Development and Examination of a Pseudo-3D Model for Density Current and Sediment Movement
|關鍵字:||分層;三維;異重流;水庫;Layer-integrated;Three Dimensional;Sediment;Density Current|
The sediment transport behavior in reservoirs can be classified into delta build-up, density current and turbidity layer. The sediment-laden flow entering into the reservoirs behaves like a jet shooting into a stilling basin, and the sediment deposits to form a delta at the entrance due to sudden deceleration. The numerical models for delta formation have been well developed and frequently used to simulate and analyze the flow phenomenon. In addition to the delta formation, the silt and clay plunging beneath the clear water due to density difference and moving toward the dam, is recognized as density current. Most of the numerical models being developed for density current were vertical 2D. However, the behaviors of flow and density current in reservoirs are indeed three dimensional. Transport behaviors of density current and its characteristics including marching speed of head, momentum exchange, sediment deposition should be analyzed by using 3D models. Under the past support of National Science Council, Hung (2007) proposed a pseudo-3D layer-integrated model in which a quadratic shape function used to approximate the velocity distribution in layer ensures the continuity of velocities and shear stress at interfaces. The water depth solved directly without the need of 2D sub-models, whereas the computation is simplified and stable. In this study, Hung’s model will be extended to cover the properties including the unstructured grid and σ-coordinate, and therefore, to allow the numerical grid adapting to complex geometries and terrains. The mixing length concept predicting vertical turbulent viscosities will be embedded into the flow model. The state function for density variation and transport mechanisms including sediment deposition, momentum exchange will be also adopted in this model. In lieu of simulation study performed for some published laboratory data and measured data from Shi-Men Reservoir, the transport mechanism of density current will be intensively analyzed as the basis for planning sediment sluicing strategies.