|
UFL/COEL -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 AND 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
ACKNOWLEDGMENT........ ............................................................. ii
AB STRACT ................................................ ..................................... .v
1 IN TRODU CTION ...................................................................................................... 1
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 monitoring ............................................................................................................ 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
111
4 .1 M o d el .................................................................................................................... 3 6
4.2 A application of M odel......................................................................................... 36
4.3 Erosional H ot 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 SUMMARY AND CONCLUSIONS ............................................................. 55
R EFER EN CES.................................................................................. ... 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 Sumerell
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 erosionall 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 causess.
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 yd3 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 north-
north-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 downdrift 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.
1948-1992
-+--1971-1992
--1982-1992
Monument Number
Figure 2. Background erosion rates
2
0
8
S. -2
w
-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-1 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 yd3 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 yd3 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 yd3, while the project area has eroded 1,104,000 yd3. The associated erosion
rate within the project area is 53,600 yd3 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 131,000 yd3 per year in the northern limit of
1971-1982
U,
Cl.
a'
0
0
U,
DISTANCE FR6M N6RTH JE TY, mILES
Figure 3. Historical volume changes (USACE 1994)
I-
-. I A J L J.X L -- rJ A ..- IJ 1lZ A. / L.
** w 1a.1 LiJ 2. 2. i U i f l 1 1. >J 41 3.1 l 4. t l -I.
DISTANCE FROM NORTH JETTY, MILES
1982-1992
" " ~ISTXNCrFR6"M N6RTi JEiY, IE L ".
1971-1992
\ Y--,
R M
i:
I:
fv -~
0
tL I
L. L e.
Ww~
9
Bathtub Reef to less than 10,000 yd3 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 yd3 per year at the northern boundary to 10,000 yd3 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 R-
20), 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
811,000 yd3 of sand (ATM 1998) eroded between the visible beach out to the -18 ft
contour, with the southernmost 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 yd3 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 (R-
110 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.us/beach/) 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 1 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
Profile Averages R9-R17
-Nov 95
-Jun_96
May_98
--Dec 99
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-110O
1070901-
R-1
R-
R
1065000-
1060000-
1055000-
1050000-
1045000-
12
13 ,
R-114
R-25
R-3-,
R-47
R-5,
R-6o
R-7
R-8 ,
R-91
R-10<
R-11
R-12,
R-134,
R-144.
R-15<
R-164
R-17\
R-184
R-19*,
R-204,
R-21
R-22,0
R-23,,
R-24&0
R-254\
R-26*
R-274\
R-28*
R-294
In P3 %A
760000
765000
770000
770000
Borrow Area
775000
Figure 5. Map of borrow area
- -~`-
0 0
V1 -
100
P 4 0 ................................................. ............ ..... .............................
60 .........................................y < ^ ^
e
C
0
-4 -2 0 2 4 6
GRAIN SIZE Iphi units
Native Sediment Borrow Combo 1
.-. 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-l, R-5, R-12, R-15, and R-23. The samples at R-l, 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 ft2 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
Change (ft)
o C,, C
o 01 0
~70
o77
CD
77
70
op
io 'ic7
''6
o7
o -w ^\
o
S
CD a2) a)E
1 0 < '<
D I ( 0 (D
CD CD 00-4(3
I ^- ^
i ^: ^
^ \ \ ^>
o ?\^^s-
I ^
\ o* -^
s ^*^
uL /& ^(
'^----r^^^"
(D 01 n ni
120
100
80
0
40% of the fill remaining. DNRBS greatly under predicted the amount of plan area lost
due to initial equilibration.
I 60 --- [-!-CDNRBS
3.2.1 June 1996
YThe immediate post-nourishment surhmvey does not reflect the planned additional
dry beach width because of the storm in March 1996. Only 75% of the project had been
complFigure 10. Percent remaining of meas ured and prior to the June survey to artiallys corresponds to June
1996
damage caused by40% of the fill remainingstorm. FiguDNRBS reatly under predicted the amount of plan area lostorm.
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-1 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 pre-
nourishment 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
R-23
20
I J Trough
Z -5)0 -10 0 1000 1500 2DO
0 -20 -
0 3-30
> -40
-50
Distance from Monument (ft)
Nov_95 Jun_96
Figure 13. Profile at R-15
R-22
Distance from Monument (ft)
Nov_95 May_97
Figure 14. Profile at R-22
z
0.)
Ig
o
.--
4-*
rt
Figure 15. Profile at R-
Figure 16. Profile at R-4
R-1
10
00
S-100 -10 10(
S -20
- -30
W-
Distance from Monument (ft)
--Nov_95 May_98
R-4
z
20
> -5)0 -10
C -20 -
"-30
U -40
w
Distance from Monument (ft)
-Nov_95 -May_98
Figure 17. Profile at R-8
Figure 18. Profile at R-2
R-8
20
Z
> -5)0 -10 -- 1500 2500
= -20
-30
-40
Distance from Monument (ft)
-Nov_95 --May_98
Figure 19. Profile at R-12
Figure 20. Profile at R-4
R-12
20
-5)0
-10
0
I -20 -
.0 -30
. -40
5
Distance from Monument (ft)
---Nov_95 -- May_99
R-4
20
> -5)0 -10 GX 1000 1500 2000
= -20 -
Cz -30
-40
Distance from Monument (ft)
Nov_95 Dec_99
Figure 21. Profile at R-8
3.3 Volumetric Changes
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
--U---Above NGVD
e 800000- -7 ----- Above -6
S- \ --- Above Clos
S\- - -ATM NGVD
w 600000- ATM-6
E
S--ATM cios
-- - - ATM Clos
400000 --
200000
0
Jun_96 May_97 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
-- May_97
AMay_98
--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 yd3. Above
the depth of closure the average volumetric density is 12.8 yd3/ft, with a gain of
287,923 yd3. These volume densities are to be compared to a design placement volume
density of 78 yd3/ft and suggest substantial survey errors.
Table 2. Volume change (yd3) between pre-nourishment (Nov. 1995) and post-nourishment
(June 1996)
ABOVE ABOVE 6 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 (yd3) between pre-nourishment (Nov. 1995) one-year post-nourishment
(May 1997)
ABOVE ABOVE 6 6 TO 12 ABOVE
NGVD CLOSURE
R-1 9,101 9,332 6348 42.472
R-2 15.707 15.456 9732 50,464
R-3 19.777 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 74,310
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 3.698 6427 12.710
R-24 10.071 3.671 14434 28.575
R-25 5.952 -3,424 20165 26.486
Total 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 yd3, the maximum documented by
all the surveys.
Table 4. Volume changes (yd3) between pre-nourishment (Nov. 1995) and two-year post-
nourishment (May1998)
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 -2,55 -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 (yd3) between pre-nourishment (Nov. 1995) and three-year post-
nourishment (May 1999)
ABOVE ABOVE 6 6 TO 12 ABOVE
NGVD CLOSURE
R-l 8,486 9,240 -8,694 -4,444
R-2 9,913 7,460 -6,354 -6,718
R-3 15,062 19,281 -2,666 20,460
R-4 15,304 17,650 -4,674 11,020
R-5 15,998 13,692 -2,823 11,140
R-6 24,240 34,825 -7,154 27,081
R-7 19,807 28,545 -3,276 23,405
R-8 13,921 12,927 -7,638 7,728
R-9 24,614 27,928 -956 33,76
R-10 24,484 34,773 2,858 56054
R-11 27,260 33,091 13,918 58,017
R-12 23,780 35,665 12,551 62,206
R-13 28,663 43,419 8713 60,076
R-14 28,233 43,958 14630 70,680
R-15 25,992 43,480 6,973 63.320
R-16 20,273 24,364 16,333 42,999
R-17 27,657 56,791 26,869 86,884
R-18 18,276 28,084 7,717 33,424
R-19 52,280 87,931 36,986 123,581
R-20 7,413 -2,700 17,952 5,431
R-21 22,299 39,650 21,655 68,486
R-23 12,765 32,805 14,689 41.386
R-24 11,766 16,600 5,670 20,963
R-25 -2,984 -8,069 1,588 -9,796
Total 475,914 673,448 169,080 909,765
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 yd3. 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 post-
nourishment (December 1999)
ABOVE ABOVE 6 6 TO 12 ABOVE
NGVD CLOSURE
R- 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 -4785 -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 Application 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
--N Pred
M--ay_98
--- Pred
3-yr Shoreline Change
100
50
0 -
10-----------------------May99o
U -50r
-100
-150
Figure 26. Three-year shoreline change
4-year Shoreline Change
100
50
E q\ -*-Dec_99
o
-U-Pred
S-50
\-
Figure 27. Four-year shoreline change
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 post-
nourishment 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
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Monument Number
Figure 28.
2 Years Post-Nourishment
V V8
... ............. ....... .. 1. 7p
R- R- R- R- 'R- R- R- R- 'R- R- R- R- R- R- R- R- R- R- R- R- R-
1 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
S50
40
0U
c 30
0 20
0
on 0
CT o
3 Years Post-Nourishment
90
80
a 70
C 50
a)
So.
40-
a)
o 20
10
o-
10 11 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.
oo00
---- --- --- --- --- - 9042
.......... -__...... ...................... ...... . . . .... .9,
..-...-.. ....k-.-. R-... -.. -. ..-. ..-. ..-., ................, .... ... I ,.- .- .....- 9.-
60
S20
U
ca 20 -
0
c
2 o.
0~O
*c
-- -- - -
1 Year Post-Nourishment
25.00
20.00
1_)5.00 -------------------*--*-----*
00 .............................................. ............ ..-------------------5
15.00
.,,
1000
30.00
2 5.00 -
o
^ lo.oo ---------4----------
00
R- 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- R-
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
-5.00------------------------------------------
Monument Number
Figure 32.
2 Years Post-Nourishment
530.00
2.o25 00
Z20 00
<15.00
0)10 00 0
5.00
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- R-
-5.00
Monument Number
Figure 33.
3 Years Post-Nourishment
Monument Number
Figure 34.
4 Years Post-Nourishment
-40.UU
Monument Number
Figure 35.
40.00
S30.00
S20.00
>. 10.00
z
0 000
*0
Q -10.00
,. -20 00
U
-30.00
0
....................... ......... ........
1 Year Post-Nourishment
Monument Number
Figure 36.
2 Years Post-Nourishment
Monument Number
Figure 37.
-20.(
U
-40.(
0
S-60.
o
U
0
0
-20.
-40.
o
,. -60.
S-80.
, -100.
-120.
0
3 Years Post-Nourishment
Monument Number
Figure 38.
4 Years Post-Nourishment
Monument Number
Figure 39.
o.oo
IO
- 0.00
0
0
> -50lo.oo00-
o
S-150.00-
^g
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 K, 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-
CE Pret Are ER
BO-- 0
S 600-
400--
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-- --........ 1982 Simulated Shoreline
1000-
-Prnject Area
800--
600-
Cn
400-
M ^
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 pre-
nourishment 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.
-- 1992 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
T 60
U
1 40
20
0
-20
-40
---GENESIS
---DNRBS
---Dec 99
Figure 43. Shoreline change after 11 years with GENESIS and DNRBS and measured changes
from December 1999
6 SUMMARY AND 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 ft 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 ft2 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.
Distance of NGVD Contour from Monument (ft)
0) l 0 0 0
0 0 0 01 0 0 0 0 0 0
CD
o
3
z
c/
0 T
0 T
o
0
N)
N ____
I, ?
S: o7- w-^~^-^
3 ^x
K? J
K? P^ ^
=f? -----^ ^ -------
REFERENCES
Applied Technology and Management, 1998, "Martin County Beach Nourishment
Project: Project Performance Report," Gainesville.
Dean, R.G., and Dalrymple, R.A, "Coastal Processes with Engineering Applications,"
Class notes, University of Florida, Gainesville (in press).
Dean, R.G., and Grant, J., 1989, "Development of Methodology for Thirty-Year
Shoreline Projections in the Vicinity of Beach Nourishment Projects,"
UFL/COEL-89/026, Gainesville, University of Florida.
Fernmndez, G., 1999, "Erosional Hot Spots at Delray Beach, Florida: Mechanisms and
Probable Causes," M.S. Thesis, Coastal and Oceanographic Engineering
Department, University of Florida, Gainesville.
Florida Oceanographic Society, 1982, "Coastal Zone Management Study of Hutchinson
Island Martin County, Florida," Stuart, Florida.
Liotta, R., 1999, "Erosional Hot Spots: Causes and Case Studies at Dade and Manatee
Counties," M.S. Thesis, Coastal and Oceanographic Engineering Department,
University of Florida, Gainesville.
United States Army Corps of Engineers, 1968, "Beach Erosion Control Study, Martin
County, Florida," South Atlantic Division, Jacksonville District.
United States Army Corps of Engineers, 1994, "Martin County, Florida Shore Protection
Project General Design Memorandum with Environmental Assessment," South
Atlantic Division, Jacksonville District.
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.
|
RSS
HIDE
