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Cross-Shore Sediment Processes

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Cross-Shore Sediment Processes 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 Birkemeier Coastal and Hydraulic Laboratory US Army Corps of Engineers Established 1977 to support the US Army Corps of Engineers’ coastal mission TheThe Outer Outer Banks Banks ofof North North Carolina Carolina Field Research FieldFacility Research Facility 600-m Pier Research Activities Cape Hatteras Beach erosion Cape Hatteras Sediment transport Nearshore waves & currents Navigation Instrumentation • Characteristics of Profiles • Surf Zone Cross-shore Transport • Modeling Cross-shore Profile Response • Sediment Transport Outside the Surf Zone • CEM Part III Outside surf zone – Sand Wind-blown – Cohesive Longshore Cross-shore –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 Elevation (m) Limits Distance (m) Distance & description Profile development Profile Line188 Sediment Budgets Volumes for 19 Feb 98 27 Jan98 1 Feb 98 Relevance of Cross-shore Transport Relevance of Cross-shore Transport When in balance, no Net transport Force Breaking Nonbreaking Waves Waves N/m2 N/m2 Constructive Average Bottom Shear Stress 0.84 0.84 (onshore Streaming Velocities 28.9 28.9 movement) Overtopping 28.6 28.6 Destructive Gravity 0.046 0.046 (offshore) Undertow: Mass Transport 28.6 28.6 Undertow: Momentum Flux 7.9 0 Constructive Suspension ? ? or Turbulence Large Small Destructive Wind Effects 0.95 0.95 Example: H=0.78 m, h=1 m, T=8 s, f=0.08, Wind Speed = 20 m/s Nearshore & Inner Shelf – Mean Processes •Just outside the surf zone, hydrodynamics driven by surf zone processes plus surface wind stress and Coriolis. -13 m •In the surf zone, mean currents driven by waves, wind stress still important From Lentz et al, JGR, Aug 15, 1999 • Important mechanism to transport • Offshore transport in rips • Onshore transport between rips Beach the zone of most concern Elevation, m NGVD -10 10 -5 5 0 0 most active Bar Zoneis 200 active, but equallysignificant active, but Zoneisless Shoreface Active Nearshore 400 Elevation (m, NGVD) -15 -10 Distance, m 10 -5 0 5 -2 -1 0 1 2 3 4 coarser 600 Median Grain Size (phi) 800 finer 1000 Cross-shore Profile: Activity & Extent Sandbars are critical to the cross-shore movement of sediment on the profile Beach Bar Zone Upper Shoreface 5 Range of bar crest position D Inner Outer V Transitional G N 0 m , n o i t 27 Aug 1982 a 3 Nov 1982 v -5 e 16 Nov 1982 l E 8 Apr 1983 -10 0 200 400 600 800 1000 Offshore Distance, m Elevation (m) Storm Change Distance (m) Distance Profile Line188 them offshore if theyexist, move create sandbars or, Storms always 19 Feb 98 27 Jan98 1 Feb 98 Elevation (m, MLW) -5 -4 -6 -3 -2 -1 0 1 100 200 300 400 500 600 700 Distance from Baseline (m) 17 Mar 1982 1 Sep1982 3 May1982 2 Mar1982 • 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 Offshore, Distance The Depth of Closure **Depth at which there is minimal vertical change in the profile Profile Line 188 27 Jan 98 1 Feb 98 19 Feb 98 ) m ( n o 27 Jan i t 1 Feb- a -1 Feb v e 19 Feb l E Very important limit in modeling: Used to terminate computations Distance (m) Prediction • Proportional to wave 10 height ) W • Event dependent L 8 M • Predictable , m • Could be shallower ( 6 C o • Related to surf zone D 4 d e width v r e 2 • Big assumption: s b •Pure cross-shore O 0 transport - not longshore 0246810 Predicted dl (m) Beach Evolution Reflective Dissipative < 1% 38% 7% 44% Duck, NC Longshore variation in shoreline change Areas that erode the most, also recover the quickest Sea Ranch Motel •Hypothesis - high-erosion zones linked to underlying geology •Process not well understood •Thursday’s field trip! 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 Equilibrium Profile Concept The profile is constantly evolves toward an equilibrium with the prevailing wave conditions 0 -1 D=0.3 mm -2 D=0.7 mm -3 Equilibrium happens! -4 m , h t -5 p e D -6 2/3 -7 -8 50 -9 0 50 100 150 200 250 300 Distance Offshore 50 – Relationship is empirical – Recent research directed to equilibrium shapes with cross-shore varying D50 0 Field Research Facility, Line 62, 331 surveys (11 years) -1 -2 -3 -4 Average Equilibrium Profile for Variable Grain Size -5 Profile Elevation, m (NGVD) -6 -7 -8 0 100 200 300 400 500 600 700 800 Distance from FRF Baseline, m Cross-shore: Physical Modeling •Based on equilibrium profile •Application of the Bruun rule •Unrealistic profile shapes 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 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 supply Wave refraction Currents Transport pathways Sandbar morphology Shoreline response Need to resolve regional processes Courtesy RobThieler, USGS Location of the Shoreface Usually outside the surf zone and bar movement zone Beach Bar Zone Upper Shoreface 5 Range of bar crest position D Inner Outer V G 0 Transitional N m 27 Aug 1982 , 3 Nov 1982 n 16 Nov 1982 o i t 8 Apr 1983 a -5 v e l E -10 0 200 400 600 800 1000 Offshore Distance, m 3 -150 Cum-100 ulative Volume Change (m /m) 100 150 200 -50 50 Slow cross-shore recovery punctuated by rapid deposition Upper ShorefaceVolumeChanges 0 1981 1983 1985 1987 1989 1991 1993 1995 1997 of Recovery Constant rate Line 188 Line 62 Date -0.7 1.0 Pressure 13 m sonar Electronics 0.8 gauge -0.6 8 m sonar 5 m sonar 0.6 -0.5 0.4 -0.4 0.2 0.0 -0.3 -0.2 -0.2 -0.4 -0.6 -0.1 Sonar -0.8 Current Meters 0.0 -1.0 0.1 -1.2 -1.4 10 0.2 4/3/98 4/4/98 4/4/98 4/5/98 5 5 m Bipod 0 8 m Bipod -5 13 m Bipod -10 Seaward CRAB survey extent -15 0 200 400 600 800 1000 1200 1400 1600 1800 Distance from baseline, m -0.3 Shallower -0.2 -0.1 0.0 0.1 13 m bipod 0.2 8 m bipod Deeper 5 m bipod 0.3 9/1/97 12/1/97 3/1/98 6/1/98 9/1/98 12/1/98 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 .
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