Estimating swash zone friction coefficients on a sandy beach

Video-based swash motions from three studies (on two separate beaches) were analyzed with respect to theoretical swash trajectories assuming plane beach ballistic motions under quadratic friction. Friction coefficient values for both the uprush and backwash were estimated by comparing measured swash space-time trajectories to these theoretical expectations given an initial velocity and beach slope. Observations were made spanning high tides, and in one case, during a light rain. Analysis of over 4500 individual swash events showed that the uprush friction coefficient was nearly constant during all three studies with a mean value of roughly 0.007 and showed no trends over a tidal cycle, In contrast, backwash friction coefficient values varied over the tidal cycles ranging between 0.01 and 0.07 with minimum values corresponding to the highest tides. Although these values are close to the theoretical estimates based on a Law of the Wall formulation and values commonly referenced in the literature, these observations show a consistent tendency for backwash friction estimates to greatly exceed uprush friction estimates, The disparity between uprush and backwash friction coefficients can be partially attributed to the exclusion of a pressure gradient term in the ballistic model. However, results indicate that backwash friction coefficients adjusted to account for this effect may be three times larger than the uprush friction values during lower tides. This tidal dependence for backwash friction coefficients is attributed to a complex interaction between swash infiltration and entrained sediment loads. These findings imply that friction estimates (necessary for sediment transport calculations and hydrodynamic predictions) based solely on grain roughness may not be correct for backwash flows. (C) 2001 Published by Elsevier Science B.V.