Effects of density and size distribution on the erosion of the adult cockle Cerastoderma edule

A series of experimental flume experiments were carried out to evaluate the passive transport of the adult cockle Cerastoderma edule with shell sizes from 15 to 35 mm. The purpose of the study was to measure the erosion of this bivalve under controlled laboratory conditions, reproducing the current velocities and the cockle size distribution found at the Lombos do Ulla river mouth in Spain during high river discharges. Increasing velocities (0.29, 0.31, 0.35, 0.40, 0.44 and 0.47 m s–1) were applied to the bivalves, analysing the influence of population density (500 and 1000 ind. m–2), activity (live vs dead individuals), and mollusc size and distribution. To assess the effect of bivalve size distribution on the cockle transport, uniform and non-uniform cockle size distributions were tested. The particle image velocimetry technique and the double-averaged methodology were used to determine velocity fields during the experiments. In the experiments cockle erosion rates were found to be directly related to the flow velocity and inversely related to the population density. The erosion behaviour of uniform and graded cockle size distributions showed differences similar to those found in the bedload transport of uniform and non-uniform sediment mixtures. Our results provide a mechanism to explain the observed physical transport of bivalve populations after large storm events.