Characteristics Of Fine Cohesive Sediment's Flocculation In The Changjiang Estuary And Its Adjacent Sea Area

Based on the In-situ particle size spectra data from LISST-100C and the synchronously measured data of suspended sediment concentration (SSC), current velocity, salinity, water temperature and dispersed particle size spectra measured between the year 2003 and 2006, (1)Spatial distribution of floc mean diameter, and characteristics of In-situ suspended particle size distribution in the Changjiang Estuary and its adjacent sea area were studied; (2)The critical diameter for the flocculation of fine cohesive sediment was analyzed; (3)The effects of SSC, current velocity, salinity and water temperature on the characteristics of flocs were discussed; (4)The method for calculating settling velocity of In-situ flocs was given. These research works give the following results:(1) There is obvious flocculation in the research area, the spatial varition of floc mean diameter is evident, floc mean diameter increases gradually from inner to outer sea area; there are several types of In-situ particle size spectra: single-peak, double-peak and multi-peak. For the whole research area, it can be divided into 3 areas: the upper area of Hengsha Island of the Changjiang Estuary and the outer areas of the Changjiang Estuary and Hangzhou Bay are weak flocculation areas; the area of Hangzhou Bay is middle flocculation area; and the area of the TM of the Changjiang Estuary is strong flocculation area.(2) The critical diameter for the flocculation of fine cohesive sediment in the Changjiang Estuary is about 32.5 microns; while that value in Hangzhou Bay is about 20.0 microns; the different values for flocculation between these two areas maybe related to the different SSC, current velocity and salinity etc.(3) Under low SSC condition, SSC has little effect on flocculation, while there is great increase of floc settling velocity when SSC increases to a certain degree; the effects of current velocity and salinity on flocculation are very significant: the critical current velocity for increasing or decreasing flocculation is about 42cm·s^-1; when current velocity is less than 42cm·s^-1,it increases flocculation, while current velocity is larger than 42cm·s^-1, it causes large flocs to break up into smaller flocs or particles; salinity causes small particles to form larger flocs; current velocity and salinity cause the quarter-diurnal and semi-diurnal variations of floc mean diameter respectively; the response of flocculation is much bigger and quicker in the condition of salinity than current velocity; because higher the water temperature, bigger the floc mean diameter, so there is great difference of floc mean diameter and In-situ particle size spectra between summer and winter.(4) The method based on the data obtained from LISST-100C for calculating floc settling velocity is given; floc settling velocities are about 0.70mm·s^-1 during flood and 0.58mm·s^-1 during ebb tide in Hangzhou Bay; while in the Changjiang Estuary, floc settling velocity is 0.82mm·s^-1 during flood tide and is 0.84mm·s^-1 during ebb tide in summer; the floc settling velocity is 0.49mm·s^-1 during flood tide and is 0.42mm·s^-1 during ebb tide in winter; the Changjiang Estuary winter's floc settling velocity is less than that of Hangzhou Bay's in summer, and Hangzhou Bay's is less than that of Changjiang Estuary in summer.