Influence of Tide and Runoff on Saltwater Intrusion in the Qiantang River Estuary, China

Qiantang River Estuary, as one of main fresh water resources for urban zone, has been threatened by salt water intrusion. Due to the combined influence of tide and runoff, the mechanism of saltwater intrusion is extremely complex. In view of this, research on the variation characteristics of chlorinity along Qiantang River Estuary is conducted based on chlorinity data. Further, analysis on effects that tide and upstream runoff exerting on chlorinity at intake is carried out to ensure urban water security. The chlorinity and tide data was obtained synchronously from automatic real-time monitoring systems at Zhakou, Qibao, Cangqian, Yanguan and Ganpu stations with sampling interval of 5 minutes. Results show that chlorinity Yanguan and Ganpu are mainly influenced by tide with period of 12.4 h; chlorinity at other stations are affected by runoff obviously. The peak chlorinity values at all observed stations increase with increase of tide ranges. The increase of upstream runoff enables to reduce the peak chlorinity. It is feasible to extract water under average tide and average runoff.