Abstract
The study is focusing on the treshold of sand motion under random waves with following current. The analysis is based on some flume experiments realized over a natural sand bed for different flow conditions. The main results comes as a map of the probabnility to exceed the treshold of sand motion, as a function of a wave and a current mobility parameter. These observations are compared to methods predicting the bed shear stress using an equivalent monochromatic wave, and links between the probability of exceeding the critical shear stress for initiation of sand motion and the calculated maximum bed shear stress are found.