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Group Title: UFLCOEL-95001
Title: Evaluation study and comparison of erosion models and effects of seawalls for coastal construction control line
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 Material Information
Title: Evaluation study and comparison of erosion models and effects of seawalls for coastal construction control line task 3, generic modelling of seawall overtopping and associated scour : summary report
Series Title: UFLCOEL-95001
Alternate Title: Task 3, generic modelling of seawall overtopping and associated scour
Generic modelling of seawall overtopping and associated scour
Physical Description: 19 leaves : ill. ; 28 cm.
Language: English
Creator: Dean, Robert G ( Robert George ), 1930-
University of Florida -- Coastal and Oceanographic Engineering Dept
Florida -- Dept. of Environmental Protection
Publisher: Coastal & Oceanographic Engineering Dept., University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1995
 Subjects
Subject: Storm surges -- Mathematical models   ( lcsh )
Coastal engineering -- Mathematical models   ( lcsh )
Sea-walls -- Models   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: by Robert G. Dean ... et al. ; prepared for Department of Environmental Protection, State of Florida.
Bibliography: Includes bibliographical references.
General Note: Cover title.
General Note: "January 3, 1995."
 Record Information
Bibliographic ID: UF00085014
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 33143138

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Title Page
        Title Page 1
        Title Page 2
    Main
        Page A-1
        Page A-2
        Page A-3
        Page A-4
        Page A-5
        Page A-6
        Page A-7
        Page A-8
        Page A-9
        Page A-10
        Page A-11
        Page A-12
        Page A-13
    The CCCL erosion model: Basis and characteristics
        Page B
        Page B-1
        Page B-2
        Page B-3
        Page B-4
    Seawall performance characteristics
        Page C
        Page C-1
        Page C-2
Full Text









UFL/COEL-95/001


Evaluation Study and Comparison of Erosion Models and
Effects of Seawalls for Coastal Construction Control Line





Task 3
Generic Modelling of Seawall Overtopping and Associated
Scour: Summary Report

by

Robert G. Dean
Lihwa Lin
Lynda Charles
Jie Zheng
and
Carol Demas


January 3, 1995


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



Summary Report



by

Robert G. Dean
Li Hwa Lin
Lynda Charles
Jie Zheng
and
Carol Demas


January 3, 1995






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


INTRODUCTION

This report provides a summary of the testing program
conducted to evaluate various characteristics of the two
dimensional interaction of seawalls with the adjacent seaward and
landward areas. Most of these results have been presented in a
series of five reports published and distributed earlier (see
REFERENCES). As the results are discussed, the reports in which the
detailed results are available will be noted. These reports usually
provide graphs portraying the profile evolution for the particular
test conditions. The intent of this report is to provide a
synthesis of the testing program and results.

This report is organized as follows. The next section
introduces the conditions tested, the modelling relationships and
the general scope of the experiments. The following (third) section
provides an overview of the test program and results. The following
section provides an overall summary and recommendations.

OVERALL TEST PROGRAM

Facilities

All of the laboratory tests were conducted in the large air-
sea tank of the Coastal Engineering Laboratory of the Department of
Coastal and Oceanographic Engineering Department of the University
of Florida. The general characteristics of this facility are shown
in Figure 1. The tank is 37 m long, 0.9 m wide and 1.2 m high and
is equipped with a programmable wavemaker. In addition, a fan is
located at the wavemaker end of the facility to direct wind over
the waves to simulate the effects of wind driven seas. The testing
program can be viewed as a modelling of several generic conditions
and also testing of several particular seawall situations.

Sand Characteristics

Two sizes of sand (0.18 mm and 0.09 mm) were used in the
testing program, although the smaller sand is scaled specifically
to approximate prototype sizes and was used in all but four of the
tests.

Wave Characteristics

The testing program evaluated the effects of both regular and
irregular waves. For the program here, the significant heights of
the irregular waves were the same as the regular wave heights.
Although these heights were the same, the energy levels and the
average heights are quite different. A discussion of the

















red
"n\ 3.




Wavemaker \ Sand Wave
Wave Bed aeat Ion of
Gauge Gauge SeawaU
CROSS-SECTION


PLAN VIEW


Figure 1. Schematic Diagram of Wave Tank Facility


Powe
Fal


Wavemaker


1.8 m








quantitative differences may assist in the interpretation of
differences in experimental results for the two types of wave
systems. For the wave height equivalency noted, the energy of the
irregular wave system is one-half that of the regular waves and the
average height of the irregular waves is 63% that of the regular
waves. Deep water wave heights of 12.3 ft. and 13.1 ft. and wave
periods of 6.5 sec and 8.3 sec were tested.

Storm Surges

Two levels of storm surges were tested with peaks of 11.8 ft
and 13.8 ft. The time variations of the storm surges were patterned
after the 100 year storm surge values developed for the CCCL in
Palm Beach County (Dean, Chiu and Wang, 1992). The predicted peaks
for Palm Beach County ranged from 11.1 ft. to 11.6 feet.

Seawall and Profile Conditions

Results presented in this report are for a vertical seawall
with the beach and nearshore profile approximately that at Range R-
192 in Palm Beach County. Three seawall crest elevations were
tested including: 16.3 ft., 17.3 ft., and 18.3 ft. The seawall
crest elevation at R-192 is 18.2 ft.

Model Scale

A nominal undistorted scale of 1:25 was adopted for the
testing program such that all linear dimensions in the model are
one twenty-fifth those in the prototype and, because Froude
criteria govern wave models, the time and velocity scale according
to the square root of the length scale, resulting in scales of 1:5,
that is, times and velocities in the model are one fifth those in
the prototype. The fall velocity of the sand was scaled in
accordance with the velocity scale resulting in a median sand size
of 0.09 mm which is about as small as can be scaled without concern
over cohesive forces.

Tests Conducted

The overall testing program is summarized in Table 1. A total
of 36 tests has been completed or are in the process of testing or
analysis and reporting. This report summarizes the results for 28
of these tests. In general, testing has evaluated toe scour, lee
scour, overtopping rates and regular and irregular waves. In
addition, the extent and rate of upland erosion due to various
degrees of seawall failure were examined.

OVERVIEW OF PROGRAM AND RESULTS

This section presents results of the interaction
characteristics of the seawall and adjacent topography, in
particular toe and lee scour characteristics.





















Table 1


Summary of Experimental Conditions


water level condition eawall condition sediment wave characteristic water
stepwse varying const. plan, vertical; median wave tpe dfepwr t r wave I volume
exp. storm surge level model elevation diameter r o wave height period over-
ID# model peak level[cm] [cm [cm waves wave wa cm topping
614.4 14.4 211ig ct122ai.4 1 ve.ticalg 1m ia iv05 1 15 11w 1e .31- -awal
# 1 #2 ip*3 IF5 I 41 #2 13 i I 2 #-r-3 *1 #2 *1 m7-em.ured
Al X X X X X X
-A--x----- ---x-- --- -x ----- ~--- "x- -
A2 X X X X X X
A3 X 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
B X X X x x x X
Be 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
C4 X X X X X X X
Cs xx x x X x X X
C5 X X X X X X X
C X__X X X X X X

cT X X X X X X
=N X X X X X -
M- X --X------ ---- X --Y----- -- X
R4 X X X X X X
-Ni X X X X X---
N2 X X X X X
F3' X X x x X
FI2 X X X X X X
F32 X X X X X X
F4 X X X X X X
FS" 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 o artificial reefing.
* F-series simulated seawall failure during the highest storm surge level.









Toe Scour


Toe scour, the localized erosion of material at the base of
the seawall due to elevated tide and wave conditions, is important
to seawall design in order that the seawall be stable under storm
action. There is not a consensus of the cause of scour in the
engineering community although scour generally occurs under storm
conditions. Dean (1986) has suggested that scour is a direct result
of and is quantitatively related to the erosion that would occur
behind the seawall if it were not present. As a result, the system
appears to erode the sand from as near to where it would have been
removed if the seawall had not been present. A number of
relationships have been proposed for predicting scour for design
purposes. A good review is provided in Fowler (1993) who concluded
that the upper limit of scour depth is the deep water wave height.
Toe scour was quantified in those experiments indicated in Table 2.

Effect of Sand Size (Report 1) Comparing the scour resulting from
Experiments A2 and B1 which have the same conditions except for
sand size, it is seen that the finer sand experiences greater toe
scour. Specifically, the toe scour for Experiment A2 is 4.9 feet
for a sand size of 0.18 mm and for Experiment B1 is 5.8 feet for a
sand size of 0.09 mm.

Effect of Regular and Irregular Waves (Report 4) The evaluation
of the effects of regular versus irregular waves on toe scour is
based on a comparison of the results from Experiment B3 (regular
waves and a toe scour of 8.2 feet) and Experiment B4 (irregular
waves and a toe scour of 6.2 feet). The greater toe scour due to
regular waves is believed to be due to the fact that although the
waves had the same nominal wave height, as noted earlier, the
average height of the irregular waves is 63% of the regular wave
height. However, the higher waves in the irregular wave system are
higher than the regular waves, specifically, by definition the
highest 33% of the irregular wave heights are greater than the
regular wave height. Thus, the irregular waves have a greater
erosion potential; however, the rate of erosion is less than that
for the regular waves.

Lee Scour

The magnitude and extent of scour behind a seawall clearly
depend on several variables, including wave, seawall and profile
characteristics. In addition to the magnitudes of the waves and
storm tides, the duration of the storm conditions is relevant. As
a first measure of the rate at which scour would progress, the
freeboard, FB, is defined as







Table 2
SUMMARY OF WAVE TANK TEST RESULTS


Exp. Sed. Storm Sea Wave Wave Reg. Max. Max. O/T
No. Size Surge Wall Ht. Per. or Toe Lee Meas? Comments
(mm) Peak Crest (ft) (sec) Irr. Scour Scour
(ft) (ft) (ft) (ft)

Al 0.18 11.8 18.3 13.1 8.3 R 1.2 1.2 N

A2 0.18 13.8 18.3 13.1 8.3 R 4.9 2.5 N
A3 0.18 13.8 18.3 13.1 8.3 R Y

A4 0.18 13.8 18.3 13.1 8.3 R Y


BO 0.09 11.8 17.3 13.1 8.3 R 2.5 0.8 N Offshore Reef

B1 0.09 13.8 18.3 13.1 8.3 R 5.8 N

B2 0.09 11.8 18.3 13.1 8.3 R 4.1 2.9 N

B3 0.09 11.8 17.3 13.1 8.3 R 8.2 0.0 N
B4 0.09 11.8 17.3 13.1 8.3 Irr 6.2 0.0 N

B5 0.09 11.8 16.3 13.1 8.3 Irr 5.7 0.0 Y

B6 0.09 11.8 16.3 13.1 8.3 R 8.2 8.6 Y

B7 0.09 11.8 16.3 13.1 8.3 Irr 4.1 6.6 N

B8 0.09 13.8 18.3 1% 8.3 Irr 5.7 9.4 N

B9 0.09 13.8 18.3 13.1 8.3 R 3.7 7.8 N









Table 2 (Continued)


SUMMARY OF WAVE TANK TEST RESULTS

Exp. Sed. Storm Sea Wave Wave Reg. Max. Max. O/T
No. Size Surge Wall Ht. Per. or Toe Lee Meas Comments
(mm) Peak Crest (ft) (sec) Irr. Scour Scour ?
(ft) (ft) (ft) (ft)

C3 0.09 11.8c 18.3 13.1 8.3 R 4.1 0.3 Y
C4 0.09 11.8c 18.3 13.1 6.5 R 4.9 0.3 Y

C5 0.09 11.8c 18.3 13.1 8.3 I 3.3 8.9 Y Storm Surge Const
C6 0.09 11.8c 18.3 13.1 6.5 I 4.1 8.9 Y Storm Surge Const
C8 0.09 11.8 17.3 13.1 8.3 R N

T3 0.09 11.8 17.3 13.1 8.3 R N
R3 0.09 11.8 17.3 13.1 8.3 R Y

R4 0.09 11.8 17.3 13.1 8.3 I Y
N1 0.09 11.8 17.3 13.1 8.3 R NA 5.3x82* N Natural Beach

N2 0.09 11.8 17.3 13.1 8.3 I NA 6.6x82 N Natural Beach
Fl 0.09 11.8 17.3 13.1 8.3 R 2.1 9.0x80 N 100% Seawl. Fail
F2 0.09 11.8 17.3 13.1 8.3 I 0.0 6.6x85 N 100% Seawl. Fail

F3 0.09 11.8 17.3 13.1 8.3 R 2.1 7.4x85 N 50% Seawl. Fail
F4 0.09 11.8 17.3 13.1 8.3 I 0.0 6.6x82 N 50% Seawl. Fail

F5 0.09 11.8 17.3 13.1 8.3 R N 25% Seawl. Fail

F6 0.09 11.8 17.3 13.1 8.3 I N 25% Seawl. Fail









H
FB = Zsw ( Zw, + )


in which Z,, is the elevation of the seawall, Z, is the elevation
of the water level, and H, is the wave height and for purposes here
the heights in Table 2 are used. The values of the freeboard ranged
from -0.1 to -2.1 and are presented in Table 3. The greatest
tendency for overtopping and lee scour occur for the more negative
values of FB.

Effect of Regular and Irregular Waves (Report 2) The evaluation
of the effects of regular versus irregular waves on lee side scour
potential are based on a comparison of the results from Experiments
C3, C4, C5 and C6 in which the peak water level of 11.8 feet was
allowed to act for a period of 9 hours prototype. Although the
waves had the same nominal wave height, as noted earlier, the
average height of the irregular waves is 63% of the regular wave
height. However, the higher waves in the irregular wave system are
higher than the regular waves. Specifically, by definition the
highest 33% of the irregular waves have heights greater than the
regular wave height. Thus, the irregular waves have a greater
erosion potential; however, the rate of erosion is less than that
for the regular waves. It is seen that although the freeboard
quantity, FB, was -0.1 for all of these cases (Table 3), the
irregular waves clearly caused greater lee side scour (8.9 ft.)
than the regular waves (0.3 ft.).

Effect of Freeboard Parameter (Table 3) The freeboard parameter,
FB, ranges from -0.01 to -2.1. It can be seen by inspecting the
results in Table 3, that, other factors being the same, there is a
rough dependency of the lee scour on the freeboard parameter.
However, the effects of the individual parameters forming FB do not
all have the same effect on the lee scour. For example, decreasing
the seawall elevation and the storm surge by the same amount would
leave the FB parameter the same, however, inspection of the
experimental results indicates that the effect would be to reduce
the lee scour, presumably due to the fact that the wave height
reduction would still be the same.

Effect of Wave Height (Report 3) The effect of varying wave
height on lee scour for irregular waves is based on comparison of
the results for Experiments B5 and B7, with wave heights of 12.3
and 13.1 feet, respectively. The associated lee scour depths are
0.0 and 6.6 feet, respectively. Thus, as might be expected, the
dependency of lee scour depth on wave height, once the threshold
has been achieved, is fairly substantial.







Table 3


FREEBOARD VALUES


Exp. Storm Sea Wave Wave Reg. Max. Max. O/T
No. Surge Wall Ht. Per. or Toe Lee Meas? FB (ft)
Peak Crest (ft) (sec) Irr. Scour Scour
(ft) (ft) (ft) (ft)

Al 11.8 18.3 13.1 8.3 R 1.2 1.2 N -0.1

A2 13.8 18.3 13.1 8.3 R 4.9 2.5 N -2.1

A3 13.8 18.3 13.1 8.3 R Y -2.1

A4 13.8 18.3 13.1 8.3 R Y -2.1


BO 11.8 17.3 13.1 8.3 R 2.5 0.8 N -1.1

B1 13.8 18.3 13.1 8.3 R 5.8 0.0 N -2.1

B2 11.8 18.3 13.1 8.3 R 4.1 2.9 N -1.1

B3 11.8 17.3 13.1 8.3 R 8.2 0.0 N -1.1

B4 11.8 17.3 13.1 8.3 Irr 6.2 0.0 N -1.1

B5 11.8 16.3 13.1 8.3 Irr 5.7 0.0 Y -2.1

B6 11.8 16.3 13.1 8.3 R 8.2 8.6 Y -2.1

B7 11.8 16.3 13.1 8.3 Irr 4.1 6.6 N -2.1

B8 13.8 18.3 V .A 8.3 Irr 5.7 9.4 N -2.1

B9 13.8 18.3 13.1 8.3 R 3.7 7.8 N -2.1








Table 3 (Continued)


FREEBOARD VALUES

Exp. Storm Sea Wave Wave Reg. Max. Max. O/T
No. Surge Wall Ht. Per. or Toe Lee Meas FB
Peak Crest (ft) (sec) Irr. Scour Scour ? (ft)
(ft) (ft) (ft) (ft)

C3 11.8 18.3 13.1 8.3 R 4.1 0.3 Y -0.1
C4 11.8 18.3 13.1 6.5 R 4.9 0.3 Y -0.1
C5 11.8c 18.3 13.1 8.3 I 3.3 8.9 Y -0.1
C6 11.8c 18.3 13.1 6.5 I 4.1 8.9 Y -0.1
C8 11.8 17.3 13.1 8.3 R N -1.1
T3 11.8 17.3 13.1 8.3 R N -1.1
R3 11.8 17.3 13.1 8.3 R Y -1.1
R4 11.8 17.3 13.1 8.3 I Y -1.1
N1 11.8 17.3 13.1 8.3 R NA 5.3x82* N -1.1
N2 11.8 17.3 13.1 8.3 I NA 6.6x82 N -1.1
F1 11.8 17.3 13.1 8.3 R 2.1 9.0x80 N -1.1
F2 11.8 17.3 13.1 8.3 I 0.0 6.6x85 N -1.1
F3 11.8 17.3 13.1 8.3 R 2.1 7.4x85 N -1.1
F4 11.8 17.3 13.1 8.3 I 0.0 6.6x82 N -1.1
F5 11.8 17.3 13.1 8.3 R N -1.1
F6 11.8 17.3 13.1 8.3 I N -1.1
_ I _ I _ I _ 11 4 _ _ 1 _ __ _






OvertonDina Rate


Overtopping is of general concern to the setting of the CCCL
since, as noted previously, it is the overtopping that is
responsible for the lee side scour. As discussed for the lee scour,
an important parameter for the overtopping rate is the freeboard
parameter, FB. The overtopping rates for the three seawall
elevations considered are shown in Figure 2. It is seen that the
overtopping rates increase rapidly with surge level. Also of
interest is that the threshold rate for overtopping for random
waves occurs at a lower surge level for the intermediate seawall
height of 17.3 feet, but for the other two seawall heights, the
threshold is about the same.

Effect of Seawall Failure

Seawall failure was induced at the mid-time of the peak water
level stage. When the wall was removed, the profile was
significantly out of equilibrium and responded rapidly. Although
the response was not entirely complete by the end of the peak storm
tide (90 minutes after seawall failure), the evolution is judged to
have been approximately 85% complete.

Effect of Toe Scour Protection

In order to simulate toe scour protection, rocks (4 ft in
diameter) overlying geotextile material were placed from the base
of the seawall to a distance of 41 ft. from the seawall (Report 5).
It was found that this protection resulted in less toe scour: 2.5
ft. with protection (Test BO) versus 8.2 ft. without toe scour
protection (Test B3). There was a slight increase in the lee scour
for the case of toe scour protection.

Effect of Offshore Reef

The presence of an offshore reef was simulated by placing
concrete at the elevation of the pre-storm profile starting at 35
feet from the seawall and extending to 140 feet from the seawall.
It was found that the presence of this reef had little effect on
the toe scour or lee scour (Report 5).

SUMMARY AND RECOMMENDATIONS

The results obtained to date have quantified certain
dependencies between the toe and lee scour and the wave, storm
surge and seawall characteristics. The maximum toe scour
encountered in these tests is significant (Test B6, 8.2 ft.) and
could cause failure for some seawall designs. Similarly, the
maximum lee scour depth measured in this program for the case of a
time-varying surge is large enough (Test B6, 8.6 ft.) to cause loss
of back anchoring.

It would be useful to carry out a detailed set of seawall-
profile interaction experiments guided by the results of this
study.








Seawall Height: 16.3 ft.


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9 10 11 12 13


9 10 11
Surge Level, (ft) NGVD


Figure 2. Measured Average Overtopping Rates Versus Storm Surge
Level.


8 9 10 11 12 13







REFERENCES


Charles, L., L. H. Lin, and R. G. Dean (1994) "Evaluation Study and
Comparison of Seawalls for Coastal Construction Control Line.
Interim Report 3", Department of Coastal and Oceanographic
Engineering, University of Florida, Gainesville, FL 32611.

Dean, R. G. (1986) "Coastal Armoring: Effects, Principles and
Mitigation", Proc., Twenty -First International Conference on
Coastal Engineering, American Society of Civil Engineers, pp.1843-
1857.

Dean, R. G., T. Y. Chiu and S. Y. Wang (1992) "Combined Total Storm
Tide Frequency Analysis For Palm Beach County, Florida" Beaches and
Shores Resource Center, Florida State University.

Demas, C., Lin, L. H. and R. G. Dean (1994) "Evaluation Study and
Comparison of Seawalls for Coastal Construction Control Line.
Interim Report 5 and Summary of Previous Results", Department of
Coastal and Oceanographic Engineering, University of Florida,
Gainesville, FL 32611.

Fowler, J. E. (1993) "Coastal Scour Problems and Methods for
Prediction of Maximum Scour", Tech. Rep. CERC-93-8, Coastal
Engineering Research Center, Waterways Experiment Station.

Lin, L. H., J. Zheng and R. G. Dean (1994) "Evaluation Study and
Comparison of Seawalls for Coastal Construction Control Line.
Interim Report 4", Department of Coastal and Oceanographic
Engineering, University of Florida, Gainesville, FL 32611.

Thompson, L., L. H. Lin, and R. G. Dean (1994) "Evaluation Study
and Comparison of Seawalls for Coastal Construction Control Line.
Generic Modelling of Seawall Overtopping and Associated Scour:
Interim Reports 1 and 2", Department of Coastal and Oceanographic
Engineering, University of Florida, Gainesville, FL 32611.
















THE CCCL EROSION MODEL:
BASIS AND CHARACTERISTICS









THE CCCL EROSION MODEL:
BASIS AND CHARACTERISTICS

INTRODUCTION

The CCCL model was developed in the 1981-1982 period and for
purposes of consistency, has not been modified. This model, as one
of a suite of models, has been used in establishing the recommended
location of the Coastal Construction Control Line (CCCL) in 22 of
the 24 coastal counties in the program. It has always been
recognized that at some time in the future, the model should be
updated through modification or replacement to reflect advances in
the state of the art since this model was developed. In fact
research has proceeded at an irregular pace toward the evaluation
of existing models and the development of new models. However, we
have maintained and still maintain that this model provides a
reasonable, appropriate and responsible basis for the purposes for
which it has been employed in the State's regulatory program. There
have been criticisms and questions relating to certain aspects of
the CCCL erosion model. Foremost among these are the application of
the "2.5" factor and the rate constant, K. In the following
paragraphs, the model is first described to provide a basis for the
reader to follow the ensuing discussion of the rationale for
inclusion of the 2.5 factor and the selection of the rate constant,
K.

BRIEF DESCRIPTION OF THE CCCL EROSION MODEL

At the time that the CCCL model was developed, no other models
were available that could be applied readily without requiring
considerable judgement in the preparation of the input data and/or
without the possibilities of instabilities which would raise
questions regarding the output. Thus, in its development, priority
was given to a model which was robust and simple in concept that
could be run with little exercise of judgement in input
preparation.

Input Preparation Because the CCCL model tracks the
displacement of various elevation grids as shown in Figure 1, it
requires a monotonic profile as input. Thus for example, bars
cannot be accommodated in the input profile. Therefore a
"smoothing" subroutine has been developed that maintains all of the
sand volume at the same elevation as occurs at its original
elevation, this is accomplished by simply displacing landward those
elements that are seaward of and at the same elevation of more
landward elements. This smoothing feature is illustrated in Figure
2.

Model Basis and Operation The basis for the CCCL model is
that, as in many other physical processes, any contour responds to
a disequilibrium by approaching the equilibrium position in a
manner that is rapid at first and much slower as its equilibrium












S= -MSL







/ hn = A(y-ymsi)2


Sirea., Grid with h and t as the independent variables and y dependent. Modified from Kriebel and Dean, 1985
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position is approached. Based on numerical modeling with the
forerunner of the EDUNE model, it had been established that this
response could be expressed approximately as

yR =y +Ayf (1 -e-KA) (1)

in which


Ay =Yeq -y1 (2)



where i denotes the contour level, K is the rate constant, andAt
is the time increment between the nth and the (n+l)th time levels.

All hydraulic and sand flow models require a "dynamic"
equation and a "continuity" equation. The dynamic equation controls
the rate and direction of the local flows and the continuity
equation accounts for the differences between the flows into and
out of a grid and changes the volume within that grid accordingly.
In the CCCL model, Equation (1) fulfills the role of a dynamic
equation. The continuity requirement, that is ensuring that the
volumes are conserved from time step to time step is accomplished
at each time by first locating the vertical origin of the "target"
profile at the current water level and then shifting the target
profile a horizontal distance such that the net sand volume change
between the two profiles is zero. This step establishes Ay_ as
defined by Equation (2). For each level, the differences in the y
values of the target (equilibrium) and actual profiles is the
quantity Ayf. Equation (1) is then applied for all active contour
levels to update the entire profile. This procedure is continued
from the initial profile until the final computational time. At the
end of this stage, there have been no adjustments to the calculated
profile.

The CCCL model can accommodate time varying forcing, ie
variations of wave height and storm tide with time.

APPLICATION OF THE "2.5" FACTOR

Following the calculations described above, the eroded
contours of the profile are adjusted in accordance with
requirements established during calibration with Hurricane Eloise
erosion data. This adjustment is applied as follows to the eroded
contours

N N N 1
Yi- 2.s= i (. 5 y-1 (1) + 5 )(3)







in which the superscript "N" denotes the final time step. The
result of applying Equation (3) is to displace the eroded contours
N 1
landward by a factor 1.5 times their displacement, yi-yi. The
justification for this increase is described below.

In the aforementioned calibration with Hurricane Eloise data,
it was found appropriate to increase the calculated erosion due to
four causes: (1) The calculated erosion was considerably less than
the measured, (2) There was substantial variability about the
average measured erosion, (3) It was felt appropriate that there be
some safety factor due to the fact that considerable portions of
the Florida shoreline are experiencing long term erosion (witness
Amelia Island, Jupiter Island, Casey Key, etc.), and (4) the
inherent flexibility present in the permitting process to address
those areas where focused studies have demonstrated that it is
justified to relax permitting requirements.

Recognizing that it is the upper portions of the profile that
are of primary concern in the establishment of the CCCL, it was
found in the calibration phase that the ratio of the average
measured to calculated recessions for the 10 and 15 ft contours
(NGVD) were 2.33 and 5.08, respectively. When the factor of 2.5 was
applied to these contours, this ratio changed to 0.87 and 0.67,
respectively. For some of the profiles and contours, there was no
calculated erosion without application of the factor.

DISCUSSION OF THE RATE CONSTANT, K

The appropriate value of K in Equation (1) was found to be
0.075 hour-1 during the calibrations with the Hurricane Eloise data.
This is equivalent to a contour eroding to 63% of its equilibrium
distance in a period of 13.3 hours for constant forcing. Other
values were tried; however, it was found that value adopted
provided best overall agreement with the recessions of the 10 ft
and 15 ft contours.

DISCUSSION

In summary, the problem of modelling profile response to storm
tides and waves is a developing technology, limited in part by the
availability of field data for evaluating and calibrating
computational models.

In considering any conservatism in the CCCL model, including
the 2.5 factor, it is worthwhile to note that there are offsetting
factors, including the breaking wave height of 10 feet used in the
model which is considerably less than would be expected during a
100 year storm. For example, off Palm Beach, the significant wave
height associated with a 21 year return period as determined from
the Wave Information Study is 12.8 feet. Also, the maximum
significant wave height measured from the Palm Beach CDN gage
during Hurricane David (1979) was 14.6 feet.
















SEAWALL PERFORMANCE CHARACTERISTICS









SEAWALL PERFORMANCE CHARACTERISTICS


INTRODUCTION

Seawalls placed along the coastline are designed to prevent
the upland from erosion and possibly flooding during storms. The
cross-sections of seawalls may vary substantially, yet by far the
most common seawall is vertically faced and is usually constructed
with steel or concrete sheet piling. For their integrity, most
seawalls depend on a minimum imbedment depth and a landward
anchorage through tiebacks and deadmen.

FAILURE MECHANISMS

Seawalls can fail by a number of mechanisms, including: (1)
Lack of minimum seawall imbedment, (2) Loss of back anchors, and
(3) General loss of seawall strength. Each of these modes is
discussed in the paragraphs below.

(1) Lack of Minimum Seawall Imbedment

If the minimum imbedment is not maintained due to various
causes, the seawall base can be displaced seaward, causing failure.
The lack of minimum imbedment can occur due to a number of causes,
including greater toe scour than anticipated during a storm,
progressive erosion lowering the profile in advance of a storm, and
simply failure in the design to provide adequate imbedment,
possibly to underestimating the wave conditions that will cause the
scour.

(2) Loss of Back Anchors

The back anchors can be lost due to overtopping of the seawall
and associated lee scour or due to erosion from adjacent property
that either does not have seawall protection or where a seawall has
failed. The incursion from adjacent shoreline segments can be
prevented by the construction of adequate return walls. Also
corrosion of the tie backs can cause loss of back anchorage.

(3) Seawall Deterioration

The salt water environment to which seawalls are exposed, can
cause fairly rapid deterioration and unless proper maintenance
measures are taken, the seawalls can lose their capability to
withstand the effects of severe storm effects. Specific causes of
deterioration usually includes either salt water effects on the
reinforcing and the attendant spelling. In such a case, the seawall
may fail due to loss of strength usually at a time during which
high loading occurs.









SEAWALL ATTRIBUTES


Based on the above, it is possible to categorize conceptual
and structural attributes of a seawall such that the intended
design purpose is accomplished and the seawall retains its
structural integrity. In the following paragraphs, these attributes
will be discussed in general terms. The quantitative
characteristics for a particular application depend, of course, on
the design conditions at the site.

Conceptual Attributes

The terminology conceptual attributes as used here relates to
the seawall characteristics that will ensure its performance during
the design event of protecting the upland against erosion and
flooding. Stated differently, this category assumes adequate
structural integrity of the seawall.

The functional performance of the seawall depends on its
height and continuity along the shoreline or adequate return walls.
The height ensures acceptable overtopping and the continuity and/or
adequate return walls addresses the possible concern of waves
attacking the property behind the seawall from the sides.

Structural Attributes

Structural adequacy relates to survival of a seawall during
the design storm. As noted, this requires an appropriate design
initially, adequate imbedment, effective tie backs, and structural
strength. A seawall, constructed in a marine environment, if not
provided with proper monitoring and maintenance, will degrade with
time with some of the deterioration difficult to restore due to the
nature of the loss of structural capability.

CONCLUDING REMARK

If the interaction characteristics of a seawall with the
design environment were known sufficiently, it would be possible to
evaluate the conceptual performance of a seawall from its design.
However, for a seawall that has been in place for a number of
years, appropriate quantification of the structural characteristics
of the seawall will require in situ examination of the seawall
condition and evaluation of any reduction in strength. This
examination may need to be carried out at several locations due to
the spatial variability of the deterioration. A documented
monitoring and maintenance program will assist in the certification
of the structural characteristics of a seawall.




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