Citation
Evaluation study and comparison of erosion models and effects of seawalls for coastal construction control line

Material Information

Title:
Evaluation study and comparison of erosion models and effects of seawalls for coastal construction control line interim report #5 and summary of previous results
Series Title:
UFLCOEL-94021
Creator:
Demas, Carol
Lin, Li-Hwa
Dean, Robert G ( Robert George ), 1930-
University of Florida -- Coastal and Oceanographic Engineering Dept
Florida -- Dept. of Environmental Protection
Place of Publication:
Gainesville Fla
Publisher:
Coastal & Oceanographic Engineering Dept., University of Florida
Publication Date:
Language:
English
Physical Description:
iii, 26 leaves : ill. ; 28 cm.

Subjects

Subjects / Keywords:
Storm surges -- Mathematical models ( lcsh )
Coastal engineering -- Mathematical models ( lcsh )
Sea-walls -- Models ( lcsh )
Genre:
bibliography ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaf 26).
General Note:
"November 1, 1994."
Statement of Responsibility:
by Carol Demas, Lihwa Lin, Robert G. Dean ; prepared for Department of Environmental Protection, State of Florida.

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
33143087 ( oclc )

Full Text
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.
November

Dean

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
2 Experimental Conditions

2.1 Laboratory Facility

Scaling

. . . . . .4

2.3 Initial Beach Profile and Seawall Configuration

2.6 Wave Conditions

. 8

2.7 SeawallFailure~onditions. . . . . .. . . . . . . .. ..t .

2.8 Seawall Toe Protection and Nearshore Reef

. 8

. 9

3 Compassion of Experimental Results

3.2 EffectofDifferentStormSurgeLevel. ....*.... ...... .

* . . . . .
. . . .

3.3 Effect of Regular versus Random Waves

3.4 EffectofoweredProfle. ... .... .. .. .. .

3.5 Effect of Energy Level of Random Waves

. . . ..




Depth of Scouring at Seawall . . . . . . . . . . . . .
Water Overtopping SeawallRate . . . . . . . . . . ..

Scour Depth

as a function of Overtopping Rate During Maximum Surge

Effect of Seawall Failure

Volume Difference from the Initial Profile

Erosion Patterns

Effects of Sediment

Size and Wind

Effect of Wave Period

3.10 Effect of Seawall Protection and Artificial Reefing.

. .* .
. .9

Summary and Conclusions References

List

of Tables

Summary of experimental cases and test parameters. Experiments compared in the present and previous reports.

List

of Figures

Schematic diagram of wave tank facility. . . . . . .




Continuous and stepped simulation of storm surge #2.

Initial profiles for seawall toe protection and nearshore reef experiments. Scour depth at seawall in relation to initial from B-series experiments. Scour depth in relation to seawall from B-series experiments. . Measured water overtopping seawall rate versus storm surge level. Scour depth versus overtopping rate during maximum surge level..

. .
. .

. .
. .*
. .9

Initial profile for no seawall experiment.

Sand volume difference from initial versus seawall failure types.

Comparison of erosion patterns from various seawall failure and no seawall experiments.

Scour depths at seawall toe for experiments A2, A3, A4, B..

Effects of seawall toe protection an offshore reef.




Evaluation Study and

Comparison of Erosion Models and

Effects of Seawalls for Coastal Construction

Interim Report #5

Scope of This

Control Line

- Physical Model Results

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. on adjacent beaches under controlled forcing conditions

The modeling examines the effects of seawalls 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.

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,

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




Table 1: Summary of experimental cases and test parameters.

water level condition

stepwise varying storm surge
model peak level[cm

seawall condition
plan, vertical; model elevation
[cm]
21.12 22.34 19.5c

sediment median diameter
"mm]

wave characteristics

wave type

regular
wavea

random waves

#2

deepwater wave height
[cm]

wave
period [s]

water volume overtopping seawall 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
Fl" 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

Experiments compared in the present and previous reports.

Interim Report Varied Experimental Parameters J Experiments Compared
,, [mo del units]
1 sediment size [0.18 mm] vs. [0.09 mm] A2 B1
2 I wave period [1.65 s] vs [1.3 s] C3 C4 C5 C6
I___ wave type [regular] vs. [random] I
3 storm surge peak [14.4 cm] vs. [16.8 cm] Al A2 B2 B9
wave type [regular] vs. [random] B6 B7 88 B9
initial profile [regular] vs. [lowered] B6 B7 B8 B9
wave energy [Ho=15 cm] vs. [Hto=16 cm] B5 B7
4 seawall failure [1/2, total failure] F1-F4
seawall status [regular seawall] vs. [no seawall] B3 B4 N1 N2 wave type [regular] vs. [random] N1 N2
B3 B4
F1 F2
F3 F4
5 seawall failure [1/4, 1/2, total failure] F1-F6
seawall status [regular seawall] vs. [no seawall] B3 B4 N1 N2
seawall height [19.9 cm, 21.12 cm, 22.34 cm] B2-B8
wind vs. no wind A2 A3 A4
sediment size[0.18mm] vs. [0.09mm] A2 B1
wave type [regular] vs. [random] N1 N2
B3 B4
F1 F2
F3 F4
F5 F6
seawall toe protection vs. no protection BO B3
artificial reef vs. no reef CO B3




Experimental

Conditions

Laboratory Facility
The laboratory experiments were conducted in the air-sea tank at the University of Florida

Coastal and Oceanographic Engineering Laboratory. long, 0.9 m wide, and 1.2 m high (Figure 1). The tai

The tank section used for the experiments is 37 m nk 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 distan

ces 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 alloted (Thompson, et al., 1993).

Two capacitance type wave gauges were installed in the tank for monitoring waves

is located 18.3 m (457.5 m prototype) seaward of the seawall and Gage

- Gage

is located 5.3 m (132.5 m

prototype)

seawar

d 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




Powered
Fan

-3.0
- 1.5

Wave Bed "' Location of
Gauge Gauge Sil

CROSS-SECTION

-3.4 m k Wavemaker

36.6 m

Wave Screens

Carrage for Wave Gauge and Profiler

Location of

Seawall

t 5.8 m--+

Basin

1.8 m

'ower

Drain Valves

Wave Gauge

Tank

Divider

Wave Bulkhead
Gauge

PLAN VIEW

Figure 1: Schematic diagram of

wave

tank facility.




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. model profile is presented in figure 2.

The profile coincides with DNR Range Number R-192.

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, Sea wall 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

?, at 22.34 cm (5.59 m prototype), is 1.22 em (model) higher than

Seawall 1 and Seawall 3, at 19.9 cm (4.98 m prototype

)is1

.22 cm (model) lower.

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 5is

a time-invariant water level which is modelled by fixing the surge water level (SWL) at 0.144 m (3.6 m




0.25 0.20 0.15 0.10 0.05
0.00 --0.05
-0.10
-1.0

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
model distance from seawall [meters]

Figure

Initial profile, modelled after DNR profile R-192,

and seawall #1.




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.

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

is 1.3 seconds in the model

.5 sec

onds 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.

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)

* I WI p-n rr E. p-n rrr p 11 I 1 1 .1




0.30

0.25
0.20 0.15
0,.1..0.05
-0.00-0.05
-0.10
-0.15
-1.0

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Distance

Figure 4: Initial profiles for seawall toe protection and nearshore reef experiments. Seawall Toe Protection and Nearshore Reef Experiments BO and C8 were conducted to determine the effects of seawall toe protection and

nearshore reefs, respec

tively. 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 H0 and C8.

Comparison

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




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.

Effect of Energy Level of Random

Waves

The effect of different energy level of random waves is investigated in a comparison of Experiments B5 versus B7 (deepwater 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 among Experiments B2, B3, B6, and

seawall toe and immediately behind the seawall were compared 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




0.10 0.05
-0.00
-0.05
-0.10
-0.15

0.10 0.05
-0.00
-0.05.
-0.10
-0.15
0.10 0.05
-0.00
-0.05
-0.10
-0.15

0.05
-0.00
-0.05
-0.10

End of Experiment Random Wave Experiment 7.8 hours elapsed(inodel).

-. rAaj

1




0.15
0.10 0.05
0.00
-0.05

B9*.**

0.10 0.05 0.00
-0.05

0.20 0.15
0.10 0.05 0.00
-0.05

B4**

0.15
0.10 0.05
0.00

End of Experiment Random Wave Experiment 7.8 hours elapsed(model)

NaVfl

* I I I I I




(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 hours. However, the scouring depth becomes moderate in front of the seawall for B2,

results slight accretion for B9 at 7
(2) Random Waves At 4.2

that in B3 at 4.2

and B6 while

hours.

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.

experiment), the erosion depth

At 7.8 hours

is decreased markedly for all Experiments B4, B5, B7

(the end of the

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 only about 6 gallon/minute at the same surge level.

wave

case

(B4) has an overtopping rate of

(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

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

10. 11. 12. 13. 14. 15. 16.

Seawaf

ifeight

21.1

10. 11. 12. 13. 14. 15. 16.

Seawall

Height

22.34

10. 11. 12. 13. 14. 15. 16.

cm




Behind Seawall Front of Seawall

0.15
0.10 0.05
0.00
-0.05
0.0

10.0 20.0 30.0

40.0

Overtopping

Rate

During Maximum Surge

[Cal/min]

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

S. *' *1 fl~ .3 .., .3.. S. **3 .3 .




Initial

Profile

Elevation

No Seawall

Cases

0.30

0.15
0.10 0.05
-0.00
-0.05
-0.10-0.15
-1.0

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

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.

N2) is shown in Figure 9.

The initial profile for the no seawall experiments (N1 and

Figure 10 shows this comparision 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 in

creases,




-1.0

No Failure 1/4-Failure 1/2-Failure Total Failure No Seawall

-1.0

No Failure 1/4-Failure (/2-Failure Total Failure No Seawall
(B2) (F6) (F4) (F2) (N2)

-0-.5"
-1.0
-1.5
n-hfl

End of Experiment
Regular Wave Experiment
7.8 hours elapsed (model)
-




0.15 I 0.10
0.05
-0.00
-0.05
-0.10
-0.15-1.0
0.15 0.10 0.05
-0.00
-0.05
-0.10
-0.15
0.15 0.10
0.0
-0.00
-0.05
-0.10
-0.15
-1.0
0.15 '0.10 0.05
-0.00
-0.05 '
-0.10
-0.15-inA

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Clfl n; 10 i; 20 2~ Rn

CL 'I

.




(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

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.

Effects of Sediment Size and Wind




Depth

Scouring

at Front

Seawall

4.8 Hours

Elapsed

A-0.02
-0.04
Le
Vn -0.06
-0.08

A2 AS A4 B1

Depth

Scouring

Front

Seawall

Hours

Elapsed

-0.02
-0.04
-0.06
-0.08

Figure 12: Scour depths at seawall toe for experiments A2, A3, A4, Bi.

Figure 12 compares these measured scour depths from the four experiments, which were all

conducted based on a 16.8 cm peak storm surge,

16 cm deepwater wave height.

a 22

.34 cm seawall, a 1.65 second wave period and a

The experiments A2, A3, and A4 have a sediment diameter of 0.1

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.

Effect of Wave Period

- *No data for A4 at 9.O hours

experiment

MUll




0.15
0.10 0.05
-0.0
-0.05
-0.10
-0.15-1.0
0.15-

0.05
-0.00
-0.05
-0.10

-0.15
-1.0

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

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

>textiles 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 to simulate a nearshore reef (see Figure 4).

.4 m (35 m to 60 m prototype) seaward of the seawall

Figure 13 shows the modelled beach erosion patterns resulted from the BO,

seawall (B3) experiments.

C8 and the normal

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.




Summary and

Conclusions

Seawall model experiments were performed at the University of Florida Coastal and Oceanographic 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 0cm,

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 aformentioned 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




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)

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 In front of the

i)

vs. B1(0.09mm)

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)

F5(1/4 fail)

F3(1/2 fail)

vs. Fl(total fail)

N1(no seawall)

In front of the seawall, the trend of erosion depth is to be greatest for a no failure condition and
Il "flfl ** r1 1 0 11 U r% .u




Random Waves: B4(no fail)

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

Coar: Fine

Sediment: A1(14.4cm)

Sediment: B2(14.4cm)

vs. A2(16.8cm) 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) Random Waves: B7(unchanged)

vs. B9(lowered 0.48cm) 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 c

onditions modelled in the experiments. Similar erosive

trends (mild scour landward and seaward of the seawall) occured 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.




B3(regular) B6(regular) B9(regular)

B4(random), B5(random), B8(random),

at 21.12 cm seawall height at 19.9 cm 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) Random: C5(1.65 s)

vs. C4(1.3 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

decreased scour depth seaward of the seawall.

as evidenced by a significantly

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 de

crease

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. is evidently an increased level of erosion behind the seawall. This e

The major impact of seawall failure rosion behavior is readily observed in




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,

., Zheng,

., 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. University of Florida, Gainesville, Florida.

COEL-94/020,

[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

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. 94/005, University of Florida, Gainesville, Florida.

COEL-