Tidal Flow Field In Qiantang River And Its Response To Structures

World-famous tidal bore in the Qiantang estuary is an important landscape of Zhejiang Province. With the economic development of Hangzhou, several bridges have been built on Qiantang River and ones will be set in the future. Although it is thought that one bridge has little effect on current, it still needs further analysis that what is the joint effect of several bridges has on the tidal current field. So the studies on the tidal current field of Qiantang Estuary and its complex response to bridges have important theoretical and practical value.The Delft3D model is used to simulate the tidal flow field in the estuarine reaches from Wenjiayan to Cangqian. The results of calculation are well verified by the measured data, from which, tidal flow field in several bends of this estuary is first analyzed. Then tidal flow field is further simulated under several combination of different bridges in the reach from Zhakou to Qibao. Comparing with those under no bridge situation, we gain some significant cognition.One characteristic of tidal flow in the bends of Qiantang estuary is the transverse swing range of mainstream during ebb tide smaller than that during flood, and the obvious bifurcation of main streamlines existing. Another characteristic is that the velocities increased in the downstream estuarine bends, which is one of important differences between tidal estuary bend and common river bend. It is caused by the effect of tidal current strengthened seaward. Because response of tidal flow to dike during flood is different from that during ebb, the scour location of dike' s head changes alternately. The local hydrodynamic intensity and the bend curvature are main factors that make the transversal slope of water surface in the lower bend larger than that in the upper bend.In the case of only one bridge on the river, the numerical simulation of tidal flow field indicates that high tide level increases in the lower reaches and reduce in the upper reaches during flood tide but it reverses during ebb tide. The effect of a bridge on flow field mainly occurs near piers. The current velocity increases in wide bridge opening while it reduces in the other regions. During flood the flow velocity in the lower reaches of a bridge tide diminishes evidently comparing with that for the scheme of no bridge, while during ebb the flow velocities in the upper and lowerreaches of the pier reduce together. The flow velocity around the bridge pier is about 1/3 to 1/2 of that in the center of the bridge opening. No matter during the flood or ebb, the flow velocity will increase in wider bridge opening for navigation while reducing in small bridge opening. It may be related to the transverse effect of waterproofing action of the bridge pier.Comparing the tidal current field calculated for the scheme of several bridges with that for the scheme of no bridge, it shows that the decrease range of flow velocity is relative to the bridges number. With the increase of bridges number, the range of lower flow velocity at the back of each bridge pier enlarges. At the same time the flow velocity in wide bridge opening decreases slowly and the flow velocity in the case of four bridges is nearly equal to that in the case of no bridge. Under the condition of several bridges, the water level at the upstream of the upper bridge must increase, and that at the downstream of the lower bridge must reduce during ebb. The more bridges on the river, the more the water level of the lower reaches will reduces.