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EQUILIBRIUM BEACH PROFILE

EQUILIBRIUM BEACH PROFILE. Conceptually the result of balancing constructive and destructive forces. Really a misnomer because equilibrium never reached. WHY? Sediment dynamics happen much slower than ever- changing hydrodynamics. EQUILIBRIUM BEACH PROFILE - APPROACHES.

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EQUILIBRIUM BEACH PROFILE

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  1. EQUILIBRIUM BEACH PROFILE • Conceptually the result of balancing constructive and destructive forces. • Really a misnomer because equilibrium never reached. • WHY? • Sediment dynamics happen much slower than ever- • changing hydrodynamics

  2. EQUILIBRIUM BEACH PROFILE - APPROACHES Kinematic: determine motion of each grain: impractical Empirical: Purely descriptive and data driven Dynamic: balance constructive and destructive forces What about details of processes? Don’t necessarily know

  3. DESTRUCTIVE FORCES Turbulence http://www.cruising-newcaledonia.com/images/SURF.JPG

  4. DESTRUCTIVE FORCES Undertow (Komar, 1998) http://www.cruising-png.com/IMAGES2/WAVE.JPG

  5. DESTRUCTIVE FORCES Gravity mgsin(b) b mg http://www.gc.maricopa.edu/earthsci/imagearchive/GSslope.jpg

  6. CONSTRUCTIVE FORCES Non-linear wave profile Net onshore stresses result from non-linear profile

  7. CONSTRUCTIVE FORCES Intermittent suspension Velocity variation under broken waves u t Wave Breaking Very rough sketch Sed concentration t Largest onshore velocities coincide with highest suspension

  8. CONSTRUCTIVE FORCES Boundary layer streaming δ1 δ2 δ3 • Flow is non-uniform in flow direction • Boundary layer thickness varies in flow direction • Induces small vertical velocity component • Time average of uw not zero since u and w not perfectly 90 degree out of phase • Finite but small additional shear stress induced

  9. EQUILIBRIUM BEACH PROFILE THEORY Turbulence is major destructive force F is wave energy flux h is water depth y’ is cross-shore coordinate (onshore-directed) D* is energy dissipation per unit volume (dependent on grain size) Solve for h(y) h varies as cross-shore coordinate to 2/3 power A is the profile scale factor (function of grain size)

  10. d = 0.1 mm d = 0.5 mm d = 1.0 mm EBPs Larger particles have steeper slopes: Can withstand energy better

  11. d = 0.1 mm d = 0.5 mm d = 1.0 mm EBPs, BEACH SLOPE UH OH. Slope goes to infinity as shoreline approached

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