The University of the West Indies Organization of
American States
PROFESSIONAL DEVELOPMENT PROGRAMME:
COASTAL INFRASTRUCTURE DESIGN, CONSTRUCTION AND MAINTENANCE
A COURSE IN
COASTAL DEFENSE SYSTEMS I
CHAPTER 2
CROSS-SHORE SEDIMENT PROCESSES
By WILLIAN BIRKEMEIER, PhD Coastal Hydraulics Laboratory
US army Corps of Civil Engineers Vicksberg, MA
Unites States of America
Organized by Department of Civil Engineering, The University of the West Indies, in conjunction with Old Dominion University, Norfolk, VA, USA and Coastal Engineering Research Centre, US Army, Corps of Engineers, Vicksburg, MS, USA.
St. Lucia, West Indies, July 18-21, 2001
Bill BirkemeierCoastal and Hydraulic Laboratory
US Army Corps of Engineers
Field ResearchFacility
The Outer Banks of North Carolina
Cape Hatteras
Field ResearchFacility
The Outer Banks of North Carolina
Cape Hatteras
Established 1977 to support the US Army Corps of Engineers’ coastal mission
600-m Pier
Research ActivitiesBeach erosionSediment transportNearshore waves & currentsNavigationInstrumentation
• Characteristics of Profiles
• Surf Zone Cross-shore Transport
• Modeling Cross-shore Profile Response
• Sediment Transport Outside the Surf Zone
Outside surf zoneWind-blown
Longshore
Cross-shore
• CEM Part III
– Sand
– Cohesive
– Mixed
Before A few days later
• Turbulence suspends sediments• Onshore: sediments deposit on the forward motion of the wave• Offshore: sediments settle out on the backward motion
• Bedload & suspended load• Gravity plays a role: downslope force & fall velocity• Offshore & onshore directed mean flows
• primarily undertow & rip currents, also upwelling & downwelling
27 Jan 981 Feb 98
19 Feb 98
Elev
atio
n (m
)
Distance (m)
Profile Line 188
Limits
Profile development& description
Volumes for Sediment Budgets
Relevance of Cross-shore Transport
Relevance of Cross-shore Transport
?Small0.95
?Large0.95
SuspensionTurbulenceWind Effects
Constructive or Destructive
Example: H=0.78 m, h=1 m, T=8 s, f=0.08, Wind Speed = 20 m/s
0.046 28.60
0.046 28.67.9
GravityUndertow: Mass TransportUndertow: Momentum Flux
Destructive(offshore)
0.8428.928.6
0.8428.928.6
Average Bottom Shear StressStreaming VelocitiesOvertopping
Constructive(onshore movement)
Nonbreaking WavesN/m2
Breaking WavesN/m2
Force
When in balance, no Net transport
Nearshore & Inner Shelf – Mean Processes
•Just outside the surf zone, hydrodynamics driven by surf zone processes plus surface wind stress and Coriolis.•In the surf zone, mean currents driven by waves, wind stress still important
-13 m
From Lentz et al, JGR, Aug 15, 1999
• Important mechanism to transport
• Offshore transport in rips
• Onshore transport between rips
Beachthe zone of most concern
Active Nearshore
0 200 400 600 800 1000-10
-5
0
5
10
Distance, m
Elev
atio
n, m
NG
VD
coarser-15
-10
-5
0
5
10
-2 -1 0 1 2 3 4
finer
Median Grain Size (phi)
Elev
atio
n (m
, NG
VD)
Bar Zone is most active
Shoreface Zone is lessactive, but equally significant
Cross-shore Profile: Activity & Extent
27 Aug 19823 Nov 1982
16 Nov 19828 Apr 1983
Bar Zone Upper ShorefaceBeach
Inner OuterTransitional
Range of bar crest position
0 200 400 600 800 1000-10
-5
0
5
Offshore Distance, m
Elev
atio
n, m
NG
VD
Sandbars are critical to the cross-shore movement of sediment on the profile
Storm Change
Storms always create sandbars or, if they exist, move them offshore
27 Jan 981 Feb 98
19 Feb 98
Elev
atio
n (m
)
Distance (m)
Profile Line 188
100 200 300 400 500 600 700
-6
-5
-4
-3
-2
-1
0
1
2 Mar 198217 Mar 1982
3 May 19821 Sep 1982
Elev
atio
n (m
, MLW
)
Distance from Baseline (m)
Dis
tanc
e O
ffsho
re, m
• The presence of an outer sandbar contributes to inshore stability
• Deep sandbar changes occur during periods of intense storm activity
• The deeper the change, the longer the recovery
The Depth of Closure**Depth at which there is minimal vertical change in the profile
27 Jan 981 Feb 98
19 Feb 98
Elev
atio
n (m
)
Distance (m)
Profile Line 188
27 Jan - 1 Feb1 Feb-
19 Feb
Very important limit in modeling: Used to terminate computations
Prediction
• Proportional to wave height
• Event dependent• Predictable• Could be shallower• Related to surf zone
width• Big assumption:
•Pure cross-shore transport - not longshore 0 2 4 6 8 10
0
2
4
6
8
10
Obs
erve
dD
oC(m
, MLW
)
Predicted d�
(m)
Beach Evolution
< 1%
44%7%
38%
Dissipative
Reflective
Duck, NC
Longshore variation in shoreline change
Sea Ranch Motel
Areas that erode the most, also recover the quickest
•Hypothesis - high-erosion zones linked to underlying geology•Process not well understood•Thursday’s field trip!
Bruun RuleBruun Rule: a barrier island will maintains its form as it migrates in
response to a rise in the adjacent ocean and lagoon
Mass is conserved, erosion = deposition
This is fundamental assumption to cross-shore models
Equilibrium Profile ConceptThe profile is constantly evolves toward an equilibrium
with the prevailing wave conditions
0 50 100 150 200 250 300-9
-8
-7
-6
-5
-4
-3
-2
-1
0
Dep
th, m
Distance Offshore
D=0.3 mmD=0.7 mm
2/3
50
Equilibrium happens!
50
– Relationship is empirical– Recent research directed to equilibrium
shapes with cross-shore varying D50
-8
-7
-6
-5
-4
-3
-2
-1
Prof
ile E
leva
tion,
m (N
GVD
)
0Field Research Facility, Line 62, 331 surveys (11 years)
0 100 200 300 400 500
Distance from FRF Baseline, m
600 700 800
AverageEquilibrium Profile forVariable Grain Size
Cross-shore: Physical Modeling•Based on equilibrium profile•Application of the Bruun rule•Unrealistic profile shapes
SBEACH: Numerical Cross-shore model
Useful for storm erosion modeling, which is more likely to be 2D
Based on equilibrium profile shape and balance of:erosion = deposition
Reality• Useful guidance• Many assumptions• Requires careful interpretation,
use of error bars
• Complex hydrodynamics– Non-linear interaction of waves and slowly varying
currents– Interaction of thin turbulent boundary layer with ripple
bed, biology cohesive or non-cohesive sediments• Sediment transport
– Primarily bedload, suspended during events– Not well understood– Normally onshore directed due to wave asymmetry.– Offshore during events and combined flow
• Important– Sediment Budget - offshore/gains and losses– Long-term impact
Influences:Sand supplyWave refractionCurrents Transport pathwaysSandbar morphologyShoreline response
Need to resolve regional processes
Courtesy RobThieler, USGS
Location of the Shoreface
27 Aug 19823 Nov 1982
16 Nov 19828 Apr 1983
Bar Zone Upper ShorefaceBeach
Inner OuterTransitional
Range of bar crest position
0 200 400 600 800 1000-10
-5
0
5
Offshore Distance, m
Elev
atio
n, m
NG
VD
Usually outside the surf zone and bar movement zone
Upper Shoreface Volume ChangesSlow cross-shore recovery punctuated by rapid deposition
Constant rateof Recovery
1981 1983 1985 1987 1989 1991 1993 1995 1997
-100
-50
0
50
100
150
200
Line 62Line 188
Date
Cum
ulat
ive
Volu
me
Cha
nge
(m3 /m
)
-150
0 200 400 600 800 1000 1200 1400 1600 1800-15
-10
-5
0
5
10
Distance from baseline, m
Seaward CRAB survey extent
8 m Bipod13 m Bipod
5 m Bipod
Current MetersSonar
Pressuregauge
Electronics
4/3/98 4/4/98 4/4/98 4/5/980.2
0.1
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
13 m sonar 8 m sonar 5 m sonar
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
9/1/97 12/1/97 3/1/98 6/1/98 9/1/98 12/1/980.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
Deeper
Shallower
13 m bipod 8 m bipod 5 m bipod
Summary
• Important to Sediment Budget• Not well understood• Sandbar formation and movement are
important to overall profile response– Many theories of sandbar location/shape
• Profile changes are 2D - only during severe storms, otherwise 3D
• Sediment grain size typically decreases with depth – important to transport
• Cross-shore models exist