Coagulation and Settling of Suspended Sediment in the Jiaojiang River Estuary, China

Abstract

The Jiaojiang River estuary, Zhejiang Province, China, receives a mean freshwater discharge of 211 m³ sec^-1 with a mean suspended sediment concentration of 0.18 g l^-1 . Mean depth is less than 4 m and mean tidal range is 4 m. The water circulation and the distribution of suspended particle size and concentration were measured along the estuary in dry weather conditions in April 1991. The estuary was partially stratified in salinity and extremely turbid, with suspended sediment concentration in the turbidity maximum zone often exceeding 10 g l^-1. The tidal currents were asymmetric with stronger peak flood than ebb tidal currents. The suspended sediment was coagulated and the floc population was bimodal with day-dominant floes and silt-dominant floes of median size 50 and 500 µm respectively. The clay-dominant floes were not destroyed by ambient turbulence and were present throughout the water column. The silt-dominant floes were very porous, and readily broken by turbulence, had a settling velocity comparable to that of clay-dominant floes 1/2 to 1/3 their sizes, and existed only for tidal current speed < 0.5 m sec^-1. Both flocculation and hydrodynamic processes sort clay particles from silt particles in the estuary.