Flocculation and sedimentation on the Po River Delta

Abstract

With the goal of improving understanding of the effect of flocculation on the formation of fine-grained deposits on continental shelves, hydrographic profiling, in situ imaging of suspended matter, and collection of surficial sediment samples were conducted at the Po River Delta in June 2001. These data show that during medium flow conditions (1920 m 3/s), sedimentation occurs rapidly immediately offshore of the main distributary, Po della Pila. Rapid sedimentation is promoted by large rapidly sinking flocs forming in the river well upstream of the mouth. The delivery of fine sediment to the seabed at the mouth of the Po is sufficient to overwhelm the erosive effects of waves and currents, leading to accumulation of mud in water depths as shallow as 4 m. On cross-shelf transects 2 km north and south of the mouth, however, suspended sediment supply from the river is reduced to the point that mud accumulates only seaward of the 8-m isobath. Along the central transect, suspended sediment concentration decreases rapidly seaward of the 6-m isobath where the emergence of a more organic-rich population of flocs along a mid-water density interface is suggested. Energetic activity along the 15-m isobath likely promotes resuspension with the potential for removal of material from the delta. Further investigation of floc properties, namely the relationship of floc size to settling velocity, is necessary to establish the degree to which the suspension is flocculated during transport and deposition.