Initiation of sand transport over coarse beds in tidal entrances

A tidal entrance often acts as a source or a sink for sand in the littoral zone. At many entrances in biologically productive regions the bed in the vicinity of the throat section (minimum flow area) is composed of sediment containing coarse material including large shells, with sand occurring in the interstitial regions between the shell-defined roughness elements. A stochastic relationship giving the critical bed shear stress for the initiation of sand transport under these conditions is applied to hydraulic and sedimentary data obtained from two entrances in Florida. The results agree well with the critical bed shear stresses derived from data based on the observation of sand movement at the bed. The entrainment function based on observations is found to be approximately three times the value obtained from Shields' criterion. A practical implication is that the use of Shields' criterion in the computation of the rate of sand transport will in general overpredict the rate through the entrance.