港湾深槽骤淤的条件探讨

Study on the conditions of rapid siltation in deep trough in the bay

深槽骤淤指深槽底部在特定条件下产生的快速淤积,通常以浮泥形式出现.综合分析杭州湾、三门湾和象山港的地形地貌、水文泥沙及深槽固定剖面风暴前后或大、小潮期间的地形变化得出港湾深槽产生骤淤的基本条件:(1)有丰富的细颗粒物质的供应源;(2)迅速衰减的动力条件;(3)存在明显的负地形.三个条件必须同时得到满足,深槽才能产生骤淤.杭州湾泥沙来源丰富,终年悬沙浓度高,东试挖槽附近受长江冲淡水次级锋面和钱塘江冲淡水锋面影响较大,因此在一个大小潮周期内水动力条件减弱时挖槽处可产生骤淤.三门湾泥沙来源较丰富,正常天气的年份内潮滩处于缓慢淤积状态,而深槽处于冲淤基本平衡状态,但在风暴等灾害性天气下发生强烈的滩、槽泥沙交换,若风暴后处于中小潮汛期则深槽发生骤淤.象山港由于环境隐蔽,水清沙少,沿岸潮滩狭窄,缺少骤淤的物源,即使风暴作用后鹰龙山深槽也没有产生骤淤.

The rapid siltation of deep trough means the suspended sediment sinking very fast to the bottom of the trough within a quite limited period, and it makes up of fluid mud layer. A comprehensive analysis is done based on the data of duplicate measurement of seabed profile depth, topography, hydrology and suspended sediment in the Hangzhou, Sanmen and Xiangshan Bays. The results show that rapid accumulation happens under three conditions: 1 fine sediment is very abundant; 2 hydrology diminishes quickly; and 3 there exists a obviously concave landform. These three factors must be satisfied at the same time, otherwise the rapid siltation cannot occur at the deep trough. Since the Hangzhou Bay is full of fine sediment, the suspended sediment concentration is high all the year and it is influenced by the secondary Changjiang plume front and the Qiantang plume front at the east trial dredged trough, the rapid deposition can be measured when the hydrology turns weak from spring to neap tidal period. The suspended sediment is plenty comparatively in the Sanmen Bay, the seabed of tidal flat is kept stable with a little siltation and the trough is nearly in the equilibrium of erosion and siltation within a normal year. Only when the storm hits the bay and the following days are between middle and neap tidal period after the storm past, a large number of suspended sediment eroded obviously from tidal flat will sink quickly to the deep trough. However, the Xiangshan Bay is sheltered with hills around the bay and islands in bay mouth, the water is very limpid and the suspended sediment is quite little, so the rapid accumulation cannot be happened in the inner bay trough even with the impact of storm.