Resuspension phenomena of benthic sediments:the role of cohesion and biological adhesion

The incipient motion conditions of benthic sediments are analyzed and an original parametrization for the critical shear stress is proposed. Starting from momentum balance considerations on a single sediment floc, the Shields dimensionless mobility parameter in incipient motion conditions is expressed also as a function of both cohesive and adhesive forces that concur to stabilize the superficial layer of the benthic sediment. The entrainment model is validated on the basis of laboratory experiments carried out on natural benthic sediments sampled from three alpine Italian lakes, having different trophic conditions and different compositions of the bed. The experimental results show that the cohesive effects decrease as the sediment water content increases. Moreover the role of bioadhesion is experimentally evaluated: the critical shear stresses of “living” sediments are higher than that measured on similar but “dead”, poisoned sediments. This stabilizing effect shows a variation in time on a seasonal time scale, accordingly to the literature indications where biological adhesion is related to phytoplankton and bacteria activity, the life and growth of which obviously depends on seasonal variations.