DATE March 22, 2016 SOLICITATION NO.: CN160225 District Two … Documents... · 2016. 3. 22. · 4.1.1 San Carlos Bay Shoreline Shorelines were plotted in AutoCAD using aerial photography

P.O. Box 398, Fort Myers, Florida 33902-0398 (239) 533-2111 Internet address http://www.lee-county.com

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DATE March 22, 2016 SOLICITATION NO.: CN160225 SUBJECT: ADDENDUM NUMBER 1 REFERENCE: Sanibel Causeway Shoreline Stabilization The following changes shall become a part of the Bid Documents and shall be as binding as if contained therein:

1. Sanibel Causeway Center Island Shoreline Stabilization Conceptual Planning Study was not attached.

Answer Attached is the study BIDDER IS ADVISED, YOU ARE REQUIRED TO ACKNOWLEDGE RECEIPT OF THIS ADDENDUM WHEN SUBMITTING A BID. FAILURE TO COMPLY WITH THIS REQUIREMENT MAY RESULT IN THE BIDDER BEING CONSIDERED NON-RESPONSIVE. ALL OTHER TERMS AND CONDITIONS OF THE BIDDING DOCUMENTS ARE AND SHALL REMAIN THE SAME. Diana Khan Diana Khan, Procurement Manager Lee County Procurement Management

BOARD OF COUNTY COMMISSIONERS

John E. Manning District One Cecil L Pendergrass District Two Larry Kiker District Three Brian Hamman District Four Frank Mann District Five Roger Desjarlais County Manager Richard Wm. Wesch County Attorney Donna Marie Collins Hearing Examiner

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SANIBEL CAUSEWAY CENTER ISLAND

SHORELINE STABILIZATION CONCEPTUAL PLANNING STUDY

Prepared for:

Lee County Board of County Commissioners

P.O. Box 398

Fort Myers, FL 33902-0398

January 14, 2015

CEC File No. 11.149

Sanibel Causeway Shoreline Stabilization Conceptual Planning Study

2

Table of Contents

1 Introduction ............................................................................................................................. 5

2 Scope of Work ......................................................................................................................... 7

3 Existing Conditions ................................................................................................................. 8

3.1 Bathymetric and Topographic Survey.............................................................................. 8

3.2 Aquatic Resource Survey ................................................................................................. 8

4 Coastal Processes and Shoreline Analysis ............................................................................ 10

4.1 Historical Shorelines ...................................................................................................... 10

4.1.1 San Carlos Bay Shoreline ....................................................................................... 10

4.1.2 Pine Island Sound Shoreline ................................................................................... 12

4.2 Wind and Wave Climate ................................................................................................ 16

4.3 Summary ........................................................................................................................ 19

5 Conceptual Plans ................................................................................................................... 20

5.1 No New Action............................................................................................................... 20

5.2 Living Shoreline ............................................................................................................. 20

5.3 Shoreline Revetment ...................................................................................................... 20

5.4 T-groin Field .................................................................................................................. 22

5.5 Detached Segmented Breakwaters ................................................................................. 24

5.6 Summary ........................................................................................................................ 27

5.7 Additional Features ........................................................................................................ 27

5.7.1 San Carlos Bay Shoreline ....................................................................................... 27

5.7.2 Pine Island Sound Shoreline ................................................................................... 27

6 Alternatives analysis .............................................................................................................. 28

6.1 Shoreline Stabilization ................................................................................................... 28

6.2 Recreation....................................................................................................................... 28

6.3 Natural Resources .......................................................................................................... 29

6.4 Fiscal .............................................................................................................................. 29

6.5 Recommendations .......................................................................................................... 29

7 Conclusions ........................................................................................................................... 30

8 References ............................................................................................................................. 32


3

Table of Figures

Figure 1. Project Location Map ...................................................................................................... 6 Figure 2. Existing Conditions Survey ............................................................................................. 9 Figure 3. Historical Shoreline Positions along San Carlos Bay between 2004 and 2014 ............ 10 Figure 4. Historical Shoreline Change Rates along San Carlos Bay between 2004 and 2014 ..... 12 Figure 5. Historical Shoreline Positions along Pine Island Sound between 2004 and 2014 ........ 13 Figure 6. Historical Shoreline Change Rates along Pine Island Sound between 2004 and 2014 . 15 Figure 7. Wind Rose at WIS-73295 Based on Hourly Hindcasts During 1980-2012 .................. 17 Figure 8. Conceptual Plan for Shoreline Revetment .................................................................... 21 Figure 9. T-groin Design Criteria (Bodge, 1998) ......................................................................... 22 Figure 10. Conceptual Plan for T-groin Field. .............................................................................. 23 Figure 11. Relationship of Dimensionless Segmented Breakwater Parameters (Pope and Dean,

1986) ........................................................................................................................... 25 Figure 12. Conceptual Plan of Segmented Breakwaters. .............................................................. 26

Table of Tables

Table 1. Historical Shoreline Positions along San Carlos Bay Between 2004 and 2014 ............. 11 Table 2. Historical Shoreline Change Rates along San Carlos Bay Between 2004 and 2014 ...... 11 Table 3. Historical Shoreline Positions along Pine Island Sound Between 2004 and 2014 ......... 13 Table 4. Historical Shoreline Change Rates along Pine Island Sound Between 2004 and 2014 . 14 Table 5. Average and Maximum Wind Speeds at WIS-73295 ..................................................... 18 Table 6. Calculated Average and Maximum Wave Heights ......................................................... 19


4

Appendix

Appendix 1: Typical Profile Drawings


5

1 INTRODUCTION

The primary purpose of the causeway islands was for the 1963 construction of the roadway

SR/CR 867) from the mainland to Sanibel. The areas alongside the roadway accommodate one

of Lee County’s more popular parks called The Causeway Islands. The combined size of the

islands is approximately 10 acres (Figure 1). The causeway bisects San Carlos Bay near the

mouth of the Caloosahatchee River and provides park visitors access to the bay and gulf. The

islands are a very popular destination since built. Lee County quickly turned the islands in a

County park and have been managing it ever since. Some of the most common activities enjoyed

are fishing, wading/swimming, picnicking, canoeing/kayaking, wind surfing, kite boarding,

shelling and sun bathing. The center island offers restrooms (San Carlos Bay side), drinking

fountains (located on each side of the road) and a few picnic tables (San Carlos Bay side).

The causeway center island shoreline has experienced chronic erosion caused by coastal forces

and stormwater runoff from the roadway. Lee County has imported fill over the years to attempt

to address the erosion and in some areas have extended the seawall and armored the shoreline

with riprap. Coastal Engineering Consultants, Inc. (CEC) was contracted by Lee County to

conduct a Study to identify the causes of erosion and develop potential solutions to address the

chronic erosion problem while preserving the functionality of the park.


6

Figure 1. Project Location Map


7

2 SCOPE OF WORK

The scope of the Conceptual Planning Study includes conducting bathymetric/topographic and

aquatic resource surveys; processing and analyzing the coastal forcing functions on the Sanibel

Causeway center island shoreline; and developing up to three (3) alternative conceptual plans to

stabilize the shoreline. The scope also includes organizing and attending a pre-application

meeting with the Florida Department of Environmental Protection, South Florida Water

Management District, and U.S. Army Corps of Engineers as applicable to discuss the goals and

objectives, present conceptual plans, and obtain input on agency permit concerns/issues. The

results shall be summarized and presented in a meeting with Lee County to address agency

permit concerns as they relate to Project permitting and design.


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3 EXISTING CONDITIONS

3.1 Bathymetric and Topographic Survey

In August 2014, CEC conducted a detailed bathymetric and topographic survey. An AutoCAD

file of the Project area was provided by the County. The survey baseline for the Study was

established utilizing the approximate centerline of the roadway as the baseline noting the

stationing was kept consistent with the AutoCAD file roadway stationing.

Real Time Kinematic (RTK) Global Positioning Systems (GPS) with Global Navigation Satellite

System (GLONASS) capability were utilized for the upland survey along with a base receiver

installed on an established control point. These systems are capable of delivering RTK positions

with coordinate accuracy of 10mm+2ppm. The upland crew also marked erosion, scouring, and

ruts along the beach along with locating Mean High Water (MHW).

The survey vessel used for the bathymetric survey was a 20-foot fiberglass hull powered by an

outboard. An Innerspace 456 single beam echo sounder was used with a side mounted

transducer. The GPS antenna utilized the same side mount bracket as the transducer to place it

directly above the transducer. A Trimble R8 GLONASS RTK GPS receiver was integrated with

the on-board computer system. The Hypack 2012 software package was the hydrographic

guidance program utilized.

Figure 2 presents the existing conditions survey depicting the elevation contours overlain on the

2008 aerial photograph obtained from Lee County. Typical profiles are presented in Appendix 1.

3.2 Aquatic Resource Survey

A survey of the seagrass was undertaken to determine extent of grasses surrounding the

causeway island. A RTK GPS with GLONASS capability with a base receiver installed on an

established control point was used to locate the edge of the seagrass beds in conjunction with a

snorkeler who visually verified the edge. Figure 2 depicts the approximate location of the

seagrass beds. On the San Carlos Bay (east) side of the center island, the seagrass beds begin at

the approximate -2 foot NAVD88 contour and extend seaward out to the approximate -7.5 foot

NAVD88 contour. The limit of the seagrass beds starts at the westernmost tip of the island and

stretches over 2,100 feet ending near Station 384+00. On the Pine Island Sound (west) side of the

center island, the seagrass beds begin at the approximate -2 foot NAVD88 contour and extend

seaward out to the approximate -7.5 foot NAVD88 contour. The limit of the seagrass beds starts

at Station 364+00 near the westernmost tip of the island and end near Station 386+00.


9

Figure 2. Existing Conditions Survey


10

4 COASTAL PROCESSES AND SHORELINE ANALYSIS

4.1 Historical Shorelines

4.1.1 San Carlos Bay Shoreline

Shorelines were plotted in AutoCAD using aerial photography from 2004 to 2008. The

shorelines were digitized following the apparent high water line on the photography and their

accuracy is dependent upon the quality of the aerial photography. The historical shorelines are

presented on Figure 3 along with the 2014 measured MWH. Shoreline positions for each year

were measured at 400-foot intervals starting at Station 364+00 and ending at Station 392+00 and

are referenced to the baseline.

Figure 3. Historical Shoreline Positions along San Carlos Bay between 2004 and 2014


11

Table 1 presents the shoreline positions for each year and Table 2 presents the shoreline change

rates for different periods between 2004 and 2014. The average shoreline change rate was

computed by subtracting the distances and dividing by the number of years between the two

measurements. Figure 4 presents the historical shoreline change rates as a histogram in

relationship to the center island.

Table 1. Historical Shoreline Positions along San Carlos Bay Between 2004 and 2014

Station 2004 Distance

(Ft)

2005 Distance

(Ft)

2008 Distance

(Ft)

2014 Distance

(Ft)

364+00 48 65 65 65

368+00 88 91 78 83

372+00 108 112 92 98

376+00 116 115 100 107

380+00 176 176 162 162

384+00 191 194 185 175

388+00 176 180 163 152

392+00 134 124 116 116

Table 2. Historical Shoreline Change Rates along San Carlos Bay Between 2004 and 2014

Station 2004-2005

Change Rate

(Ft/Yr)

2005-2008

Change Rate

(Ft/Yr)

2008-2014

Change Rate

(Ft/Yr)

2004-2014

Change Rate

(Ft/Yr)

364+00 17.0 0.0 0.0 -0.5

368+00 3.0 -4.3 0.8 -0.4

372+00 4.0 -6.7 1.0 -0.1

376+00 -1.0 -5.0 1.2 -1.1

380+00 0.0 -4.7 0.0 -1.5

384+00 3.0 -3.0 -1.7 -1.9

388+00 4.0 -5.7 -1.8 -2.5

392+00 -10.0 -2.7 0.0 -1.1

A comparison of the shoreline change data along with review of available aerial photography

indicates the following trends since 2004. The shoreline segment from approximate Station

362+00 to Station 372+00 experienced minimal erosion over the years. The shoreline segment

from approximate Stations 380+00 to 390+00 experienced continual erosion, ranging from 14

feet to 24 feet with an average of 18 feet since 2004. The shoreline from approximate Station

390+00 to Station 394+00 experienced erosion until the shoreline reached the seawall. During

this period significant storm events impacted the Project area. For example, in 2004, Hurricanes

Charley, Frances, Ivan and Jeanne impacted Southwest Florida. However, with the exception of

Stations 376+00 and 392+00, the rates for the 2004 to 2005 period were accretional. It is noted

the imported material may have offset some percentage of the historical erosion; thus the actual

rates may be higher than computed.


12

Figure 4. Historical Shoreline Change Rates along San Carlos Bay between 2004 and 2014

4.1.2 Pine Island Sound Shoreline

A similar analysis was performed along the Pine Island Sound shoreline. The historical

shorelines are presented in Figure 5 along with the 2014 measured MWH. Note the vertical axis

for Figures 5 and 6 have been oriented to match the direction of the change in shoreline position.


13

Figure 5. Historical Shoreline Positions along Pine Island Sound between 2004 and 2014

Table 1 presents the shoreline positions for each year and Table 4 presents the shoreline change

rates for different periods between 2004 and 2014. Figure 6 presents the historical shoreline

change rates as a histogram in relationship to the center island.

Table 3. Historical Shoreline Positions along Pine Island Sound Between 2004 and 2014

Station 2004 Distance

(Ft)

2005 Distance

(Ft)

2008 Distance

(Ft)

2014 Distance

(Ft)

364+00 149 146 144 147

368+00 135 140 136 151

372+00 131 132 126 139

376+00 117 124 117 118

380+00 143 147 152 144

384+00 169 182 185 174

388+00 166 175 167 158

392+00 57 60 56 56


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Table 4. Historical Shoreline Change Rates along Pine Island Sound Between 2004 and

2014

Station 2004-2005

Change Rate

(Ft/Yr)

2005-2008

Change Rate

(Ft/Yr)

2008-2014

Change Rate

(Ft/Yr)

2004-2014

Change Rate

(Ft/Yr)

364+00 -3.3 -0.5 0.4 -0.2

368+00 4.7 -1.3 2.6 1.7

372+00 1.6 -2.0 2.0 0.8

376+00 7.5 -2.6 0.2 0.1

380+00 4.7 1.5 -1.2 0.2

384+00 13.0 0.8 -1.7 0.5

388+00 9.8 -2.7 -1.6 -0.8

392+00 3.0 -1.6 0.0 -0.2

The Pine Island Sound shoreline segment has remained relatively stable over the years (2004 to

2014). Over the 10 year period of analysis, at Station 364+00 and between Stations 388+00 to

392+00 are the only reaches to experience a net erosion trend noting the magnitude is less than

one foot per year. It is noted the imported material may have offset some percentage of the

historical erosion; thus the actual rates may be higher than computed.


15

Figure 6. Historical Shoreline Change Rates along Pine Island Sound between 2004 and

2014


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4.2 Wind and Wave Climate

Wind and wave data were sourced from the Wave Information Studies (WIS) conducted by the

US Army Corps of Engineers (USACE) Coastal Hydraulics Laboratory and Engineer Research

and Development Center (Hubertz, 1992). WIS data provides directional wave climate

information for shorelines of the Gulf of Mexico based on a 32-year hindcast using the 2nd

generation wave model WISWAVE. The current hindcast covers the period 1980 to 2012, and

provides hourly hindcasts of the significant wave height, peak spectral period, dominant wave

direction, wind speed, and wind direction. WIS Station 73295 is located in the Gulf of Mexico

approximately 8 miles south of Sanibel Causeway.

Statistical analysis of the wind data at WIS-73295 (Figure 7) indicates that there are three

dominant directions (22.5° directional bands) that the waves enter the computational domain

from: 90° (clockwise from true North), 112.5° and 67.5°, which occur in 14.9%, 12.3% and

12.0% of all cases, respectively. Overall, the quadrant between 45° and 135° accounts for more

than 57% of all occurrences. The average and maximum wind speeds for all wind directions are

presented in Table 5.


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Figure 7. Wind Rose at WIS-73295 Based on Hourly Hindcasts During 1980-2012 (source:

http://wis.usace.army.mil/)


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Table 5. Average and Maximum Wind Speeds at WIS-73295

Wind Direction,

Counterclockwise

from

Wind Occurrence

Frequency, %

Average Wind Speed

(mph)

Max Wind Speed

(mph)

0° (North) 5.7% 15.0 62.9

22.5° 6.7% 15.2 45.4

45° (Northeast) 9.0% 14.3 54.1

67.5° 12.0% 13.6 48.5

90° (East) 14.9% 12.5 71.1

112.5° 12.3% 11.9 58.2

135° (Southeast) 9.0% 11.4 38.9

157.5° 5.8% 11.0 62.4

180° (South) 4.3% 11.0 43.8

202.5° 3.2% 10.5 40.9

225° (Southwest) 2.7% 10.7 54.6

247.5° 2.4% 11.0 41.4

270° (West) 2.3% 11.6 40.9

292.5° 2.4% 13.0 49.9

315° (Northwest) 3.3% 14.3 48.1

337.5° 4.1% 14.5 58.2

The wind data were used to estimate wave height at the center island based on the wind speed

adjustment and wave growth formulation provided in USACE’s Automated Coastal Engineering

System (ACES) computer program. The program uses wind speed and direction data and fetch

lengths (distances over water for wind generated waves) to calculate wave heights. Table 6

presents the calculated wave heights corresponding to average and maximum wind speed for

each direction. The wave heights based on the average wind speed range from 0.2 feet to 1.0

feet, noting that the most dominant wind direction (90°) yields a wave height of 0.9 feet. The

wave heights based on the maximum wind speed range from 1.6 feet to 4.1 feet with the most

dominant wind direction (90°) yielding a wave height of 4.1 feet.


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Table 6. Calculated Average and Maximum Wave Heights

Wind Direction,

Counterclockwise

from

Wind

Occurrence

Frequency, %

Fetch

Length

(miles)

Wave Height Based

on Average Wind

Speed

(feet)

Wave Height

Based on Max

Wind Speed

(feet)

0° (North) 5.7% 1.9 0.8 3.4

22.5° 6.7% 3.3 0.7 2.3

45° (Northeast) 9.0% 1.7 0.6 2.5

67.5° 12.0% 2.8 0.7 2.6

90° (East) 14.9% 4.5 0.9 4.1

112.5° 12.3% 8.2 1.0 4.0

135° (Southeast) 9.0% 17.0 1.0 3.0

157.5° 5.8% 0.8 0.9 4.1

180° (South) 4.3% 0.7 0.7 2.8

202.5° 3.2% 0.7 0.4 1.9

225° (Southwest) 2.7% 0.7 0.2 1.6

247.5° 2.4% 0.7 0.4 1.7

270° (West) 2.3% 1.0 0.5 2.0

292.5° 2.4% 4.0 0.6 2.5

315° (Northwest) 3.3% 2.5 0.8 2.8

337.5° 4.1% 7.5 0.9 3.4

4.3 Summary

The San Carlos Bay shoreline segment between approximate Stations 378+00 to 392+00

experienced erosion between 2004 and 2014 and is recommended for the stabilization efforts to

protect the upland infrastructure (e.g., restrooms). A section of this segment is armored

including a seawall and/or riprap. The historical shoreline change analysis indicates the

shoreline erosion is chronic versus episodic. This is supported by the coastal processes analysis

that determined the predominant wind and wave direction is from the south to east. Further, this

eroding section of shoreline is adjacent to the deep water channel separating the two causeway

islands; therefore, no buffer from the wind and waves exists from the south and east.

While not to the same degree as the San Carlos Bay shoreline, the Pine Island Sound shoreline

segment between approximate Stations 386+00 to 392+00 experienced erosion between 2004

and 2014 and is recommended for the stabilization efforts. As confirmed by the coastal processes

analysis, the wind and wave climate is milder on the Sound side than the Bay side. Further, there

are no major upland improvements on the Sound side. Thus the degree or extent of the

stabilization efforts for the Pine Island Sound shoreline may be less compared to the San Carlos

Bay shoreline.


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5 CONCEPTUAL PLANS

5.1 No New Action

Based on personal knowledge of the project area and communications with the County, the

approach employed by the County to date, specifically import fill and/or riprap on an as-needed

basis, is not acceptable for the long-term solution for stabilizing the shorelines along the center

island. This approach has had minimal success in addressing the erosion problem and has

adversely affected the utilization of one of the more active beach areas adjacent to the restroom

facility. Further, this option requires frequent maintenance.

Specific to the prior actions by the County, based upon site observations and surveys, it is

recommended that the existing riprap along the San Carlos Bay shoreline be removed as it is

undersized and frequently displaced. Further, it is recommended that the existing riprap along the

seawall between approximate Stations 388+00 to 392+00 be improved with appropriately sized

rock to reduce reflective wave energy off the seawall. For the Pine Island Sound shoreline, it is

recommended the existing riprap along the seawall and shoreline between approximate Stations

390+00 to 392+00 be improved as well.

5.2 Living Shoreline

Creating a living shoreline feature along the center island shoreline would consist of planting

native dune and wetland plants along the beach and nearshore areas, respectively. The vegetation

would serve to buffer the effects of the coastal forcing functions on the shoreline and reduce the

erosion rate. However, based on the coastal processes analysis, the project area is not conducive

to a living shoreline as a solitary feature. It is anticipated the wind and wave conditions will

adversely affect the opportunity to establish plant growth and sustainability along the shoreline.

Further, the presence of existing healthy seagrass beds along a majority of the nearshore zone in

effect already provides a “living shoreline” at the park.

5.3 Shoreline Revetment

Revetments are structures that are built along a shoreline and separate land and water primarily

to prevent erosion and other damage due to wave action. Revetments absorb some of the wave

energy and reflect the waves. Revetments typically prevent erosion landward of the revetment

but usually increase the steepness of the beach profile as sediment is carried off the beach. A

revetment also typically transfers erosion to the downdrift shoreline. Figure 8 presents a

conceptual plan for the San Carlos Bay shoreline revetment extending between Stations 380+00

and 388+00.


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Figure 8. Conceptual Plan for Shoreline Revetment


22

5.4 T-groin Field

Groins are relatively short structures that extend perpendicularly from the shore. Usually

constructed in groups called groin fields, their primary purpose is to trap and retain sand,

nourishing the beach compartments between them. T-groins are groins with a T-shaped

attachment at their seaward end designed to better trap and anchor sediment trapped by the groin

structure. A T-groin field typically transfers erosion to the downdrift shoreline. To determine T-

groin field design parameters and its effect on the shoreline, a combined wave and current

modeling study would have to be performed to predict changes in circulation, wave pattern, and

shoreline response caused by the T-groin field.

For the conceptual design, the “one-third” rule (Bodge, 1998) was used, which requires that T-

groin head H = 2(G/3) where G is the gap width between the heads (Figure 9). Based on

professional judgment and experience, and review of T-groin design criteria (e.g., Bodge, 1998),

it was assumed the T-groin heads would be 110 feet long and spaced 160 feet apart (parameters

H and G in Figure 9, respectively). The San Carlos Bay shoreline segment to be stabilized is 800

feet, thus four (4) T-groins would be required. The T-groin heads would be oriented

perpendicular to the dominant wave direction which is from the east. For the purposes of the

planning level analysis, the lengths of the trunk were assumed to be 60 feet from the 2014 MWH

shoreline, equal to double the amount of average erosion experienced between Stations 378+00

and 390+00 since 2004, to allow for importing sand to recreate and sustain the shoreline. A more

detailed analysis will be required to establish final design criteria. Figure 10 presents a

conceptual plan of the T-groin field.

Figure 9. T-groin Design Criteria (Bodge, 1998)


23

Figure 10. Conceptual Plan for T-groin Field.


24

5.5 Detached Segmented Breakwaters

Detached breakwaters are generally shore-parallel structures constructed in the nearshore along

the shoreline. They reduce the amount of wave energy reaching the protected area. The reduction

in wave energy slows the littoral drift and produces sediment deposition in the sheltered area

often creating a salient or tombolo behind the breakwaters. Some longshore sediment transport

may continue along the shoreline in the lee of the nearshore breakwaters.

Detached breakwaters may have downdrift impacts resulting from disruption of natural littoral

drift. They may also impact navigation. Further, if tidal currents along the shoreline are

significant placing the breakwaters too close to the shore may cause an increase in currents

behind them causing erosion and adversely impacting the shoreline. To determine detached

breakwater design parameters and their effects on the shoreline, a combined wave and current

modeling study would have to be performed to predict changes in circulation, wave pattern, and

shoreline response caused by the breakwaters.

For the planning level analysis, the Pope and Dean (1996) formulation was used to develop

breakwater parameters. The formulation is based on the relationship between the distance from

breakwaters to depth ratio and the breakwaters length to gap ratio (Figure 11). Assuming the 8.5-

foot depth (ds in Figure 11) and 300-foot distance to shore (X in Figure 11) yields the

dimensionless ratio of 35 on the vertical axis. To achieve a non-sinusoidal shoreline response to

the breakwaters (i.e., without salient and tombolo formation between breakwaters and shore), the

ratio between the breakwater length (Ls in Figure 11) and gap between the breakwaters (Lg)

should be approximately 0.5. For the purposes of the planning level analysis, it was assumed the

breakwaters would be 115 feet long and spaced 230 feet apart. A more detailed analysis will be

required to establish final design criteria. The San Carlos Bay shoreline segment to be stabilized

is 800 feet, thus three (3) breakwaters would be required. The breakwaters would face (i.e., be

perpendicular to) the dominant wave direction which is from the east. Figure 12 presents a

conceptual plan of the segmented breakwaters.


25

Figure 11. Relationship of Dimensionless Segmented Breakwater Parameters (Pope and

Dean, 1986)


26

Figure 12. Conceptual Plan of Segmented Breakwaters.


27

5.6 Summary

The No New Action option is not recommended for consideration in the alternatives analysis due

to the limited success to date with importing fill and riprap along with the high maintenance

requirements. The Living Shoreline option is not recommended for consideration in the

alternatives analysis due to the challenges associated with establishing vegetation in the

nearshore environment under the identified coastal forcing functions, plus the presence of

existing healthy seagrass beds along a majority of the park in effect already provides a “living

shoreline.” The Shoreline Revetment, T-groin Field, and Segmented Breakwaters options are

recommended for consideration for the San Carlos Bay shoreline segment in the alternatives

analysis due to their technical feasibility to address the erosion problem while maintaining some

functional beach and recreational use of the park. None of the major shoreline stabilization

solutions are recommended for the Pine Island Sound shoreline segment in the alternatives

analysis in consideration of the milder wave climate and lack of upland improvements requiring

protection.

5.7 Additional Features

5.7.1 San Carlos Bay Shoreline

In addition to the three options, the following additional features are recommended for

consideration on the San Carlos Bay shoreline by the County in their long-term plan for park

improvements.

Remove riprap from the shoreline as it is undersized and frequently displaced as

described above

Import riprap of sufficient size and place it along the seawall between approximate

Stations 388+00 and 392+00 to improve the current armoring

Import fill to regain a percentage of land lost between Stations 378+00 and 388+00

Stabilize the areas that have been scoured due to stormwater runoff. Specifically,

excavate the rutted areas on the order of one-foot below existing grade, install geogrid,

and fill and regrade with a minor swale to channel stormwater runoff over the geogrid.

Further, install additional landscaping along roadway to help attenuate runoff from the

bridges and complement the swale construction.

5.7.2 Pine Island Sound Shoreline

The following features are recommended for consideration on the Pine Island Sound shoreline by

the County in their long-term plan for park improvements.

Import riprap of sufficient size and place it along the seawall between approximate

Stations 390+00 and 392+00 to improve the current armoring

Import fill to regain a percentage of land lost between Stations 386+00 and 390+00

Stabilize the areas that have been scoured due to stormwater runoff. Specifically,

excavate the rutted areas on the order of one-foot below existing grade, install geogrid,

and fill and regrade to create minor swales to channel stormwater runoff over the geogrid.

Further, install additional landscaping along roadway to help attenuate runoff from the

bridges and complement the swale construction.


28

6 ALTERNATIVES ANALYSIS

To compare and contrast the alternatives under consideration for the San Carlos Bay shoreline,

the following parameters were analyzed for the Shoreline Revetment, T-groin Field, and

Segmented Breakwaters conceptual plans: shoreline stabilization, recreation, natural resources,

and fiscal. All three alternatives were evaluated for technical issues such as constructability and

contractor accessibility, and were found to be technically feasible.

6.1 Shoreline Stabilization

As the primary goal of the Project is to protect the upland infrastructure and the coastal processes

analysis identified the shoreline segment from Station 378+00 to 392+00 as critically eroding,

the alternative’s effectiveness on stabilizing this shoreline segment is the measure of its benefit.

The Shoreline Revetment Alternative will in essence armor this shoreline segment to prevent

further erosion thus it is considered highly effective. It is noted this alternative may have a

downdrift impact on the adjacent shoreline segment.

The T-groin Field Alternative is considered to be effective in stabilizing the shoreline as it will

also serve as armoring of the shoreline noting the gaps between the structures will allow for

natural response and shoreline adjustment under the influence of seasonal coastal forcing

functions. It is noted this alternative may also have a downdrift impact on the adjacent shoreline

segment.

The Segmented Breakwaters Alternative is considered to be effective in stabilizing the shoreline

as it will reduce incoming wave energy thereby reducing shoreline erosion noting the segmented

structures will allow for natural response and shoreline adjustment under the influence of

seasonal coastal forcing functions. It is noted this alternative may have a downdrift impact on

the adjacent shoreline segment; however, the design of this alternative could allow for some

alongshore sediment transport to bypass sand to the downdrift shoreline to mitigate this potential

impact.

6.2 Recreation

The benefits and impacts on the recreational use of the park were considered in terms of beach

access, swimming or wading, boating, and new opportunities.

The Shoreline Revetment Alternative was not viewed as providing an opportunity to enhance the

recreational use of the park. By armoring the shoreline, this alternative will impact beach access

and swimming or wading along one of the more highly utilized areas within the park. This

alternative will not have an impact on boating in the Project area. The Shoreline Revetment does

not provide any new recreational opportunities.

The T-groin Field Alternative was not viewed as providing an opportunity to enhance the

recreational use of the park. By installing the structures, this alternative will reduce beach access

and swimming or wading along one of the more highly utilized areas within the park. This


29

alternative will not have an impact on boating in the Project area. The Shoreline Revetment does

not provide any new recreational opportunities.

The Segment Breakwaters Alternative will not impact beach access and swimming or wading

along the park shoreline. To avoid impacting boating in the Project area, appropriate signage

should be installed. This alternative may provide the opportunity for a recreational amenity in

terms of attracting fish and other marine life, thus it is considered to have a positive benefit.

6.3 Natural Resources

All three alternatives can be constructed utilizing best management practices for control of

turbidity and sedimentation during fill placement and structure installation. Both the Shoreline

Revetment and Segmented Breakwaters Alternatives can be constructed to avoid the existing

seagrass beds. The T-groin Field Alternative will have a direct impact on the existing seagrass

beds; which may require mitigation from the regulatory agencies.

6.4 Fiscal

Order of Magnitude Construction Budgets were prepared for each alternative based upon

professional judgment, experience, and recent bid tabulations for similar coastal construction in

Southwest Florida. The construction budgets include mobilization/demobilization, foundation

layer material (e.g., filter fabric or geogrid), bedding stone, and armor stone. The construction

budgets exclude the design and permitting costs because they will be similar for each alternative,

and they exclude the additional features as they are proposed for all three alternatives.

The order of magnitude construction budget for the selected Alternatives is as follows:

Shoreline Revetment: $940,000

T-groin Field: $383,000

Segmented Breakwaters: $853,000

6.5 Recommendations

Based upon the alternatives analysis, the Segmented Breakwaters is the recommended

alternative. This alternative will provide for shoreline stabilization of the critically eroding

shoreline segment while allowing for some variable shoreline response without complete

interruption of the natural alongshore transport. The structures will not impact the recreational

use of the park, can be marked appropriately to minimize impacts to boating within the Project

area, and could provide a benefit, that is, a new opportunity for fishing. Lastly, the structures will

not have a direct impact on the existing seagrass beds.


30

7 CONCLUSIONS

The primary purpose of the causeway islands was for the 1963 construction of the roadway

SR/CR 867) from the mainland to Sanibel. The areas alongside the roadway accommodates one

of Lee County’s more popular parks called The Causeway Islands which has become a popular

destination for fishing, wading/swimming, and other typical beach recreational activities. The

center island, which offers restrooms, drinking fountains and picnic tables, has experienced

chronic erosion caused by coastal forces and stormwater runoff. This Study was conducted to

identify the causes of erosion and develop potential solutions to address the chronic erosion

problem while preserving the functionality of the park.

A detailed Bathymetric and Topographic Survey and a Natural Resources Survey were

conducted to characterize the existing conditions of the center island. Utilizing historical aerial

photography provided by Lee County, a shoreline change analysis was completed to document

the erosion along the causeway shorelines. This analysis in conjunction with a coastal processes

analysis determined the shoreline segment along San Carlos Bay between approximate Stations

378+00 to 392+00 experienced erosion between 2004 and 2014 and was recommended for

structural stabilization efforts.

Various options to stabilize the shoreline were examined including No New Action, Living

Shoreline, Shoreline Revetment, T-groin Field, and Segmented Breakwaters. For the San Carlos

Bay shoreline, the Shoreline Revetment, T-groin Field, and Segmented Breakwaters options

were carried forward to the alternatives analysis.

While it is the second most expensive alternative, based upon the alternatives analysis, the

Segmented Breakwaters is the recommended alternative for the San Carlos Bay shoreline. This

alternative will provide for shoreline stabilization of the critically eroding shoreline segment

while allowing for some variable shoreline response without complete interruption of the natural

alongshore transport. The Segmented Breakwaters will be installed in the nearshore and will not

impact the recreational use of the park or impact the existing seagrass beds. The structures can be

marked with appropriate signage to minimize impacts to boating within the project area, and

could provide a new recreational opportunity for fishing. The order of magnitude construction

budget for the Segmented Breakwaters is $853,000. Several additional features are

recommended for the long-term management of the park including remove riprap from the

shoreline as it is undersized and frequently displaced, import riprap of sufficient size and place it

along the seawall between Stations 388+00 and 392+00 to improve the current shoreline

armoring, import fill to regain a percentage of land lost between Stations 378+00 and 388+00;

and stabilize the areas that have been scoured due to stormwater runoff utilizing geogrid and

installing additional landscaping. The order of magnitude construction budget for the additional

features is $467,000. A budget of 15% of the construction cost is recommended for design,

permitting, and construction management. In total, the recommended improvements for the San

Carlos Bay shoreline are on the order of $1,518,000.

For the Pine Island Sound shoreline, the shoreline change data indicate this shoreline has been

relatively stable and the coastal processes analysis indicated this segment is exposed to a milder


31

wind and wave climate than the San Carlos Bay side. Thus, major structural stabilization

alternatives are not warranted.

Several additional features are recommended for the long-term management of the Pine Island

Sound shoreline, that being stabilization of the areas that have been scoured due to stormwater

runoff utilizing geogrid and installing additional landscaping, improvement of the armoring

along the seawall between station 390+00 and 392+00, and import of fill to regain a percentage

of land lost between Stations 386+00 and 390+00. The order of magnitude construction budget

for the additional features is $204,000. A budget of 15% of the construction cost is

recommended for design, permitting, and construction management provided it is done

concurrent with the San Carlos Bay shoreline segment. In total, the recommended improvements

for the Pine Island Sound shoreline are on the order of $235,000.

Combined, the recommended improvements for both the San Carlos Bay and Pine Island Sound

shoreline segments are on the order of $1,753,000.


32

8 REFERENCES

Bodge, K. R., 1998. Beach Fill Stabilization with Tuned Structures; Experience in the

southeastern USA and Caribbean. In: Coastlines, Structures, and Breakwaters, N.W.H. Allsop,

Ed. Thomas Telford Publishing, pp. 82-93.

Hubertz, J. M. 1992. User’s Guide to the Wave Information Studies (WIS) Wave Model: Version

2.0, WIS Report 27, US Army Corps of Engineers, Waterways Experiment Station, Vicksburg,

MS, 41 pp.

Pope, J., and Dean, J. L. 1986. Development of Design Criteria for Segmented Breakwaters,

Proceedings, 20th International Coastal Engineering Conference, November 9-14, Taipei,

Taiwan, American Society of Civil Engineers, pp. 2144-2158.

APPENDIX 1

TYPICAL PROFILE DRAWINGS

-2

2

-

2

1

-

2

0

-20

-

1

9

-

1

9

-19

-

1

8

-

1

8

-18

-17

-

1

7

-17

-17

-16

-

1

6-16

-1

6

-

1

5

-

1

5

-15

-

1

5

-14

-1

4

-1

4

-14

-

1

4

-1

3

-

1

3

-

1

3

-13

-

1

3

-13

-12

-

1

2

-

1

2

-

1

2

-

1

2

-12

-

1

2

-12

-12

-1

2

-1

1

-11

-11

-11

-1

1

-11-11

-11

-

1

1

-

1

1

-

1

1

-1

1

-11

-1

1

-11

-11

-

1

1

-1

1

-

1

1

-

1

1

-1

1

-11

-1

0

-10

-10

-

1

0

-10

-10

-10

-10

-10

-

1

0

-1

0

-10

-10

-10-1

0

-10

-10

-10

-

1

0

-1

0

-

1

0

-10

-

1

0

-

1

0

-

9

-9

-9

-9

-9

-9

-9

-9

-9

-9

-9

-

9

-

9

-9 -9

-9

-9

-9

-9

-

9

-

9

-9

-9

-

9

-

9

-9

-

9

-

9

-

9

-

8

-

8

-

8

-8

-8-8

-8

-

8

-8

-

8

-

8

-

8

-

8

-

8

-8-8

-8

-8

-8

-8

-8

-8

-8

-

8

-8

-8

-8

-8

-

8

-

8

-

8

-8

-

8

-8

-8

-8

-8

-8

-8

-8

-7

-7

-7

-7

-

7

-7

-7

-7

-

7

-

7

-7

-7

-7

-7

-7

-7

-

7

-7

-7

-7

-7

-7

-7

-7

-

7

-7

-7

-7

-7

-7

-

7

-7

-

7

-7

-7

-

7

-7

-7

-7

-7

-7

-7

-

7

-7

-7

-7

-7

-7

-7

-7

-7-

7

-

7

-

7

-

7

-7

-

7

-7

-

7

-6

-6

-6

-6

-6

-6

-6

-6

-

6

-6

-6

-6

-6

-6

-6

-

6

-

6

-6

-

6

-6

-6

-

6

-6

-

6

-

6

-

6

-6

-6

-

6

-6

-6

-

6

-6

-6

-6

-6

-6

-6

-6

-6

-

6

-6

-6

-6

-

6

-6

-6

-

6

-5

-5

-

5

-5

-5

-5

-5

-

5

-5

-5

-5

-5

-5

-5

-

5

-5

-5

-

5

-5

-

5

-5

-5

-5

-5

-

5

-5

-5

-5

-

5

-5

-

4

-4

-4

-4

-4

-4

-4

-

4

-4

-4

-

4

-4-4

-

4

-4

-3

-3

-

3

-3

-3

-3

-3-3

-3

-3

-3

-3

-2

-2

-2

-2-2

-2

-2

-2

-2

-2

-2

-2

-1-1

-1

-1-1

-1

-1

-1

-1

-1

-1

-1

00

0

0

0

0

0

0

0

0

0

0

SANIBEL CAUSEWAY

SAN CARLOS BAY

MEAN HIGH WATER

(AUGUST 2014)

PINE ISLAND SOUND

ALIGNMENT=364+00 E

STATION=0+00.00

ALIGNMENT=372+00 E

STATION=0+00.00

ALIGNMENT=384+00 E

STATION=0+00.00

ALIGNMENT=392+00 E

STATION=0+00.00

ALIGNMENT=392+00 W

STATION=0+00.00

ALIGNMENT=384+00 W

STATION=0+00.00

ALIGNMENT=372+00 W

STATION=0+00.00

ALIGNMENT=364+00 W

STATION=0+00.00

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NOTES

1. INFORMATION SHOWN HEREON REFLECTS CONDITIONS AS THEY EXISTED ON THE SURVEY DATE SHOWN AND CAN ONLY BE CONSIDERED INDICATIVE OF CONDITIONS

AT THAT TIME.

2. TOPOGRAPHIC & BATHYMETRIC SURVEYS CONDUCTED BY COASTAL ENGINEERING CONSULTANTS, INC. JULY 31 & AUGUST 7, 2014.

3. SURVEY ACCURACY STANDARDS, QUALITY CONTROL, AND QUALITY ASSURANCE REQUIREMENTS WERE FOLLOWED DURING THIS SURVEY IN ACCORDANCE WITH

USACE EM 1110-2-1003, HYDROGRAPHIC SURVEYING MANUAL, 1 JAN 02.

4. ELEVATIONS SHOWN HEREON ARE IN FEET AND TENTHS AND REFERENCED TO THE NORTH AMERICAN VERTICAL DATUM OF 1988 (NAVD 1988).

5. COORDINATES SHOWN HEREON ARE BASED ON THE NORTH AMERICAN DATUM 1983/1990 ADJUSTMENT, FLORIDA MERCATOR WEST ZONE.

6. AERIAL PHOTOGRAPHY OBTAINED FROM LEE COUNTY AND DATED 2013.

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= JULY 31, 2014 SURVEY

= LIMIT OF EXISTING SEAGRASS

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