A Preliminary Review of
Beach Profile and
Hardbottom Interactions
Douglas W. Mann, P.E., D.CE.
CB&I
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Presentation Goal
 Lead to a better understanding of the challenges
regarding the estimation of sediment transport in
the vicinity of hardbottom resources
 Review the basic understanding of typical beach
profiles
 Review some tools to describe beach profile
behavior during wave events
 Review some information regarding wave
transformation over obstacles
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Introduction
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Offshore Beach Profiles
Dean (1977)
 Reviewed 502 beach profiles on the U.S. east
coast
 Y= A x2/3 where Y is the water depth , X is
the distance offshore , and A is a coefficient
with units of Length1/3
 Balance between gravitational forces and
onshore directed wave forces
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Offshore Beach Profiles
 Moore (1982) determined that coefficient A
is a function of the mean grain size. Larger
grain size yields larger A values and steeper
beaches.
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Select Offshore Profiles
Typical Offshore Beach Profiles (after Dean, 1977)
0
0
500
1000
1500
2000
2500
Depth (feet)
-10
-20
-30
-40
Distance Offshore (feet)
'd=0.20 mm'
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'd=0.40 mm'
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Valid Range of Coefficient A
Jaspar Beach, ME
Chenier Ronquille, LA
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Offshore Beach Profiles
• Munoz-Perez et al. (1999) evaluated A
values landward of coral reefs in Spain and
found that profiles with reefs had a greater
A value (steeper profiles) than that
predicted by grain size alone.
Munoz-Perez, J., et al., “Equilibrium Profile for Reef-Protected Beaches”, JCR, Vol 15, No. 4, pg. 950-957, 1999.
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Depth of Closure, after Hallermeier (1981)
 Depth of closure is a depth where there is no net
sediment transport during a particular time frame
 Doc = 2.28Hs -68.5( Hs2/gT2; ), relative to MLW
 Hs is the significant wave that occurs for 12 hours
per year
Hallermeier, R. J. (1981). “A profile zonation for seasonal sand beaches from wave climate,” Coastal Engineering 4, 253-277.
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Depth of Closure, after Houston (1995)
Alternatively,
 Doc=8.9 Hs, MLW
 Hs = mean annual significant wave.
Houston, J. R. (1995). “Beach-fill volume required to produce specified dry beach width,” Coastal Engineering Technical Note 11-32, U.S. Army
Engineer Waterways Experiment Station, Vicksburg, MS.
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Typical Offshore Profiles and Example Depth of Closure
Typical Offshore Beach Profiles
0
0
500
1000
1500
2000
2500
Depth (feet)
-10
-20
-30
-40
Distance Offshore (feet)
'd=0.20 mm'
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'd=0.40 mm'
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Illustrative Example No. 1
Typical Offshore Beach Profiles
0
0
500
1000
1500
2000
2500
Depth (feet)
-10
-20
-30
-40
Distance Offshore (feet)
'd=0.20 mm'
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'd=0.40 mm'
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Illustrative Example No 2.
Typical Offshore Beach Profiles
0
0
500
1000
1500
2000
2500
Depth (feet)
-10
-20
-30
If the hardbottom is landward of the depth of closure, then
• From the Doc definition, there is a probability of sediment transport
which may bury some of the hardbottom under wave action.
-40
AND…
Distance Offshore (feet)
'd=0.20
mm'
'd=0.40 mm'
• From the equilibrium force
balance,
there
is a probability that the
net onshore wave forces which will clean the hardbottom.
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Engineering Tool
Storm Induced Beach Change (SBEACH) developed by Larson, Kraus and
Byrnes (1990)
Waves,
initial beach
profile
SBEACH, an empirical cross
shore sediment transport
model
Final beach
profile
Larson, M, Kraus,N., and Byrnes,M. “SBEACH: Numerical Model for Simulating Storm-Induced Beach change, Report 2
Numerical formulation and Model Tests”, Technical Report CERC89-9, May 1990.
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SBEACH Goal
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Cross Shore Beach Performance
1. Do the wave conditions indicate the profile
should erode?
Ho/Lo> 0.00070 (Ho/wT)3
3/2/2017
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Cross Shore Sediment Transport
2. What is the magnitude and spatial limits of the
sediment transport?
Wave Height
Water Depth
Larson, (1989)
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Determination of Offshore Transport Limits
Transport relative to Transport at Breakpoint
Relative transport decay seaward of the breakpoint
1.00
0.75
0.50
0.25
0.00
0
5
10
15
20
25
30
35
40
45
-0.25
-0.50
-0.75
-1.00
Distance Seaward of the Breakpoint (meters)
Offshore Transport
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Onshore Transport
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SBEACH Results
 Therefore, if we know the wave conditions, the
profile shape, and the sediment size, we can
calculate:
– the magnitude,
– direction,
– and cross shore spatial limits of the sediment
transport.
 Determine the likelihood of impacts to
hardbottom.
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Wave Transformation over Obstacles
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Schematic Wave Obstacle Interaction
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Profile with Multiple Hardbottom Ridges
10
5
Elevation( Ft NAVD)
0
0
500
1000
1500
2000
2500
3000
-5
-10
-15
-20
-25
-30
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Distance from Monument (Ft)
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Wave Science
 Grilli and Horrillo (1999) investigated wave
propagation and decay over submerged
obstacles. They found:
– For steep waves, the increase in depth (inshore)
results in highly nonlinear wave decomposition
– Fully nonlinear wave theory is required to
estimate wave heights over and beyond the
obstacles
Grilli, S. and Horrillo, J., “Shoaling of Periodic Waves over Barred Beaches in a Fully Nonlinear
Numerical Wave Tank”, International Journal of Offshore and Polar Engineering, Vol 9 (4), Pg
257-263, 1999.
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Wave Transformation over Obstacles
Which means…..
The prediction of wave properties
(height, orbital velocities, etc.) at the
inshore edge of hardbottoms is complex.
Predicting erosion or deposition of
sand at the edges of hardbottom
resources is challenging.
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Conclusions of this Preliminary Review
 Nearshore hardbottom resources located
landward of the depth of closure
– Are susceptible to burial
– Are subject to onshore sediment transport resulting in
self cleaning forces
 Using cross shore models, such as SBEACH, for
prediction of offshore sediment transport limits
– Results in finite transports seaward of the wave
breaking depth
– Suggests onshore sediment transport occurs from
distances further offshore than offshore transport for a
given wave height
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Conclusions of this Preliminary Review
 Estimation of wave properties after propagating
over obstacles (hardbottom) requires nonlinear
wave theory.
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Looking Forward
 A complete review of the science is
recommended.
 Interactions between beach profiles and
hardbottoms should be reflect a scientific
perspective.
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Questions?
Douglas W. Mann, P.E., D.CE.
CB&I
2481 Boca Raton Blvd.
Boca Raton, FL 33431
[email protected]
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