Sediment entrainment on a wave-graded shelf, Roussillon, France

Abstract

The sands of the inner shelf off the Roussillon coast of the microtidal Golfe du Lion display a seaward-fining texture and merge, in water deeper than 20–30 m, with midshelf muds. There is a qualitative correspondence between the textural gradient and the seaward-attenuating, annual (surface gravity) wave power expended by tangential stress on the shelf surface. Various equations, used to quantify the relation of sediment texture to fluid energy, predict the trend of the observed textural gradient, but overestimate its value. An empirical threshold equation (Komar and Miller, 1975a, b) and a theoretical null point equation (Johnson and Eagleson, 1966) give comparable results, implying that both may be describing the same state. There is a disparity between the predictions of an incipient motion equation (Johnson and Eagleson, 1966) and the threshold equation, probably because static instability of grains occurs prior to dynamic instability of grains in an increasing fluid velocity field. Theory forecasts that, in water shallower than 30 m, the sands are frequently entrained by shoaling waves, particularly during the winter months; the threshold of movement of sands in 5 m of water is exceeded for more than 40% of the year. Hence the grain size of the shallow-water sands does not reflect the maximum energy level of the environment.