Impacts of Huge Amount of Sand Dredging on Riverbed Morphology and Tidal Dynamics of the Lower Reaches of the Dongjiang River and the Dongjiang River Delta
大量采沙对东江下游及东江三角洲河床地形和潮汐动力的影响

The sand dredging and its impacts on riverbed evolution and changes of the tidal dynamic in the Dongjiang River network and lower reaches of the Dongjiang River are analyzed in this paper.The huge amount of sand,totally 0.332 billion m3 from 1980 to 2002,was mined from the riverbed of the Dongjiang River network and lower reaches of the Dongjiang River.The main impacts of sand dredging on the riverbed evolution include increasing substantially of the channel capacity,lowering of the average riverbed elevation,and deepening of the water depth and decreasing of the longitudinal riverbed gradient.The main tidal dynamic changes are as follows:(1) In the upper part of the Dongjiang River network and lower reaches of the Dongjiang River,the tidal water level drops apparently,the tidal range widens,the flood tidal duration goes longer and the amplitudes of the main tidal components become bigger.(2) The tidal wave spreads faster.(3) The limits of the tidal level,tidal current and salt water move upwards.(4) The tidal dynamic in the upper part of the Dongjiang river network and lower reaches of the Dongjiang River intensifies greatly.The above changes are mainly caused by the dramatic change of riverbed topographic alteration shaped as a result of the huge amount of sand dredging.