长江口北槽抛泥流速和悬沙浓度时空分布观测

Field observations of current and suspended sediment concentration during sediment disposal in the Changjiang (Yangtze) Estuary

河口泥沙运动有其独特的规律,需要采用高分辨率的观测手段进行系统的现场观测,以此发现河口流速和泥沙分布结构,进而探讨其形成机制,应用声学多普勒流速剖面仪和声学悬浮泥沙观测系统,通过定点和走航式观测长江河口不同潮型和流态下流速和悬浮泥沙浓度时空分布发现:(1)不同潮型出现高浓度“事件”的次数和成因存在差异,中潮型出现高浓度“事件”的可能性最大;(2)抛泥泥沙浓度垂向分布至少有3种结构类型,即上小下大的“L”型、指数型和上大下小的“漂浮”型;(3)受抛泥泥沙输移的影响,断面流场形成低流速区,它们的强度随落潮流的扩散逐渐减弱;(4)不同潮型的落潮流表现出不同的输移行为,大、小潮型落潮流偏北,中潮型落潮流偏南;(5)在落潮流和颗粒重力共同作用下抛泥泥沙同时存在输移扩散和沉降过程,小潮型抛泥泥沙主要就近扩散和沉降,中潮和大潮型抛泥泥沙输移扩散范围较远。

Acoustic Doppler current profile and acoustics concentration profiler are applied respectively via fixed-point serial and sailing observations to measure temporal and spatial variations of current and suspended sediment concentration at the North Passage in the Changjiang (Yangtze) Estuary. Field observations show that (1) near-bed high concentration events occur in various tides, with different frequencies and formation mechanisms. They take place frequently in a moderate tide, especially at the slack or maximum tides. (2) At least three types of vertical profiles of suspended sediment concentration are found out, that is, L-type with the upper concentration smaller and the lower larger, exponential type, and floating type with the upper concentration larger and the lower smaller. (3) low-velocity zones occur owing to the effect of dredged sediment disposal on the flow, which are located at the surface column or through the whole column, for the latter case, forming flow fronts. The intensity of the zones tends to decrease with the diffusion and settling of the disposal sediment. (4) The behavior of ebb currents differs at various tidal phases. The ebb currents are directed mainly to the north in the spring and neap tides, while they are directed mainly to the south in the moderate tide. (5) Both sediment transport diffusion and settling coexist during dredged sediment disposal, under the combined action of ebb current and gravity. Disposal sediment deposit is immediate in the neap tides, and it disperses far away in the moderate and spring tides.