Evolution Of Mouth Bar And Its Response Of Human Activities In Qiantang Estuary

Mouth bar is a production of runoff and tidal dynamics, it is located at the throat on the way from river to sea. The change of runoff and tidal dynamics and sediment is quite big in estuary area. At the same time, human society and economy are more developed in this area, thus estuary always has been concerned. Mouth bar located at inside or outside of the river mouth induced by the difference of the runoff/tide ratio of the estuary. This article analyzed the annual transformation and long term evolution of the Qiantang mouth bar based on long time data in the estuary, and built the relationship between natural factor and the mouth bar elevation, try to set up a self-adjustment system of the estuary riverbed and the balance of the mouth bar.Qiantang mouth bar is mainly combined with fine-grained sand. The vertex of the mouth bar located from Qibao to Yanguan, The evolution of mouth bar was an important factor to keep the balance between runoff and tide in the self-adjustment system of the estuary riverbed. Qiantang mouth bar was eroded during flood season and deposited during dry season in a year, and was eroded during flood years and deposited during dry years in a long period. The vertex of the mouth bar was promoted to the downstream in continuous dry years, and back upstream in continuous flood years. The channl elevation and the average elevation of the mouth bar were compared. The result shows that the average elevation can better express the regularity of the riverbed evolution. The location of the mouth bar was discontinuous because of the the existing of several bend in the estuary, thus it won't have a good relationship with the runoff/tide dynamics.The response of the mouth bar on human impact was studied. After the Building of the Xin'anjiang Reservoir, the mouth bar was eroded and the transformation decreased. After the reclaiming in the estuary, the mouth bar was eroded and the transformation increased. A 1D numerical model was built to analyze the quantitative relationship between the runoff, the erosion/deposition evolution and the mouth bar. Different hydrodynamics corresponding to certain balance of the mouth bar. Human activities will destroy the original balance, and form a new balance. M was defined as a coefficient of the runoff/tide ratio at the cross section in the estuary. The vertex elevation of the mouth bar was mainly controlled by the runoff according to the analysis of the relationship between runoff and tidal dynamics and the mouth bar. The tidal current is just a factor to keep the balance of the mouth bar. The elevation of the mouth bar changed with the runoff, and it caused the change of the low tide level, then the tidal prism changed, the runoff/tide ratio of the estuary changed at the end.