CROSS-SHORE SEDIMENT PROCESSES

Transcription

1 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

2 Bill Birkemeier Coastal and Hydraulic Laboratory US Army Corps of Engineers

3 Established 1977 to support the US Army Corps of Engineers coastal mission The The Outer Outer Banks Banks of of North North Carolina Carolina Field Research Field Facility Research Facility 600-m Pier Research Activities Beach erosion Sediment transport Nearshore waves & currents Navigation Instrumentation Cape Hatteras Cape Hatteras

4 Characteristics of Profiles Surf Zone Cross-shore Transport Modeling Cross-shore Profile Response Sediment Transport Outside the Surf Zone

5 Outside surf zone Wind-blown CEM Part III Sand Longshore Cross-shore Cohesive Mixed

6 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

7 Elevation (m) Profile Line Jan 98 1 Feb Feb 98 Profile development & description Limits Volumes for Sediment Budgets Distance (m)

8 Relevance of Cross-shore Transport

9 Relevance of Cross-shore Transport

10 When in balance, no Net transport Force Breaking Waves Nonbreaking Waves N/m 2 N/m 2 Constructive Average Bottom Shear Stress (onshore movement) Streaming Velocities Overtopping Destructive (offshore) Gravity Undertow: Mass Transport Undertow: Momentum Flux Constructive or Destructive Suspension Turbulence Wind Effects? Large 0.95? Small 0.95 Example: H=0.78 m, h=1 m, T=8 s, f=0.08, Wind Speed = 20 m/s

11 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

12 Important mechanism to transport Offshore transport in rips Onshore transport between rips

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14 Beach the zone of most concern

15 Elevation, m NGVD Elevation (m, NGVD) Active Nearshore Bar Zone is most active coarser finer Distance, m Median Grain Size (phi) Shoreface Zone is less active, but equally significant

16 Elevation, m NGVD Cross-shore Profile: Activity & Extent Sandbars are critical to the cross-shore movement of sediment on the profile Beach Bar Zone Upper Shoreface 5 0 Range of bar crest position Inner Transitional Outer Aug Nov Nov Apr Offshore Distance, m

17 Elevation (m) Storm Change Profile Line Jan 98 1 Feb Feb 98 Storms always create sandbars or, if they exist, move them offshore Distance (m)

18 Elevation (m, MLW) Mar Mar May Sep Distance from Baseline (m)

19 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 Distance Offshore, m

20 Elevation (m) The Depth of Closure **Depth at which there is minimal vertical change in the profile Profile Line Jan 98 1 Feb Feb Jan -1 Feb 1 Feb- 19 Feb Very important limit in modeling: Used to terminate computations Distance (m)

21 Observed DoC (m, MLW) Prediction Proportional to wave height Event dependent Predictable Could be shallower Related to surf zone width Big assumption: Pure cross-shore transport - not longshore Predicted d (m)

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23 Beach Evolution Reflective Dissipative < 1% 7% 38% 44% Duck, NC

24 Longshore variation in shoreline change Areas that erode the most, also recover the quickest Sea Ranch Motel

25 Hypothesis - high-erosion zones linked to underlying geology Process not well understood Thursday s field trip!

26 Bruun Rule Bruun 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

27 Depth, m Equilibrium Profile Concept The profile is constantly evolves toward an equilibrium with the prevailing wave conditions D=0.3 mm D=0.7 mm -3-4 Equilibrium happens! / Distance Offshore

28 50 Relationship is empirical Recent research directed to equilibrium shapes with cross-shore varying D 50

29 0-1 Field Research Facility, Line 62, 331 surveys (11 years) Profile Elevation, m (NGVD) Equilibrium Profile for Variable Grain Size Average Distance from FRF Baseline, m

30 Cross-shore: Physical Modeling Based on equilibrium profile Application of the Bruun rule Unrealistic profile shapes

31 SBEACH: Numerical Cross-shore model Based on equilibrium profile shape and balance of: erosion = deposition Useful for storm erosion modeling, which is more likely to be 2D

32 Reality Useful guidance Many assumptions Requires careful interpretation, use of error bars

33 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

34 Influences: Sand supply Wave refraction Currents Transport pathways Sandbar morphology Shoreline response Need to resolve regional processes Courtesy RobThieler, USGS

35 Elevation, m NGVD Location of the Shoreface Usually outside the surf zone and bar movement zone Beach Bar Zone Upper Shoreface 5 0 Range of bar crest position Inner Outer Transitional Aug Nov Nov Apr Offshore Distance, m

36 Upper Shoreface Volume Changes Slow cross-shore recovery punctuated by rapid deposition Line 62 Line 188 Constant rate of Recovery Cumulative Volume Change (m 3 /m) Date

37 Pressure gauge Electronics m sonar 8 m sonar 5 m sonar Current Meters Sonar 5 m Bipod Seaward CRAB survey extent /3/98 4/4/98 4/4/98 4/5/98 8 m Bipod Distance from baseline, m 13 m Bipod

38 -0.3 Shallower Deeper 13 m bipod 8 m bipod 5 m bipod 0.3 9/1/97 12/1/97 3/1/98 6/1/98 9/1/98 12/1/98

39 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