SEDIMENT FLUXES TO THE SEA AS INFLUENCED BY SEDIMENT DELIVERY OF THE LOWER YELLOW RIVER

The lower Yellow River in the North China Plain is a large size sediment sink in the drainage basin At historical timescales,almost all Quaternary deposits in the North China Plain belonged to this sediment sink; in the modern times,according to the hydrometric data,400 Mt of sediment was deposited in this sink annually,representing 25 % of the total annual sediment load of the Yellow River Thus,sediment deposition in the lower Yellow River plays an essential role in controlling sediment fluxes to the sea To quantify the effect of this sediment sink,the ratio of sediment flux to the sea to the total sediment inflow of the lower Yellow River (RSF) is studied,on the basis of the data collected at the hydrometric stations The RSF has been found to decrease with the 0 05 mm percentage in suspended sediment,but increase with the 0 025 mm percentage Because the sediment carrying capacity of streamflow is lower for coarse fractions of sediment load than for fine ones,therefore,fine sediment has more opportunities to be carried to the sea The RSF was closely related with the combinations of runoff and sediment; the higher the event maximum suspended sediment concentration and the incoming sediment coefficient were,the lower the RSF The hyperconcentrated flows also have some influence on the sediment fluxes to the sea When hyperconcentrated flows occurred,large quantities of sediment may be deposited in the lower Yellow River,especially in the braided reach above Gaocun Thus,the sediment that could be carried to the sea decreased significantly,leading to a decline in the RSF This study shows that,when the event maximum suspended sediment concentration was higher than 300kg/m 3,the RFS became lower than 0 50 Changes in RSF can be closely related to different water and sediment source areas in the Yellow River drainage basin Four major runoff and sediment source areas have been identified in the upper and middle Yellow River basin and have different effects on the RSF The runoff from the two areas where relatively clear runoff is generated may enhance sediment carrying capacity and thereby more sediment is transported to the sea and the RSF increases On the contrary,the sediment from two major sediment source areas may greatly increase the sediment load of the lower Yellow River,leading to a decreased RSF Based on the annual and flood based data,a series of statistical relationships have been established,which may be used to explain and predict the effect induced by the changes in runoff,sediment load and sediment composition In the past 30 years,the RSF tended to decrease,reflecting the result of channel adjustment in response to the decreasing runoff Due to the continuously decreased runoff and sediment load to the lower Yellow River,the river channel shrunk and its sediment transporting ability declined As a result,although the sediment load into the lower Yellow River has shown a tendency of decrease,the sediment deposited in the lower Yellow River does not decrease significantly This resulted in a decrease in the RSF