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Martin County, Florida beach nourishment project

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Title:
Martin County, Florida beach nourishment project
Series Title:
Martin County, Florida beach nourishment project
Creator:
Sumerell, Heather Renn
Place of Publication:
Gainesville, Fla.
Publisher:
Coastal & Oceanographic Engineering Dept. of Civil & Coastal Engineering, University of Florida
Language:
English

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University of Florida
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University of Florida
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All applicable rights reserved by the source institution and holding location.

Full Text
UFIJCOEL -2000/010

MARTIN COUNTY, FLORIDA BEACH NOURISHMENT PROJECT: EVALUATION OF PROJECT PERFORMANCE AND PREDICTION CAPABILITIES by
Heather Renn Sumerell
Thesis

2000




MARTIN COUNTY, FLORIDA BEACH NOURISHMENT PROJECT:
EVALUATION OF PROJECT PERFORMANCE ANT) PREDICTION CAPABILITIES
By
HEATHER RENN SUMERELL

A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA

2000




ACKNOWLEDGMENT

I would like to thank Dr. Robert Dean for guiding me through my time at the University of Florida. I greatly value his opinion and have benefited from his insight. I would also like to thank Dr. Robert Thieke and Dr. Daniel Hanes for serving on my committee and for their guidance in the classroom. I also would like to thank Becky Hudson and Helen Twedell for all their help.
I would like to thank all my friends in the Civil and Coastal Engineering Department, especially those with whom I started. They have made the past two years unforgettable.
Finally, I would like to thank my family and friends, especially Mom and Dad who have always supported me.




TABLE OF CONTENTS
A CKN OW LED GM EN T ............................................................................ ii
AB STRA CT .......................................................................................... v
I INTRODU CTION ...................................................................................................... I
1.1 Problem Statem ent .................................................................................................. 1
1.2 Objectives and Scope .............................................................................................. 2
2 DESCRIPTION OF MARTIN COUNTY BEACH NOURISHMENT PROJECT ... 4
2.1 Site D escription ....................................................................................................... 4
2.2 Background ............................................................................................................. 5
2.3 Project D escription .................................................................................................. 9
2.4 Previous Study ...................................................................................................... 10
2.5 M onitoring ............................................................................................................ 11
2.6 D ata Sources ......................................................................................................... 11
2.7 Borrow Area .......................................................................................................... 12
3 PROJECT PERFORM AN CE ................................................................................... 17
3.1 Site V isit ............................................................................................................... 17
3.2 Shoreline Changes ................................................................................................ 18
3.2.1 June 1996 ...................................................................................................... 20
3.2.2 M ay 1997 ...................................................................................................... 20
3.2.3 M ay 1998 ...................................................................................................... 21
3.2.4 M ay 1999 ...................................................................................................... 21
3.2.5 D ecem ber 1999 ............................................................................................. 21
3.3 V olum etric Changes .............................................................................................. 27
3.3.1 Post-N ourishm ent ......................................................................................... 30
3.3.2 One-Year Post-N ourishm ent ......................................................................... 31
3.3.3 Tw o-Y ear Post-N ourishm ent ........................................................................ 32
3.3.4 Three-Y ear Post-N ourishm ent ...................................................................... 33
3.3.5 Four-Y ear Post-N ourishm ent ........................................................................ 34
4 M OD ELIN G ............................................................................................................. 36
iii




4.1 M odel .................................................................................................................... 36
4.2 Application of M odel ............................................................................................ 36
4.3 Erosional Hot Spots .............................................................................................. 37
4.3.1 Shoreline Changes ........................................................................................ 41
4.3.2 Volumetric Changes ...................................................................................... 42
5 DESIGN AND PREDICTION ................................................................................. 49
5.1 Calibration of GENESIS ....................................................................................... 49
5.2 Verification of GENESIS ..................................................................................... 52
5.3 Simulation of Project Performance ....................................................................... 53
6 SUM M ARY AND CONCLUSIONS ....................................................................... 55
REFERENCES ...................................................................................... 58
BIOGRAPHICAL SKETCH ....................................................................... 59




Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science
MARTIN COUNTY, FLORIDA BEACH NOURISHMENT PROJECT:
EVALUATION OF PROJECT PERFORMANCE AND PREDICTION CAPABILITIES By
Heather Renn Sumnerell
December 2000
Chairman: Dr. Robert G. Dean
Major Department: Civil and Coastal Engineering
The Martin County Beach nourishment project was completed in April 1996. The project did not perform as well as was modeled using Generalized Model for Simulating Shoreline Change (GENESIS) or Department of Natural Resources, Beaches and Shores (DNRBS). This project was designed by the Jacksonville District of the United States Army Corps of Engineers (USACE) and was later analyzed by Applied Technology and Management (ATM). As of December 1999, the shoreline had receded beyond the baseline established for the planned renourishment interval of eleven years.
Erosional hot spots are areas of the shoreline that have eroded beyond that which is predicted. These are important areas to analyze because the public, who pays for nourishment projects, evaluates the relative success of a project by the amount of beach that can be seen and used. Determining causes for erosional hot spots and better predictive capabilities will ultimately help the future of all beach nourishment projects.
v




The Martin County beach nourishment project consisted of placing 1.5 million cubic yards of compatible sand on 3.75 miles of shoreline. The amount of additional sand in the immediate post-nourishment survey was affected by a storm that hit the project area during construction. This storm affected the profile slope which was at an unnatural steeper construction slope. Depressions in the surfzone were present after the storm. This is thought to be the major factor in the unexpected performance of this project.




1 INTRODUCTION

1. 1 Problem Statement
Beach erosion can be a significant concern for coastal communities. The beach system is a very dynamic area, with shoreline retreat the result of tides, waves, currents, wind, and sea level rise. In some areas, human influence may contribute to erosional pressures. As the beach retreats, it is often met by development along the shoreline. In these cases, the dune system, which provides stabilization to the beach, is unable to retreat with the shoreline. If the coastal environment is not preserved many benefits could be lost such as storm protection to adjacent structures, including hotels, homes, and roads, a recreational benefit for citizens and tourists, and the environmental benefits of turtle nesting.
Beach nourishment is the most "natural" solution to the erosion problem. Other methods of beach stabilization such as groins, seawalls, and headlands do not add sand to the littoral system, and can cause negative effects downdrift of the project area.
Beach nourishment projects require a great deal of planning, involving environmental, financial and engineering considerations. One of the most important concerns when designing a project is estimating its longevity. The renourishment interval is always a factor in budgeting a project. Public support is a major factor when presenting plans for a beach nourishment project, and a major public concern is the project's budget. The public judges a project on how long the sand stays visible on the




2
beach. It is true that a project may be successful by engineering standards, however, if the sand only lasts several years, or more volume resides underwater than anticipated, the public may consider the project a failure. This thesis examines a project in Martin County, FL that has not met longevity expectations.
Localized erosion is a major concern when designing a beach nourishment project. These areas are called "erosional hot spots" (EHS) and are important areas to consider more closely when evaluating the performance of a project so that the problems can be averted in future projects. Erosional hot spots must be separated from causes of beach erosion for which the physics are understood such as end losses, losses due to a change in the littoral system such as groins, and the background erosion within the project area. This thesis will evaluate the performance of the Martin County beach nourishment project, and will describe and analyze the unexpected performance and attempt to determine its cause(s).
1.2 Objectives and Scope
A beach nourishment project was completed on Hutchinson Island, in Martin County, FL in April 1996. Approximately 1.5 million yd' of compatible sand was placed on 21,600 ft of shoreline, between DEP Monuments R-1 to R-25. The Martin County Board of County Commissioners filed an application with the Florida Department of Environmental Protection (DEP), on February 23, 1994, for a 4-mile beach nourishment project in Martin County. The project was designed and authorized by the U.S. Army Corps of Engineers (USACE) in June 1994. Applied Technology and Management, Inc. (ATM) was the permitting agent for this project. On March 27, 1995, the DEP Bureau of Beaches and Coastal Systems approved the construction permit. Construction began on




3
December 13, 1995 and was completed on April 10, 1996. The project was administrated by the Jacksonville District office of the USACE, with Great Lakes Dredge and Dock Company serving as the primary contractor.




2 DESCRIPTION OF MARTIN COUNTY BEACH NOURISHMENT PROJECT
2.1 Site Description
Hutchinson Island is 24.5 miles long, 1 mile or less wide, and is separated from the mainland by inlets and the Indian River Lagoon. The St. Lucie River joins the Indian River Lagoon north of the St. Lucie Inlet, which is just south of the project area. Nearshore barrier reefs and rock outcroppings parallel, sometimes quite close, the shoreline starting about R-20 (see Figure 1) and continuing south. These reefs are composed of various coquina limestones with polychaete worms that form a sand-tube "rock on the limestone." "Bathtub Reef," located at the southern end of the island, forms a "lagoon" when the reef is exposed at low tide. Hutchinson Island is oriented northnorth-west to south-south-east. The project area includes 3.75 miles of shoreline on Hutchinson Island, from the St. Lucie County line south to the Martin County DEP Monument R-25, just south of Stuart Public Beach Park. Figure 1 presents a map of the project area in Martin County as well as the borrow area.
2.2 Background
The Martin County Nourishment Project is, in part, a response to an erosional trend during the period 1982 to 1992. Examining the background erosion rate for different periods of time presents a different picture of the dynamics of the system. As shown in Figure 2 the period 1948 to 1992 shows erosion between R- 110, in St. Lucie County, to R-7, in Martin County, shoreline change variability between R-7 and R- 18,
4




Figure 1. Map of project area (ATM 1998)

and accretion along the remainder of the project area. However, the periods 1971 to 1992 and 1982 to 1992 show erosion over the entire project length. The more recent period shows a much greater yearly erosion rate.
Improvements to St. Lucie Inlet were completed in the early 1980's, however, the inlet is downdraft of the area of interest and its influence to the project area is lessened by Bathtub Reef The erosional trend could be caused by a change in localized waves, however, there is no long- term offshore wave data by which to evaluate this possibility.




--0-1948-1992
----1971-1992
--<-1982-1992

Monument Number
Figure 2. Background erosion rates

2 0
.4
#, -2
-6
o
o Inl
.4
-6
-8




7
The prevailing winds are from the northeast through the southeast, with the southeast being the dominant direction. Southeast tradewinds dominate the summer months, which are also characterized by tropical weather systems. These tropical storms can be very damaging to the shoreline. During the winter months, winds are generally from the northwest through northeast.
Historic volumetric changes were computed in USACE 1994 using surveys conducted in Martin County between Monuments R-1I to R-42 by the Florida Department of Environmental Protection (FDEP) in 1971, 1982 and by the Jacksonville District of the USACE in 1992. According to the data, the shoreline accreted 2,781,200 yd' along 42,400 ft of shoreline between 1971 and 1982. Erosional and accretional patterns were observed above mean low water along the study area. The majority of accretion occurred in the offshore portion of the beach profiles. Figure 3 shows the erosional trend which occurred during the period 1982 to 1992. The study area lost 4,357,400 yd' of sand during this period. The only construction during this time was an extension to St. Lucie's north jetty and the construction of a breakwater. These improvements only affected the southern portion of the study area, which does not include the project area. Two major storms, 1984 Thanksgiving Day and 1990 Halloween Day northeasters, contributed to the erosion during this time period. Between 1971 and 1992, the study area has eroded 1,508,700 yd', while the project area has eroded 1,104,000 yd'. The associated erosion rate within the project area is 53,600 yd' of sand per year. The report (USACE 1994) also determined the littoral transport using the numerical model GENESIS. The results suggest differences in the localized net littoral drift patterns. The net longshore transport rate is to the south and varies between 13 1,000 yd' per year in the northern limit of




1971-1982

U
A
0
4
O,

S. isTKNc FR6d 6RT JUy, mi
Figure 3. Historical volume changes (USACE 1994)

I-

o ,--rr rar J fi l i.. ill 1/ 1/ L.I- Y A .l/ Al/L I t 4. .L 3 .1 1. .3 to 4.t
DISTANCE FROM NORTH JETTY, MILES 1982-1992
"t "M iSTXNCFR 'M W6RTf JEY, IL a U 4.
1971-1992
x rn
Vj /
/ / / A

0
> I

to "

L.Q Ll e




9
Bathtub Reef to less than 10,000 yd' per year in the vicinity of the rock outcrops for the period 1971 to 1976. The period 1976 to 1982 resulted in a net longshore transport to the south that varied from 102,000 yd' per year at the northern boundary to 10,000 yd' per year in the area of the rock outcrops north of Bathtub Reef.
2.3 Project Description
The original project length of 4 miles was reduced to 3.75 miles because of "hard bottom" located relatively close offshore of Monuments R-21 through R-25. The new project called for fill to be added between Monuments R-2 and R-21, with gradual tapers to R-l and R-23 on the north and south ends of the project, respectively. The project was designed for 1.5 million cubic yards of fill to be placed within the project area.
The planned template for the project included a dune elevation of 12.5 ft above MSL and a top width of 20 ft. The berm was planned at an elevation of 8 ft above MSL and 35 ft wide. The planned slope was 1:8.5 to MLW and then a 1:20 slope below the water.
The existing shore protection structures within the project area include a concrete seawall at the Jensen Beach Public Beach (between R-4 and R-5), two rock revetments at the Little Ocean Club and Place (near R-19) and at Islander 12 Condominium (near R20), and a wooden bulkhead at a private residence (near R-20). None of these structures appear to have major effects on the shoreline.
During construction, between March 11 and 13, 1996, a storm impacted the project area. Construction had been completed from Monuments R-7 through R-25. After the storm, the project was completed and additional fill was added between R-20 and R-23. While no nearshore wave data are available, the significant wave heights were




10
estimated between 15 and 18 ft based on storm tide elevations and offshore bar formation. Wave run-up overtopped the dunes. Since the sand had just been placed, the beach still maintained a construction slope, which is steeper than the equilibrium slope. This storm had a great effect on the equilibration of the project. An estimated 811,5000 yd' of sand (ATM 1998) eroded between the visible beach out to the -18 ft contour, with the southernost mile of the project receiving the worst damage. Most of the erosion occurred between the -6 ft contour and the -18 ft contour, which lost 785,000 yd' of sand.
2.4 Previous Study
ATM completed a Project Performance Report in July 1998. The report was based on the monitoring survey data for November 1995, June and December 1996, and May1997, as well as the "condition" survey taken in March 1998. The "condition" survey lines were spaced 2,000 ft apart, while the monitoring survey lines were spaced 1,000 ft apart. ATM concluded that the longevity of the project was greatly influenced by the storm in March 1996, which occurred during construction. The storm created large depressions in the nearshore region, which were later infilled by sand from the nourishment project. This additional equilibration process removed sand from above NGVD. The "hot spots" in the south end of the project correspond with the areas that lost the greatest amount of sand in the storm. After one-year post-nourishment, the project area lost 23 percent more beach width than the projected adjusted widths determined by the USACE. ATM determined that 25 percent of the project area, or 5,400 ft of shoreline, could be considered erosional hot spots. These areas eroded more than was predicted in the permitted plans. The hot spots included the region R-20 to R-25. ATM




11
states that this was anticipated due to end losses along the termination of the project, the increased easterly offset of the shoreline resulting in higher wave energy, and the placement of the jack up booster pump located along Stuart Beach. The volume changes calculated by ATM will be examined in Chapter 3.
2.5 Monitoring
A monitoring program is required to comply with the conditions of the Project construction permits. The components of the program include beach profile surveys, optional sand compaction testing, and sea turtle monitoring. The surveys include DEP Monuments R-1 to R-25, 6 monuments north of the project area in St. Lucie County (R110 to R-115), and 5 monuments south of the project area (R-26 to R-30). The total length of surveyed shoreline is 31,879 ft.
2.6 Data Sources
Survey profiles were contracted by the USACE Jacksonville District on the following dates: November 1995, April 1996, June 1996, December 1996, May 1997, May 1998, May 1999, and December 1999. The Florida Department of Environmental Protection (FDEP)'s website (http://www.dep.state.fl.usibeach/) list data for many counties in Florida, including historic shoreline change drawings, profile change drawings and bathymetric maps for Martin County. The data used in this study was obtained from the USACE Jacksonville District office. The central portion of the project, Monuments R-9 to R-17, were averaged, Figure 4, for each survey date to assist in evaluating the accuracy of the data. The surveys were averaged using Beach Morphology Analysis Package (BMAP). The indicated survey lines were plotted on the same graph for each survey date. The zero NGVD crossing-point for each monument was then




12
aligned to the same point on monument R-9. This alignment was performed for each of the survey dates. BMAP then averaged the profiles using a I ft horizontal increment. The survey lines do not close out at the depth of closure, which, on average, is 20 feet. This indicates that the surveys' offshore portions of the profiles are not accurate when comparing different survey dates.
2.7 Borrow Area
The borrow site utilized for this project is Gilbert Shoal, which is 3 miles north of St. Lucie Inlet and about 3,000 ft offshore. The area is irregularly shaped with a maximum length of 6,750 ft and about 3,000 ft wide. The site contains approximately 6 million cubic yards of clean sand, with a deposit thickness about 10 to 16 feet and a silt content less than 5%. Figure 5 shows the location of the borrow area in relation to the project area. The borrow area is located in a depth of approximately 42 feet, as seen in Figure 6. The area chosen was in a pre-dredged depth of about 26 feet. Figure 7 shows contour map comparison between the November 1995, pre-nourishment survey, and the June 1996, immediate post-nourishment survey. On average, 10 feet of sand was excavated from the central portion of the borrow area.
The mean grain size of the borrow material is 0.38 mm and the native sand of the project area has a mean grain size of 0.27 mm; therefore the borrow area material is very compatible. The Jacksonville District of the USACE conducted geophysical surveys and sampled 25 vibracore borings. Figure 8 shows an analysis of the native sand found within the project area compared to the sediment analysis of the borrow area. USACE (1994) determined that the borrow material is coarser than the native material and has




13
Profile Averages R9-R17
--420
5 -1010 -Nov 95
o -20-Dec 99 .2 -20
-30
-40
Distance from Monument
Figure 4. Profile averages R-9 through R-17 approximately the same sorting as the native material, therefore no overfill volume was required in the beach nourishment plans.




R-110, 1070 00F
R-1 R-' R
10650001060000105500010500001045000-

II
I2&, I13.\
-114*
R.1154

'-I

7>

R-1., N
R-2, R-34, R-4,
R-5i, R-6#,
R-7#
R-8i,,
R-96
R-106,
R-110
R-124, R-1 2# R-13&
R-144,
R-15,
R-164,
R-174,,
R-184..
R-19* R-20, R-21 R-221. R-23*, R-24,, R-25*, R-260, R-276
R-28, R-296

760000
760000

765000
765000

Borrow Area

770000

Figure 5. Map of borrow area




1051000-

1501050000- go I a
1049000 104900010460- 104800010470 1047000IN
1046000-1046000104500 \ 1045000
774000 775000 776000 777000 774000 775000 776000 777000
Figure 6. Contour chart of borrow area Figure 7. Contour difference between pre-nourishment and postnourishment




16
i00
C
I
V
e
P 0 .. . . . . . . . . . . . . . I ..,.. . . . .. . . . . .
e
r
c
0
-4 -2 0 2 4 6
GRAIN SIZE lphi units
- Native Sediment Borrow Combo I
Borrow Combo 2 Borrow Combo 4
Figure 8. Comparison of borrow and native sediment (USACE 1994)




3 PROJECT PERFORMANCE

Survey data from November 1995, June 1996, May 1997, May 1998, May 1999, and December 1999 were analyzed for this thesis. The data were in state plane coordinates, which are in feet referenced to some point in Florida. Using Excel, the coordinates were then converted to distance from the monument from which each survey line was taken. For each survey date, the individual survey lines were placed into a program called Beach Morphology Analysis Package (BMAP). The individual survey lines could then be compared to the different survey dates. The volume changes between the dates were calculated using this program. Survey data of the borrow area were also taken in November 1995, June 1996, May 1997, May 1998 and May 1999. These data were also in state plane coordinates. The data were analyzed using Surfer, which has the ability to plot contour maps, as well as comparing two different surveys by subtracting one from the other, resulting in volume changes.
3.1 Site Visit
A site visit was conducted on May 10, 2000. Beach cusps were present at several locations along the project area. The beach also appeared to be rather steep and the sand appeared coarse. The northern end of the project appeared to be in good shape with plenty of dry beach width. Stuart Public Beach Park, located at the south end of the project, showed evidence of erosion, including beach walk-overs which were closed because the walk-over did not extend down to the beach. There was also hard bottom




18
exposed within the breaking area. Sediment samples from the dune, berm, and foreshore were taken at R-1, R-5, R-12, R-15, and R-23. The samples at R-1, R-12, and R-23 were analyzed to determine the mean diameter, as shown in Table 1.
Table 1. Mean grain sizes (mm)
DUNE BERM FORESHORE
R-1 0.316 0.636 0.361
R-12 0.433 0.490 0.367
R-23 0.468 0.460 0.574
3.2 Shoreline Changes
The public judges the success of a beach nourishment project based on the amount of additional dry beach area. An analysis of the shoreline position changes within the project area provides a good basis for determining whether a project will be perceived as a success or failure. The shoreline changes from pre-nourishment conditions along the project area are presented in Figure 9. The average shoreline changes within the project area for June 1996, immediate post-nourishment, is 96.5 ft. This can be compared to the average shoreline change within the project area for December 1999, which is 35 ft. This is a loss of almost 65% of the 2,152,309 ft' of plan area that was added within the project area. Figure 10 shows a graph of the percent plan area remaining within the project area verses time. The graph also shows the DNRBS (Dean and Grant 1989) predicted plan area remaining, which is discussed in Chapter 4. The percentage of plan area remaining one year post-nourishment is only 47%. The remaining years hover around




Figure 9. Shoreline position change from November 1995

S50

-- Jun96
-- May_97
-A--May_98
--May_99
--Dec 99




120 100 80
S60 --- N B
40 20 0
0 1 2 3 4
Years After Nourishment
Figure 10. Percent remaining of measured and predicted plan area zero years corresponds to June 1996
40% of the fill remaining. DNRBS greatly under predicted the amount of plan area lost due to initial equilibration.
3.2.1 June 1996
The immediate post-nourishment survey does not reflect the planned additional dry beach width because of the storm in March 1996. Only 75% of the project had been completed, and additional fill was added prior to the June survey to partially repair the damage caused by the storm. Figure 11 shows the trough that was created by the storm.
3.2.2 May 1997
R- 1 shows little change from the post-nourishment survey. A trough begins to form in depths of -3 to -8 ft between Monuments R-8 to R-12, as shown in Figure 12. In this area, the shoreline position has already retreated more than halfway through the




21
added width. A trough between depths of -12 to -18 ft is apparent at Monuments R-15 (Figure 13) and R-16. Figure 14 shows the trough at R-22, which begins to form again around R-21, and by R-23, the shoreline has retreated back to the pre-nourishment position.
3.2.3 May 1998
Figures 15 and 16 show a trough from NGVD to the -12 ft contour present at R-1 and R-4. This trough is present from R-I to R-4, it then is seen between the -3 ft and -8 ft contours from R-5 through the rest of the project area. Figure 17 shows R-8 which has receded to the pre-nourishment survey.
3.2.4 May 1999
The profiles at the northern end of the project are very similar to the prenourishment profiles, as can be seen in Figure 18. Beginning with R-12, Figure 19, a trough in the pre-nourishment surveys, located between the -5 ft and -10 ft contours, has filled in the May 1999 surveys.
3.2.5 December 1999
A trough is evident throughout the project area. R-1 and R-2 have a trough between the -5 ft and -15 ft contours. Starting at R-3 and preceding through R-13, the trough is located between NGVD and the -15 ft contour, as shown at R-4 and R-8 in Figures 20 and 21. From R-21 through R-25, the trough is located between the -3 ft and
-10 ft contours.




Figure 11. Profile at R-23

Figure 12. Profile at R-9




Figure 13. Profile at R- 15

R-22
-20
-5)0 -10 1-5 1 1000 1500 2C0 0-20
-30
05
-40
Distance from Monument (ft) =Nov 95 May_97

Figure 14. Profile at R-22




Figure 15. Profile at R-1

Figure 16. Profile at R-4

R-1
z
20
0 -1(00 01000 2000 3000
-0 -10 -20
&-30
40
Distance from Monument (ft) Nov_95 --May_98

R-4
20
-5)0 1500 2500
. O-20
-30
&-40
50
Distance from Monument (ft) Nov 95 May_98




Figure 17. Profile at R-8

Figure 18. Profile at R-2

R-8
20 10
> -5)0
2 2 -10 5
a -20
-30
-40

Distance from Monument (ft)

Nov_95 May98




Figure 19. Profile at R-12

Figure 20. Profile at R-4

R-12
20
0 -5)0 V-10
o
-20 o -30
;>-40

Distance from Monument (ft)

Nov_95 May_99

R-4

> -5
O .2...
. C..
z>

Distance from Monument (ft)
Nov 95 Dec_99




Figure 2 1. Profile at R-8

3.3 Volumetric Changzes
After any beach nourishment project, the main concern is the equilibration of the beach profile. Sand is initially placed at a steeper slope than is natural for the ocean and sediment characteristics. The natural slope is determined by the action of the waves as well as the sediment size. Coarser grain sand equilibrates at a steeper slope than finer sand. The sand is redistributed both cross- and along-shore. Sand can move offshore to build sandbars, or alongshore towards adjacent beaches. These changes are best evaluated through volumetric changes at different depths.
Volumetric calculations were performed using Beach Morphology Analysis Package (BMAP). Volume changes at each profile were taken between the monument and NGVD, -6 ft contour, -12 ft contour, and depth of closure, which ranges between




28
-18 ft and -24 ft and was approximated separately for each survey line. As can be seen in Figure 22, the volume above closure gained sand after the post-nourishment survey,
Total Volume Change
1400000 1200000
1000000 .
----Above NGVD
S800000 --Above -6
- -4-- Above Clos
-- U--ATM NGVD
D 600000 -- A - ATM -6
E
- -* - ATM Clos
400000 200000
0 Ju
Jun_96 May97 May_98 May_99 Dec_99
Figure 22. Volume change from pre-nourishment
where the volumes above NGVD and the -6 ft contour lost sand. This indicates a loss of sand in the nearshore region and an accretion beyond the -6 ft contour. Survey
inconsistencies are also likely as indicated by the profile averages in Figure 4. The December 1999 survey shows a leveling off trend for the volume changes above NGVD, which would tend to indicate that the beach face has returned to an equilibrium profile. The volumetric changes from ATM 1998 are also presented in Figure 22. The ATM report only had surveys through March 1998, which was data that was not used in this study because the surveys were only taken at every other monument. The March 1998




29
and May 1998 volumetric changes above the -6 ft contour and depth of closure are very different, which could be caused by survey error. The profiles do not close-out at a depth of closure as seen in profiles of November 1995 and May 1998 (Figures 15, 16, and 17). There are inconsistencies between the volume changes in ATM 1998 and those in this
Cumulative Volume Change (cu yd.)

*Jun_96
--m- May_97 May_98
-e-- May_99
- Dec 99

Figure 23. Volume change from pre-nourishment verses depth

thesis above NGVD for June 1996 and May 1997 which cannot be explained. The volume calculations in this thesis were calculated from the monument seaward. Figure 23 shows the cumulative volume change from pre-nourishment at NGVD, -6 ft contour,
-12 ft contour, and depth of closure, -20 ft. There is a large gain above the -6 ft contour in June 1996. In May 1997 and 1998, the greatest gain occurs above closure, which




30
would indicate an accretion in the region between -12 ft and depth of closure. In May 1999, the accretion trend in this region is no longer apparent. By December 1999, the area beyond the -12 ft contour is no longer gaining sand because the volume change in this region is equivalent to the change above NGVD.
3.3.1 Post-Nourishment
Using the June 1996 survey as a basis, the volumetric changes were calculated within the project area for above NGVD, the -6 ft contour, -12 ft contour, and depth of closure. The results of the volumetric changes can be seen in Table 2. The average volumetric density above NGVD is 38 yd3/ft. The average volumetric density above the
-6 ft contour is 50.74 yd3/ft. The area between the -6 ft and -12 ft contours showed substantial erosion in the post-nourishment interval, with a loss of 362,906 yd 3. Above the depth of closure the average volumetric density is 12.8 yd 3/ft, with a gain of 287,923 yd3. These volume densities are to be compared to a design placement volume density of 78 yd 3/ft and suggest substantial survey errors.




Table 2. Volume change (yd3) between pre-nourishment (Nov. 1995) and post-nourishment (June 1996)
ABOVE ABOVE6 6 TO 12 ABOVE
NGVD CLOSURE
R-1 8,637 2,162 -22,141 -30,688
R-2 22,181 25,565 -14,758 -2,924
R-3 35,511 42,943 -12,864 4,941
R-4 36,495 49,547 -1,671 33,370
R-5 36,236 48,330 -4,940 17,743
R-6 50,192 73,633 -4,921 46,949
R-7 29,918 34,058 -17,314 -6,646
R-8 28,728 39,446 -10,506 9,666
R-9 34,204 44,649 -9,450 10,297
R-10 27,421 33,115 -21,477 -6,066
R-11 29,333 36,517 -20,593 -5,693
R-12 38,889 58,891 -1,979 27,229
R-13 39,420 52,327 -13,980 8,745
R-14 48,462 62,070 -10,351 25,171
R-15 49,698 72,584 -10,432 44,260
R-16 38,374 53,735 -15,399 8,130
R-17 44,749 64,963 -11,643 23,123
R-18 40,301 52,631 -21,540 9,711
R-19 46,388 67,101 -12,910 28,395
R-20 30,975 36,679 -17,430 -1,658
R-21 39,499 54,477 -14,744 19,001
R-22 35,092 50,850 -16,526 32,188
R-23 33,561 47,798 -24,677 15,102
R-24 22,203 32,484 -25,662 1,710
R-25 8,583 6,566 -24,999 -24,131
Total 855,049 1,143,116 -362,907 287,924

3.3.2 One-Year Post-Nourishment

The May 1997 survey data are shown in Table 3. The May 1997 survey shows that the area above NGVD lost sand compared with the June 1996 survey. The amount remaining within the project area above NGVD is 62% (535,785 yd3) of the original 855,049 yd3 added above this datum. The average volumetric density above NGVD is 23.8 yd3/ft, compared to an average design placement density over the entire profile of

78 yd3/ft.




Table 3. Volume change (yd 3) between pre-nourishment (Nov. 1995) one-year post-nourishment (May 1997)
ABOVE ABOVE6 6 TO12 ABOVE
NGVD __ __CLOSURE
R-1 9,101 9,332 6348 42,472
R-2 15,707 15,456 9732 50,464
R-3 19,77 26,668 11642 72,997
R-4 19,045 17,172 5421 52,835
R-5 21,244 25,826 6824 53,524
R-6 29,706 46,071 10623 74,3 10
R-7 19,656 24,962 -1581 32,509
R-8 15,421 13,533 12476 42,257
R-9 18,023 13,608 5509 30,610
R-10 19,933 17,122 -1585 30,083
R-11 19,356 17,751 6548 46,457
R- 12 17,941 18,470 5956 33,112
R- 13 28,057 39,865 228 38,419
R- 14 34,237 42,864 7884 61,381
R- 15 25,954 27,427 -9198 14.872
R-16 31,511 37,296 -537 31,649
R-17 31,416 42,333 1756 54,871
R-18 29,780 35,442 -7445 38,888
R-19 34,100 43,207 1870 52,234
R-20 21,110 21,618 6490 27,059
R-21 27,196 28,901 16858 53,633
R-22 20,482 33,426 10231 56,347
R-23 11.011 31698 6427 12,710
R-24 10,071 3,671 14434 28,575
R-25 5,952 -3,424 20165 26,486
Total 1 535,785 602,296 147,083 1,058,752

3.3.3 Two-Year Post-Nourishment

The May 1998 volume changes are presented in Table 4. The volumetric change between the November 1995 and May 1998 surveys was calculated. Fifty-four percent of the sand which was present above NGVD in June 1996 still remains above NGVD, while only 36% of that added above the -6 ft contour remains above the -6 ft contour. The total volume change above the depth of closure is 1,212,918 yd', the maximum documented by

all the surveys.




Table 4. Volume changes (yd3) between pre-nourishment (Nov. 1995) and two-year postnourishment (May 1998)
ABOVE ABOVE 6 6 to 12 ABOVE
NGVD CLOSURE
R-1 9,481 4,644 -16,088 21,155
R-2 15,869 12,078 -1,977 41,933
R-3 17,729 16,735 -439 62,213
R-4 17,480 11,890 -9,126 45,227
R-5 23,821 25,275 1,443 54,412
R-6 23,678 23,323 -7,356 46,887
R-7 17,276 14,666 10,074 60,976
R-8 9,568 -2,555 -2,305 31,507
R-9 24,348 23,558 4,060 63,039
R-10 20,635 16,231 -2,944 56,224
R-11 30,788 39,329 21,871 104,659
R-12 27,714 32,079 8,560 77,400
R-13 26,129 29,074 5,317 61,132
R-14 28,954 33,655 10,940 82,508
R-15 21,835 24,011 -6,407 44,341
R-16 22,345 26,405 -7,711 34,943
R-17 17,466 12,850 29,736 58,293
R-18 21,783 15,707 4,576 31,107
R-19 12,172 4,354 10,629 33,543
R-20 11,792 -1,324 18,055 28,085
R-21 21,905 19,135 19,277 53,877
R-22 15,097 18,599 3,151 37,500
R-23 10,265 12,051 7,028 28,923
R-24 18,261 16,746 16,313 49,433
R-25 -3,974 -18,812 2,529 3603
Total 462,418 409,705 119,205 1,212,918

3.3.4 Three-Year Post-Nourishment

The May 1999 survey data are presented in Table 5. There is very little change above NGVD from the May 1998 survey. The area above the -6 ft contour has gained sand since May 1998, with 59% of the fill remaining. The volume above the depth of closure has decreased from May 1998 to a change of 909,765 yd3 from pre-nourishment.




Table 5. Volume changes (yd') between pre-nourishment (Nov. 1995) and three-year postnourishment (May 1999)
ABOVE ABOVE 6 6 TO12 ABOVE
_____ NGVD ___CLOSURE
R-1 8,8 9,40 -869 -4,444
R-2 9,1 7,460 -635 -6718
R-3 15,02 1928 -266 2460
R-4 15,34 17,65 4,64 11,020
R-5 1598 13,62 -2,83 11,140
R-6 4,240 34,85 -7,154 27,081
R-7 1987 28,45 -3,76 3,405,
R-8 13,92 12,97 -7,68 7,728
R-9 4,614 27,98 -956 3,7
R-10 24,484 4,73 2,5 56,05
R-12 23,78 35,65 12,55 62,0
R-13 2866 43419 6007699L
R- 14 28,23 43,58 O j O
R-1 5 25,99 i 4 348 63 320
R-20~ J~J~ ~ ~ 673 63 320
R-22 41 7~462206i
R-16 20,27 24,36 16,33 42,99
R-14 27,6 56,79 26,869 8688
R-252,6 32,80 14689 41 386
T otal 475,914 673,4481 169,0801 909,765

3.3.5 Four-Year Post-Nourishment

The December 1999 volume changes are presented in Table 6. Fifty percent of the fill remained above NGVD, while only 41% remained above the -6 ft contour. There was a substantial loss of sand in the -6 ft to -12 ft region, a volume difference of
-36,516 yd 3. This region had shown an accretional trend after the initial post-nourishment

survey.




Table 6. Volume changes (yd3) between pre-nourishment (Nov. 1995) and four-year postnourishment (December 1999)
ABOVE ABOVE6 6TO12 ABOVE
NGVD CLOSURE
R-1 4,310 6,069 -11,783 -13,307
R-2 5,574 4,313 -10,908 -8,630
R-3 13,073 7,521 -7,254 721
R-4 14,796 10,168 -15,286 -12,524
R-5 7,359 -2,442 -14,816 -16,754
R-6 26,262 35,492 -10,233 12,494
R-7 20,048 31,001 -15,594 14,621
R-8 5,332 436 -19,024 -28,747
R-9 23,886 19,474 -10,946 9,537
R-10 23,735 28,496 -11,018 27,773
R-11 26,446 40,095 -3,635 45,354
R-12 21,364 17,687 2,125 18,208
R-13 22,951 23,930 -5,736 17,222
R-14 20,007 20,149 -592 27,740
R-15 22,691 38,628 -1,611 31,774
R-16 26,775 43,736 4,436 47,278
R-17 38,365 66,193 8,145 79,827
R-18 22,991 31,174 -1,505 28,819
R-19 55,416 57,443 19,415 76,033
R-20 7,928 -5,225 6,642 -14,446
R-21 9,280 2,484 29,460 44,821
R-22 6,690 13,347 18,850 42,300
R-23 9,069 6,314 14,315 13,776
R-24 -6,987 -20,035 7 -17,012
R-25 -103 -10,975 4,824 -6,881
Total 427,259 465,471 -36,516 419,997




4 MODELING

4.1 Model
The model used to predict the beach nourishment project performance in this thesis was developed by Dean and Grant (1989). They developed a methodology for predicting the thirty-year shoreline projection for nourishment projects and can be applied for other purposes. The model determines the sediment transport given certain data such as sediment transport coefficient, K, average wave conditions, shoreline orientation, background erosion rates, and the geometry of the beach fill. The model then outputs calculated yearly shoreline positions. The model is called DNRBS, which stands for Department of Natural Resources, Beaches and Shores, the sponsor of the model. The model is written in Fortran language.
4.2 A12plication of Model
Data used in the program was gathered from reports from the Army Corps of Engineers (1968, 1994). The data required for program input include sediment size, wave characteristics, location of coastal structures, project length, and the volume of sand placed per unit beach length.
The mean sediment size used to determine the sediment transport coefficient, K, was that found in the foreshore region of the beach. This is the most active portion of the beach face, therefore, the sediment found there would have the greatest effect on sediment transport. The mean diameter used was 0.36 mm, which corresponds to a K 36




37
value of 1.00 (Fernandez 1999). The berm height and depth of closure values, taken from profile surveys, are 8 ft and 20 ft, respectively. Thus, for compatible sand, for each cubic yard of compatible sand added, the equilibrated shoreline added would be 0.96 ft.
4.3 Erosional Hot Spots
The changes in shoreline position taken from survey results are very different from the changes predicted by the DNRBS model. The initial 1-year change from placement is compatible from R- 110 to R-4, south of which the Measured changes are much greater (shoreline recession) than those predicted. The model under predicted the erosive shoreline change, as seen in Figures 24-27. The 2-year change comparisons are better, with compatibility extending to R-6. The 3-year and 4-year post-nourishment surveys show much better agreement. The measured and predicted lines approach each other, except in the southern end of the project area. These results could indicate the presence of an erosional hot spot located on the southern. end. An erosional hot spot occurs when an area of beach erodes more than the rest of the beach, or erodes more than was predicted by a model. There are essentially eight potential causes for erosional hot spots.
*Dredge Selectivity: In this case, the finer grain fill is pumped the longest distance, and the coarser sand is placed closest to the borrow area. Pumping the coarser material the shortest distance decreases the operation cost. This would not be a factor in this project for two reasons: a) the mean grain size of the borrow area was coarser than the native sand, and b) the borrow area was closest to the southern end of the project, which would have placed the coarser sand on the southern end.




1-year Shoreline Change

Figure 24. One-year shoreline change
2-yr Shoreline Change

Figure 25. Two-year shoreline change

-+--May 97
----Pred

--- May_98
---Pred




3-yr Shoreline Change

Figure 26. Three-year shoreline change
4-year Shoreline Change

Figure 27. Four-year shoreline change

--*-- May 99
-- Pred

-4- Dec99
- Pred




40
" Residual Structure: Structures present on the beach affect the bathymetry around
them. The contours around a structure are forced into a certain position, where the adjacent contours equilibrate to the wave direction. Adding fill can cause reduced shoreline advancement at the structure because the fill at the structure fills to the adjacent contours. Since there are no structures which would have affected the
bathymetry within the project area, this is not a probable cause.
" Borrow Pit Location: Salients or erosional areas can form between the borrow pit
location and the adjacent shoreline. This can occur if the borrow pit is located too
close to the shoreline. This is unlikely at the project site.
* Breaks in bars or reefs: Areas behind breaks in bars or reefs can experience a higher
concentration of wave energy. This could be a possible cause of the high erosion rates at the southern end. There is a reef which runs parallel to the shoreline at the
southern end of the project area and could be a cause of erosion at the southern end.
" Mechanically Placed Fill: Sand which is mechanically placed is less compacted than
if placed hydraulically. The template can be met by filing mechanically, but the more compacted, hydraulically-filled sand, will eventually become less compact, creating a
greater area covered by the fill. However, this project was not mechanically filled.
* Seawalls: The elevation of the mean sea level at a seawall can be lowered due to the
lack of sufficient volume to form a dry beach. When a seawalled-beach is filled, enough sand must be added first to create an incipient beach in front of the seawall. If this is not done, the sand added will simply fill in the area below MSL up to the seawall. There are not seawalls within the project area and thus this could not be a
cause.




41
" Headlands: Headlands work like a seawall, in that and incipient beach must first be
created in order for any of the fill material to create a dry beach area. The reef which
begins at R-20 and heads south could act like a headland.
" Residual Bathymetry: Irregular bathymetry can be created underwater during the
nourishment process. These formations can cause wave transformation, including diffraction or refraction. This could cause a concentration of wave energy either directly adjacent to the shoreline or downdrift of the formation. Examining the postnourishment survey bathymetry, there are no apparent irregularities in the bathymetry,
thereby negating this is as a likely cause.
One approach to identifying the presence of an erosional hot spot is by comparison of the measured shoreline or volumetric changes with the change predicted by DNRBS or other model. The difference between the two values is then plotted and the standard deviation is calculated. Fernandez (1999) proposed that an erosional hot spot or cold spot occurs when the longshore differences lie persistently above or below, respectively, the standard deviation lines.
4.3.1 Shoreline Changes
Analyzing the shoreline change comparisons in Figures 28-31 there are no apparent erosional hot spots, since a hot spot is considered an area which exceeds the standard deviation for several monuments. Note that in Figures 28-31, the standard deviation lines are located around the mean difference since the averages of the predicted and measured changes are different. There are several erosional cold spots (ECS), or areas which performed better than expected. The One Year Post-Nourishment graph shows an ECS at Monuments R-1 through R-5. The same ECS is evident in the second




42
year. The third year shows no evident erosional cold or hot spots. The fourth year shows an ECS located between Monuments R-15 and R-17. An EHS is also located at Monuments R-23 through R-25.
4.3.2 Volumetric Changes
An analysis of the volumetric change differences both above NGVD and above closure, do not indicate the presence of erosional hot spots. The volume change differences are above zero for the area above NGVD, which means the model under predicted the amount of erosion that occurred within the project area. This is the same case as the predicted shoreline versus the measured shoreline. Conversely, the volume change differences above depth of closure are below zero, which means the model under predicted the accretion that occurred.




1 Year Post-Nourishment

I 2 3 4 5 6 7 8 9 10 I1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Monument Number Figure 28.

2 Years Post-Nourishment

.................. ........ 18.7p

R- 'R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Monument Number
Figure 29.

'-" 80
70
C-) 60 50
40
c 30 o 20
0
on 0




3 Years Post-Nourishment

90 80 0) 70 60 S50, C 40U
.) 30
o 20
10

10 it 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Monument Number

Figure 30.
4 years Post-Nourishment

Monument Number

Figure 31.

t00

......................_.................. ......... .......... .........9,
- -.. ... . ,. . . . . . ..-. h . - R . .. . . . ..-... . .. . . .. . . . I .. .. . . .. . -... ..- .

60
40
0)
0.
0 0)




1 Year Post-Nourishment 25.00
20.00
0
................... ................................................. -5
15.00 .,,,
1000
00
> 0.00 1 -' R-2' R-3' R-4' R-' R-6' R-7' R-8' R.9' R-' R- R- R- R- R- I R- R- R- k -R R R- R- .R- .'
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Monument Number Figure 32.
2 Years Post-Nourishment IV30.00
Q.25.00
0)
Z20.00
0
0.00
R-I R-2 R-3 R-4 R-5 R-6 R-7 R-8 R9R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R10 11 12 13 14 15 16 17 19 19 20 21 22 23 24 25 Monument Number

Figure 33.




3 Years Post-Nourishment

Monument Number
Figure 34.

4 Years Post-Nourishment

-400.UU
Monument Number Figure 35.

40.00
30.00
I 20,00
. 10.00
0
z
O 0.00
,1 -20.00
U
C1)
-30.00
0

....................... ......... ........




1 Year Post-Nourishment

Monument Number
Figure 36.

2 Years Post-Nourishment

Monument Number
Figure 37.

0.(
-20.(
U
40.(
0 CO
,.0
-60.(
o80.(
-100.(
0

0.
-20
-40.
0
,. -60.
0.
S-100.
-120.
0




3 Years Post-Nourishment

Monument Number
Figure 38.

4 Years Post-Nourishment

Monument Number

Figure 39.

o.oo
0.) I
0 50.00
0
-100.00to
-150.00
0




5 DESIGN AND PREDICTION

The Jacksonville District of the U.S. Army Corps of Engineers used GENESIS (Generalized Model for Simulating Shoreline Change) to predict the performance of the project. The calculated renourishment interval was 11 years, which is the amount of time for the advanced fill to erode. The advance fill is the sediment added to the project plans to account for the erosion in between renourishments. The renourishment interval is chosen to optimize the project costs. The annual erosion rate was computed to be 53,600 yd3 per year, resulting in an advanced nourishment of 589,600 yd3. The model was run with a fill of 125 ft additional beach width along the entire project area. The berm height and depth of closure used were 11 and 29 feet, respectively. This results in a project volume of 3,666,667yd3 of sand added to the project, which is more than double that actually added (-1,500,000 yd3). In order to lose only the advanced nourishment amount, the percent remaining after 11 years would have to be equal to 84%. DNRBS was used to model the beach nourishment project with the same input conditions as the GENESIS model and obtained a renourishment interval of only 4 years.
5.1 Calibration of GENESIS
GENESIS allows calibration of the model to improve computation of sediment transport. The site-specific conditions are calibrated using shoreline surveys, wave data, sediment analysis, offshore bathymetry, the locations of shoreline structures and beach fills. The wave data are used in the longshore transport equation in each time step to




50
calculate movement through the cells. The longshore transport equation uses two coefficients, K, and K2, which can be adjusted based on historical shoreline changes. Because of the complex offshore bathymetry, a wave propagation model, RCPWAVE (Regional Coastal Processes Wave Propagation Model), was used to convert deep water wave conditions to that used as input in GENESIS. The wave model uses linear wave theory because the results are relatively accurate in first-order solutions at a low cost. The cell spacing for RCPWAVE was 500 ft alongshore and 250 ft cross-shore. The area modeled included 48 profile lines (St. Lucie County R- 110 to R- 115 and Martin County R-1 to R-42). The total modeled distance for RCPWAVE and GENESIS was 43,500 ft of shoreline.
Historical shoreline data were used to compare the model for calibration purposes. Survey data were obtained by the Florida Department of Environmental Protection (FDEP) on October 12, 1971, January 28, 1976, February 16, 1982, and by the Jacksonville District of the USACE on May 21, 1992. The area north of the project area in St. Lucie County was surveyed by the FDEP in 1966, June 1, 1972, February 11, 1987 and by the Jacksonville District of the USACE on May 21, 1992. The St. Lucie County surveys were converted to the same years as the Martin County surveys by interpolating between the surveys. The resulting four shorelines for the study area were 1971, 1976, 1982, and 1992.
Calibration of GENESIS was performed for the period October 12, 1971 through January 28, 1976. The longshore sediment transport calibration coefficients, K, and K2, were both set to a value of 0.10. It is noted that the usual value of K1 is approximately 0.8. The active berm height was taken to be 11 ft MLW, the median diameter of the sand




51
was 0.27 mm, and the average depth of closure was 29 ft MLW. The calculated volume change during this period was an accretion of 10,000 yd3 per year over the entire modeled area, the project area showed an accretion of 83,000 yd3 per year. The modeled shoreline changes were within an average of 25 ft of the measured over the entire study area. The comparisons between the measured 1971 and 1976 and the 1976 simulated shorelines are shown in Figure 40.
1400
1971 Shoreline
- - 1976 Shoreline 1200 1976 Simulated Shoreline
1000
",. Prpiert Area
- ~ 800600
400
ZOO
0-t
0 20 40 60 80 100 120 140 160 180
CELL NUMBER
Figure 40. Shoreline changes GENESIS calibration (USACE 1994) cell numbers increase to the south




5.2 Verification of GENESIS
Verification of the GENESIS model was performed by inputting identical data to that used in the calibration phase. The shoreline change from 1976 to 1982 was then predicted using the model and compared with the actual change. Figure 41 shows the comparison between the actual shoreline position and that predicted by GENESIS. The average difference between the measured and predicted shorelines was 21 ft. The volumetric changes during this period were calculated to be an accretion of 85,000 yd3 per year over the entire study area and an accretion of 69,000 yd3 per year within the project area.
1400
1976 Shoreline
- - 1982 Shoreline 1200- ......... 198Z Simulated Shoreline
1000
Project Area
- 800600
C 40

Figure 41. Shoreline changes GENESIS verification (USACE 1994)




5.3 Simulation of Project Performance
GENESIS was used to predict the performance of the beach nourishment project using the calibrated data from the period 1971 to 1982. A 1995 shoreline, or prenourishment shoreline, had to be extrapolated using the 1992 survey and the erosion rates from 1982 to 1992. The model was run for an 11 year time frame with a beach fill of 125 ft added to the mean high water line. The results are presented in Figure 42. The project area is located between cells 24 through 114. The model was run without the addition of the beach fill in order to evaluate the effect of the project as it relates to shore protection. Figure 43 compares the GENESIS model after an 11 year simulation to that of the DNRBS model. The December 1999 survey is added to the graph to show a comparison of both models prediction of the performance after 11 years to the actual performance after not quite 4 years. Both models greatly under-predicted the amount of erosion after the added nourishment. The actual additional fill did not average out to be 125 ft, which could account for some of the differences.




199Z Shoreline
- - 1995 Shoreline .......... 2006 Shoreline w/o Fill
2006 Shoreline u Fill

CELL NUMBER

Figure 42. GENESIS shorelines with or without beach fill (USACE 1994)

140 120 100 80 c 60
U
Z 40
20
-20 -40

R-2 R-3 R-4 R-5 R-6 R-7 R-8 R-9 R- R- R- R- R- R- R- R10 11 12 13 14 15 16 17

Figure 43. Shoreline change after 11 years with GENESIS and DNRBS and measured changes from December 1999

G EN ESIS ,DNRBS
-A-Dec_99




6 SUMMARY AN]) CONCLUSIONS

The Martin County beach nourishment project did not perform according to the expectations based on predictions from GENESIS or DNRBS. The GENESIS model was calibrated using surveys and wave data collected prior to 1982. As shown in Figure 2, an erosion trend occurred after 1982. Calibrating a model during an accretional period would cause an overestimation of the performance of a beach fill. The DNRBS model used the background erosion rate for the period 1982 to 1992, which would take into account the erosional period. Using the background erosion rate for the period 1982 to 1992, with an average erosion rate of 2 ft per year, would only account for 8 ft of erosion in 4 years. Both models are unable to simulate changes in profile due to storms. Since a severe storm occurred during project construction, the beach profiles were not equivalent to the planned profiles. The storm caused major cross-shore sediment movement, which is not predicted by either model.
Considering only the change in the position of the 0 ft NGVD contour (Figure 44) the entire project, especially the southern end, did not perform' to expectations. In December 1999, the shoreline is approximately the same as the pre-nourishment shoreline south of R- 19 within the project area.
The accuracy of the survey data, especially that beyond wading depth, appears inadequate for volumetric computations. The May 1998 surveys do not match beyond the depth of closure. The profile averages in Figure 4 show other possible inaccuracies in




56
the various surveys. This raises the question of whether only the surveys above the -6 ft contour can be used with confidence. Figure 22 shows large and unrealistic positive volumetric changes above the depth of closure. The volumetric changes indicate a large amount of sediment movement in the nearshore region. Taking into account the possible error in the survey data beyond wading depth, the best possible criteria for evaluating this project would be to examine the changes in plan area (above NGVD). Figure 10 indicates that there was a large loss of sediment above the 0 R NGVD contour after one year post-nourishment, which could be accounted for by the profile equilibrating after the storm in March 1996. The shoreline change from pre-nourishment conditions ranges from 96.5 ft in June 1996 to 35 ft in December 1999. This is a reduction of almost 65% of the 2,152,309 ft' of plan area that was added within the project area.
It is difficult to determine the exact cause for the poor performance of this project. There are several factors which could have affected the project, including the March 1996 storm, the location of the borrow area, and factors which were already contributing to the erosion within the project area. Due to the possible survey errors, it is difficult to draw solid conclusions about the cross-shore transport of sand.




400
F 350
E
300
0
E
0
250
0
oo 200
z
0 150
C
o 100
50
0
R-1 R-2 R-3 R-4 R-5 R-6 R-7 R-8 R-9 R- R- R- R- R- R- R- R- R- R- R- R- R- R- R- R10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Monument Number

Figure 44. Distance of 0 ft NGVD contour from monument (ft)




REFERENCES

Applied Technology and Management, 1998, "Martin County Beach Nourishment
Project: Project Performance Report," Gainesville.
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BIOGRAPHICAL SKETCH

Heather Sumerell was born in Raleigh, North Carolina in 1975. She attended North Carolina State University and received a Bachelor of Science degree in environmental engineering. While attending school she worked two summers for the North Carolina Department of Transportation in the Geotechnical Section. Her third summer was spent in Boston working for the Massachusetts Water Resources Authority.
After graduating, she lived in San Diego, CA, where she worked for Coastal Environments. She was involved with several ongoing beach erosion studies. She began her studies at the University of Florida in August 1998.