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