Saltwater Intrusion And Its Response To Topographical Descent In Oiantang River

The Qiantang River is a typical macro tidal estuary which is famous for tidal bore. Under the influence of strong tidal salt water intrudes upstream very far, sometimes intrudes into the water source area. In dry season as the runoff is weak, salinity near water intake remains high, water works can’t take water as usual, which is a big threaten to water safety of HangZhou. So the research of salt water intrusion in Qiantang River and providing methods to avoid salt water intrusion in drinking water sources is very which can not only guarantee the safety of water supply and improve the quality of life for residents but also promote urban economic development.Tide and salinity data is monitored continuously at7stations near the Qiantang River, which is used in this article to study salinity variation and distribution. Shortcomings such as low measuring frequency of salinity data and long interval is made up by numerous data. By analyzing the salinity data which is measured every five minutes,"Increasing and Decreasing Time of Salinity" is calculated accurately. The ratio of "Increasing and Decreasing Time of Salinity" at Qibao, Yanguan, Ganpu and Zhapu is15.71,1:1.67,1:1.12,1:1.08. According to these data, it is revealed that the ratio of rising time and declining time increases downstream. Further research shows that mutations happens in increasing process of salinity and usually last for5to10minutes. This phenomenon is notable at Yanguan station and it is related to tide bore. Except these mutations, salinity increasing rate is low. At mutation point of Yanguan station salinity increment is2.63, which is27.2%of the difference between maximum and minimum value, the time of mutation is only2.1%of increasing time. It is found that the variation of salinity increases gradually from Wenjiayan to Yanguan and decreases from Yanguan to Zhapu. The variation of salinity at Yangguan is the fiercest which is12.6times of Cangqian.A2D numerical model is used in this research. It shows that the maximum depth averaged water velocity at Qibao curve is1.85m/s, water level of concave bank is10cm higher than convex bank. It is revealed that salinity of concave bank is higher than convex bank by0.52. Research also shows that, when runoff at Wenjiayan border is 823m3/s and tidal range at Cangqian border is3.52m, there is a salinity front at Qibao at the time of strongest salt water intrusion, and the variation character of the front is analyzed in this article, which shows the front gradient increases when salt water goes upstream, and decreases when salt water goes downstream.Keeping the discharge at Wenjiayan and tidal range at Cangqian unchanged, a numerical simulation is conducted to exam the influence of topography on salinity intrusion. By analyzing the result of these two different topography, we found that after topographical descent, the tidal prism increased by3.7million m3every tidal cycle. A concept of "Counteract Discharge" is proposed in this article. When tidal range at Cangqian is3.52m and runoff at Wenjiayan is993m3/s, the "Counteract Discharge" turns out to be330m3/s. By using this "Counteract Discharge", salinity front moves downstream to its original position, but front gradient is larger than before, which causes the salinity from Zhakou to Qibao became smaller and0.5iso-salinity-line moves downstream by4km.