Erosion And Deposition Of Local Seabed In Hangzhou Bay

The operational life of water intake of nuclear power plant is influenced directly by erosion and deposition of seabed. To analyze and forecast the erosion and deposition near the water intake of nuclear power plant not only has theoretical significance of estuary dynamics but also provides important practical value for ensuring normal operation of plant. Based on the fifteen measurements of seabed morphology near the water intake of Qinshan Nuclear Power Plant and the corresponding data on runoff and tidal range, large scale mathematical model of tidal flow, digital analysis of seabed and BP network model of erosion and deposition are used to analyze evolution mechanism and the variational law for erosion and deposition of local seabed in Hangzhou Bay.There are pools and shallow shoals near the north bank of Hangzhou Bay (Section A), where the water intake of Qinshan Nuclear Power Plant is located. Its mechanism of erosion and deposition in Section A is different from that in the other research area (Section B). In Section A, the pool scours while the shallow shoal silts during flood tide, while the result is opposite during ebb tide. The ratio of flood velocity to ebb velocity determines the net erosion or deposition in pools and shallow shoals. In Section B, the flood current brings scouring while the ebb current provokes silting, and the governing factors of seabed evolution are the peak value of erosion and deposition during flood and ebb tide.Digital analysis of erosion and deposition on the seabed indicates that the increase of upstream runoff will promote silting in research area while the increase of local tide range trends to scour. For the same conditions of runoff and tidal range, the amount of deposition in the pools increases after water intake built which proves that the water intake will accelerate silting.Applying BP network model for the quantitative calculation of erosion and deposition on the seabed, its thickness of deposition reaches to 0.502 m in 2003. This result is consistent with the mean annual deposition of 0.41 m from digitial analysis of erosion and deposition during the three years of topography surveying. If the upperstream and downstream boundary conditions do not change obviously, the thickness of deposition forecasted by BP model will reach 4 to 5 meters in future 10 years.A new concept, the time-dependent sediment transport capacity, is proposed in order to reflect the time-varying properties of flow and sediment transport in estuaries. It is suggested that sediment concentration achieves maximum (or minimum) at a critical equilibrium point between non-equilibrium states of sediment-carrying shortage (excess) and sediment-carrying excess (shortage) during acceleration (deceleration) of tidal flow. Based on this equilibrium mechanism the hydraulic parameters and concentrations are chosen from the unsteady processes of flow and sediment in several estuaries. A general formula for time-dependent sediment transport capacity is developed, based on which a formula for the calculation of erosion and deposition on the seabed with clear physical concept is proposed. It provides a new way for numerical calculation of seabed evolution.