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

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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
Portion of title:
Task 3, generic modelling of seawall overtopping and associated scour
Portion of title:
Generic modelling of seawall overtopping and associated scour
Creator:
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:
19 leaves : ill. ; 28 cm.

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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.
General Note:
Cover title.
General Note:
"January 3, 1995."
Statement of Responsibility:
by Robert G. Dean ... et al. ; prepared for Department of Environmental Protection, State of Florida.

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University of Florida
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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
33143138 ( oclc )

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
I NTRODUCTITON
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 airsea 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 condi 'itions 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 cof the




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quantitative differences may assist in the interpret at --'-on 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 1.1.8 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 R192 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.




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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 great er 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 ii 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
F1 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
_ _ _ _ __ __ ____I __ ___ ___ __ __ ___ __ ___ ___ ___




FB = Z (1 +H
in which ZSW is the elevation of the seawall, ZW1 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%1 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, FE, 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 FE, 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 (Re-port 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
Bi 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 I 1L 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.Ox8O 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 _ ___I _ I _I_ _I_ _




Overtonnina 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, FE. 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 850- 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 seawallprofile interaction experiments guided by the results of this study.




Seawall Height: 16.3 ft.

8 9 10 11 12 13

Seawall Height: 17.3 ft.

9 10 11
Surge Level, (ft) NGVD

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




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"1, Department of Coastal and Oceanog~raphic 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"1, 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"1, 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 shculd 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 "12.5"1 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 ina Figure
2.
Model Basis and operation The basis for the CCCL model is that, as in many other physical processes, any contour resp onds to a disequilibrium by approaching the equilibrium position in a manner that is rapid at first and much slower as its equilibrium




MSL
Sh = A(yyms
__ / hn = A(y-ymst)2/Z3

e {. Grid with h and t as the independent variables and y dependent. Modified from Kriebel and Dean,

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wr 2 14 fr4Ia-h's a Y +k of (rt k w




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
n+l n -KA L)
+Y Yi +A ( (1)
in which
Ay =ye. -y (2)
where i denotes the contour level, K is the rate constant, andA t 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 Ayi 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 iN
Y'(.5 (Y) +l.5J M) (3)




in which the superscript 'IN" denotes the final time step. The result of applying Equation (3) is to displace the eroded contours landward by a factor 1.5 times their displacement, yf-yl. 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-' during the calibrations with the Hurricane Eloise data. This is equivalent to a contour eroding to 631 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 spalling. 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.