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UFL/COEL-TR/117
IMPACTS ON THE INLET-BEACH SYSTEM OF
EBB TIDAL SHOAL MINING
Final Report
by
Michael E. Trudnak
Hsiang Wang
December, 1997
Sponsored by
Coastal Engineering Research Center
U.S.A.E. Waterways Experiment Station
Engineering Application Unit (CEWES-CD-SE)
Contract No. DACW39-95-K-0084
COASTAL & OCEANOLORAPHC ENINEERIN DEPATME T
University of Florida Gainesville, Florida 32611
UFL/COEL-TR/117
IMPACTS ON THE INLET-BEACH SYSTEM OF
EBB TIDAL SHOAL MINING
Final Report
by
Michael E. Trudnak
Hsiang Wang
December, 1997
Sponsored by
Coastal Engineering Research Center
U.S.A.E. Waterways Experiment Station
Engineering Application Unit (CEWES-CD-SE)
Contract No. DACW39-95-K-0084
REPORT DOCUMENTATION PAGE
1. report lo. 2. 3. Recipient's AccesLioea o.
4. Title and Subtitle 5. Report Date
IMPACTS ON THE INLET-BEACH SYSTEM OF EBB TIDAL December, 1997
SHOAL MINING 6.
FINAL REPORT
7. Author(s) S. Performing Organisation Report No.
Michael E. Trudnak UFL/COEL-TR-117
Hsiang Wang
9. Performing Organitioa Name and Address 10. Project/Task/Work Unit No.
Department of Coastal and Oceanographic Engineering
University of Florida 11. contract or crant o.
336 Weil Hall, P.O. Box 116590 DACW39-95-K-0084
Gainesville, FL 32611-6590 13. T .p of port
12. Sponsoring Organization Name and Addres
Coastal Engineering Research Center Final Report
U.S.A.E. Waterways Experiment Station
Engineering Application Unit (CEWES-CD-SE)
Vicksburg, MS 39180-6199 14.
15. Supplemntary Notes
16. Abstract
The ebb tidal shoal is an attractive source of compatible sand for beach renourishment.
However, ebb shoal mining has not become common practice, for little is known about the effects
of removing the ebb tidal shoal. Physical movable bed modeling was performed in order to determine
the impacts on the inlet-beach system of removing a portion of the ebb-tidal shoal. The main focuses
were on the downdrift erosion and the regeneration process of the borrow area. Two models were
run, one with and one without removal of the ebb tidal shoal, using an idealized inlet of rectangular
cross-section representing a typical mid-sized Florida east coast inlet. The laboratory results showed
that utilizing ebb tidal shoal mining for downdrift nourishment is potentially feasible from the point
of view of renourishment and borrow area regeneration requirements. Methods were proposed to
evaluate the feasibility and potential of ebb shoal mining utilizing laboratory data. The laboratory ebb
tidal shoal characteristics were compared with the characteristics of three small to medium sized
Florida east coast inlets. The results demonstrate that ebb tidal shoals similar to those occurring in
nature can be duplicated in the laboratory and showed the potential of parameterizing ebb shoals, both
in the laboratory and in nature.
17. Origiator's Key Words 1. Availability Statement
Ebb-tidal shoal
Inlet
Inlet mining
Mobile bed
Sediment transport
19. U. S. Security Claseif. of the Report 20. U. S. Security Clasitf. of This Plag 21. No. of Pages 22. Price
Unclassified Unclassified 109
UFL/COEL-TR/117
IMPACTS ON THE INLET-BEACH SYSTEM OF
EBB TIDAL SHOAL MINING
FINAL REPORT
by
Michael E. Trudnak
Hsiang Wang
Sponsored by
Coastal Engineering Research Center
U.S.A.E. Waterways Experiment Station
Engineering Application Unit (CEWES-CD-SE)
Contract No. DACW39-95-K-0084
December, 1997
ACKNOWLEDGEMENTS
The authors gratefully acknowledge support of this study by the Coastal Engineering Research
Center, U.S.A.E. Waterways Experiment Station, Engineering Application Unit, Vicksburg, MS,
Contract Number DACW39-95-K-0084. Mr. Jack Davis is the project monitor. His assistance and
guidance throughout the project is sincerely acknowledged.
TABLE OF CONTENTS
ACKNOWLEDGEMENTS .....................................................................................................
LIST O F FIG U RE S .................................................................................................................
LIST O F TA B LE S ....................................................................................................................
CHAPTERS
1 INTRODUCTION ..........................................................................................
2 LITERATURE REVIEW .............................................................................
3 LABORATORY CONSTRAINTS AND MODELING LAWS ......................
3.1 Considerations and Constraints ............................................
3.2 Scaling Laws ....................................................................
4 LABORATORY EXPERIMENTS .......................................... ................
4.1 Design of Initial Inlet-Beach Model ..........................................
4.2 Test Conditions .......................................................................
4.3 Design of Ebb Shoal Mining ................................... ................
4.4 Test Procedures .............................................................
5 LABORATORY EBB TIDAL SHOAL DEFINITION AND
CALCULATIONS ........................................................ ...............................
5.1 Defining the Ebb Tidal Shoal in the Laboratory .............................
5.2 Ebb Tidal Shoal Volume Calculations ...........................................
6 EXPERIMENTAL RESULTS .....................................................................
6.1 Ebb Tidal Shoal Growth ...................................................................
6.2 Beach Erosion ...........................................................................
6.3 Inlet Channel Shoaling ............................................. ................
6.4 Accumulation of Sand at Downdrift Boundary and Inside Inlet.....
7 COMPARISONS WITH PROTOTYPE DATA ..........................................
7.1 Volumetric comparisons ........................................ ................
7.2 Ebb Tidal Shoal Location ...................................... ...............
7.3 Geometric Shape ......................................................................
8 EVALUATION OF EBB TIDAL SHOAL MINING.........................................
9 SUMMARY AND CONCLUSIONS ................................... ..............
APPENDICES
A
B
C
D
E
F
G
H
I
REFERENCES
CROSS-SHORE PROFILES FOR EC ......................................... .............
CROSS-SHORE PROFILES FOR EC2 ........................................ ...........
BATHYMETRY SURVEY FOR EC ........................................... .............
BATHYMETRY SURVEY FOR EC2 ...................................... ..........
CHANGES IN BATHYMETRY FOR EC 1 ............................................
CHANGES IN BATHYMETRY FOR EC1 .....................................
CROSS-SHORE PROFILES FOR SEBASTIAN INLET .............................
CROSS-SHORE PROFILES FOR JUPITER INLET ..................................
CROSS-SHORE PROFILES FOR BOCA RATON INLET .........................
................................................................................. ........................................
LIST OF FIGURES
1. Tidal Prism-Ebb Shoal Volume Relationship for Florida's East Coast Inlets ....... ... 9
(after Marino, 1986).
2. Location Map of Nineteen Inlets Along Florida's East Coast ............................................ 10
3. Jonson's Flow Regime Chart. ................................................................... 13
4. Sediment Transport Modes Diagram (after Shibayama and Horikawa, 1980) ............. 13
5. Two Different Beach Profile Regions Scheme. ................................................................. 16
6. Schematic Setup for the Movable Bed Inlet Model. ........................................................ 19
7. Current M easurements at Sebastian Inlet. ........................................................................ 21
8. Changes in Bathymetry in EC1 after 3200 min. ................................................................ 24
9. Bathymetry Contours for EC1 after 0 min, 1600 min, and 3200 min ........................... 27
10. Bathymetry Contours for EC2 after 0 min, 1600 min, and 3200 min .......................... 28
11. Changes in Bathymetry for ECI after 800 min, 1600 min, and 3200 min. .................. 29
12. Changes in Bathymetry for EC2 after 800 min, 1600 min, and 3200 min. ................... 30
13. Volume of Ebb Tidal Shoal Versus Time for ECI and EC2 ............................................ 32
14. Rate of Ebb Tidal Shoal Growth Versus Time for EC 1 and EC2 ................................... 32
15. Volume of Downdrift Erosion Versus Time for EC and EC2 ................................... .. 33
16 Rate ofDowndrift Erosion Versus Time for EC1 and EC2 .......................................... 33
17. Volume of Accumulation at Downdrift Boundary Versus Time for EC 1 and EC2 .......... 36
18 Rate of Accumulation at Downdrift Boundary Versus Time for EC 1 and EC2 ........... 36
19. Volume of Accumulation Inside Inlet Versus Time for EC1 and EC2 ............................ 37
20. Rate of Accumulation Inside Inlet Versus Time for EC 1 and EC2 ................................. 37
21. Sebastian Inlet Bathymetry Surveyed in 1989. ............................................. ........... 40
22. EC Ebb Tidal Shoal Bathymetry After 3200 min. ........................................ .......... 40
23. Jupiter Inlet Bathymetry (after Coastal Planning & Engineering, 1989) .......................... 41
24. Boca Raton Inlet and Offshore Bathymetry Chart
(after Coastal Planning & Engineering, 1991). ............................................. ........... 42
25. Boca Raton Beach Topographic and Offshore Bathymetry Survey
(after Coastal Planning & Engineering, 1991). .............................................. ........... 43
26. Sebastian Inlet Ebb Tidal Shoal Above Uninfluenced Downdrift Profile ....................... 46
27. EC1 Ebb Tidal Shoal Above Initial Profile After 3200 min .......................................... 46
28. Jupiter Inlet Ebb Shoal Above Uninfluenced Downdrift Profile ....................................... 47
29. Boca Raton Inlet Ebb Tidal Shoal Above Uninfluenced Downdrift Profile ...................... 48
30. Baseline Definition for Determining the Radial Distance and Bearing Angle ................... 49
31. Geometric Shape of Sebastian Inlet Ebb Tidal Shoal. ................................... ......... 51
32. Geometric Shape of the Laboratory Inlet Ebb Tidal Shoal ...................................... 51
33. Geometric Shape of Jupiter Inlet Ebb Tidal Shoal. ..................................... ........... 52
34. Geometric Shape of Boca Raton Inlet Ebb Tidal Shoal. ................................... ......... 52
35. Idealized Ebb Tidal Shoals for Sebastian, Jupiter, Boca Raton,
and Laboratory Inlets. .................................................................................................. 53
LIST OF TABLES
1. Entrances Where Ebb Delta Mining has been Performed ........................................... 3
2. Benefits, Adverse Impacts, and Monitoring at Entrances as Given in Table 1.................. 4
3. Summary of Fall Velocity Distorted Models. .................................................................. 15
4. M modified M modeling Law ....................................................................................................... 17
5. Inlet Model Experimental Conditions. ......................................................................... 20
6. Volume of Ebb Shoal and Rate of Ebb Shoal Growth for EC 1 and EC2 ......................... 34
7. Volume and Rate of Downdrift Erosion for EC1 and EC2 ............................................ 34
8. Volume and Rate of Accumulation at Downdrift Boundary for EC and EC2 ............... 38
9. Volume and Rate of Accumulation Inside Inlet for EC and EC2 ...................................... 38
10. Calculated Ebb Tidal Shoal Volumes and Locations for Sebastian, Jupiter,
Boca Raton, and Laboratory Inlets. ..................................................................................... 49
11. Geometrical Parameters for Sebastian, Jupiter, Boca Raton, and Laboratory Inlets ........... 50
CHAPTER 1
INTRODUCTION
As offshore sand sources with reasonable reclaiming costs for beach renourishment
diminish, other methods of supplying sand must be utilized. In Florida, severe erosion occurs mainly
along the downdrift side of tidal inlets. Thus, mining an inlet's ebb tidal shoal is becoming an
attractive alternative for renourishing adjacent beaches. The ebb tidal shoal is created from the
combined deposition of sand eroded from adjacent beaches and the inlet channel together with
longshore sediment transport that is entrained by the ebb tidal currents. Because sand stored in the
ebb tidal shoal theoretically is deprived from the downdrift beach, it is only natural to return the sand
to the beach. In addition, owing to the more energetic flow environment over the ebb shoal, the
material stored there is usually of good quality compatible to the downdrift beach. However, such
practice is not prevalent as there is general reluctance to dredge the ebb tidal shoal. This is simply
because the formation of the ebb tidal shoal is a natural process and little is known about the impact
on the inlet-beach system due to such a man made disturbance. Mehta et al. (1996) studied the
limited cases along the east coast of the United States where a portion of the ebb tidal shoal has been
removed. Their results are summarized in Tables 1 and 2. It can be seen that presently the
knowledge on ebb shoal dynamics as well as the knowledge of its effects on adjacent shorelines is
poor and post-dredging monitoring has not been extensively performed.
An ebb tidal shoal is formed due to the accumulation of sediment outside the inlet entrance
under the combined influence of waves and currents. It can be expected to grow to a volume in
equilibrium with the prevailing tidal prism and wave climate (Walton and Adams, 1976). Once the
ebb shoal attains this equilibrium size, the shoal acts as a sand "bridge" to allow for natural
bypassing of sediment across the mouth of the inlet (Dean, 1988). Dredging a large portion of the
ebb tidal shoal can create a sediment sink, decrease natural bypassing, alter inlet hydrodynamics, and
expose previously sheltered parts of the downdrift shoreline to increased wave energy (Walther and
Douglas, 1993) resulting in more downdrift erosion. These potential detrimental effects have
aroused controversies concerning removal of the ebb tidal shoal as a sand source for downdrift
renourishment.
The goal of this study is to examine the effects on the inlet-beach system due to ebb tidal
shoal mining through physical models, focusing primarily on the effects on downdrift erosion and the
regeneration process of the ebb tidal shoal. Physical modeling is a useful tool to achieve these goals,
for the models can be executed at accelerated time scales and the results can be used to determine the
effects of ebb shoal removal including shoreline changes and ebb shoal borrow area response.
However, the reliability of the results could be severely affected by the physical size limitations in the
laboratory and techniques used to extrapolate the results to prototype scale. Therefore, in the design
of laboratory experiments of this kind one must first address the scaling problem and have a
reasonable understanding of the limitations and the validity of the test results.
The modeling laws used in this investigation for the inlet-beach model were determined in a
previous study by 2-D wave tank and 3-D wave basin experiments. The results were reported by
Wang et al. (1994). The feasibility and usefulness of physical models to study the ebb tidal shoal
evolution process and the corresponding shoreline changes for natural and improved inlets were
further demonstrated by Wang et al. (1995). The present study was performed with a more refined
scope to shed insight on ebb tidal shoal borrow area response and the effects on the inlet-beach
system due to dredging a portion of the ebb tidal shoal.
Table 1: Entrances Where Ebb Delta Mining has been Performed (Mehta et al., 1996)
Entrance
John's Pass, FL
Longboat Pass, FL
New Pass, FL
Redfish Pass, FL
Boca Raton, FL
Jupiter FL
Nassau Sound, FL
Port Royal Sound, SC
Fripp, SC
Mining Site
Seaward side ofnorthem delta lobe
Seaward side of delta
Seaward side of delta
Center of delta from channel to seaward
extent of delta
Top of delta
Seaward side of delta
Outer, updrift and relict portion of delta
Seaward edge of delta
North delta of entrance
Year
1988
1993
1993
1981
1988
1985
1995
1994
1990
1974
1980
Volume (nm)
405,000
1,020,000
720,000
501,000
1,220,000
169,000
392,000
2,140,000
596,000
469,000
1,080,000
Placement Location
Updrift beach
Downdrift beach
Updrift beach
Downdrift beach
Downdrift beach
Downdrift beach
Updrift beach
Downdrift beach
Updrift beach
Method of Dredging
61 cm Cutterhead
Dustpan Dredge
Dustpan Dredge
53 cm Cutterhead
61 cm Cutterhead
76 cm Cutterhead
76 cm Cutterhead
76 cm Cutterhead
51-61 cm pipeline
Port Isidro, SC Landward edge of delta 1990 524,000 Beach eroded by entrance flood 76-84 cm pipeline
channel
Captain Sam's, SC Closed off migrating old entrance, creating a 1983 134,000 Operation meant to nourish Earthmovers and Bulldozers,
new entrance updrift ofthe old one (by tides and waves) downdrift beach 99,000 m' to close old channel
Hog. Sc Shore-attached delta 1990 288,000 Updrift beach hydraulic hoe at low tide
Townsend's, NJ Updrift entrance swash bar complex 1978 483,000 Updrift beach 76 or 91 cm Cutterhead
1983 626,000
Townsend's, NJ Downdrift entrance swash bar complex 1987 1,030,000 Surrounding beaches 76 or 91 cm Cutterhead
Great Egg, NJ Undetermined portion of delta 1992 to 1994 4,900,000 Downdrift beach Dustpan or Hopper dredges
Absecon, NJ Spit attached to north jetty 1986 76,500 Downdrift beach 76 or 91 cm Cutterhead
Table 2: Benefits, Adverse Impacts, and Monitoring at Entrances as Given in Table 1 (Mehta et al., 1996)
Entrance Benefits Adverse Impacts Monitoring
John's Pass, FL Beach was restored; reduced shoaling in entrance channel due to Increased beach erosion in the vicinity due to reduced sand Borrow area shoaled at the rate of 24,000 m' from 1988 to 1992
sand trapping in borrow area bypassing
Longboat Pass/ Calculated wave refraction patterns showed a reduction of sediment None were predicted to occur, no impacts were monitored Between Dec. '91 and Dec. '92 Longboat Pass borrow area volume
New Pass, FL "trapped" by deltas and a more even spreading of wave energy to increased by 150,000 m'; however, between Dec. '92 and April'93
the south of each entrance (due to March 13-14 storm) borrow area volume decreased by
41,000 m'
Redfish Pass, FL Erosion protection for downdrift shoreline; accretion to the north of No specific studies were performed Approximately 80,000 m' of fine-grained material was carried into
the entrance and south of the project the borrow area within 18 months after 1981 project; between 1989
and 1991 35,000 m' filled both (1981 and 1988) borrow area
Boca Raton, FL Beach Erosion contained, improved navigation conditions over delta Feeder beach within 600 m of entrance eroded critically, beach Beach I km south ofentrance grew by an average of 12m; the
and maintenance of water quality in Lake Boca Raton nourishment project planned for 1995 using delta sediment entire ebb delta including borrow area exceeded pre-project volumes
Jupiter, FL Erosion protection for downdrift shoreline None were predicted to occur, including focusing of wave energy on Pre- and post-project surveys of fill area to be canied out
jetties, changing of littoral pattern or increased salinity in
Loxahatchee River
Nassau Sound, FL Refraction analysis for borrow area showed a reduction of wave None were predicted to occur No monitoring of the relict delta region
energy on shoreline and an increase of energy in the sound,
resulting in decreased sediment transport
Port Royal Sound, SC Mitigated chronic erosion problem with a predicted 8 year project None were predicted to occur No monitoring of delta
life
Fripp, SC Temporary Beach Nourishment No studies were performed; no impacts were monitored Rapid recovery of borrow area, approximately 153,000 m'
accumulated in delta since 1980
Port Isidro, SC Dredging the ebb delta moved the channel 125 m offshore, None were predicted to occur, no impacts were monitored Sediment filled the borrow area, and the channel slowly migrated
removing the source of scour and renourishing the beach landward towards the equilibrium position
Captain Sam's, SC Relict delta "pushed" ashore by wave action nourished the beach at None were predicted to occur, no impacts were monitored Entrance began migrating south at its previous rate
the rate of 130,000 m'/yr between Mar. '83 to May'85; totally
1,150,000 m' by 1993
Hog, SC Emergency nourishment for heavily armored sections of shoreline Delta filled with sediment at the expense of deltas further offshore Position of Hog Inet channel did not shift toward Myrtle Beach
following Hurricane Hugo and the downdrift shoreline; 7,000 m' ofremedial nourishment shoreline; six months after the project, 95 % of the dry beach was
became necessary to guard downdrift shoreline recovered due to nourishment and seasonal effects
Townsend's, NJ Beach Renourishment Mining redirected the ebb channel northward through the delta, Critical beach erosion along the Avalon shoreline, and growth of a
resulting in changed channel hydraulics large spit in the interior of the entrance where the channel once
occurred
Townsend's, NJ Emergency Renourishment, redirection of channel No studies were done; no impacts were monitored Monitoring to verify channel position
Great Egg, NJ Beach Renourishment No studies were performed Monitoring of delta system planned
Abecon. NJ Beach Renourishment No studies were performed 43% of fill remained in 1991, some sediment was washed offshore;
no monitoring ofdelta
CHAPTER 2
LITERATURE REVIEW
Although the ebb tidal shoal is an appealing source of high quality sand compatible for
renourishment of the downdrift beach, the physical and ecological impacts to the natural system of
removing this sand must be assessed. Likewise, the feasibility of modeling the ebb tidal shoal must
be considered. However, information on the above topics is scarce. Laboratory experiments
performed recently by Wang et al. (1994, 1995) at the University of Florida have shed some light on
modeling the ebb tidal shoal in the laboratory, and other researchers have investigated various
aspects of ebb tidal shoal mining. A brief review of those works most relevant to the present study is
presented in this chapter.
Wang et al. (1995) studied the ebb tidal shoal evolution process in the laboratory under
storm wave conditions with a natural inlet and an improved inlet with porous and impervious jetties.
The ebb shoal evolution process was documented for all cases, and a prediction of the sediment flux
patterns was attempted using a new empirical eigen function approach. Results indicated that ebb
tidal shoals similar to those found in nature can be established in the laboratory.
Wang et al. (1994) investigated the modeling laws to be used in laboratory beach modeling.
Several different modeling laws were tested and compared using two-dimensional wave tank models
and a three-dimensional wave basin model. A modified modeling law was derived based on the work
of Wang (1990) and proved to be the most accurate. This modeling law was adopted by Wang et al.
(1995) and in the present study.
6
Sill (1981) and Hayter (1988) investigated ebb tidal shoal dynamics in the laboratory using a
small scale movable-bed inlet model. The models mixed prototype-scale sand and tidal period with
laboratory-scale geometry and waves. The small scale yielded highly distorted model conditions. The
questions on modeling laws and the morphological time scale on ebb tidal shoal evolution could not
be addressed. The results showed that the volume and shape of the laboratory ebb tidal shoal
resembled those occurring in nature.
Mehta, Dombrowski, and Devine (1996) addressed research needs for developing site
selection criteria for ebb shoal mining and examined the role of waves in ebb tidal shoal growth. A
review of ebb shoal mining undertaken at several tidal inlets showed that the choice of mining
location and the method of mining have been specific to those inlets, and it is unclear whether general
guidelines for determining the site and volume of mining can be developed. An analytic method was
used to show that the rate of ebb shoal growth for a newly opened tidal inlet depends on a parameter,
B, representing the ratio of wave power to tidal power. Observations suggested that an ebb shoal
may never reach a true equilibrium size. However, a quasi-equilibrium volume may be predicted
given the B value characteristic of a particular tidal inlet representing the long term wave and tidal
conditions of that inlet.
Walther and Douglas (1993) studied the ebb shoal borrow area recovery rate. A transport
ratio method was developed to quantify the trapping rates and sediment transport rates over a mined
ebb shoal. Values calculated with this method were reasonably accurate compared to measured data
from Boca Raton Inlet, Redfish Pass, and John's Pass in Florida. The results demonstrated that a
shallower cut will decrease the bypassing rate less initially, however, a deep cut will result in
approximately the same bypassing rate over a longer period of time.
CHAPTER 3
LABORATORY CONSTRAINTS AND MODELING LAWS
In conducting laboratory experiments, the constraints must be acknowledged and considered
to determine the range of experimental parameters. There are three basic kinds of physical properties
one needs to address: the geometrical parameters, the sediment properties, and the natural forces. The
geometrical parameters include such quantities as bathymetries, shoreline configurations, and inlet
geometries. The most relevant sediment properties are grain sizes, specific gravity, and others such
as shapes and porosity as well as the dynamic properties such as the rate and direction of sediment
supplies from the boundaries. The important natural forces to be modeled include ocean waves, tidal
currents, and water level changes. Ocean waves are a vital force in mobilizing bottom sediment and
producing longshore and cross-shore sediment transport. Tidal current is a primary force in shaping
the ebb tidal shoal due to its sediment transport capacity and its strong interactions with nearshore
waves and bathymetric features. The water level defines the boundary affected by the dynamic forces
and modifies the nearshore current and wave conditions. It is evident that the inlet ebb tidal shoal
evolution process is extremely complicated owing to the large number of physical parameters
involved. In laboratory modeling one must first simplify the process to be tested. This is done by
imposing constraints through modeling laws. The methodologies are discussed in this chapter.
3.1 Considerations and Constraints
The inlet model is located in the wave basin in the University of Florida's Coastal and
Oceanographic Engineering Department laboratory. The basin is approximately 25 m wide, 30 m
8
long, and 1 m deep and is equipped with a snake-type wave-maker consisting of 88 independent
paddles each 24 cm wide. Waves of varying angles of incidence can be produced by adjusting the
phase of each individual paddle. Because of the basin's lateral constraints, angles greater than 15
degrees are unsuitable. Depending on the water depth, which is limited to 75 cm, wave heights
ranging from 1 to 15 cm and periods from 0.9 to 1.9 seconds can be produced without difficulty.
The physical dimensions of the wave basin and the scaling laws required limit the size of the
ebb tidal shoal that can be accurately simulated in the laboratory. The basic model scale, defined as
the prototype to model ratio of horizontal scale, must be large enough to accurately reproduce the ebb
tidal shoal volumes found in nature. Thus, data on ebb tidal shoal characteristics found in nature
must be known.
Inlet characteristics in nature vary widely as do the ebb tidal shoal shapes and volumes.
Walton and Adams (1976) and Marino and Mehta (1986) compiled ebb shoal volumes for 15 inlets
along the east coast of Florida and proposed different empirical relationships between the ebb tidal
shoal volume and tidal prism. The results of Marino and Mehta (1986) and the location of the tidal
inlets are shown in Figures 1 and 2 respectively. The ebb tidal shoal volumes generally decrease from
north to south in these samples. The volumes of the four northernmost ebb tidal shoals are similar
and significantly greater than those in the southern and middle coastline. The majority of ebb tidal
shoal volumes in the southern and middle coastline range from 0.1 to 10 million cubic meters which
is considered to be small to moderate in size. These inlets can generally be characterized as mixed
energy type in which both waves and currents change inlet morphology in time scales of engineering
interest from days to decades. Matanzas Inlet and Nassau Sound are the only natural inlets while the
rest have been improved with jetties.
The laboratory inlet model was chosen to represent the inlets in the latter group, for the
group includes more than two thirds of Florida's east coast inlets and the ebb shoal volumes are
9
within the constraints imposed by the basin dimensions and modeling laws. The ebb tidal shoal
volumes associated with the smaller inlets in this group can be simulated in the laboratory wave
basin with a horizontal scale of 40 to 80, and a horizontal scale of 100 can accommodate the mid-
sized inlets. The model inlet design is based on an idealized inlet configuration with the general
hydraulic characteristics of this group. The range of tidal current strength and inlet cross-sectional
area can be estimated from Figure 1 which plots tidal prism versus ebb tidal shoal volume.
100 1 1 1 1 I I I
o _- V s 5.59 x 10-p1" (Marino, 1986)
S---V 6.08 10 p23 (Walton and Adam, 1976)
10- /0
x St Augustine
X Ft. George
5 Nassau St. Johns
Lake Worth x Sound
S/St. x Ft.Plerce
Baers Hiaulover x Lucle Ponce do Leon
1 In x Mantanzas -
0.1 05 1.0 5 10 50 100 500 1000
EBB SHOAL VOLUME, V (x 10 S3 )
Figure 1: Tidal Prism-Ebb Shoal Volume Relationship for Florida's East Coast Inlets
(after Marino and Mehta, 1986).
The selection of experimental wave conditions is also limited due to the laboratory
restrictions and time limitations. In nature, wave conditions are random in magnitude, period, and
direction. Wang et al. (1995) devised a simple plan to select the experimental wave conditions by
examining the effects of waves on the sediment transport and associated bathymetry changes. They
10
assumed that along a coast waves can be separated into two groups: those from a dominant wave
direction and the rest lumped into non-dominant direction. Four categories of waves were then
analyzed including waves from the dominant wind direction causing beach erosion, waves from the
dominant wind direction causing beach accretion, waves from the non-dominant wind direction
causing beach erosion, and waves from the non-dominant wind direction causing beach accretion.
Along the east coast of Florida, the dominant wind is from the northwest, and the non-dominant wind
direction is from the southeast. For Florida's mid east coastal region, it is estimated that 75% of the
time waves are from the dominant direction, and 25% of the time waves are either from the non-
dominant direction or negligibly small in magnitude (Wang et al., 1992).
I Z': St Mary$
I Nassau Sound
S. Fl. F Ge-orge
S. -SLt. Johns
.* StL Augustine
once do Lion
00
Port Canaveral
Sbastlan
041 Pierce
o/ SL Lucie
Co I k'- L Worth
L .....South Lake Worth
Figure 2: Location Map of Nineteen Inlets Along Florida's East Coast.
-A .-'
11
Based on laboratory experiments and field observations, the sediment transport rate and
associated bathymetry changes are governed by extreme waves from either the dominant or non-
dominant direction. Thus, the effects of storm waves from the dominant weather direction were tested
first by Wang et al. (1995). However, a major constraint in movable-bed physical modeling is the
compatibility of the flow regime and modes of sediment transport between field and laboratory
scales. During a storm wave event under natural conditions, sediment transport is governed by the
suspended load and the flow is mainly turbulent. Therefore, these modes must be preserved in the
laboratory.
Laboratory flow conditions are determined using Jonsson's (1966) flow regime chart shown
in Figure 3 as a guideline. The flow regime consists of three different flow zones and three transition
zones. The flow condition is determined by two parameters: a roughness parameter
a,/k,
and Reynolds number
R Ubam
a V
where a, and ub are the amplitudes of the fluid particle displacement and velocity respectively, v is
the kinematic viscosity, and k, is the roughness length generally considered to be on the order of the
sand grain size.
Sediment transport conditions are classified using a diagram proposed by Shibayama and
Horikawa (1980) shown in Figure 4. The diagram consists of two parameters: the relative fall
velocity
UIW
and the Shields parameter
fu
m2sgd
where Wis the fall velocity, ub is the bottom flow velocity, f, is the bottom friction coefficient, s is
the sediment specific gravity, d is the particle size, and g is the gravitational constant.
To preserve turbulent flow and suspended sediment transport in the laboratory model while
maintaining horizontal scales in the range of 40 to 100, there exists some flexibility in selecting the
combinations of sediment material and vertical geometrical scale. Different materials have been
proposed and used in movable-bed model experiments, however, the most common one is natural
quartz sand because it closely resembles the natural beach material and is easier to obtain at low cost.
To use natural sand as bottom material, however, vertical geometrical scale distortion appears to be
necessary. The degree of distortion is addressed in the following section in Scaling Laws.
The final constraint considered in physical modeling is the time scale. In prototype, the ebb
tidal shoal evolution and regeneration are of long-term morphological process taking years or
decades. These processes need to be accelerated with a different time scale in the laboratory model.
Based on Froude number consideration, the time scale can be shown to be inversely proportional to
the square root of the vertical scale. However, this time scale is not sufficient to describe the ebb
shoal process, for one year prototype time would require nearly two months model run time in a
laboratory with an undistorted model of vertical scales ranging from 40 to 100. However, as
mentioned previously, the sediment transport rate and the associated bathymetric changes in the
nearshore environment are dominated by storm events. Hence, the experiments can be conducted
under storm wave conditions in order to accelerate the processes. The results of Sebastian Inlet
movable-bed model testing conducted by Wang et al. (1992) support this reasoning.
1o'
10'
10
102
ia
0 No movement
1 Bed load(BL)
2 Bed Load-Suspendd load
Internndlete (BSI)
3 Suspended load(SL)
4 Sheetflow(SF)
2 2 2
0 1
I L-|
No movement jBL' SI
Transilon
4 SF
10li
10
*2
.0I
10"
Shields Parameter
Figure 4: Sediment Transport Modes Diagram (after Shibayama and Horikawa, 1980).
id 10o 1id 1
Re,Reynolds Number
Figure 3: Jonsson's Flow Regime Chart.
I I
14
In the experiments of Wang et al. (1992), bathymetry changes were examined for a six-day
NE storm wave attack with a wave height of 1.8 meters and a wave period of 8 seconds in prototype
equivalents, followed by an eight-day ENE swell condition with a wave height of 0.6 meters and a
wave period of 16 seconds. The horizontal length scale, vertical length scale, hydrodynamic time
scale, and morphological time scale used in the testing were Nx=60, N8=41, N=9.5, and Nt=6.3
respectively. The experimental results indicated that the six-day storm waves produced a prototype
equivalent sediment transport of 1700 m3/day at the downdrift side boundary as opposed to 370
m3/day in the following eight-day swell period. A marked ebb shoal topographic change (contour
increment of 25 cm in prototype equivalent) occurred only during the six-day storm event. This trend
was also observed for the net sediment loss into the inlet. The commonly accepted sediment transport
formulas also support this condition. It is evident that the impacts from the swell conditions were
insignificant, therefore, all the present model experiments are conducted under storm wave conditions
in order to accelerate the processes governing the morphological change of the nearshore
environment.
3.2 Scaling Laws.
As mentioned in the previous section, under storm conditions the nearshore flow is mainly
turbulent and sediment transport is dominated by suspended load. The modeling law must have the
flexibility to accommodate these conditions in the laboratory, therefore, a distorted vertical scale may
be necessary in order to fulfill the requirements using quartz sand as the bottom material. The
modeling law selected was derived by Wang et al. (1994). It was originally developed for studying
morphological changes in a plane beach model and was later applied to an inlet model (Wang et al.
1995) for describing the generation of the ebb tidal shoal. A brief review of the modeling law
derivation is presented here.
15
The work of Wang et al. (1994) entailed two-dimensional wave tank and three-dimensional
wave basin modeling on beach profile response carried out at different geometric scales. Four
different modeling laws proposed by Le Mehaute (1970), Vellinga (1982), Hughes (1983), and
Wang et al. (1990), as shown in Table 3, were evaluated at horizontal scales of 20, 30, and 40 with
vertical distortions specified by the modeling laws. A parallel set of experiments with undistorted
scales proved to be unsuccessful. The results were compared with data from a prototype scale
experiment performed in the German Large Wave Tank (GWK) test (Dette and Uliczka, 1986). The
comparison of wave tank results with GWK data was presented separately (Wang et al., 1994).
Table 3: Summary of Fall Velocity Distorted Model Laws.
Author Geometric Hydrodynamic Morphological
_Distortion Time Scale Time Scale
Le Mehaute (1970) Na=(N)Vo)2/3 NT="N, Nt =N6
Vellinga (1982) N=NO44O8 N=N, N,7N
Hughes (1983) N, =(NAx)2/3 NT=NxN, Nt=N N
Wang et al. (1994) N=(N,)0.4N,. NT=N/, Nm N
N= prototype to model scale ratio
W = fall velocity scale
S = sediment specific gravity scale
X = horizontal length scale
8 = vertical length scale
H = wave height scale
T= hydrodynamic time scale
Since the intent of Wang et al. (1995) was to extend the beach profile modeling laws to also
cover the offshore shoal region, the model evaluation criteria were extended to include that region.
Dune Region Bar Region
------^^*^^*K -- *1 --- --- -
SWL
-I
-- -\ --------------------------
-- Initial Profile
Storm Profile
Figure 5: Two Different Beach Profile Regions Scheme.
tank stopped at the offshore bar which includes a portion of the ebb tidal shoal. The evaluation of
the modeling laws was carried out in two different beach profile regions: the dune region (shore
region) and bar region (offshore region) as shown in Figure 5. The modeling laws were evaluated
based on five criteria including dune erosion volume, nearshore profile, bar volume, bar crest
location, and geometrical location. The results from the two-dimensional wave tank tests indicated
the following:
For dune erosion, all four existing modeling laws were reasonably adequate to predict the
final erosion volume but over predict the erosion rate before reaching the final experimental
stage.
Wang's and Vellinga's modeling laws performed better for nearshore profile.
All the modeling laws predicted the main bar location closer to the shoreline than the
prototype data.
One probable cause for the latter is that all existing models treated wave height scale the same as the
vertical scale. However in the nearshore zone it is known that wave breaking is affected by water
depth as well as local beach slope. Waves tend to break earlier (at a larger water depth) on a gentle
17
slope than on a steeper slope. A general breaking criterion incorporating slope effect can be given as
Hb=y(m)hb
where H, and hb are the wave height and water depth at breaking, respectively, and y is the breaking
index here expressed as a function of slope, m. In general the value of y increases with increasing
beach slope. In other words, when the slope becomes exaggerated in a distorted model, the wave
height scale should also be enhanced accordingly in order to preserve the surf zone width. Therefore,
if the wave height is simply scaled according to the vertical scale, the surf zone width in the model
when scaled up to prototype will be narrower than that found in nature. Hence, the breaking bar
location from the model prediction will also be closer to shore than bars occurring in nature. To solve
this problem, a modified modeling law was proposed with wave height scaling enhanced as follows:
Na
where N, and N, are the vertical and horizontal scale ratios, respectively. The quantity in the
parenthesis can also be viewed as the breaking index scale (Wang et al., 1994). Accordingly, the new
set of equations that were established for the modified modeling law are shown in Table 4. This
modified modeling law was found to adequately scale both nearshore and offshore regions in the 2-D
wave tank tests and proved effective in 3-D tests.
Table 4: Modified Modeling Law.
Geometric Wave Height Hydrodynamic Morphological
Distortion Distortion Time Scale Time Scale
N =(NNw 0.N8 NT=Nx N,=Nx NtfN
CHAPTER 4
DESIGN OF EXPERIMENTS
4.1 Design of Initial Inlet-Beach Model
The laboratory model design considered the constraints and modeling laws addressed in the
previous chapter. The model design is shown schematically in Figure 6. Tidal currents were
generated by recirculating water through the channels as depicted. The flow discharge is controlled
by the weir boxes located on either side of the basin. Water is supplied from the upper basin weir
boxes (flood flow weirs) to create the flood current and from the lower basin weir box (ebb flow
weir) to create the ebb current. The test section is bounded on the sides by semi-perforated wave
guides formed by concrete blocks to allow flows in and out of the test section. The downdrift wave
guide has an opening in the nearshore zone to allow the longshore sediment transport to deposit in
the catch channel. For this inlet model configuration, the wave generator is located about 27 m from
the shoreline based on an average water depth of 0.35 m.
An idealized inlet of rectangular cross-section was constructed cutting through a plane beach
made of natural beach sand with Ds0=0.19mm. The plane beach consists of a flat back shore.segment
and a mild-sloped offshore profile which extends to about 6 meters offshore before merging with the
flat concrete basin floor. The beach profile approximates an equilibrium shape h =AxO' where h is the
water depth and x is the offshore distance from the shoreline. The overall length of the beach from
the updrift end to the downdrift end is approximately 19 meters. The inlet is a straight rectangular
channel with impervious jetties defining an inlet channel with a uniform width and depth of 1.2 m
and 0.2 m respectively. The updrift and downdrift jetties are parallel and of equal length extending
19
0.7 meters seaward of the initial shoreline. The jetty height is about 10 cm above the flood tide water
elevation, and the jetty width is about 20 cm. The inlet is located offset from the center towards the
updrift end creating an updrift beach length of 4.5 m and a downdrift beach length of 12 m.
Sand is supplied to the test section using a curved feeder beach at the updrift end, therefore,
the sand supply to the downdrift is purely wave-induced transport. This design allows for uniform
sediment supply, yet the feeder beach has to be replenished from time to time during the intervals of
conducting beach surveys.
MOVERBLE-BED INLET MODEL
0 2 4 6 8 10
SCALE IN METERS
Figure 6: Schematic Setup for the Movable-Bed Inlet Model.
20
4.2 Test Conditions
Two models were run to study the effects of ebb shoal removal on the inlet-beach system.
The test conditions and run times are given in Table 5. The models have the same inlet configuration
and differ only in the initial bathymetry. The first model, EC1, is the case without ebb shoal removal,
and the second model, EC2, is the case where the ebb shoal was partially removed. The wave maker
at the offshore boundary generated storm condition waves of 7 cm height and 1 sec period and an
approach angle of 7.5 degrees.
The tidal currents are simulated by alternating the ebb and flood cycles every 40 minutes.
This time interval roughly corresponds to a semi-diurnal tidal period at 1:80 geometric scale ratio
based on Froude criterion. The tidal current condition can be simulated with equal flood-ebb
Table 5: Inlet Model Experimental Conditions.
Case Mean Incident Wave Conditions Beaches Slope Ebb Test
Water Tidal Time
Depth Wave Wave Wave Foreshore Offshore Shoal (min)
Period Height Angle Mining
EC1 35 cm 1 sec 7 cm 7.50 1:2.9 1:14.5 No 3200
EC2 35 cm Isec 7 cm 7.50 1:2.9 1:14.5 Yes 3200
discharge, equal flood-ebb current strength, or unequal discharges or current strengths at the inlet
throat. Equal discharge, which usually results in stronger ebb current, was adopted for the laboratory
models based on field measurements at Sebastian Inlet (Wang et al., 1991) and other inlets. In the
present study, the cross-sectional averaged flood and ebb currents in the inlet were 0.12 m/sec and
0.14 m/sec, respectively. The discharge was kept constant at 0.04 m3/sec within each ebb and flood
period. The ebb and flood currents were simulated alternatively in step-wise fashion, instead of
sinusoidal or other types, based on current measurements from Sebastian Inlet, shown in Figure 7.
SI I
0 1
I i I .
>1 I
-0.5 \.
10 10.5 11 11.5 12
January,1990
Figure 7: Current Measurements at Sebastian Inlet.
It is evident that the current variations within each ebb or flood cycle can be reasonably
approximated by uniform step function.
The significant effect of water level on beach erosion must also be considered. In the present
study, no attempt was made to simulate storm surges, however, the periodical water level change due
to tidal cycles was included. The water level is higher for flood tides as water is pulled towards the
inlet, whereas water jets away from the inlet during ebb tides. The simulated tidal range is 3 cm in the
experiment with an inlet water depth of 0.2 m for the flood tide and 0.17m for the ebb tide.
22
4.3 Design of Ebb Shoal Mining
The testing procedures and conditions were identical for EC 1 and EC2 with the exception of
i
the initial model bathymetry. The initial bathymetry for EC1 was described in section 4.1. The initial
bathymetry for EC2 was obtained by modifying the final (after 3200 min) bathymetry of EC1 by
mining the ebb shoal and renourishing the downdrift beach. The ebb tidal shoal mining was designed
so that sand would only be removed from the seaward side of the ebb tidal shoal which has been
common practice in actual ebb shoal mining cases. The ebb tidal shoal was dredged to a depth of -40
cm in the model. This sand was used in all for renourishing the downdrift beach in the preparation of
the initial topography for EC2. The volume of sand mined from the ebb tidal shoal was not sufficient
to complete the renourishment of the downdrift beach, thus, additional sand from outside the model
was required.
4.4 Test Procedures
The laboratory experiments were conducted according to the following procedures:
(1) Prepare model initial bathymetry.
(2) Survey initial profiles at the 20 cross-sections..
(3) Adjust water level and discharge to specified design values. Start experiment with ebb cycle
first.
(4) Start wave generator with pre-calibrated settings. The experiment is interrupted at intervals of
40 min. for the change of tidal conditions between ebb and flood cycles.
(5) Conduct bottom profile surveys at selected time intervals at 40 min, 80 min, 120 min, 160
min, 480 min, 800 min, 1120 min, 1600 min, and 3200 min.
(6) Measure sand accumulated outside the downdrift boundary and inside the inlet.
CHAPTER 5
LABORATORY EBB TIDAL SHOAL DEFINITION AND CALCULATIONS
5.1 Defining the Ebb Shoal in the Laboratory.
The goal of this study is to determine the impact on the inlet-beach system due to ebb tidal
shoal mining. The main focuses are on the downdrift shoreline erosion and the regeneration process
of the ebb tidal shoal after removal. In order to examine the ebb shoal characteristics of EC1 and
EC2, it is necessary to first clearly define the ebb tidal shoal in the laboratory.
The ebb tidal shoal is defined as the accumulation of sediment above a specified reference
contour in the region under the influence of the ebb tidal current. In reality, specifying the reference
contour can be subjective. The reference contour is usually chosen as the updrift or downdrift
shoreline not affected by the tidal inlet. Large discrepancies in volume calculations can exist in nature
when the updrift and downdrift shorelines are notably offset and when the bathymetry is significantly
complicated by bottom undulations (Mehta et al., 1996). In the laboratory, the task is considerably
easier by simply using the initial bathymetry of the respective model tests.
A more difficult task is the determination of the region of influence of the ebb tidal current.
Often the ebb tidal shoal will merge with the channel shoals and the offshore bars as they grow. In
the present study, the ebb tidal shoal and channel shoals were separated by defining the channel
shoals as the accumulation of sediment in the inlet channel within the confines of the jetties. Since
the experiments were conducted mainly under storm wave conditions, offshore bars induced by
breaking waves were present. These offshore bars would eventually merge with the ebb tidal shoal.
To separate them is not always easy. In the model, the breaking wave bar is defined by a minimum
24
accumulation of 4 cm and a maximum accumulation of approximately 8 cm. However, the points of
+8 cm accumulation of the bar are far enough updrift and downdrift of the inlet to assume that they
are not part of the ebb shoal. Thus, the location of the ebb tidal shoal boundary was chosen to
exclude the updrift and downdrift portions of the bar and include the accumulation of sediment above
the +4 cm isoline within the vicinity of the inlet. The downdrift limit was chosen as the survey line
#15 (approximately 3.5 meters downdrift of the inlet). This line is far enough downdrift from the
jetty yet not too close to the physical limit of the model. The updrift limit of the ebb tidal shoal is
defined by a shore perpendicular line drawn from the tip of the updrift jetty. By inspecting the
contour plot for EC1 after 3200 min shown in Figure 8, one sees that there is no significant
accumulation of sediment outside the defined boundaries.
Contour Changes in EC1 After 3200 min
I 1-- Ebb Shoal Boundary
E5- 4
04-
3 ............
02l ~ ~. ~" "1 --1
I I f__fil_, Ii
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure 8: Changes in Bathymetry in EC1 after 3200 min.
5.2 Ebb Tidal Shoal Volume Calculations
After defining the reference contour and the horizontal limits, the ebb shoal volumes can be
calculated. Each of the survey lines contained 80 equally spaced data points. By viewing the cross-
25
shore profiles for each survey with respect to the initial bathymetry profile, the amount of
accumulation at each survey line is evident (as is the amount of erosion). The volumes were
calculated after every survey by first calculating the area of accumulation for each survey line within
the region of influence. The areas were calculated by summing the product of the differential height
at each data point and the data point spacing. The areas of every two adjacent survey lines were then
averaged and multiplied by the longshore distance between the two survey lines to obtain the
estimated volume between those two survey lines. The total volume of the ebb tidal shoal is the
summation of these partial volumes throughout the region of influence. Appendix A and Appendix B
contain cross-shore profiles at every survey line after 3200 min for EC1 and EC2 respectively.
CHAPTER 6
EXPERIMENTAL RESULTS
Experiments EC1 and EC2 were designed to determine the impact on the inlet-beach system
due to ebb tidal shoal mining. Both experiments were run for a total of 3200 min (40 complete tidal
cycles) under the test conditions specified in section 4.2. In prototype scale, this time is equivalent to
20 days of storm wave conditions. The experiments were terminated at this time due to the downdrift
erosion being too severe to continue. In both cases, the downdrift shoreline retreated close to the
model's onshore boundary. Bathymetry surveys were conducted at 0 min, 40 min, 80 min, 120 min,
160 min, 480 min, 800 min, 1120 min, 1600 min, and 3200 min in order to monitor downdrift beach
erosion, regeneration of the ebb shoal, and inlet shoaling. Figure 9 and Figure 10 illustrate, as
examples, the results of bathymetry surveys at 0 min, 1600 min, and 3200 min for EC1 and EC2
respectively. Appendix C and Appendix D contain the complete set of results from the bathymetry
surveys for EC 1 and EC2. The longshore sediment transport and net upchannel sediment transport
inside the inlet were estimated by collecting sand outside the downdrift boundary and inside the inlet
at the survey times specified above.
The impacts on shoreline erosion, ebb shoal establishment, and inlet channel shoaling can be
quantified by comparing the changes in bathymetry between the surveys. Figure 11 and Figure 12
show the changes in bathymetries at 800 min, 1600 min, and 3200 min with respect to initial
condition (0 minute) for EC 1 and EC2 respectively. The complete set of bathymetry changes with
respect to the initial surveys for EC 1 and EC2 are given in Appendix E and Appendix F.
1I
0 2 4
6
3
0 2 4 6
Iongshor
0 min
. ... 1i-40 ,
-30
.20
8 10 12 14 16
600 min
-30
I -40 p -
-10
8 10 12 14 16
200 min
S- 430
8 10 12 14 16
e distance (m)
Figure 9: Bathymetry Contours for ECI after 0 min, 1600 min, and 3200 min.
1
3:
0 2 4 6
Iongshor
0 min
-40
------- --
-20
8 10 12 14 16
600 min
-40
8 10 12 14 16
200 min
-40
8 10 12 14 16
re distance (m)
Figure 10: Bathymetry Contours for EC2 after 0 min, 1600 min, and 3200 min.
800 min
0 2 4
-4
~Vfc~~--~--=--
6 I I1 1I I
6 8 10 12 14 1
E5
54
3
0
a2
0)
4
5
6
o2
0)
0 2 4
6 8
10 12 14 16
3200 min
0 2 4 6 8 10
longshore distance (m)
12 14 16
Figure 11: Bathymetry Changes for EC1 after 800 min, 1600 min, and 3200 min.
1600 min
~-4
-< -- ..- --.. -
._ ,- -* -... _-- =
800 min
5 I I
co
0 2 j = = -0
I) I
44
1-
r -------
0 2 4 6 8 10 12 14 16
1600 min
6I
6 -4------ --- --
0 2 4 6 8 10 12 14 16
6
E5
.a
52
0o
i1
3200 min
9
--
-4
--16 ----..
--_ -4
-16 ----
T"" ae a
0 2 4 6 8 10
longshore distance (m)
12 14 16
Figure 12: Bathymetry Changes for EC2 after 80 0 min, 1600 min, and 3200 min.
31
6.1 Ebb Tidal Shoal Growth
The degree of ebb tidal shoal growth can be quantified using the changes in bathymetry
generated from the surveys. Using the definition of the ebb shoal and ebb shoal boundaries discussed
in section 5.1, the volume of the ebb shoal can be calculated. Figure 13 compares the calculated ebb
tidal shoal volume versus elapsed time for EC1 and EC2, and Figure 14 illustrates the rate of ebb
shoal growth in both experiments. In the first 800 min, the ebb tidal shoal in EC2 accumulated less
volume than the shoal in ECI implying a slower rate of growth in EC2. However, from 800 min to
3200 min, the ebb shoal growth approaches a steady rate at approximately 0.017 m3/hr in both
experiments. The volumetric changes fluctuate greatly in the first 160 min reflecting the effects of
individual ebb and flood tidal cycles during this initial period. These results are also presented in
tabular form in Table 6. As can be seen during the initial stage, the ebb tidal shoal grows during ebb
cycle but diminishes during flood cycle, although the net effect is the accumulation of sediment in the
form of ebb tidal shoal.
6.2 Beach Erosion
Downdrift beach erosion is defined as the volume of sediment eroded from the beach
between the downdrift jetty and the downdrift model boundary. Using the same method described in
section 6.1 for calculating the ebb tidal shoal, the volume of sand eroded between survey lines 9-20
can be calculated. The calculated volumes of downdrift beach erosion and the rates of erosion are
included in Table 7. These results are illustrated graphically in Figure 15 and Figure 16 which show
the accumulated downdrift erosion versus elapsed time and the rate of downdrift beach erosion
respectively for both EC1 and EC2. Extensive downdrift beach erosion occurred in the first 160 min
of the two experiments, with a greater erosion rate in EC2 than in EC1. After 160 min, the downdrift
beach was eroded at about an equal rate in EC1 and EC2. From 480 min to 3200 min, the downdrift
0 500 1000 1500 2000 2500 3000 3500
elapsed time (min)
Figure 13: Volume of Ebb Tidal Shoal Versus Time for EC1 and EC2.
0 500 1000 1500 2000 2500 3000 3500
elapsed time (min)
Figure 14: Rate of Ebb Tidal Shoal Growth Versus Time for ECI and EC2.
0 500 1000 1500 2000 2500 3000 3500
elapsed time (min)
Figure 15: Volume of Downdrift Erosion Versus Time for ECI and EC2.
0.25
C
| 0.2
UJ
S0.15
a:
Figure 16: Rate of Downdrift Erosion Versus Time for EC1 and EC2.
Table 6: Volume of Ebb Shoal and Rate of Ebb Shoal Growth for EC1 and EC2.
EC1 EC2
Elapsed Time
(min) Volume of Ebb Rate of Ebb Volume of Ebb Rate of Ebb
Shoal Shoal Growth Shoal Shoal Growth
(ms) (m3/hr) (m') (m3/hr)
40 0.0948 0.1422 0.0402 0.0603
80 0.0805 -0.0214 0.0805 0.0604
120 0.1033 0.0342 0.0855 0.0075
160 0.1078 0.0068 0.0880 0.0037
480 0.2667 0.0298 0.1999 0.0210
800 0.3917 0.0234 0.2783 0.0147
1120 0.4743 0.0155 0.3053 0.0051
1600 0.6006 0.0158 0.4321 0.0158
3200 0.9399 0.0127 0.8589 0.0160
Table 7: Volume and Rate of Downdrift Erosion for EC1 and EC2.
ECI EC2
Elapsed Time
(min) Volume of Rate of Erosion Volume of Rate of Erosion
Erosion (m3) (m3/hr) Erosion (m3) (m3/hr)
40 0.2521 0.3782 0.1819 0.2729
80 0.3139 0.0927 0.4351 0.3798
120 0.4359 0.1830 0.5006 0.0983
160 0.4988 0.0944 0.6395 0.2083
480 0.8748 0.0705 1.1037 0.0870
800 1.1513 0.0518 1.3552 0.0472
1120 1.3755 0.0420 1.5282 0.0324
1600 1.7336 0.0448 1.9671 0.0549
3200 2.3164 0.0219 2.7105 0.0279
35
erosion continued but at a decreasing rate from approximately 0.08 m3/hr at 480 min to 0.025 m'/hr
after 3200 min in both experiments. Neither EC 1 nor EC2 appeared to have reached an equilibrated
state at 3200 min.
6.3 Inlet Channel Shoaling
The inlet channel shoaling is defined as the accretion of sediment in the inlet within the
confines of the jetties. The inlet channel shoaling, which occurred mainly in the inlet channel near the
updrift jetty, was noticeable in both EC1 and EC2. This inlet channel shoaling was more severe in
EC2 than in EC1. The total sand volumes of inlet channel shoaling evaluated at 3200 min. in EC1
and EC2 were 0.022 and 0.115 m3, respectively.
6.4 Accumulation of Sand Outside Downdrift Boundary and Inside Inlet
Sediment transport across the downdrift boundary and sand carried into the inlet by the
combined currents and waves were determined by the amount of sand collected outside the downdrift
boundary and inside the inlet in EC1 and EC2. Tables 8 and 9 contain the volume of sand
accumulation and rate of accumulation at the downdrift boundary and inside the inlet, respectively,
for ECI and EC2. These results are expressed graphically in Figures 17 and 18 which show the
accumulated volume and rate of sand accumulation versus time at the downdrift boundary and
Figures 19 and 20 which illustrate the same parameters for inside the inlet. It is apparent that the
longshore sediment transport at the downdrift boundary was significant in both EC1 and EC2, with
more transport in EC2 than in EC1. The rate of sand accumulation outside the downdrift boundary
generally decreased with time for both EC1 and EC2. Inside the inlet, there was not a significant
amount of sand accumulated in EC2 compared to a much greater amount in EC1. The rate of sand
accumulation inside the inlet decreased with time for EC 1 and increased, though not significantly, for
EC2.
0 500 1000 1500 2000 2500 3000 3500
elapsed time (min)
Figure 17: Volume of Accumulation at Downdrift Boundary Versus Time for EC1 and EC2.
1500 2000
elapsed time (min)
Figure 18: Rate of Accumulation at Downdrift Boundary Versus Time for EC1 and EC2.
U.16
0.14 ---"
0.12 -
/
/
0.1 /
0.08 /
/
0.06 / __ EC1
/ EC2
0.04
0.02
0 'i'
0 500 1000 1500 2000 2500 3000 3500
elapsed time (min)
Figure 19: Volume of Accumulation Inside Inlet Versus Time for EC1 and EC2.
035
).03
025
1.02 __ EC1
EC2
015 I
.01 -
)05 /
0 ~~
0 I
0 500 1000 1500 2000 2500 3000 3500
elapsed time (min)
Figure 20: Rate of Accumulation Inside Inlet Versus Time for EC1 and EC2.
o
0
E
ZB
0.
-0.
(o
E
S0.
S0
I
0.0
Table 8: Volume and Rate of Sand Accumulation at Downdrift Boundary for EC 1 and EC2.
EC1 EC2
Elapsed Time
(min) Volume of Rate of Volume of Rate of
Accumulation Accumulation Accumulation Accumulation
(m3) (m3/hr) (m3) (m3/hr)
40 0.030 0.045 0.060 0.090
80 0.060 0.045 0.105 0.068
120 0.080 0.030 0.145 0.060
160 0.105 0.037 0.185 0.060
480 0.255 0.028 0.475 0.054
800 0.380 0.023 0.655 0.034
1120 0.505 0.023 0.855 0.038
1600 0.650 0.018 1.135 0.035
3200 0.810 0.006 1.167 0.0012
Table 9: Volume and Rate of Sand Accumulation Inside Inlet for EC1 and EC2.
EC1 EC2
Elapsed Time
(min) Volume of Rate of Volume of Rate of
Accumulation Accumulation Accumulation Accumulation
(m3) (m3/hr) (m3) (m3/hr)
40 0.020 0.030 0.0 0.0
80 0.024 0.006 0.0 0.0
120 0.032 0.012 0.0 0.0
160 0.036 0.006 0.0 0.0
480 0.056 0.004 0.001 0.0002
800 0.096 0.008 0.004 0.0006
1120 0.116 0.004 0.009 0.0009
1600 0.136 0.003 0.014 0.0006
3200 0.156 0.0008 0.039 0.0009
CHAPTER 7
COMPARISONS WITH PROTOTYPE DATA
In the previous chapters it was shown that an ebb tidal shoal can be successfully created in a
laboratory setting. In this chapter, an attempt is made to compare the laboratory results with field
data. Three small to medium sized inlets along the east coast of Florida were chosen for the analysis
which include Sebastian Inlet (Wang et al., 1992), Jupiter Inlet (Coastal Planning & Engineering,
1989), and Boca Raton Inlet (Coastal Planning & Engineering, 1991). These three east-coast Florida
inlets were chosen based on their relatively recent bathymetry surveys that indicated distinct ebb tidal
shoals. Figures 21-25 illustrate the bathymetries for these three inlets. The laboratory data from EC1
were used here for comparisons because it represents the ebb tidal shoal formation starting from an
initial condition of a plane beach form.
As discussed previously, the characteristics of ebb tidal shoals can vary widely and there are
no clear-cut criteria on which to base this comparison. Several simple indices were chosen for the
present effort which include the volume, location, base dimensions and geometrical shape of the ebb
tidal shoal. These comparisons should at least show whether on a semi-quantitative basis the
laboratory results can portray that occurring in nature.
7.1 Volumetric Comparisons
The volumes of the ebb tidal shoals for the three inlets mentioned above were calculated via
the same method used for the laboratory models in order to maintain consistency. From the
hydrographic survey maps, profiles were first constructed across the contour plots perpendicular to
200
c150
a
CO
(D
0 50 100 150 200 250 300 350 400
Longshore Distance (m)
Figure 21: Sebastian Inlet Bathymetry Surveyed in 1989
EC1 Bathymetry After 3200 min
3 4 5 6 7
longshore distance (m)
Figure 22: ECI Bathymetry After 3200 min..
,I-
//
I~ -
*-:-.-)
;~r~Lcfl
,ai ~WP~~tf M~ UYOU O EL ~ .9k .t~k'( ,4
.-- 9 !
Figuc 23Jupier nictBatbmetr (aLer *Coata Planning &C1 Engincrng,199)
I .1 .
BOCA RATON INLET- .
AND
.OFFSHORE SHOAL BATHYMETRIC CHART -.
APRIL 18. 1990'
"... en.., .... 4_____ -,__ ".p ..' c '. ... ,.
-""-, .... ..... ...." '- --- .. -
-* ----; --. .... ----,--.,. -.- -- "----- --* ..
-"...,....... .
--*-
"*S " ","I, U .w LI 11 __
"- .. ,,. :-.L_...-,,.- "- --1, .
I -.4 !--I
SMIET 1--
Figure 24: Boca Raton Inlet Ebb Shoal Bathymetry (after Coastal Planning & Engineering, 1991).
BOCA HATON BEACH
TOPOGRAPHIC ANO OFFSHORE
OCTOBER. 21.1990
I ,
BATHYMETRIC SURVEY
1 I f a
SCA0 i 400flc
(CutM In rE
N-
C asa i nos..
sast ..,
Figure 25: Boca Raton Inlet Bathymetry (after Coastal Planning & Engineering, 1991).
I
I
44
the downdrift shoreline. The configurations of ebb tidal shoal were then established by plotting these
contours in reference to the updrift and downdrift shoreline profiles uninfluenced by the inlets.
Appendix G-I contain the profiles for Sebastian Inlet, Jupiter Inlet, and Boca Raton Inlet
respectively.
Large discrepancies occurred between the volume calculations obtained using the updrift
and downdrift uninfluenced profiles as reference contours for both Jupiter and Boca Raton Inlets.
This was due to the significant offset of the updrift and downdrift shorelines. The downdrift
uninfluenced profile was selected here as the reference contour. For Jupiter Inlet, a volume of
315,000 m3 was calculated using the uninfluenced downdrift profile which compares favorably with
the 300,000 m3 volume calculated by Marino and Mehta (1996). A volume of 440,000 m3 was
calculated using the uninfluenced downdrift profile for Boca Raton Inlet ebb tidal shoal which is
much less than the 800,000 m3 obtained by Marino and Mehta (1996). For Sebastian Inlet, a volume
of lx106 m3 was calculated using the uninfluenced downdrift profile. This volume agrees with the
volume calculated by Wang et al. (1992) where the updrift profile was used as a reference.
The laboratory experiments were of generic nature with no designated scale to represent any
specific inlet in nature. As noted earlier, the results are likely to be valid only if the horizontal scale is
in between 40 to 100. Volumes were computed based on two horizontal scales of 60 and 100. The
equivalent prototype volumes of EC for a scale of 60 and 100 were computed to be 89,500 m3 and
374,200 m3, respectively. These values are seen to be generally smaller than that of the three
prototype inlets but are of the same order of magnitude. However, the time scale in the experiment
reflected only between twenty to thirty days of storm wave conditions, depending upon whether the
horizontal scale is 60 or 100. It was estimated by Wang et al. (1994) that along the east coast of
Florida the net longshore sediment transport produced by twenty storm days from the dominant wave
direction is approximately equivalent to the annual longshore transport rate. With this duration the
45
ebb tidal shoal has not yet reached its equilibrium condition, hence, the corresponding ebb tidal shoal
volume is also likely to be smaller than its potential capacity. It is, however, difficult to estimate the
volume that the ebb shoal could actually achieve should the experiment be continued until
equilibrium is reached. Thus, given that the ebb tidal shoal did not reach an equilibrium size in the
experiment, the laboratory ebb shoal volume seems reasonably representative of ebb tidal shoals
associated with small inlets found in nature.
7.2 Ebb Tidal Shoal Location
Contours of the ebb tidal shoal were constructed using the accumulation of sediment above
the reference contours discussed above for each inlet. The location of the ebb tidal shoal with respect
to the inlet entrance can be approximated from these contours which are shown in Figures 26-29 for
Sebastian Inlet, Jupiter Inlet, and Boca Raton Inlet respectively. The locations discussed here are
determined by a radial distance and a bearing angle. The origin of the baseline is set at the mid point
between the tips of the two jetties and the orientation of the baseline is determined as parallel to the
updrift jetty near the entrance. This definition is shown in Figure 30. The radial distance is measured
from the center of the ebb tidal shoal defined by the point of maximum accumulation. The bearing
angle is defined as positive in the clockwise direction. Table 10 contains the calculated volumes and
locations for Sebastian Inlet, Jupiter Inlet, Boca Raton Inlet, and EC1.
It can be seen that the radial distance to the ebb tidal shoal at Sebastian Inlet, Boca Raton
Inlet, and EC 1 are in the same order but is considerably longer at Jupiter Inlet. The bearing of the
laboratory ebb tidal shoal is much smaller than that of the three prototype inlets. This indicates the
ebb tidal shoals of the three prototype inlets are located further downdrift from the inlet than the ebb
tidal shoal in the model. One could speculate that a young ebb tidal shoal probably will form closer
to the inlet but gradually moves downdrift when it becomes more mature.
1989 Sebastian Inlet Ebb Shoal Above Downdrift Reference Contour
100 150 200 250 300
Longshore Distance (m)
Figure 26: Sebastian Inlet Ebb Tidal Shoal.
EC1 Ebb Tidal Shoal After 3200 min
3 4 5 6 7
longshore distance (m)
Figure 27: EC1 Ebb Tidal Shoal After 3200 min.
Jupiter Inlet Ebb Shoal Above Downdrift Profile
800
700
600
E
a)
c 500
CO
- 400
300
200
100
0..5
-S
-^- ^ s--
200
300 400
Longshore Distance (m)
Figure 28: Jupiter Inlet Ebb Tidal Shoal Above Uninfluenced Downdrift Profile.
900
N
4..
-0 z-
-~4-
---N^ ^-O
100
500
600
ioal Contours Above Downdrift Profile
350
300
E
a,
O 250
o
200
| 200
- -
100
0 50 100 150 200 250 300
Longshore Distance (m)
Figure 29: Boca Raton Inlet Ebb Tidal Shoal Above Uninfluenced Downdrift Profile.
Boca Raton Inlet
*- - --1
6-
5.5
8 Siradial distance
0 3.5 -
3 4 5 6 7
longshore distance (m)
Figure 30: Baseline Definition for Determining the Radial Distance and Bearing Angle.
Table 10: Calculated Ebb Tidal Shoal Volumes and Locations for Sebastian, Jupiter,
Boca Raton, and Laboratory Inlets
Inlet Ebb Tidal Shoal Radial Distance to Bearing Angle to
Volume Ebb Tidal Shoal (m) Ebb Tidal Shoal (m)
(m')
Sebastian 1,000,000 160 32
Jupiter 315,000 615 27
Boca Raton 440,000 180 40
EC1 (1:60) 89,500 155 8
EC1 (1:100) 374,200 310 8
50
7.3 Geometrical shape
The geometrical shapes of the four cases are shown in Figures 31-34. It can be seen that with
reference to a natural beach profile, the shapes of the ebb tidal shoal for all four cases are similar and
can be roughly described as cone shaped. The shape of the base varies from near circular (EC1 and
Sebastian Inlet) to elliptical (Jupiter Inlet and Boca Raton Inlet). For the elliptical shaped base, the
major axis is approximately parallel to the shoreline. The geometrical parameters including the major
and minor axes, the vertex height, and aspect ratios are given in Table 11. The three dimensional
plots of these four idealized ebb tidal shoal are shown in Figure 35.
Table 11: Geometrical Parameters for Sebastian, Jupiter, Boca Raton, and Laboratory Inlets
Inlet Major Axis Minor Axis Height Minor / h / Minor
(m) (m) (m) Major
Sebastian 95 95 4 1 0.04
Jupiter 340 175 2 0.51 0.01
Boca Raton 200 95 2.5 0.48 0.026
EC1 (1:60) 78 78 3.2 1 0.04
EC1 (1:100) 130 130 4.8 1 0.037
0 50 100 150 200 250 300 350 400
Longshore Distance (m)
Figure 31: Geometric Shape of Sebastian Inlet Ebb Tidal Shoal.
6-
5.5
5 4
3--4
2.5- jJ J
2--
1.5 *^- B
3 4 5 6 7
longshore distance (m)
Figure 32: Geometric Shape of the Laboratory Inlet Ebb Tidal Shoal.
0 100 200 300 400 500 600
Longshore Distance (m)
Figure 33: Geometric Shape of Jupiter Inlet Ebb Tidal Shoal.
0 50 100 150 200 250 300
Longshore Distance (m)
Figure 34: Geometric Shape of Boca Raton Inlet Ebb Tidal Shoal.
3-D View (Exagerated vertical scale)
Sebastian Inlet
Jupiter Inlet
Boca Raton Inlet
Laboratory Inlet
Figure 35: Idealized Ebb Tidal Shoals for Sebastian, Jupiter, Boca Raton, and Laboratory Inlets.
Plan View
CHAPTER 8
EVALUATION OF EBB TIDAL SHOAL MINING
In this chapter the subject of applying the experimental results to assess the potential of ebb
tidal shoal mining is addressed. The question of whether ebb shoal mining is viable for downdrift
beach nourishment has to be evaluated in terms of feasibility and benefit under a set of economical
and environmental constraints. Here we illustrate how the laboratory results can be applied to address
some of these questions excluding the environmental constraints.
The question of economic feasibility is based mainly on the requirement of renourishment
frequency. The period of renourishment can be computed by equating the mined volume from the ebb
shoal to the cumulative volume of erosion in the nourished downdrift region. The time required to
reach this cumulative volume (or a fraction/multiple of it) is the required period of renourishment.
This cumulative volume can be computed in two different ways; one is an absolute volume and the
other is a relative volume. The absolute volume is the cumulative downdrift erosional volume
measured with respect to the fixed initial condition. This volume can be read directly from the
ordinate in Figure 15 for different elapsed time and is defined as Ta. As an example, the mined
volume from the ebb shoal in the laboratory is approximately equal to 2.1 m3. This volume was
placed on the downdrift beach segment. From Figure 15, for a cumulative volume of 2.1 m3, the
corresponding elapsed time can be obtained from the EC2 curve as equal to 2,100 min. This is the
time period in which the nourished quantity of 2.1 m3 placed on the downdrift side from mining has
been eroded away.
55
The relative volume, on the other hand, is computed with respect to the no-mining condition.
Since the downdrift beach is also erosional under the no-mining condition (a background erosion so
to speak), this volume is clearly different from the absolute volume. This volume also depends upon
the timing of ebb shoal mining. If the mining takes place when the ebb shoal is at a young stage the
background erosion is higher than that at a mature stage. This relative volume is more relevant in
terms of economic evaluation as it is measured with respect to the existing condition. The present
experiment was carried out in sequence, therefore, the erosion rate at the end of EC1 (at 3200 min)
should apply. From Table 7, this rate is obtained as 0.0219 m3/hr, or 3.67x 10- m3/min. The
following equation then approximates the background erosion:
V = 3.67x10 -4t
where V. is the volume of the background erosion in m3 based on the no-mining case and t is time in
minutes. Similarly the post nourishment cumulative downdrift erosion volume is given by
Vm= 1.9+4.7x10-4(t-1500) for t>1500
where V, is in m3 and t is in min. Thus, relative to the no-mining case, the return period for a net
volume loss of 2.1 m3 becomes,
2.1= V V
which yields T, = 8800 min and is defined as the relative renourishment period.
Based upon the present experiments, there is a significant difference in magnitude between
T, and T,. If one assumes here that a 3200 min laboratory test time is equivalent to one year
prototype, then the prototype absolute renourishment period is equal to 8 months whereas the relative
renourishment period is equal to 2 years and 9 months. The absolute renourishment period is clearly
56
unrealistically low. One of the major factors that influenced this low value is the laboratory test
condition using storm waves throughout. This storm wave test condition caused rather drastic beach
profile adjustment during the initial period in the form of an offshore bar. This contributed to the
bulk of the beach erosion. Since the test contained no recovery process the volume stored in the
offshore bar though still in the limits of the nourished beach segment is not accounted for.
Another way of estimating the renourishment period is by comparing the volume lost
downdrift of the nourished region with the volume of the mined quantity. In this way, sediment
retained in the offshore bar is not counted as a loss but preserved in the system. Using the same
approach as the first method, the volume lost downdrift can be estimated by
Vd= 1.0+.x104(t-1,400) for t> 1,400
where Vd is the volume lost downdrift in m3 and t is time in min. The renourishment period can be
calculated by equating the mined volume to this downdrift volume and solving for t. The
renouishment period, Td, so computed is equal to 12,400 min, or approximately equivalent to 4
years in prototype. This method appears to be more reasonable for defining the renourishment period
The next question to be addressed is whether it is feasible for repeated ebb tidal shoal
minings. This depends on whether the ebb tidal shoal can be regenerated within the required
renourishment period. Again using the same approach given above, the ebb shoal regenerating
volume can be calculated based on the following empirical relationship which is derived from Figures
13 and 14, or values given in Table 6:
V, = 0.31+2.64x10-4(t-1,200) for t > 1,200
where V, is the volume of ebb shoal regeneration in m3 and t in minutes. For V, to regenerate to 2.1
m3, the required time period is T, = 7,980 min., or 2 years 6 months prototype equivalent.
57
It should be remarked here that material presented in this section is more for illustrating the
methodology rather than provide quantitative prediction for an actual inlet. Although the values as
presented appear to be reasonable, there is no field evidence to support them.
CHAPTER 9
SUMMARY AND CONCLUSIONS
The present study aimed at determining the impacts on the inlet-beach system due to partial
mining of ebb tidal shoals through laboratory experiments. The experiments consisted of studying a
generic inlet with main focuses on downdrift beach erosion and ebb shoal borrow area regeneration.
The effect on downdrift erosion is of obvious importance as the goal of ebb shoal mining is to use the
mined sand to renourish and protect the downdrift beach. Mining the ebb tidal shoal would not be
worthwhile if the erosion rate increased dramatically, thus quickly negating the benefits of the
renourishment. The regeneration process is of interest to determine whether the borrow area will
reattain its post-dredging configuration and at what rate. This is important in order to assess whether
or not or how often the ebb tidal shoal can be mined repeatedly. In the present study, these effects
were analyzed based on the removal of a seaward portion of the ebb tidal shoal. The major findings
from the experiments are as follows:
1. Ebb shoal mining increased the volume of downdrift erosion. This increase is
mainly due to the increase of erosion rate in the early stage right after mining the ebb shoal.
However, the rate of erosion soon becomes closer to but still slightly larger than the rate of erosion in
the natural case.
2. In the regeneration process, the mined ebb shoal initially grew at a slower rate than
the natural case. However, the rate of growth soon equaled and slightly exceeded the rate of growth
compared with the natural case.
3. Inlet channel shoaling increased due to ebb tidal shoal removal.
59
4. Downdrift longshore transport volume as well as transport rate also increased due to
ebb tidal shoal removal. However, the rate of downdrift transport gradually decreased and became
less than that of the natural case as time progressed.
The laboratory ebb tidal shoal characteristics were compared with the characteristics of three
small to medium sized Florida east coast inlets. The similarities among them were discussed. These
results combined with those by Wang et al. (1995) who studied the evolution process of the ebb tidal
shoal in similar laboratory conditions as the present study demonstrate that ebb tidal shoals similar to
those occurring in nature can be duplicated in the laboratory. The results presented here also showed
the potential of parameterizing ebb shoals, both in the laboratory and in nature.
Methods were proposed to evaluate the feasibility and potential of ebb shoal mining utilizing
laboratory data. The methods address two issues: the renourishment intervals based on downdrift
erosion and the ebb shoal regeneration intervals to meet the required volume. These methods were
applied to the present model study.
In summary, the present study demonstrated the feasibility and effectiveness of simulating
the inlet-ebb tidal shoal processes in the laboratory. Partial success was also achieved in determining
the effects on the inlet-beach system due to mining the ebb tidal shoal. The laboratory results showed
that utilizing ebb tidal shoal mining for downdrift nourishment is potentially feasible from the point
of view of renourishment and regeneration requirements. However, the study is only exploratory; any
quantitative extrapolation for field application at this stage is not recommended. Clearly, more
comprehensive work is necessary on the subject. As mentioned earlier, the laboratory test conditions
did not realistically represent the environmental conditions in nature for long term predictions. The
parameters tested were also very limited as only one set of geometry was tested with a limited time
duration and under one set of wave-current combinations.
60
In the present study, the seaward portion of the ebb shoal was removed, however, there are
many aspects that can be analyzed for future studies of ebb shoal mining. One suggestion is to
analyze in more detail the effects due to various borrow area locations such as a seaward portion
(present study), landward portion, top of ebb shoal, center of ebb shoal, or the sides of the ebb shoal.
Another suggestion is to determine the effects due to the percentage of the ebb shoal volume mined
and whether a cutoff percentage exists below which adverse impacts on downdrift erosion are
minimal. The effects on channel hydraulics and channel shoaling are also important in ebb shoal
mining.
One critical area where more study is badly needed is physical modeling. In the present
study, the modeling laws proposed by Wang et al. (1994) were adopted. They are basically the
extension of modeling laws for beach profile evolution process. Although they were evaluated by
Wang et al. (1995) for the application to 3-D inlet experiments and the present study appeared to
yield reasonable results based on them, the modeling laws are restrictive and strictly speaking should
apply only to the downdrift profile development. Refinement is needed to address the temporal scales
of shoal evolution and channel shoaling. Similarly, restrictions were imposed by using unrealistic test
conditions, of which the most serious one is the absence of beach recovery process. Hence, much
work remains to be done to understand ebb tidal shoal dynamics and improve movable bed physical
modeling of the ebb tidal shoal.
APPENDIX A
CROSS-SHORE PROFILES FOR EC1 AFTER 3200 MIN
Appendix A shows cross-shore profiles for EC1 after 3200 min at every survey line
compared to the initial bathymetry of EC1.
EC1 Profiles 1-5
0.5,
0-
-0.5 '
0 1 2 3 4 5 6 7
0.5
0
-0.5
0.5
0
-0.5
0.5
0
-0.5
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
-r
3 4
offshore distance (m)
-I
FI C I I I
Figure Al. Cross-shore Profiles for EC 1 after 3200 min for Survey Lines 1-5
(Dashed Line Represents Initial Profile).
62
-n
. 0
0
63
EC1 Profiles 1-5
0.5
-0.5 ,--,
0 1 2 3 4 5 6
0.5,,,,
0-
-0.5
0 1 2 3 4 5 6
n c;--- --------------- --
-A
0 1 2 3 4 5 6
S---
.
0
3 4
offshore distance (m)
Figure Al. Cross-shore Profiles for EC1 after 3200 min for Survey Lines 1-5
(Dashed Line Represents Initial Profile).
-n
s
L ------ -----
--e
5
p
I I I I t I
64
EC1 Profiles 6-10
0 1 2 3 4 5 6 7
0
E
.0
--0.5
0 1 2 3 4 5 6 7
0
.0
| -0.5
0 1 2 3 4 5 6 7
0.5 ,-,
-0.5
0 1 2 3 4 5 6 7
0.5 1 I -Ii-I
0 1 2 3 4 5 6
offshore distance (m)
Figure A2. Cross-shore Profiles for EC1 after 3200 min for Survey Lines 6-10.
(Dashed Line Represents Initial Profile).
EC1 Profiles 11-15
;.
0 1 2 3 4 5 6
S0-
S0
. 0
U.0
0~ ---- ------ ------------
-0.5 '
0 1 2 3 4 5 6 7
0.5
_n g IIIIII
3 4
offshore distance (m)
0
Figure A3. Cross-shore Profiles for EC1 after 3200 min for Survey Lines 11-15.
(Dashed Line Represents Initial Profile).
-n
I I
-I
-n
-n
i I
-I
I i i 1
I I 1 II I I
SI I
--
--
----
66
EC1 Profiles 16-20
0
0..3 4
3 4
offshore distance (m)
Figure A4. Cross-shore Profiles for EC1 after 3200 min for Survey Lines 16-20.
(Dashed Line Represents Initial Profile).
3-
rL
0
-0.i
51 1 .
)I I I I I I -
I
-0.!=
APPENDIX B
CROSS-SHORE PROFILES FOR EC2 AFTER 3200 MIN
Appendix B shows cross-shore profiles for EC2 after 3200 min at every survey line
compared to the initial bathymetry of EC2.
68
EC2 Profiles 1-5
-A
0 1 2 3 4 5 6
n -
0
n r
-0.5'
C
1 2 3 4 5 6
0-
-0.5
0 1 2 3 4 5 6 7
0.5
0-
-0.5
0 1 2 3 4 5 6 7
offshore distance (m)
Figure B1. Cross-shore Profiles for EC2 after 3200 min for Survey Lines 1-5
(Dashed Line Represents Initial EC2 Profile).
-
- I I I
SI I I I
I I I I ------I
B E I I m Ill I I
r'
_n
',
*.
-.n
)
69
EC2 Profiles 6-10
0 1 2 3 4 5 6 7
S 1 2 3 4 5 6 7
-I
3 1 2 3 4 5 6 7
S1 2 3 4 5 6 7
) 1 2 3 4 5 6 7
1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
3 4
offshore distance (m)
Figure B2. Cross-shore Profiles for EC2 after 3200 min for Survey Lines 6-10
(Dashed Line Represents Initial EC2 Profile).
0
._o
1-0.5
0
Z -0.5
0.5
0
-0.5
I I i I I
-0.5
0
I
EC2 Profiles 11-15
0.5
0
-0.5 i 4 5 6
0 1 2 3 4 5 6
-n
-n i
0
-I
S0 1 2 3 4 5 6 7
0.5
0
-0.5 7--
0 1 2 3 4 5 6 7
3 4
offshore distance (m)
Figure B3. Cross-shore Profiles for EC2 after 3200 min for Survey Lines 11-15
(Dashed Line Represents Initial EC2 Profile).
0 -
5;
* I
[ I I
-I- 7
-0.5
0
Ik 1 .. I.111 1... 11 11 1. I .. I .I I . I I ,.,. ., .... 1.1,,1
71
EC2 Profiles 16-20
c;.
-n0
-n
'I
0
-I
0
-0.5'
C
U.0 I
0- L
-0.5 I
0 1 2 3 4 5 6
:ii
.1 -
_______________
0 3 4
3 4
offshore distance (m)
Figure B4. Cross-shore Profiles for EC2 after 3200 min for Survey Lines 16-20
(Dashed Line Represents Initial EC2 Profile).
1 2 3 4 5 6
.0
SI I I I
SI
)
-0.
APPENDIX C
BATHYMETRY SURVEYS FOR EC
Appendix C shows the results from each bathymetry survey in EC 1 after every time interval.
0 min
E,5 -- -
0 2 4 6 8 10 12 14 16
40 min
6-
E5-
-30
c 4
3 -
1ihi
62
0 2 4 6 8 10 12 14 16
80 min
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure Cl: Bathymetry Contours for ECI after 0 min, 40 min, and 80 min.
74
120 min
0 2 4 6 8 10 12 14 16
160 min
6
52
-30
4
-20
3 --
o0
0 2 4 6 8 10 12 14 16
480 min
6:- -40 -
13
24 --
02
~1
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure C2: Bathymetry Contours for EC1 after 120 min, 160 min, and 480 min.
800 min
6 -40
E 5
-30
353
02
4 1- -
0 2 4 6 8 10 12 14 16
1120 min
6-
0-30
. 6= J ."40 -
---- ---il-I..J------- ---- -------i----
0 2 4 6 8 10 12 14 16
1600 min
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure C3: Bathymetry Contours for EC1 after 800 min, 1120 min, and 1600 min.
3200 min
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure C4: Bathymetry Contours for EC1 after 3200 min.
APPENDIX D
BATHYMETRY SURVEYS FOR EC2
Appendix D shows the results from each bathymetry survey in EC2 after every time interval
0 min
0 2 4 6 8 10 12 14 16
40 min
0 2 4 6 8 10 12 14 16
80 min
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure Dl: Bathymetry Contours for EC2 after 0 min, 40 min, and 80 min.
120 min
I -40
or
2-
0 2 4 6 8 10 12 14 16
160 min
6-40
5- '
0 2 4 6 8 10 12 14 16
Iongshore distance (m)
480Figure D2: Bathymetry Contours for EC2 after 120 inn, 160 mi, and 480 mi
,-40
2 4 6 8 10 12 14 16
longshore distance (m)
Figure D2: Bathymetry Contours for EC2 after 120 min, 160 min, and 480 min.
800 min
2 I
0 2 4 6 8 10 12 14 16
1120 min
-40
U)
c4
U)
0 2 4 6 8 10 12 14 16
1600 min
6-
-40
E5
3
62
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure D3: Bathymetry Contours for EC2 after 800 min, 1120 min, and 1600 min.
3200 min
S-40
3
02-
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure D4: Bathymetry Contours for EC2 after 3200 min.
APPENDIX E
CHANGES IN BATHYMETRY FOR EC 1
Appendix E illustrates the changes in bathymetry with respect to the initial bathymetry for
EC1 after all the surveys.
40 min
6
E5
a)
4
0
52
ol
0 2 4 6 8 10 12 14 16
80 min
6
E5-
04
: 4q 2
0 2 4 6 8 10 12 14
0 2 4 6 8 10 12 14 16
120 min
E,5
0)
04
-3
02
0 2 4 6 8 10
longshore distance (m)
12 14 16
Figure El: Changes in bathymetry for EClafter 40 min, 80 min, and 120 min.
84
160 min
4 =. 4
4 I4 I 4
0 2 4 6 8 10 12 14 16
480 min
4
4
4
4
44
[ 1 ,7 ,0
Ei
I1
C'
cc
53
I-
(
o
2o
03
(
O
4
__ILIII I I
0 2 4 6 8 10 12 14 1
longshore distance (m)
Figure E2: Changes in bathymetry for EC1 after 160 min, 480 min, and 800 min.
0 2 4 6 8 10 12 14 16
800 min
>
I
t
5
4
3
2
1
6
1
3
3
6
5
3
1120 min
4
3 o
a -- L- L-
0 2 4 6 8 10 12 14 16
1600 min
8II I -4
L I-1. -i'---'- ---- ----
0 2 4 6 8 10 12 14 16
3200 min
( 12
___ ILi 1
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure E3: Changes in bathymetry for ECI after 1120 min, 1600 min, and 3200 min.
APPENDIX F
ACCRETION AND EROSION PATTERNS FOR EC2
Appendix F illustrates the accretion and erosion patterns with respect to the initial
bathymetry for EC2 after all the surveys.
6
4,
o2
0)
.W
=53
87
40 min
0 2 4 6 8 10 12 14 16
80 min
6I
5 I
I I
3- 44
2- 24 -- 6 8 1
10 2 4 6 8 10 12 14 16
0 2 4 6 8 10 12 14 16
120 min
0 2 4 6 8 10
longshore distance (m)
12 14 16
Figure DI: Changes in Bathymetry for EC2 after 40 min, 80 min, and 120 min.
160 min
E 5 I
I -I
s -4
3 34 4
-2
0 2 4 6 8 10 12 14 16
480 min
6 -* '
l5 I I
|.- 3- -^
4 -
3-
0 2 4 6 8 10 12 14 16
800 min
0 2 4 6 8 10 12 14 16
longshore distance (m)
Figure D2: Changes in Bathymetry for EC2 after 160 min, 480 min, and 800 min.
89
1120 min
0 2 4 6 8 10 12 14 16
1600 min
0 2 4 6 8 10 12 14 16
3200 min
-4
l---l
i 5.T2 -
+-:.;- 'i -
0 2 4 6 8 10
longshore distance (m)
12 14 16
Figure D3: Changes in Bathymetry for EC2 after 1120 min, 1600 min, and 3200 min.
E5
,--
0
t: 4
03
2
02
-r
I)
APPENDIX G
CROSS-SHORE PROFILES FOR SEBASTIAN INLET EBB TIDAL SHOAL
Appendix G shows cross-shore profiles for Sebastian Inlet ebb tidal shoal based on a 1989
survey by Wang, et al. (1992). The profiles in this appendix include every other survey line between
the downdrift jetty and the downdrift limit. The profile of the updrift-most and the downdrift-most
survey lines are included for comparison.
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