Bed shear stresses, tau, are determined from the gradients of the modeled flow field as, tau = (du/dz+dw/dx). A simple means of visualizing the shear stress at each cross-shore location for the simulation time is obtained through a bed shear stress time stack. The time stack is created by calculating the stresses at each cross-shore position and entering into a matrix where each column is a given time. Upon display, it is trivial to observe the bed shear stress for each passing wave and swash (FIGURE 1; irregular wave case from the LSTF). Here, narrow regions of high onshore directed stress are observed to occur alternately between broad regions of smaller magnitude offshore-directed stress provided by skewed wave motions in the surf zone. Variations in the stress magnitude are also observed along the wave propagation direction and are related to the variations in wave dissipation at the bed and likely a lower-frequency wave component in the wave basin.
FIGURE 1. Shear stress time stack for irregular wave case. Color scale is the bed shear stress in N/m^2. Positive values are onshore-directed. Negative values are offshore-directed.
We are now in the process of running additional simulations over planar bathymetry using variable wave conditions to obtain shear stress variability under spilling, plunging and surging breakers. More information will follow as additional data is obtained.
For additional information see Puleo at al. ICCE proceedings (2004) or Puleo et al. Coastal Engineering Journal (submitted).
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