Grid stirred turbulence: Applications to the initiation of sediment motionand lift-off studies

A laboratory experimental set - up for studying the behaviour of sediment i n presence of a turbulent field with zero mean flow is presented. Particula r interest is shown on the initiation of sediment motion and in the sedimen t lift; - off. Some examples of the results obtained with this set-up are s hown. A turbulent velocity u' lower than that u estimated by the Shield diagram i s required to start sediment motion. The minimum u' required to start sedim ent Lift - off, is a function of sediment size, cohesivity and resting time . The lutocline height depends on u', and the vorticity at the lutocline se ems constant for a fixed sediment size. Combining grid stirring and image a nalysis, sediment vertical fluxes and settling speeds could be measured. (C ) 2001 Elsevier Science Ltd. Ali rights reserved.