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UFL/COEL-94/021
EVALUATION STUDY AND COMPARISON OF
EROSION MODELS AND EFFECTS OF SEAWALLS
FOR COASTAL CONSTRUCTION CONTROL LINE
Interim Report #5 and Summary of Previous Results
by
Carol Demas
Lihaw Lin
Robert G. Dean
November 1, 1994
Prepared for:
Department of Environmental Protection
State of Florida
Evaluation Study and Comparison of Erosion Models and
Effects of Seawalls for Coastal Construction Control Line
Interim Report #5 and Summary of Previous Results
by
Carol Demas
Lihwa Lin
Robert G. Dean
November 1, 1994
Prepared for:
Department of Environmental Protection
State of Florida
PREFACE
This is the fifth in a series of reports presenting experimental results from the physical modelling
of beach erosion in the vicinity of a seawall for a DEP Coastal Construction Control Line study. The
beach and the associated seawall are modelled after a DNR profile at Range Number R-192 in Highland
Beach, Palm Beach County, Florida. In this report, the effects of the model beach profile responses with
and without seawall, and with seawall failure during the peak surge level period of the modelled storm
surge were investigated.
Contents
1 Scope of This Report 1
2 Experimental Conditions 4
2.1 Laboratory Facility ........................................ 4
2.2 Scaling ...................................... ......... 4
2.3 Initial Beach Profile and Seawall Configuration ..... . . . . . . . . 6
2.4 Storm Surge ............................................. 6
2.5 Sediment Size ................... .... ............. ... ....... 8
2.6 Wave Conditions ........ . ........................... 8
2.7 Seawall Failure Conditions . . . . . . . ..... . . . . . . . . . 8
2.8 Seawall Toe Protection and Nearshore Reef . . . . . . . .... . . . .. 9
3 Comparision of Experimental Results 9
3.1 Introduction ...................................... ....... 9
3.2 Effect of Different Storm Surge Level . . . . . . . ..... . . . . .. 9
3.3 Effect of Regular versus Random Waves . . . . . . . .......... .. 10
3.4 Effect of Lowered Profile ...... ....... ..... . ............. . .. 10
3.5 Effect of Energy Level of Random Waves .............. .......... 10
3.6 Effect of Seawall Height ........ ...... ..................... 10
3.6.1 Depth of Scouring at Seawall ................... ......... 10
3.6.2 Water Overtopping Seawall Rate . . . . . . . .......... .. 13
3.6.3 Scour Depth as a function of Overtopping Rate During Maximum Surge . . 15
3.7 Effect of Seawall Failure ............. ............ ........ 15
3.7.1 Volume Difference from the Initial Profile . . . . . . . . . . 16
3.7.2 Erosion Patterns ................... ............... .. 19
3.8 Effects of Sediment Size and Wind ......... ....... ................ 19
3.9 Effect of W ave Period ...................................... 20
3.10 Effect of Seawall Protection and Artificial Reefing . . . . . . . ..... . . 21
4 Summary and Conclusions 22
5 References 26
List of Tables
1 Summary of experimental cases and test parameters. . . . . . . . ... .... 2
2 Experiments compared in the present and previous reports. . . . . . . . . 3
List of Figures
1 Schematic diagram of wave tank facility. . . . . . . . . . . . . 5
2 Initial profile and Seawall #1, modelled after DNR profile R-192. . . . . . . . 7
3 Continuous and stepped simulation of storm surge #2. . . . . . . .... ... 7
4 Initial profiles for seawall toe protection and nearshore reef experiments. . . . . . 9
5 Scour depth at seawall in relation to initial from B-series experiments. . . . . ... 11
6 Scour depth in relation to seawall from B-series experiments. . . . . . ..... 12
7 Measured water overtopping seawall rate versus storm surge level. . . . . . ... 14
8 Scour depth versus overtopping rate during maximum surge level. . . . . . ... 15
9 Initial profile for no seawall experiment. . . . . . . . ... . . ..... 16
10 Sand volume difference from initial versus seawall failure types. . . . . . . ... 17
11 Comparison of erosion patterns from various seawall failure and no seawall experiments. 18
12 Scour depths at seawall toe for experiments A2, A3, A4, B. . . . . . . .... 20
13 Effects of seawall toe protection an offshore reef. . . . . . . . ..... . . 21
Evaluation Study and Comparison of Erosion Models and
Effects of Seawalls for Coastal Construction Control Line
Interim Report #5 Physical Model Results
1 Scope of This Report
This is the fifth in a series of reports presenting results from wave tank experiments performed
at the University of Florida Coastal and Oceanographic Engineering Laboratory as part of the physical
modeling of a DEP Coastal Construction Line study. The modeling examines the effects of seawalls
on adjacent beaches under controlled forcing conditions. The beach profile used in the experiment is
modelled after DNR Range Number R-192 located in Highland Beach, Palm Beach County.
This project is sponsored by the Beaches and Shores Center at Florida State University. The
technical monitor of the project is Dr. T. Y. Chu.
Table 1 summarizes the experimental cases and the test parameters used in the wave tank studies
completed to date. These experiments were designed to serve as sensitivity tests for individual experi-
mental parameters. Table 2 provides a summary of the parameters varied for comparison of experimental
results presented in this and previous reports.
Previous reports aimed at investigating the effect of varying sediment diameter (A2, B1), the
effect of regular waves versus random waves (C-series), the effect of different storm surge conditions
(A1,A2,B2,B9), and the effects of lowering the model initial beach profile and lowering the wave energy
level (B-series).
This report examines the effect of seawall failures on the adjacent beach profile in terms of scour
depth at seawall, beach profile elevation difference and volume difference from initial for three types of
seawall failure. The results were compared to the experiments with the normal seawall and without the
seawall for both random and regular waves. The rate at which water overtops the seawall for various
seawall height at different storm surge level and the associated scour depth during the maximum surge are
also investigated. Additionally, tests were also conducted to examine the effects of toe scour protection
and the presence of a nearshore reef. At the end of this report, the conclusions of the findings from this
and previous reports for the Highland Beach seawall and beach model experiments were presented.
Table 1: Summary of experimental cases and test parameters.
water level condition seawall condition sediment wave characteristics water
stepwise varying const. plan, vertical; median wave type deepwater wave volume
exp. storm surge level model elevation diameter regular random win wave height period over-
ID# model peak level[cm [cm] [cm] mm waves waves waves [cm] [s] topping
14.4 16.8 14.4 21.12 22.34 19.9 0.18 0.09 16 15 1.65 1.3 seawall
#1 | #2 #3 #4 #5 # #2 #3 #2 1 2 #3 # 2 #1 #2 measured
Al X X X X X X
A2 X X X X X X
A3 X X X X X X X
A4 X X X X X X X X
BOt X X X X X X
B1 X X X X X X
B2 X X X X X X
B3 X X X X X X
B4 X X X X X X
B5 X X X X X X X
B6 X X X X X X X
B7 X X X X X X
B8 X X X X X X
B9 X X X X X X
C3 X X X X X X X
C4 X X X X X X X
C5 X X X X X X X
C6 X X X X X X X
C8I X x X X X X
T3 X X X X X X
R3 X X X X X X X
R4 X X X X X X X
N1 X X X X X
N2 X X X X X
FI" X X X X X X
F2* X X X X X X
F3* X X X X X X
F4* X X X X X X
F5* X X X X X X
F6* X X X X X X
t seawall toe was protected by rocks.
t beach modelled with a small section of artificial reefing.
F-series simulated seawall failure during the highest storm surge level.
Table 2: Experiments compared in the present and previous reports.
storm surge peak [14.4 cm] vs. [16.8 cm]
wave type [regular] vs. [random]
initial profile [regular] vs. [lowered]
wave energy [Ho=15 cm] vs. [Ho=16 cm]
seawall failure [1/2, total failure]
seawall status [regular seawall] vs. [no seawall]
wave type [regular] vs. [random]
seawall failure [1/4, 1/2, total failure]
seawall status [regular seawall] vs. [no seawall]
seawall height [19.9 cm, 21.12 cm, 22.34 cm]
wind vs. no wind
sediment size[0.18mm] vs. [0.09mm]
wave type [regular] vs. [random]
seawall toe protection vs. no protection
artificial reef vs. no reef
F1-F6
B3 B4 N1 N2
B2-B8
A2 A3 A4
A2 B1
N1 N2
B3 B4
F1 F2
F3 F4
F5 F6
BO B3
CO B3
I
2 Experimental Conditions
2.1 Laboratory Facility
The laboratory experiments were conducted in the air-sea tank at the University of Florida
Coastal and Oceanographic Engineering Laboratory. The tank section used for the experiments is 37 m
long, 0.9 m wide, and 1.2 m high (Figure 1). The tank is equipped with a hydraulic powered wavemaker
which generates regular and irregular waves and is controlled by a Wavetek Model 110 computer system.
The wavemaker and a powered fan used to generate wind waves are located at one end of the tank. A
wave energy absorbing basin is located at the far end of the wave tank where the model beach is located.
Above the wave tank there is an electrically powered trolley capable of moving over the entire
length of the wave tank. This trolley is used in measuring the model beach profiles in the following way.
Along the entire length of the model beach, a grid is marked on the top of the wave tank wall. The trolley
is equipped with a graduated vertical rod which can be raised or lowered manually. By stopping the cart
at predetermined increments along the grid, model beach elevations are then measured by lowering the
graduated rod so that it just rests on the sand and an elevation is read from the graduated survey rod.
Horizontal distances are adjusted with zero datum at the seawall location. Elevations are referenced to
the DNR vertical datum of NGVD. The model beach profile surveys were conducted at various time levels
for each experiment depending on the objectives and time allowed (Thompson, et al., 1993).
Two capacitance type wave gauges were installed in the tank for monitoring waves Gage 1
is located 18.3 m (457.5 m prototype) seaward of the seawall and Gage 2 is located 5.3 m (132.5 m
prototype) seaward of the seawall.
The volume of water overtopping the seawall was measured during some experiments by collecting
the water in a movable catch pan located immediately landward of the seawall. Measurement of this
overtopping volume requires that the experiment be performed twice. Since the profile response behind
the seawall is also of interest, the same experiment needs to be performed once for each measurement.
2.2 Scaling
A model to prototype length scale ratio of 1:25 and time scale ratio of 1:5 are used in all
experiments. This means that, for instance, a distance of 1 m in the model corresponds to a distance of
red
"n -3.0
-I I H 1- 1.5
C~~~.~0 _3
Wavemaker
IC
Wavemaker
1.8 m
Wave
Gauge
Sand /
Bed Gauge
CROSS-SECTION
Location of
Seawall
36.6 m
Location of
Carriage for Wave Locatn of
! Wave Screens /Gauge and Profiler \
* A1 L MXIX K KK
Hydro | 1
Power Drain Valves
Unit
\
Wave
Gauge
'I I r~' WV V ~
, p~Ip
Tank Wave
Divider Gauge
Bulkhead
7.9 m
PLAN VIEW
Figure 1: Schematic diagram of wave tank facility.
Powe
Fa
-> -5.8 m--+
SBasin o
" ..
1_0'/
/ hY~I~wYrMvr
25 m in the prototype or actual case being modelled. Similarly, a one second time duration in the model
corresponds to 5 seconds in the prototype. Dimensions in this report are given primarily in model units.
2.3 Initial Beach Profile and Seawall Configuration
The initial beach profile used in the experiments is modelled after an actual beach profile in
Highland Beach in Palm Beach County. The profile coincides with DNR Range Number R-192. The
model profile is presented in figure 2.
Three simple vertical seawalls were used in the Highland Beach model experiments. Since these
seawalls varied only in seawall height, they were constructed out of the same piece of 2.54 cm thick
plywood which was placed at three different elevations to create the three model seawalls. The seawall
location along the profile was fixed for all the experiments listed in Table 1 which involve seawalls. Here,
Seawall 1 is modelled after the actual seawall located at R-192 which has a model elevation of 21.12 cm
(5.28 m prototype). The Seawall 2, at 22.34 cm (5.59 m prototype), is 1.22 cm (model) higher than
Seawall 1 and Seawall 3, at 19.9 cm (4.98 m prototype ) is 1.22 cm (model) lower.
2.4 Storm Surge
Four time-varying severe surge conditions are modelled in the experiments listed in Table 1.
They are referred to as Storm Surges 1, 2, 3, and 4. These severe surge conditions are numerically
determined combined total storm tides which have been established for various portions of Palm Beach
County (Dean, et al., 1992). These total storm tides include storm surges, astronomical tide, and dynamic
wave set-up which occurs primarily in the inner surf zone (landward of wave breaking). The storm tides
are physically modelled in the wave tank by raising the water level in a stepwise fashion. Figure 3 is a
schematic of the continuous storm surge profile of the prototype and the stepped simulation which was
applied in the model experiment.
In contrast to the time-varying water level of the aforementioned storm surges, Storm Surge 5 is
a time-invariant water level which is modelled by fixing the surge water level (SWL) at 0.144 m (3.6 m
protype). This time-invariant water level is the predicted peak value for Highland Beach in Palm Beach
County. The constant storm surge was used in the Series C experiments.
0.25 -
Q 0.20
Z 0.15 -
0.10 -
.2 0.05 -
- 0.00 -
I -0.05 -
-0.10
-1.0
Figure 2: Initial profile, modelled after DNR profile R-192, and seawall #1.
Figure 3: Continuous and stepped simulation of storm surge #2.
0.0 0.5 1.0 1.5 2.0
model distance from seawall [meters]
seawall
---
initial profile
NGVD
-- 0 m elapsed -------------
I .. - ..p I ... ..... ... .. ... ..
--- ......elpe `--------
2.5 3.0 3.5 4.0
1
2.5 Sediment Size
Median diameters of the two sediment sizes used in the experiments are 0.18 mm for sand in
the A-series experiments and 0.09 mm for all other experiments (B,C,N, and F-series). These model
diameters correspond to prototype values of 0.65 mm and 0.21 mm, respectively.
2.6 Wave Conditions
Both regular and random waves are modelled in the wave tank experiments listed in Table 1.
The regular waves are modelled with a deepwater wave height of 0.16 m for all the experiments completed
thus far. This model wave height corresponds to a prototype value of 4 m. In A4, the regular wave height
was slightly higher than 0.16 m since the experiment includes the effects of wind.
The random waves are modelled by a Pierson-Moskowitz spectrum with a deepwater significant
wave height of 0.16 m (4 m prototype) for all the random wave experiments except in B5 where a
significant wave of 0.15 m (3.75 m prototype) was used.
Two different wave periods are modelled in the regular wave experiments Wave Period 1
is 1.65 seconds in the model (8.25 seconds prototype) and wave period 2 is 1.3 seconds in the model
(6.5 seconds prototype). The modal wave period of 1.65 seconds (8.25 seconds prototype) has been used
in all the experiments except in C4 and C6 where a modal wave period of 1.3 seconds (6.5 seconds
prototype) was utilized.
2.7 Seawall Failure Conditions
The F-series experiments were conducted to determine the effect of seawall failure on the model
beach profile immediately adjacent to the seawall. Three different types of failures are investigated.
Type I failure denotes a total seawall failure in which the entire seawall fails. Experiments F1 (regular
waves) and F2 (random waves) are of the Type I failure. Type II failure denotes the failure of the upper
half portion of the seawall above zero NGVD. Experiments F3 (regular waves) and F4 (random waves)
involve this Type II failure. Type III failure signifies a lesser seawall failure in which only the upper
quarter portion of the seawall above zero NGVD is removed. Experiments F5 (regular waves) and F6
(random waves) utilize the Type III failure.
0.30
0.25 -
0.20
S0.15
0..1.0.
.2 0.05
.j
> -0.
3 -0.
-0.
-0.
0u
05- Concr
10
15- Normal Profile:B- Geotextile:BO ---- Concrete:CB
15 I
-0.5 0.0 0.5
1.0 1.5
Distance [m]
2.0 2.5 3.0 3.5
Figure 4: Initial profiles for seawall toe protection and nearshore reef experiments.
2.8 Seawall Toe Protection and Nearshore Reef
Experiments BO and C8 were conducted to determine the effects of seawall toe protection and
nearshore reefs, respectively. Experiment BO used small rocks (5 cm in diameter) above geotextiles in the
area from seawall to 0.5 m (12.5 m prototype) seaward of the seawall, and Experiment C8 used concrete
in the area 1.4 m to 2.4 m (35 m to 60 m prototype) seaward of the seawall to simulate a nearshore reef.
Figure 4 displays the initial profile configurations of the experiments of BO and C8.
3 Comparision of Experimental Results
3.1 Introduction
Prior to describing the test results obtained in the experiments outlined in this report, a brief
review of previously reported results is presented.
3.2 Effect of Different Storm Surge Level
The effect of different storm surges has been shown by comparing Experiments B1 and B2 in
Interim Report #3.
Rocks
a,,
-1.0
'c-
3.3 Effect of Regular versus Random Waves
The effect of regular versus random waves was investigated in Interim Reports #2 and #3 in a
comparison of results from experiments B3 and B4, B6 and B5, and B9 and B8 for time-varying storm
surges, and from C-series experiments for invariant storm surges. More discussion of this effect is included
in Sections 3.6 for varying seawall height and in Section 3.7 for different seawall failure in the model.
3.4 Effect of Lowered Profile
Experiments B8 and B9 involved lowering of the beach profile and the results were reported in
Interim Report #3.
3.5 Effect of Energy Level of Random Waves
The effect of different energy level of random waves is investigated in a comparison of Experi-
ments B5 versus B7 deepwaterr significant wave heights are of 15 cm and 16 cm, respectively, in these
experiments). Discussion of the two experiments and results have been presented in Interim Report #3.
3.6 Effect of Seawall Height
Three different seawall heights (19.9 cm, 21,12 cm, 22.34 cm) are utilized in the present model
experiments. The effect of seawall height is evaluated here in terms of the depth of scouring at the seawall
location.
3.6.1 Depth of Scouring at Seawall
The depths of scouring at the seawall toe and immediately behind the seawall were compared
among Experiments B2, B3, B6, and B9 (regular waves), and B4, B5, B7, and B8 (random waves).
Figure 5 shows this simple comparison of scour depth. Figure 6 shows the same information with the
initial and final elevations surveyed in relation to the seawall height. All elevation differences refer to
erosion except as noted.
0.1
E
0.0
0.
-o0.
-0.
-0.
0.1
OJ
-0.
0.
-0.
a -0.
-n
0.10
0.05
' -0.00
S-0.05
S-0.10
-0.15
0.10
S0.05
0
o' -0.00
0 -0.05
$ -0.10
-0.15
B4 B5 B7 B8
End of Experiment
Random Wave Experiment
7.8 hours elapsed(model).
Figure 5: Scour depth at seawall in relation to initial from B-series experiments (negative values denote
erosion).
0
End of Peak Surge Level 0 Behind Seawall
Regular Wave Experiment
5 4.2 hours elapsed (model) Front of SeawalT
00
05-
10
15
B2 B3 B6 B9
0
End of Experiment
Regular Wave Experiment
)5 7.8 hours elapsed (model)
00
05.l
10-
1-
i -
I
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
-0.
0.4
0.3
0.3
0.
0.!
0.1
0.1
0.
0.
-n
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
B4** B5 B7 B8*
End of Experiment
Random Wave Experiment
7.8 hours elapsed(model)
L0
10 -- --------------------
15 End of Peak Surge Level
Regular Wave Experiment
10 4.2 hours elapsed (model) 0 Behind Seawall
0 Front of Seawall
!5
.0 -
15
0
)5
05
B2 B3 B6 B9*,**
C0
35 End of Experiment
Regular Wave Experiment
30 7.8 hours elapsed (model)
!5 -
20 -
L5 -
10
05 --
End of Peak Surge Level
Random Wave Experiment
4.2 hours elapsed (model)
Figure 6: Scour depth in relation to seawall from B-series experiments solid lines denote the seawall; the
blocks to the left and right represent the change of profile elevations behind and in front of the seawall.
*Initial profile lowered 2.4cm, **B9(front, 7.8 hrs), B4(behind) show accretion.
(1) Regular Waves Behind the seawall, the depth of scouring is greatest for B6 and B9, moderate
for B2, and very slight for B3 at 4.2 hours (the end of the peak surge level). Little change is evidenced
from 4.2 hours to 7.8 hours (the end of the experiment). In front of the seawall, the depth of scouring is
greatest for B3 and B6 with B2 and B9 displaying scour depth about half as large as that in B3 at 4.2
hours. However, the scouring depth becomes moderate in front of the seawall for B2, B3, and B6 while
results slight accretion for B9 at 7.8 hours.
(2) Random Waves At 4.2 hours behind the seawall, large depths of erosion for B7 and B8, slight
erosion for B5, and a small amount of accretion for B4 were observed. In front of the seawall, moderate
scour depths for B4, B5, and B8, and a lesser scour depth were observed. At 7.8 hours (the end of the
experiment), the erosion depth is decreased markedly for all Experiments B4, B5, B7, and B8.
3.6.2 Water Overtopping Seawall Rate
Figure 7 shows water overtopping seawall rate measured versus storm surge level for three
different seawall heights.
(1)Seawall height 19.9 cm: For regular waves (B6) the overtopping rate is about 35 gallon/minute
at a surge level of 15.5 cm. For random waves (B5), the maximum surge level is about 20 gallon/minute
at the same surge level.
(2)Seawall height 21.12 cm: The regular wave case (B3) achieves a maximum value of about
17 gallon/minute at a surge level of 14.4 cm while the random wave case (B4) has an overtopping rate of
only about 6 gallon/minute at the same surge level.
(3)Seawall height 22.34 cm: The case of regular waves with wind input (A4) achieves an over-
topping rate of about 35 gallon/minute at a surge level of 16.7 cm. In the case of regular waves (A3)
alone, the rate is moderately decreased from the wind case at 28 gallon/minute. Note that both A3 and
A4 are using coarse sand (median size 0.18 mm) in the experiments. Based on the C-series experiment
(fine sand, median size 0.09 mm) the overtopping rates range from about 1 to 4 gallon/minute at a surge
level of 14.4 cm.
Seawall Height 19.9 cm
Seawall H-eight 21.12 cm
Regular (BS) --- Random (B4)
.10. 11. 12. 13. 14. 15. 16. 17.
Seawall Height 22.34 cm
Wind Waves + Regular (A4) --- Regular (A3)
I
I/
y I
s
I I
9. 10.
11. 12. 13. 14.
Surge Level [cm], NGVD
15. 16. 17.
Figure 7: Measured water overtopping seawall rate versus storm surge level.
O Behind Seawall
E Front of Seawall
10.0 20.0 30.0
Overtopping Rate During Maximum Surge [Gal/min]
40.0
Figure 8: Scour depth versus overtopping rate during maximum surge level.
3.6.3 Scour Depth as a function of Overtopping Rate During Maximum Surge
Only cases B3 to B6 are examined. Figure 8 shows the comparison of the scour depth versus
the water overtopping seawall rate during the maximum surge level. The lowest overtopping rate and a
moderate scour depth are displayed in B4 (21.12 cm seawall height, random waves). At the same seawall
height, when regular waves are induced (B3), a higher overtopping rate and a larger scour depth are
evidenced. In B5 (19.9 cm seawall height, random waves) the overtopping rate was larger than the one
in B4 but the amount of scour behind and in front of the seawall was similar to that in B4. Finally,
experiment B6 (19.9 cm seawall height, regular waves) displays the largest overtoppping rate, the greatest
scour depth behind the seawall, and a considerably large scour depth in front of the seawall.
3.7 Effect of Seawall Failure
The responses of the model profile associated with seawall failures were investigated in the F-
series experiments. The seawall failure was simulated in the model at the midpoint of the maximum storm
surge interval (see Figure 3). The evaluation of the effect is based on the volume changes behind and
Initial Profile Elevation for No Seawall Cases
0.30
,-, 0.25
0.20 -
S0.15 \ seawall
S0.10 location
S-0.10 -
0.05
-0.05
-- N:Regular Waves --- NZ:Random Waves
-0.15 I --- I - I-
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Figure 9: Initial profile for no seawall experiment.
in front of the seawall from initial to erosive profiles measured in the surrounding areas of the modelled
seawall. The volume change was computed for the areas behind seawall from the seawall to -0.8 m and
in front of the seawall from the seawall to 1.5 m seaward of the seawall. The evaluation of the seawall
failure effect also included the comparison with the volume changes from the cases of no failure (B2 and
B3) and no seawall (N1 and N2) experiments. The initial profile for the no seawall experiments (N1 and
N2) is shown in Figure 9. Figure 10 shows this comparison for the volume changes computed for the
F-series and the cases of no failure and no seawall experiments at the end of the peak surge level and at
the end of the experiment. Figure 11 shows the comparison of erosion patterns of profiles from various
seawall failures, no failure, and no seawall case for the area from -1 m behind seawall to 4 m in front of
the seawall.
3.7.1 Volume Difference from the Initial Profile
In Figure 10, four subfigures are shown. The upper two plots are for regular waves and the
lower two plots depict random wave cases. The volume changes shown are the ones at the end of the
peak surge level (4.2 model hours) and at the end of the experiment (7.8 model hours).
(1) Regular waves: For the no failure case (B3), slight accretion behind the seawall and a large
amount of erosion in front of the seawall were observed. As the degree of failure of seawall increases, the
amount of erosion increases behind the seawall but decreases in front of the seawall. For the case of no
seawall (N1), a moderate erosion behind the seawall and a least amount of erosion in front of the seawall
were observed.
End of Peak Surge Level
Regular Wave EXperiment. Behind Seawall.
4.2 hours elapsed (model) 0 Front of Seawall
1/4-Failure
(F5)
1/2-Failure
(F3)
Total Failure No Seawall
(Fl) (N1)
End of Peak Surge Level
Random Wave Experiment
4.2 hours elapsed (model)
No Failure 1/4-Failure 1/2-Failure Total Failure No Seawall
(B2) (F6) (F4) (F2) (N2)
End of Experiment
Random- Wave- Experiment
7.8 hours elapsed(model)
Figure 10: Sand volume difference from initial versus seawall failure types.
-1-.5
1, r%
No Failure
(B3)
1.0
S0.5
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model distance from seawall [m]
Figure 11: Comparison of erosion patterns from various seawall failure and no seawall experiments.
0 :_NG -.. ,
seNormal Seawall:3 Falure:F1 - Failure:F3 No Sawall:NI
15
1 0 -
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
End of Experiment
Regular Wave Experiment
7.8 hours elapsed (model)
(2) Random waves: Behind the seawall, the scour depth was seen to increase with the degree
of larger failure of the seawall. In front of the seawall, the scouring observed did not clearly exhibit
correspondence between degree of seawall failure and amount of material eroded. However, the no failure
case (N2) displays the most erosion in front of the seawall while others show either small erosion or even
some accretion (F6).
3.7.2 Erosion Patterns
(1) Regular Waves
Behind Seawall: At 4.2 hours the case of no failure of seawall (B3) displays very slight accretion
(Figure 11). The case with no seawall (N1) shows moderate erosion, and the failure cases Fl (total
failure) and F3 (1/2-failure) show more erosion than the normal seawall case (B3). At the end of the
experiment there is nearly no erosion for the normal seawall case, and moderate erosion for the case of
no seawall, but large erosion for both cases of the total and 1/2 failures.
Front of Seawall: At 4.2 hours the no failure experiment (B3) displays more erosion than all
other cases in comparison. From 4.2 hours to 7.8 hours, all the cases compared were seen to continue the
erosion pattern but with much slower rate of erosion.
(2) Random Waves
Behind Seawall: At 4.2 hours the normal seawall case shows little erosion while all the failure
cases and the case of no seawall show extensive erosion. The erosion patterns from all the cases compared
remain unchanged from 4.2 hours to 7.8 hours.
Front of Seawall: Both failure cases and the case of no seawall show similar mild erosion while
the no failure case has significant erosion in the area from seawall to 1.5 m seaward of the seawall.
3.8 Effects of Sediment Size and Wind
The effects of sediment size and additional wind input, besides the normal wave forcing, were
investigated by comparing the scour depths in front of seawall from the regular wave experiments A2,
A3, A4, and B1. Among these four experiments, only A4 includes the additional stimulus of wind in the
Depth of Scouring at Front of Seawall 4.8 Hours Elapsed
A2 A3 A4 BI
Depth of Scouring at Front of Seawall 9.0 Hours Elapsed
I VA
*No data for A4 at 9,0 hours
Figure 12: Scour depths at seawall toe for experiments A2, A3, A4, B1.
experiment. Figure 12 compares these measured scour depths from the four experiments, which were all
conducted based on a 16.8 cm peak storm surge, a 22.34 cm seawall, a 1.65 second wave period and a
16 cm deepwater wave height. The experiments A2, A3, and A4 have a sediment diameter of 0.18 mm
while B1 has a 0.09 sediment diameter. At 4.8 hours all these experiments display moderate erosion just
seaward of the seawall. However, at 9 hours A2 and A3 exhibit slight accretion while the fine sediment
experiment B1 has shown the greatest erosion in front of the seawall.
3.9 Effect of Wave Period
The effect of different wave periods has been shown by comparing the experiments C3 and C4
for regular waves, and C5 and C6 for random waves in Interim Report #2.
0.02
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fn fna
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a
15 seawall
/ location End of Peak Surge Level
10 Regular Wave Experiment
4.2 hours elapsed (model)
35 -
05
10
Normal Seawall:B3 ---- Geotextle:B0 ---- Concr.te:C8
15 I I I I I I
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Figure 13: Effects of seawall toe protection an offshore reef.
3.10 Effect of Seawall Protection and Artificial Reefing
The effects of seawall protection and nearshore reef were examined from the experiments BO
and C8, respectively. The experiments BO has protected the seawall toe by placing small rocks (5 cm in
diameter) above geotextiles in the area from seawall to 0.5 m (12.5 m prototype) seaward of the seawall,
while C8 uses poured concrete in the area 1.4 m to 2.4 m (35 m to 60 m prototype) seaward of the seawall
to simulate a nearshore reef (see Figure 4).
Figure 13 shows the modelled beach erosion patterns resulted from the BO, C8 and the normal
seawall (B3) experiments. The model results are found to be similar between experiments C8 and B3.
The use of the artificial reef in C8 did not improve model performance in terms of seawall protection.
On the other hand, the experiment BO exhibits good protection to the seawall as evidenced by the very
small amount of scour seaward of the seawall.
0.5 1.0 1.5 2.0 2.5 3.0
3.5 4.0
4 Summary and Conclusions
Seawall model experiments were performed at the University of Florida Coastal and Oceano-
graphic Engineering Laboratory to evaluate the effects of seawalls on adjacent beaches under controlled
extraneous forces. All the experiments completed so far were based on the beach profile modelled after
DNR Range Number R-192 in Highland Beach, Palm Beach County. The seawall model utilized in the
experiment is undistorted in the sense that the horizontal and vertical length scales are equal. The length
scale used in the model is 1:25 (model to prototype ratio) and the corresponding time scale ratio is 1:5
based on a Froudian model law.
In this and previous reports the seawall effects were investigated subject to the conditions of
variation of seawall height, sediment size, storm surge level, and different wave energy levels under
regular and random wave inputs. In summary, three seawall heights (19.9cm, 21,12cm, 22.34cm, NGVD,
in model scale), two sediment sizes (median diameters of 0.09mm, 0.18mm), four different storm surge
configurations, and two different wave energy contents have been utilized in the experiments. Additional
experiments were conducted to examine the effects of seawall failure occurring during the peak storm surge
interval. The seawall failure experiments simulated the seawall failure at three different heights at Ocm,
10.56cm, 15.84cm, NGVD. Also, experiments were conducted for a model beach profile with no seawall
present in the model, and for the models to evaluate the effects of scour protection and a nearshore reef.
Table 1 presents a list of all the experiments completed upon the modelled DNR R-192 beach
profile. Among these experiments, the A-series experiments were based on the coarse sediment size
(0.18mm) whereas the B,C,R,T,F, and N series were based on fine sediment (0.09mm). The B-series are
designed mainly for sensitivity testing of varying seawall heights. The R and T-series repeat some of the
B-series for reproduceability testing of the experiments. The F and N-series are performed for the seawall
failure and no seawall experiments, respectively.
Among the aforementioned experiments, special description is needed for A4, B8, and B9. In A4,
wind was generated in addition to the regular wave input in the model to demonstrate how the combined
wind and wave forcing can influence model behaviors. In B8 and B9, although both seawall and storm
surge configuration are the highest among different seawalls and storm surges tested in the B-series, the
two experiments are equivalent to the condition of lowering simultaneously the seawall, storm surges, and
initial beach profile. Therefore, the experimental results from B8 and B9 can be compared to those from
B6 and B7 for evaluation of the effect of lowering the initial beach profile in the model studies.
Conclusions from this and previous reports are summarized in following:
(1) Seawall Height
Regular waves: B2(22.34 cm) vs. B3(21.12 cm) and B6(19.9 cm)
Behind the seawall, the depth of scour adjacent to the seawall and the nearby erosion are least for
the highest seawall and greatest for the lowest seawall. Erosion patterns behind the seawall are observed
to be dependent upon the initial profile prepared. In front of the seawall, the greatest scour depth occurs
for the lowest seawall at maximum surge level.
Random Waves: B4(21.12 cm) vs. B7(19.9 cm)
Behind the seawall, much greater erosion occurs for the lower seawall case. Again, erosion
patterns depend on the initial profile. In front of the seawall, mild erosion similar to that in the regular
wave experiments is observed.
(2) Sediment Size
A2,A3(0.18mm) vs. Bl(0.09mm)
In front of the seawall, both the coarse sand and fine sand exhibit the similar mild erosion pattern
at the end of maximum surge level (4.8 hours). However, from the end of the maximum surge level to
the end of the experiment (9 hours), the coarse sand shows a significant build-up in front of the seawall
while the fine sand still displays the mild erosion pattern. In our case, the fine sand model is deemed to
simulate the Highland Beach better since the Highland Beach sand has a diameter of 0.21 mm which is
more closely simulated by the fine sand of 0.09 mm.
(3) Seawall Failure
Regular Waves: B3(No fail) vs. F5(1/4 fail) vs. F3(1/2 fail) vs. Fl(total fail) vs. Nl(no seawall)
In front of the seawall, the trend of erosion depth is to be greatest for a no failure condition and
decreases with increasing degree of failure. In the case of no seawall erosion pattern is mild. Behind the
seawall, the trend displays that erosion depth is least for the no failure case and increases to the condition
of total failure. The no seawall case shows mild erosion.
Random Waves: B4(no fail) vs. F6(1/4 fail) vs. F4(1/2 fail) vs. F2(total fail) vs. N2(no seawall)
Conclusions are similar to the regular wave experiments.
(4) Storm Surge Level
Coarse Sediment: Al(14.4cm) vs. A2(16.8cm)
Fine Sediment: B2(14.4cm) vs. B9(16.8cm)
Greater water overtopping seawall rates accompany higher surge levels. For the case of coarse
sediment, a higher surge level produces a greater depth of erosion behind the seawall. In front of the
seawall, scour is observed only during the maximum storm surge level with the higher storm surge
exhibiting deeper scour. The fine sediment experiments display erosion patterns simliar to the coarse
sediment experiments. Erosion patterns are mild in front of the seawall and extensive behind the seawall.
(5) Lowering of the Initial Profile
Regular Waves: B6(unchanged) vs. B9(lowered 0.48cm)
Random Waves: B7(unchanged) vs. B8(lowered 0.48cm)
In Interim Report #3, it concluded that lowering the initial profile has little influence on the
profile evolution given the time-varying severe sea conditions modelled in the experiments. Similar erosive
trends (mild scour landward and seaward of the seawall) occurred in all four experiments but more erosion
behind the seawall was observed in Experiment B8.
(6) Wind
A4(wind, regular waves) vs. A3(regular waves)
A larger water overtopping seawall rate is observed with wind because wind adds extra energy to
the prevailing waves and carries water splashed upward over the seawall. Consequently, greater erosion
and scour depth were observed behind the seawall during the maximum surge level in A4 than A3.
(7) Wave Characteristics
(i) Regular vs. Random Waves
B3(regular) vs. B4(random), at 21.12 cm seawall height
B6(regular) vs. B5(random), at 19.9 cm
B9(regular) vs. B8(random), at 22.34 cm
Scour depths in front of and behind the seawall are greater in the case of regular waves because
the regular wave experiments provide slightly more energy than corresponding random wave cases.
(ii) Wave Energy Content
B5(15.0cm) vs. B7(16.0cm), both random wave experiments
Scour depths are greater in B7 which contains greater wave energy.
(iii) Wave Period
Regular: C3(1.65 s) vs. C4(1.3 s)
Random: C5(1.65 s) vs. C6(1.3 s)
Interim report #2 concluded that the effect of different wave periods is relatively insignificant.
(9) Seawall Toe Protection and Offshore Reef
Seawall toe protection results in improved model performance as evidenced by a significantly
decreased scour depth seaward of the seawall. Nearshore reef does not improve model performance in
terms of erosion adjacent to the seawall.
The addition of a seawall to a beach results in an increased amount of erosion in front of and
a decreased amount behind the seawall as compared to a beach with no seawall. Thus the area behind
the seawall is afforded some protection from erosion. This decrease in erosion behind the seawall as
compared to the no seawall case is most dramatic under the condition of regular waves with a high
seawall. If the seawall survives a storm (no failure condition), it will protect the portion of the beach
behind the seawall but can cause substantial erosion in front of it. The major impact of seawall failure
is evidently an increased level of erosion behind the seawall. This erosion behavior is readily observed in
both the regular and random wave experiments. The depth of erosion immediately behind the seawall
is proportional to the degree of failure in the seawall. A completely failed seawall can produce more
significant erosion than the no fail case.
5 References
[1] Dean, R.G., Chiu, T.Y., and Wang, S.Y., 1992. Combined total storm tide frequency analysis for
Palm Beach County, Florida, Division of Beaches and Shores, Department of Natural Resources, 58pp.
[2] Lin, L., Zheng, J., and Dean, R.G., 1994. Evaluation study and comparison of erosion models and
effects of seawalls for CCCL, Physical Modelling Program Report- Interim Report 4. COEL-94/020,
University of Florida, Gainesville, Florida.
[3] Thompson, L.L., Lin, L., and Dean, R.G., 1993. Evaluation study and comparison of erosion models
and effects of seawalls for the CCCL, Physical Modelling Program Report- Interim Reports 1 and 2.
COEL-94/003, University of Florida, Gainesville, Florida.
[4] Thompson, L.L., Lin, L., and Dean, R. G., 1993. Evaluation study and comparison of erosion models
and effects of seawalls for the CCCL, Physical Modelling Program Report- Interim Report 3. COEL-
94/005, University of Florida, Gainesville, Florida.
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