarding beach profile conditions, shoreline positions, nearshore seagrass mapping, offshore waves, and sediment characteristics in the study area.

The existing, available data sources utilized include: •

Historical beach profiles and other survey data collected and archived by Florida Department of Environmental Protection (FDEP).

•

Historical reports and sets describing historical beach nourishments.

•

Sand source borrow area limits and capacity for the 2002 beach nourishment project.

•

Historical meteorological and oceanographic data including winds, tides and waves.

•

Previous coastal engineering reports and studies.

•

Previous permitted beach management documentation

M&N compiled, reviewed and analyzed the wind, wave and profile data. The annual shoreline changes and profile volume changes since 1997 were estimated, and the analyzed results were compared with the data in the 1997 Long Range Beach Nourishment Plan (1997 Report) prepared by other consultants.

This 1997 Report was the most recent long term study completed for the project area. The net sand loss on the project area was reviewed and summarized relative to longshore sediment transport. The evaluation includes a summary of beach management projects constructed on Key Biscayne since 1969, as well as graphics from the 1997 Plan referencing the impacts of Government Cut on the downdrift shoreline.

2.2.

Beach Nourishment Conceptual Design

The maintenance of beach width along the project shoreline is for shore protection, marine turtle nesting habitat, and recreation. Studies have been completed over the years with the first beach nourishment constructed in 1969. The Long Term Beach Management Plan adopted by the Village in 1997 resulted in a larger, 120,000 cubic yard beach nourishment, that was constructed in 2002. The plan contemplated maintenance projects at regular intervals. M&N conducted a conceptual design of a 100,000+ cubic yard beach nourishment project. The beach and berm profile volumes and geometries were determined and compared with the minimum shore protection established by the U.S. Army Corps of Engineers for the 1987 beach nourishment project. Desktop coastal engineering methods were utilized to establish the beach and berm design template at the FDEP R-monument sections, and the resulting volume of beach fill determined. This optimum beach fill template was evaluated relative to adjacent nearshore seagrass bed, and an area of impact estimated. The estimated impacted area was compared with the estimated seagrass impacts from the 1987 project. Seagrass beds that were impacted from the 1987 project have since recruited / recovered, despite sand placement in various projects from 2002-2017.

9699-04

Moffatt & Nichol | Engineering Study Approach

3

Key Biscayne Beach Management Feasibility Study

2.3.

Village of Key Biscayne

Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

In conjunction with a large-scale beach nourishment, the Village has requested evaluation of coastal structures to reduce erosion rates and to facilitate shoreline stabilization. The costs of truck haul nourishment projects are increasing, and the nearest upland mine with suitable beach compatible material is in the Moore Haven area of Florida, just south of Orlando. During recent Council meetings, M&N has discussed conceptual coastal structural approaches to shore protection that have included groins, shore-attached breakwaters, and detached breakwaters. Based on feedback from the Village, offshore detached submerged breakwaters are the preferred option for consideration. In addition, the joint coastal permit for beach management from the FDEP is expiring in 2020, and approaches to longer term beach management and shore protection need to be evaluated in conjunction with these environmental permits that will need to be processed.

The preliminary design of these submerged breakwaters for estimating the geometry and approximate structural footprint depends on the design wave conditions. The Wave Information Studies (WIS) hindcast wave data at station of 63471 were used to conduct the wave transformations of offshore deep water waves to the nearshore zone in the project area. The WIS extreme wave data were analyzed utilizing the WIS time series wave hindcast data from 1980 to 2014. M&N applied the MIKE 21 Spectral Wave (SW) numerical model to transform the offshore waves to the nearshore project area.

The wave dissipation for waves propagating over a proposed submerged breakwater were estimated using a numerical flume wave model and a desktop method. The effects of the top elevation and the width of the proposed submerged breakwater on the waves were estimated.

2.4.

Preliminary Submerged Breakwater Design

Based on the numerical modeling results and experience in the project area, M&N prepared a preliminary design for the proposed submerged breakwaters. A plan with the location of the breakwaters was prepared along with a typical cross section. The layout was based on coastal structure design guidelines in the Coastal Engineering Manual (CEM) published by the U.S. Army Corps of Engineers. The initial layout and typical cross sections were utilized to estimate rock construction quantities for the cost estimates. In addition, the structure footprint was estimated to evaluate area of potential impacts to marine resources. A preliminary analysis was performed to develop the size of armor stone utilizing the method outlined in the CEM. The conceptual layout was prepared for initial discussion purposes with the Village. The preliminary submerged breakwater design will need to be refined with further coastal numerical modeling, studies, and coastal engineering. The coastal structure configuration will also need to be evaluated with the environmental regulatory agencies.

9699-04

Moffatt & Nichol | Engineering Study Approach

4

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

3. Existing Data and Literature Review Coastal process studies were first conducted along Key Biscayne in 1961, and beach nourishments conducted periodically since 1969. The following is a brief summary of nourishment projects and Figure 2 includes a graphical representation of beach management projects: Table 1: Key Biscayne Beach Nourishment History Date Volume (CY) Borrow Source Completed 1969 196,300 Offshore 1987 1998 2002 2008 2012 2017 • •

•

•

•

•

420,000 34,000 121,000 2,400 37,500 26,100

Offshore Upland Offshore Upland Upland Upland

Location

Length (Mi.)

R92.5-R96 and R99-R101 R101-R113.7 R105-R106 R101-R108 R103-R107 R101.3-R107.6 R101.75R107.75

1.0 2.4 0.3 1.3 0.75 1.2 1.1

1969 – Restored 50-foot wide berm at elevation +7 ft NGVD along R92.5-R96 and R99-R101 using 196,300 cy sand from borrow area immediately offshore 1987 – In 1985, under provisions of the 1965 Rivers and Harbors Act, Key Biscayne Shore Protection Project was federally authorized. The project was a Section 103 project through the continuing authorities program (CAP), and was a one-time project authorization. The project restored 2.4 miles (excluding R111-R112.3) with 420,000 cy of sand from an offshore borrow area located one mile southeast of Cape Florida. Restored 25 foot wide berm at Key Biscayne and a 20-foot wide berm at Cape Florida State Park, both +7 ft MLW, and provided seven years of advance nourishment.

1998 – truck haul maintenance project constructed in the vicinity of the Ocean Club development (R105 to R106) as part of a Coastal Construction Control Line (CCCL) permit.

Sand from an upland source was placed generally above the Mean High Water (MHW) line, and no documentation is available to confirm the actual quantity of material placed.

2002 – Non-federal beach nourishment from R101-R108 was completed along 1.3 miles of beaches using 121,000 cy of sand from offshore borrow site approximately 4,000 ft offshore from the southern tip of Key Biscayne. Project had a construction berm width 0f 35 ft at +7 ft NGVD. Physical and environmental monitoring performed to verify no nearshore seagrass impacts. The project was funded the state and by the County (75%).

2008 – Significant erosion damage along occurred along Key Biscayne from Hurricanes Rita and Wilma in 2005. The Village renourished portions of the dune in 2008 from an upland sand source. This project was funded by FEMA as an Engineered Beach.

2012 – approximately 37,500 cy of sand was placed between R101+300 and R-107+600 from an upland source. The renourishment project was to renourish erosion occurred from Hurricane Sandy. This project was funded by FEMA as an Engineered Beach.

9699-04

Moffatt & Nichol | Existing Data and Literature Review

5

Key Biscayne Beach Management Feasibility Study

•

Village of Key Biscayne

2017 – The most recent beach renourishment project occurred in 2017 between R101+750 and R107+750 due to impacts from Hurricane Matthew. This renourishment project placed 26,100 cy of sand from an upland source. Funding reimbursement has been applied for through FDEP.

Figure 2: Historic Beach Renourishment Projects on Key Biscayne, Virginia Key and Fisher Island 9699-04

Moffatt & Nichol | Existing Data and Literature Review

6

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Available historical data sets including historical beach profiles, shorelines, wave and wind conditions, tide elevations and sediment characteristics were compiled and analyzed. These data sets were utilized for wave transformation modeling, longshore sediment transport and shoreline evolution, and beach profile storm response and benefit analysis.

3.1.

Tide and Storm Surge

Tidal information is available from a National Ocean Service (NOS) tidal bench mark, and can be used for reference in the study.

Table 2: Tidal Elevations at Station 8723214 Mean High Water (MHW) North American Vertical Datum (NAVD) Mean Sea Level (MSL) North Geodetic Vertical Datum (NGVD) Mean Low Water (MLW)

Tidal Elevation (ft, MLLW) 2.13 1.97 1.10 0.39 0.11

Storm surge values from the Federal Emergency Management Agency (FEMA) are referenced in table 3 from the Miami-Dade County Flood Insurance Study (FIS).

Table 3: Storm Surges Return Period (year) 10 25 50

3.2.

Storm Surge (ft, MLLW) 5.28 5.85

Storm Surge (ft, MSL) 4.18 4.75

6.58

5.48

Resiliency

Beach management projects in Florida need to account for Sea Level Rise (SLR). The Southeast Florida Regional Climate Change Compact developed the “Unified Sea Level Rise Projection for Southeast Florida.” The updated projection, published in 2015, estimates that southeast Florida can expect to see average sea levels 6 to 10 inches higher by 2030 than they were in 1992, 14 to 34 inches higher by 2060, and 31 to 81 inches higher by 2100. This change in average sea levels will amplify the risks of storm surge, and hurricane activity has increased since 1995. FEMA flood zone maps are being updated for Miami-Dade County, and the Miami-Dade County Climate Change Advisory Task Force (CCATF) has provided recommendations for planning and design of infrastructure as well as coastal zone management. The Village held town hall meetings in February, 2017 to review ongoing resiliency planning. Addressing SLR relative to coastal zone management is beyond the scope of services for this feasibility study, however the planned beach management projects described in this study, as well as other coastal zone management projects, should account for SLR and the recommendations of the County’s CCATF.

9699-04

Moffatt & Nichol | Existing Data and Literature Review

7

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

4. Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis 4.1.

WIS Hindcast Wave Data

Offshore wave and wind data were extracted from data sets of the Wave Information Studies (WIS).

WIS is a U.S. Army Corps of Engineers sponsored project that generates consistent hourly, long-term wave climates along all U.S. coastlines, including the Great Lakes and U.S. island territories. Unlike a forecast, a wave hindcast predicts past wave conditions using a computer model and observed wind fields. By using value-added wind fields, which combine ground and satellite wind observations, hindcast wave information is generally of greater accuracy than forecast wave conditions and is often representative of observed wave conditions.

The hindcast historical wave and wind data at WIS station 63471 as illustrated in Figure 3 were extracted and analyzed. Station 63471 is located at 25.75o latitude and -79.92o longitude in a water depth of approximately 1,280 feet. The analyzed seasonal wave roses and annual wind rose based on time series of wind and wave data during January 1980 and December 2014 are illustrated in Appendix A. The annual wave rose is illustrated in Figure 4. The annual prevailing offshore waves are from north-northeast which predominate over 18.4 percent of the time.

Figure 3: Location of WIS Station 63471 9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

8

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 4: Annual Wave Rose at WIS Station 63471 9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

9

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

A relationship between significant wave height and peak wave period was developed from the offshore data pairs, and this exponential relationship is shown in Figure 5 . The equation in Figure 5 was used to assign peak wave periods to wave heights. The storm event return period of 35-yr (1980-2014) is shown in Figure 6.

Exponential Relationship between Hs and Tp 9

Wave Period (sec)

8 7 6 5 y = 5.4029x0.3033

4 3 2 1 0 0

0.5

1

1.5

2 2.5 Wave Height Hs (m)

3

3.5

4

Figure 5: Exponential Relationship between Wave Heights and Wave Periods at WIS Station 63471

Figure 6: Storm Event Return Period of 35-yr (1980-2014) at WIS Station 63471 9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

10

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

The analyzed normal wave conditions and extreme wave conditions at WIS station 63417 are presented in Table 4. The analyzed normal wind conditions and extreme wind conditions at WIS station 63417 are presented in Table 5.

Table 4: Design Wave Conditions at WIS Station 63417 Hs Wave Event Annual 90-percentile non-exceedance 4.63 ft (1.41 m) 25-year Return Period 22.60 ft (6.89 m)

Tp, sec 6.0 9.7

Table 5: Design Wind Conditions at WIS Station 63417 Wind Speed Wind Event Annual 90-percentile non-exceedance 29.86 ft/sec (9.1 m/sec) 95.80 ft/sec (29.2 m/sec) 25-year Return Period

4.2.

Nearshore Wave Transformation

The nearshore wave transformation study was conducted utilizing a third-generation, spectral wave numerical model. The MIKE 21 Spectral Waves (SW) software was applied to calculate wave conditions approaching Key Biscayne, using winds and offshore wave boundary conditions as discussed in the above sections. MIKE 21 SW simulates the growth, decay and transformation of windgenerated waves and swells in offshore and nearshore coastal areas.

A regional wave model was developed to transform waves from offshore to the nearshore project area.

The tidal elevations were predicted using MIKE 21 tide prediction tool based on global tide model data. The wave model domain, computational mesh resolution and model bathymetry are illustrated in Figure 7 and Figure 8, respectively. The model bathymetry was based on profile survey data from March 2017 and NOAA nautical chart data. The high resolution of the data set and correlated high resolution of the model computational mesh resolves the complex nearshore bathymetry from FDEP reference monuments R-91 to R-113 along Key Biscayne.

The calibrated model parameters of a spectral wave model for a shoreline management project in Broward County, Florida were used for the Key Biscayne regional wave model. The Broward County shoreline management project is located at approximately 43 miles to north of the Key Biscayne project. The WIS Station 63417 wave data was applied as an offshore open boundary conditions, and the wind data was applied over the entire model area. Waves were computed for the continuous period from January 1980 through December 2014. The computed wave model results were extracted for Key Biscayne.

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

11

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 7: Wave Model Mesh and Bathymetry

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

12

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 8: Bathymetry along Key Biscayne The directional wave roses along the contours of -3.3 ft (-1.0 m), -6.6 ft (-2.0 m), and -9.9 ft (-3.0 m), MLLW as illustrated in Figure 9 through Figure 11 were developed based on the wave transformation modeling results. The following wave conditions and potential longshore sediment transport at the nearshore project area were obtained based on the annual (1980-2014) wave roses: •

The wave heights at the south beach area (R-104.5 to R-108) are higher than the wave heights at the north beach area (R-102 to R-104.5).

•

At the north beach area, the annual net longshore sediment transport direction is from south to north. At the south beach area, the annual net longshore sediment transport direction is from north to south.

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

13

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

•

The annual net longshore rate along the south beach area is larger than the rate at the north beach area.

•

The typical annual net longshore sediment transport trends along the Key Biscayne Beach are illustrated in Figure 12.

•

The erosion “hotspot” along the Key Biscayne shoreline is located at between R-104.5 to R106.5. A “hot spot” is an area with historically higher erosion rates than nearby adjacent beaches.

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

14

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 9: Wave Roses along -3.28 feet (-1 m), MLLW contour (1980-2014)

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

15

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 10: Wave Roses along -6.56 feet (-2 m), MLLW contour (1980-2014)

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

16

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 11: Wave Roses along -9.84 feet (-3 m), MLLW contour (1980-2014)

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

17

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 12: Typical Annual Net Longshore Sediment Transport Pattern

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

18

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

The normal and extreme wave conditions were computed using the regional wave model. Figure 13 illustrates the simulated wave height distributions along Key Biscayne for the annual 90-percentile non-exceedance wave conditions. Figure 14 through Figure 18 show the simulated wave height distributions along Key Biscayne for the 25-year return period wave event.

Figure 13: Wave Height Distributions (Annual 90-percentile non-exceedance) at Project Site

Figure 14: Wave Height Distributions when Incident Waves from Northeast (25-year Return Period)

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

19

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 15: Wave Height Distributions when Incident Waves from East-northeast (25-year Return Period)

Figure 16: Wave Height Distributions when Incident Waves from East (25-year Return Period)

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

20

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 17: Wave Height Distributions when Incident Waves from East-southeast (25-year Return Period)

Figure 18: Wave Height Distributions when Incident Waves from Southeast (25-year Return Period)

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

21

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

The design wave conditions are summarized in Table 6.

Table 6: Design Wave Conditions along Key Biscayne Wave Height, Hs Wave Condition Annual 90-percentile non-exceedance 25-year return period

4.3.

3.44 ft (1.05 m) 6.56 ft (2.0 m)

Wave Period, Tp (sec) 6.05 10.6

Wave Dissipation on the Submerged Breakwaters

A desktop method and a coastal numerical model were applied for the wave transmission analysis.

Usually, the wave dissipation over a submerged breakwater is relative to the breakwater crest width, breakwater crest elevation, water depth, and the incident wave conditions. The proposed breakwater crest elevation is located at mean low water (0.11ft, MLLW), and the breakwater crest width varies between 6.6 feet and 32.8 feet (2 m and 10 m).

The parameter used to measure the effectiveness of a submerged breakwater in terms of wave attenuation is the transmission coefficient, đ??žđ??žđ?‘Ąđ?‘Ą =

đ??ťđ??ťđ?‘Ąđ?‘Ą đ??ťđ??ťđ?‘–đ?‘–

Where đ??žđ??žđ??žđ??ž is the wave transmission coefficient, đ??ťđ??ťđ??ťđ??ť is the transmitted wave height on the lee of the structure, and đ??ťđ??ťđ??ťđ??ť is the incident wave height on the seaward side of the structure.

Friebel and Harris (2004) derived an empirical wave transmission formula from the data collected from five physical model studies, đ??šđ??š

đ??žđ??žđ?‘Ąđ?‘Ą = −0.4969đ?‘’đ?‘’đ?‘’đ?‘’đ?‘’đ?‘’ďż˝đ??ťđ??ťďż˝ − 0.0292

đ??ľđ??ľ â„Ž đ??ľđ??ľ đ??šđ??š − 0.4257 − 0.0696 log ďż˝ ďż˝ + 0.1359 + 1.0905 đ?‘‘đ?‘‘ đ?‘‘đ?‘‘ đ??żđ??ż đ??ľđ??ľ

Where đ??ťđ??ť is the incident wave height, đ??šđ??š is the freeboard above the crest of the structure, đ??ľđ??ľ is the crest width of the structure, â„Ž is the height of the structure, đ?‘‘đ?‘‘ is the water depth, and đ??żđ??ż is the wave length.

The DHI MIKE 21 Boussinesq Wave (BW) model includes wave shoaling, refraction, diffraction, wave breaking, bottom friction, non-linear wave-wave interaction, frequency spreading and directional spreading. A flume wave model was developed for the wave dissipation analysis. The flume was represented using a uniform square grid with horizontal grid resolution of 3.28 feet (1.0 m). The normal wave condition (annual 90-percentile non-exceedance) presented in Table 6 was utilized as wave open boundary conditions for the wave dissipation analysis.

Table 7 presents the predicted wave transmission coefficients for different breakwater crest widths.

Figure 19 illustrates the relationship between the predicted wave transmission coefficients and the proposed submerged breakwater crest widths. For the crest widths between 6.6 feet (2 m) and 32.8 feet 9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

22

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

(10 m), the predicted wave transmission coefficients are between 0.41 and 0.49 using Friebel and Harris’s formula, and the predicted wave transmission coefficients are between 0.32 and 0.45 from the BW flume wave model.

Table 7: Predicted Wave Transmission Coefficients Kt (Friebel and Harris) Breakwater Crest Width (ft) 6.6 13.1 19.7 26.3 32.8

0.49 0.48 0.45 0.43 0.41

Kt (BW Wave Model) 0.45 0.40 0.36 0.34 0.32

Wave Transmission Coefficient, Kt

1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

Breakwater Crest Width (ft)

Friebel & Harris

BW wave model

Figure 19: Wave Transmission Coefficient, Kt vs. Breakwater Crest Widths

9699-04

Moffatt & Nichol | Preliminary Nearshore Wave Transformation and Wave Dissipation Analysis

23

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

5. Beach Nourishment Conceptual Design This section presents the proposed beach nourishment design for the Key Biscayne project with the associated engineering and environmental considerations. In 1987, the U.S. Army Corps of Engineers (USACE) constructed a beach nourishment project to provide the minimum shore protection. The fill template of the 2002 beach nourishment project coincides with the permitted construction footprint of 1987 project. The proposed beach nourishment conceptual design is intended to restore the Village beach to the similar footprint as specified in the USACE 1987 construction template. Environmental concerns are addressed including potential impact to the nearshore seagrass beds.

5.1.

Beach Nourishment Development

Criteria for the conceptual beach nourishment design was developed based on the historical erosion rates, critical areas of erosion, limit of seagrass beds, and permitting considerations. M&N conducted a conceptual design of an approximate 100,000+ cy beach nourishment. The beach fill design is located within the Village from monuments R-102 to R-108. The 1998 and 2017 survey profiles, 1987/2002 projects and 2017 seagrass locations are illustrated in Figure 20 through Figure 25. The berm elevation of the proposed nourishment profile is approximately +5.5 feet NAVD with a construction foreshore slope of 10:1 (H:V) extending offshore to the toe of the fill. Table 8 presents the dimensions of the proposed beach nourishment project. After placement of the construction fill, the proposed beach nourishment profile is expected to be adjusted towards an equilibrium profile.

Table 8: Dimensions of the Proposed Beach Nourishment Project Profile

Berm Elevation (ft, NAVD)

Foreshore Slope (H:V)

Fill Density (cy/ft)

R-102 R-103 R-104 R-105 R-106 R-107 R-108

5.5 5.5 5.5 5.5 5.5 5.5 -

10:1 10:1 10:1 10:1 10:1 10:1 -

5.12 10.69 11.67 36.56 27.75 14.29 -

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

24

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Elevation in Feet (NAVD)

R-102 10

Jul-98

8

Mar-17

6

2017 Seagrass

4

2002 Project

2

Proposed Beach Fill

0 -2 -4 -6 -8 -10

0

100

200

300

400

500

Distance in Feet from R-102 Figure 20: Beach Fill Design at R-102

R-103 10

Jul-98

Elevation in Feet (NAVD)

8

Mar-17

6

2017 Seagrass

4

2002 Project

2

Proposed Beach Fill

0 -2 -4 -6 -8 -10

0

100

200

300

400

500

Distance in Feet from R-103 Figure 21: Beach Fill Design at R-103

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

25

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Elevation in Feet (NAVD)

R-104 10

Jul-98

8

Mar-17

6

2017 Seagrass

4

2002 Project

2

Proposed Beach Fill

0 -2 -4 -6 -8 -10

0

100

200

300

400

500

Distance in Feet from R-104 Figure 22: Beach Fill Design at R-104

Elevation in Feet (NAVD)

R-105 10

Jul-98

8

Mar-17

6

2017 Seagrass

4

2002 Project

2

Proposed Beach Fill

0 -2 -4 -6 -8 -10

0

100

200

300

400

500

Distance in Feet from R-105 Figure 23: Beach Fill Design at R-105

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

26

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Elevation in Feet (NAVD)

R-106 10

Jul-98

8

Mar-17

6

2017 Seagrass

4

2002 Project

2

Proposed Beach Fill

0 -2 -4 -6 -8 -10

0

100

200

300

400

500

Distance in Feet from R-106 Figure 24: Beach Fill Design at R-106

R-107 10

Jul-98

8

Mar-17

Elevation in Feet (NAVD)

6 4

2017 Seagrass

2

2002 Project

0

Proposed Beach Fill

-2 -4 -6 -8 -10

0

100

200

300

400

500

Distance in Feet from R-107 Figure 25: Beach Fill Design at R-107

5.2.

Potential Impacts on the Nearshore Seagrasses

The impacts of the proposed beach nourishment project to the nearshore seagrasses primarily involve the area of nearshore beds that may be covered by the toe of the initial fill with the potential for additional coverage through the equilibrium spreading of the fill.

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

27

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

The seaward extent of the post-nourishment equilibrated profile, or the ETOF, can be predicted to evaluate the potential for burial and/or impacts to indirect impacts to nearshore marine resources. The following section examines the potential ETOF locations utilizing the Dean Method.

Dean Equilibrium Profile Method The Dean Method is summarized in the “Beach Nourishment, Theory and Practice” (Dean, 2002). By using the Dean Method, the beach profile contour advancement can be evaluated based on beach fill volumes, fill grain sizes, design berm heights and the depths of closure. Therefore, the ETOF locations can be estimated by using the evaluated beach profile contour advancements. Appendix B presents the more detailed information regarding the Dean Method.

The procedure utilized to estimate the ETOF locations for the proposed beach renourishment Project using the Dean Method entailed the following: •

Evaluate the beach fill volume, obtain the fill grain size and the native beach sand grain size, and determine the design berm height.

•

Determine the nourished beach profile type based on the fill grain size and the native beach sand grain size.

•

Calculate the additional dry beach width, which is the equilibrium shoreline advancement from the existing shoreline, after beach nourishment.

•

Calculate the difference in cross-shore distance between the existing shoreline and the post nourishment shoreline to a particular depth contour.

•

Add the calculated distance difference to the existing profile and obtain the fill equilibrium profile.

•

Determine the ETOF locations based on the existing profile and the Dean fill equilibrium profile.

In the Dean Method, the pre-construction beach profiles, the native beach sand sizes (d50=0.30mm), the fill material sizes (d50=0.30mm, assumed from an offshore borrow source), the fill berm heights, the fill volumes and the depths of closure were utilized to evaluate the Dean’s equilibrium profiles at the reference monuments. The A-value for the native beach is approximately 0.186 ft1/3, and the A-value for the fill material is approximately 0.186 ft1/3.

Results The ETOF locations were estimated at the reference monuments. Figure 26 through Figure 30 illustrate in cross section the pre-construction profile (March 2017), the estimated Dean equilibrium profile, the beach fill, the landward limit of seagrass (February 2017) and the estimated ETOF for the proposed beach nourishment project. Table 9 presents the predicted direct and indirect impacted areas on the nearshore seagrasses.

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

28

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Table 9: Impacted Areas on the Nearshore Seagrasses Impacted Area (acre) Project Direct Burial Indirect Burial 28.6 1987 2.6 2002 4.1 4.5 Proposed 13.4

Figure 26: Predicted Equilibrium Profile at R-103

Figure 27: Predicted Equilibrium Profile at R-104

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

29

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 28: Predicted Equilibrium Profile at R-105

Figure 29: Predicted Equilibrium Profile at R-106

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

30

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Figure 30: Predicted Equilibrium Profile at R-107

9699-04

Moffatt & Nichol | Beach Nourishment Conceptual Design

31

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

6. Preliminary Submerged Breakwater Design For longer term shore protection, the Village and their consultant team have requested an evaluation of submerged breakwaters. Offshore breakwaters were evaluated in previous studies by the U.S. Army Corps of Engineers in 1984, however the coastal protection alternative was not recommended due to the high construction costs ($4.9M – 1983 costs). The proposed submerged breakwaters will provide protections for the Key Biscayne shoreline. An initial submerged breakwater design was proposed for the discussion purposes. A typical breakwater cross-section with breakwater crest elevation, crest width, foreshore slope and footprint was proposed.

6.1.

Design Criteria

The design methods and design parameters outlined in the Coastal Engineering Manual were used for the coastal structural design for the project.

• • • • •

6.2.

The high-quality armor stone will be used for the proposed submerged breakwaters.

25-year return period storm event is used to design the armor rock sizes.

The wave transmission coefficient is approximately between 0.36 and 0.45.

The crest elevation of the proposed submerged breakwater is located at MLW of 0.11 feet (MLLW) NAVD. The breakwater crest width is approximately 19.7 feet (6 m).

Submerged Breakwater Design

Four submerged breakwaters located at nearshore approximately seaward 400 feet to 500 feet from the existing shoreline are illustrated in Figure 31.

Figure 31: Initial Submerged Breakwater Locations 9699-04

Moffatt & Nichol | Preliminary Submerged Breakwater Design

32

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

The Hudson formula (1984) outlined in the CEM was used to evaluate armor rock stable sizes. The submerged breakwaters are designed to have damage of 0% to 5% for the 25-year design wave event.

Table 10 presents the armor rock design parameters. The design armor rock sizes and underlayer stone sizes for the breakwaters are summarized in Table 11 and Table 12, respectively.

Table 10: Armor Rock Design Parameters Parameter Design water level (ft, MSL) Design water depth (ft, MSL) Design wave Height, Hs (ft) Design wave height, H1/10 (ft) Armor unit density (pound/foot3) Density of water (pound/foot3) Stability coefficient, Kd Breakwater slope Stable armor rock mass (pound) Equivalent cube length (ft)

Value 4.76 14.07 6.56 8.33 156.07 64.30 3.00 2:1 (H:V) 5181 3.22

Table 11: Design Armor Rock Sizes Item Equivalent cube length (ft) Armor rock mass (pound)

Value 2.92 to 3.48 3,885 to 6,473

Table 12: Underlayer Stone Sizes Item Equivalent cube length (ft) Underlayer stone mass (pound)

Value 1.15 to 1.41 243 to 450

Table 13: Material Quantities for Beach Management Project Value Material Sand (cy) 101,700 Armor rock (tons) 31,170 8,000 Underlayer stone (tons) 86,330 Geotextile (ft^2)

9699-04

Moffatt & Nichol | Preliminary Submerged Breakwater Design

33

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

7. Opinion of Probable Construction Cost The below table summarizes the construction costs estimated for the project. The concepts prepared and estimated quantities were utilized to compile “order of magnitude� construction budgets for the major line items. Additional field data collection, engineering design, and cost estimating is required for further refinement of these budgets. Three (3) sand sources were considered to evaluate alternatives and costs for this component of the Project. The most economical source of sand is the borrow source that is to the southeast of the Project. This source of sand was utilized for both the 1987 and 2002 projects. Unit costs were utilized to estimate budgets for the truck haul beach nourishment as was recently implemented in 2012 and in 2017. The third alternative considered was for the use of aragonite sand from the Bahamas. This option would entail a large trailing suction hopper dredge that would dredge beach compatible sand in the Bahamas, and then hydraulically unload the hopper dredge for placement of sand along the project area. Total costs for the proposed beach nourishment, including contingencies, range between $4.2 and $7.2M. The construction of the offshore breakwaters is estimated at $10.3M. Utilizing the lowest cost for the beach nourishment, the total project estimated cost is $14.5M. This budget does not include soft costs (i.e. field data collection, planning, legal/lobbying support, permitting, engineering, construction administration, and monitoring). The budget also does not include mitigation for seagrass impacts from the beach nourishment and from the breakwater construction. Total direct impacts estimates are anticipated to be in the range of 16-18 acres. A line item of $5.1M for seagrass mitigation was included in budget estimates prepared for the Village in 2011 for a 75,000 cy beach nourishment.

Seagrass mitigation for this Project could exceed $10M. However, after litigation there was no mitigation required from the 1987 beach nourishment project that impacted over 20 acres of seagrass. Further studies and evaluation are required to estimate seagrass impacts and the required mitigation.

9699-04

Moffatt & Nichol | Opinion of Probable Construction Cost

34

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

OPINION OF PROBABLE CONSTRUCTION COST(2)

QUANTITY(1) NUMBER UNIT

ITEM DESCRIPTION Beach Nourishment - Borrow Source(3) Mobilization/Demobilization Place beach compatible sand Turbidity Control and Monitoring Vibration Control and Monitoring Beach Tilling Construction Surveying Geotechnical Testing Staging Area Temporary Construction Endangered Species Observer

1 101700 1 1 9 1 1 1 1

Performance and Payment Bonds General Conditions Engineer Contingency

LS CY LS LS ACRE LS LS LS LS

1 LS 1 LS 10%

UNIT COST

$ 1,900,000.00 $ 14.50 $ 80,000.00 $ 50,000.00 $ 2,300.00 $ 38,000.00 $ 15,000.00 $ 20,000.00 $ 110,000.00 Subtotal $ $

$ $ $ $ $ $ $ $ $ $

1,900,000 1,474,650 80,000 50,000 20,700 38,000 15,000 20,000 110,000 3,708,350

50,000 $ 70,000 $ $

50,000 70,000 370,835

$

4,199,185

$ $ $ $ $ $ $ $ $

180,000 5,491,800 80,000 50,000 20,700 38,000 15,000 100,000 5,975,500

120,000 $ 100,000 $ $

120,000 100,000 597,550

$

6,793,050

$ $ $ $ $ $ $ $ $

3,500,000 2,847,600 80,000 50,000 20,700 38,000 15,000 60,000 6,611,300

100,000 $ 70,000 $ $

100,000 70,000 661,130

$

7,442,430

$ $ $ $ $ $ $ $ $

1,400,000 57,600 1,200,000 5,922,300 80,000 20,000 450,000 60,000 9,189,900

120,000 $ 70,000 $ $

120,000 70,000 918,990

$

10,298,890

BEACH NOURISHMENT - BORROW SOURCE SUBTOTAL

Beach Nourishment - Truck Haul Source Mobilization/Demobilization Deliver, place and survey beach compatible sand Turbidity Control and Monitoring Vibration Control and Monitoring Beach Tilling Construction Surveying Geotechnical Testing Staging Area Temporary Construction

1 101700 1 1 9 1 1 1

Performance and Payment Bonds General Conditions Engineer Contingency

LS CY LS LS ACRE LS LS LS

1 LS 1 LS 10%

$ 180,000.00 $ 54.00 $ 80,000.00 $ 50,000.00 $ 2,300.00 $ 38,000.00 $ 15,000.00 $ 100,000.00 Subtotal $ $

BEACH NOURISHMENT - TRUCK HAUL SUBTOTAL Beach Nourishment - Alternate Sand Source(4) Mobilization/Demobilization(5) Place beach compatible sand Turbidity Control and Monitoring Vibration Control and Monitoring Beach Tilling Construction Surveying Geotechnical Testing Endangered Species Observer

1 101700 1 1 9 1 1 1

Performance and Payment Bonds General Conditions Engineer Contingency

LS CY LS LS ACRE LS LS LS

1 LS 1 LS 10%

$ 3,500,000.00 $ 28.00 $ 80,000.00 $ 50,000.00 $ 2,300.00 $ 38,000.00 $ 15,000.00 $ 60,000.00 Subtotal $ $

BEACH NOURISHMENT - ALTERNATE SAND SOURCE SUBTOTAL

Marine Works - Offshore Breakwaters Mobilization/Demobilization Furnish and Install Geotrextile Furnish and Install Underlayer Stone Furnish and Place Armor Stone Aids to Navigation Construction Surveying Turbidity Control and Monitoring Endangered Species Observer

Performance and Payment Bonds General Conditions Engineer Contingency MARINE WORKS - OFFSHORE BREAKWATERS SUBTOTAL

9699-04

1 9600 8000 31170 1 1 1 1

LS SY TON TON LS LS LS LS

1 LS 1 LS 10%

$ 1,400,000.00 $ 6.00 $ 150.00 $ 190.00 $ 80,000.00 $ 20,000.00 $ 450,000.00 $ 60,000.00 Subtotal $ $

SCHEDULED VALUE

Moffatt & Nichol | Opinion of Probable Construction Cost

35

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Notes: 1) 2)

3) 4) 5)

6)

Quantities are based on conceptual design in feasibility report from Moffatt & Nichol Report dated December, 2017.

The cost estimates herein are made on the basis of Moffatt & Nichol's experience and qualifications, and represent Moffatt & Nichol's best judgement as an experienced and qualified professional generally familiar with the industry. However, Moffatt & Nichol has no control over the cost of labor, materials, equipment, or services furnished by others, or over the Contractor's methods of determining prices, or over competitive bidding or market conditions. Moffatt & Nichol is unable to guarantee that proposals, bids, or actual Construction Costs will not vary from the above estimates.

Borrow areas southeast offshore Key Biscayne utilized for 2002 Project.

Budget provided by Dredging International nv for use of aragonite sand source from the Bahamas. Assumes Trailing Suction Hopper Dredge.

Dredging International nv mobilization budgetary estimate ranges from $3.5M - $6.5M depending on location of equipment.

Higher mobilization cost associated if equipment is in Europe, however lower cost range would be combined with other projects in the region.

8. Conclusions and Recommendations M&N and EAC Consulting, Inc. were retained by the Village to provide a study and conceptual engineering designs of beach nourishment and submerged breakwaters for long term beach management on Key Biscayne. For longer term shore protection, the Village and their lobbyist have requested an evaluation of submerged breakwaters (coastal structures). The objective of the Project is to provide a conceptual beach nourishment design and an initial submerged breakwater design, and to provide estimates of construction costs.

Historical beach management projects were summarized and graphically represented for projects completed between 1969 and 2017. The available data sets including historical beach profile and shoreline position, aerial photography, hydrographic data, publicly available wave, tide, and wind data, and prior coastal engineering studies were reviewed and compiled. M&N performed a nearshore wave transformation study and a wave dissipation study. A regional wave model was developed to simulate wave transformation from offshore to nearshore project site. The normal and extreme wave conditions were computed utilizing the MIKE 21 SW wave model for the 35 years wave data from January 1980 to December 2014. The wave roses at Project site were developed based on the simulated 35 years of wave data. A potential annual net longshore sediment transport was developed. The wave dissipation for waves propagating over a proposed submerged breakwater were estimated using the MIKE 21 BW wave model along with desktop methods.

Based on the results of the coastal process evaluation, M&N prepared a conceptual beach nourishment design. The beach and berm profile volumes and geometries were determined and compared with the minimum shore protection established by the U.S. Army Corps of Engineers for the 1987 beach nourishment project. The potential direct and indirect impacted areas of the proposed beach nourishment project to the adjacent nearshore seagrass beds were estimated using a desktop method.

M&N prepared a preliminary design for the proposed nearshore submerged breakwaters. A plan with the location of the proposed submerged breakwaters was prepared along with a typical cross section.

The armor rock size and quantity, and underlayer stone size and quantity were estimated.

9699-04

Moffatt & Nichol | Conclusions and Recommendations

36

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

The estimated quantities from the conceptual design of the beach nourishment and breakwaters were utilized to provide a probable opinion of construction costs for the Project.

8.1.

Conclusions

•

Beach management projects have been constructed along the beaches of Key Biscayne beginning in 1969. A total of 837,300 cy of sand have been placed between 1969 and 2017.

The Village has managed and implemented projects between 1998-2017. Prior to 1998, projects were managed by the U.S. Army Corps of Engineers with Miami-Dade County as the local sponsor.

•

Previous studies estimated annual losses (erosion) of 40,000 cy between 1919-1960 along the shoreline of Key Biscayne, with the annual loss estimated at 22,000 cy along developed areas.

This estimate was further refined in 1984 to 35,000 cy along the southern 2.4 miles, and to 12,000 cy/year based on 1987-1996 survey data. These erosion trends will continue due to the lack of littoral drift south of Government Cut.

•

SLR will continue to influence coastal zone management along the Project area.

•

Coastal process evaluation summary: Wave heights at the south beach area (R-104.5 to R-108) are higher than wave heights at the north beach area (R-102 to R-104.5).

At north beach area, the annual net longshore sediment transport direction is from south to north. At south beach area, the annual net longshore sediment transport direction is from north to south.

Annual net longshore sediment transport rate at south beach area is larger than that at north beach area.

Erosion “hotspot” at the Key Biscayne beach is located at between R-104.5 to R-106.5.

Annual 90-percentile non-exceedance wave height is approximately 3.44 feet with a corresponding perk wave period of 6.05 seconds at project site.

For the 25-year return period wave event, the extreme wave height is approximately 6.56 feet with a corresponding perk wave period of 10.6 seconds at Project site.

•

Proposed submerged breakwaters consist of a crest width of between 6.6 feet (2 m) and 32.8 feet (10 m), the predicted wave transmission coefficients are between 0.32 and 0.49.

•

The conceptual beach nourishment design with sand fill volume of approximately 101,700 cubic yard is to restore the Village beach to the similar footprint as specified in the USACE 1987 construction template. The estimated direct and indirect impacted areas to the existing nearshore seagrasses are approximately 4.5 acres and 13.4 acres, respectively.

9699-04

Moffatt & Nichol | Conclusions and Recommendations

37

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

•

An initial submerged breakwater design along the Key Biscayne beach between reference monuments R-104 and R-107 was prepared. The proposed submerged breakwaters are located at nearshore approximately seaward 400 feet to 500 feet from the existing shoreline. The proposed breakwater crest is located at MLW, 0.11 feet MLLW. The proposed initial submerged breakwater crest width is approximately 19.7 ft (6 m). The estimated direct impacted seagrass area by the proposed submerged breakwaters is approximately 1.70 acres.

•

Probable opinion of construction for the offshore breakwaters is estimated at $10.3M. Utilizing the lowest cost alternative for the beach nourishment, the total Project estimated cost is $14.5M. This budget does not include soft costs (i.e. field data collection, planning, legal/lobbying support, permitting, engineering, construction administration, and monitoring). The budget also does not include mitigation for seagrass impacts from the beach nourishment and from the breakwater construction.

8.2.

Recommendations

The project as presented is feasible and will provide a long-term coastal management project.

Additional studies that would include field data collection in the form of marine resource, geotechnical, bathymetry, and beach profiles are required to further refine the conceptual design as presented in this feasibility study. Further coastal engineering studies that would include numerical modeling as well as limited physical modeling are also required. The submerged breakwater locations, breakwater length and width, breakwater crest elevation, and gap width between breakwaters need to be optimized utilizing detailed modeling that may include 2D/3D sediment transport numerical modeling.

Once the conceptual design is further refined, pre-application meetings need to be conducted with the federal, state and local environmental agencies that would have jurisdiction over the Project to evaluate the environmental permitting feasibility. Refined construction cost estimates would need to be completed breakwater design will be refined with further coastal numerical modeling and/or physical modeling, studies, and coastal engineering. The implementation of the project would require an experienced, capable team of coastal engineers, environmental scientists, attorneys with environmental backgrounds, as well as lobbyists. There is a track record of cost-sharing with funding beach management projects on Key Biscayne with support from local, state and federal sources. In conjunction with the planning/engineering for the Project, the funding for the Project, along with periodic nourishment (and or post storm recovery projects) needs to be evaluated.

9699-04

Moffatt & Nichol | Conclusions and Recommendations

38

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

9. References Coastal Systems International, 1997. Long Range Beach Nourishment Plan for Village of Key Biscayne, Dade County, Florida. Technical Report.

Coastal Systems International, 2011. Village of Key Biscayne Beach Renourishment Maintenance Event, Equilibrium of Toe of Fill Report. Technical Report.

Dean, R.G., 2002. Beach Nourishment, Theory and Practice. Advanced Series on Ocean Engineering – Volume 18, World Scientific.

Federal Emergency Management Agency, 2009. Flood Insurance Study, Miami-Dade County, Florida. FEMA, Flood Insurance Study Number: 12086CV000A. Florida Department of Environmental Protection, 2015, Strategic Beach Management Plan, Southeast Atlantic Coast Region. Division of Water Management.

Flynn, B.S., Blair, S.M. and Markley, S.M., 1991. Environmental Monitoring of the Key Biscayne Beach Restoration Project. Proceedings of the 1991 National Conference on Beach Preservation Technology.

Friebel, H., and Harris, L., 2004. A new wave transmission coefficient model for submerged breakwater. Proceedings 29th International Conference on Coastal Engineering. Lisbon Portugal.

Moffatt & Nichol, April 2017. Beach Nourishment, Village of Key Biscayne, Florida – Equilibrium Toe of Fill Report. Technical Report.

U.S. Army Corps of Engineers, 1961. Beach Erosion Control Report on Cooperative Study of Virginia and Biscayne Keys, Florida. Technical Report.

U.S. Army Corps of Engineers, 1984. Section 103 Detailed Project Report Key Biscayne Florida.

Technical Report U.S. Army Corps of Engineers, 2006. Coastal Engineering Manual.

9699-04

Moffatt & Nichol | References

39

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Appendix A: Wind and Wave Roses at WIS Station 63417

9699-04

Moffatt & Nichol | References

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Appendix B: Dean’s Method

9699-04

Moffatt & Nichol | References

Key Biscayne Beach Management Feasibility Study

Village of Key Biscayne

Appendix C: Preliminary Submerged Breakwater Design

9699-04

Moffatt & Nichol | References