This paper presents the results of a hydrodynamic study of a cockle population from the Ulla's estuary in Galicia (Spain). An analysis of the parameters has made it possible to determine the hydraulic characteristics of whole cockles during their threshold of movement and suspension.
Dynamic analysis of the forces exerted on the submerged cockles led to a detailed characterization of a final population of 11 cockles, including their characteristic dimensions in addition to shell and body density. The friction angle between the existing sediment in the Ulla's estuary and the cockle shells was also recorded. The particle image velocimetry (PIV) laser technique was successfully applied to obtain an accurate calculation of the threshold velocity fields. This experimental tool provides the means to experiment with various approaches of the velocity governing the onset of motion and to obtain a more detailed definition of the parameters involved. Applying the PIV technique, three drag entrainment coefficients were obtained with different velocity profiles. Using the depth-averaged velocity profile, a constant entrainment drag coefficient value of 0.43 was obtained for this cockle family, whereas the use of the upper tangential velocity on the cockle resulted in a value of 0.56 with a wider range of variation. This velocity may be considered a more intrinsic parameter of the movement of the cockle.
The process of cockle sedimentation was also monitored, enabling us to ascertain the settling velocity and to calculate the settling drag coefficient. Both parameters present constant values in the group of cockles analyzed, with mean values of 0.36 m s−1 and 1.08, respectively. The settling drag coefficient was also calculated. A comparison of these two experimental values with other existing reports exhibited similar results, thus validating both the procedures used and the effect of the shape factor on the settling drag coefficient value.