THE FLUVIAL PROCESSES OF THE BIG SAND BAR INSIDE THE CHIEN TANG CHIANG ESTUARY
NING CHIEN, HAN-SIANG SIE, CHIH-TEH CHOW QUAN-PIN LEE (Institute of Hydrotechnical Research) (Institute of Hydraulic Research, Chekiang Province)
There exists an enormously large sand bar with a total length of 130km and a peak of 10 m above the baseline inside of the estuary of the Chien Tang River. Its formation is closely related to the evolution of the Hang-Chow Bay. Following the development of a funnel-shaped bay, the sea tide is greatly strengthened and the zone in which the fresh water and the tidal flow balance each other shifts inland. The large amount of sediment originating from the outside sea is pushed into the estuary and de-posited there to form the big sand bar. Based on the field data collected from 22 different estuaries, it appears that the ratio between the fresh water discharge (bed-forming discharge), Q1, and the mean tidal dis-charge, Q2, is the main factor governing the position of sediment accretion. For Q1/Q2 larger than 0.10, the sand bar is formed outside of the estuary or at the neighborhood of the channel outlet. For Q1/Q2 smaller than 0.02, the sand bar is pushed into the estuary. For Q1/Q2 = 0.02-0.10, a transition between the two seems to exist-there is a seasonal movement of sediment up and down of the estuary without resulting in permenant bar formation. The extent of the sand bar depends on the amount of sediment supply from the outside source. The sand bar will assume great prominence only if there are sediment-carrying rivers entering the sea within that area, which continuously bring in sediment to be deposited on the shallow sea floor. If, on the other hand, the river itself brings a large quantity of sediment into the estuary, then the funnel-shaped bay cannot be kept in existence, thus undermining the foundation on which the sand bar is formed. From engineering view point, the sand bar at the Chien Tang Estuary is essentially at equilibrium. And yet the sediment continuously shifts up and down of the bar, resul-ting in a very unstable stream channel. On flood seasons, the apex of the sand bar is lowered and moves seaward. After Spring tides, the sand bar is again elevated with its apex pushing landward. There exists a quantitative correlation between the elevation and location of the peak of the sand bar and the Q1/Q2 prevailing at that period. Fol-lowing the changes in dynamic conditions of the flow, a distinct seasonal variation of the channel bed above Chang-Chien can be noticed. The existence of a big sand bar within the estuary makes the Chien-Tang Estuary different in many respects with others. The vigorous deformation of the sea tide, the formation of the notorious bore above Chi-Shan, the voluminous aggradation and degra-dation of the stream channel, the great variability of the location of the thalweg, and the backwater effect felt by the Fu-Tsun River upstream are all related in one way or another with the presence of the sand bar inside of the estuary.