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Title: One-Dimensional Modeling of Dam-Break Flow over Movable Beds

item WANG, S.S.Y.

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2005
Publication Date: 1/1/2007
Citation: Wu, W., Wang, S. 2007. One-Dimensional Modeling of Dam-Break Flow over Movable Beds. Journal of Hydraulic Engineering, ASCE, 133(1):48-58.

Interpretive Summary: A 1-D model has been developed to simulate the fluvial processes in flow during dam failure over movable material. The hydrodynamic model adopts the generalized shallow water equations, which consider the effects of sediment transport and bed change during the flow event. The sediment model computes the nonequilibrium transport of bed load (coarse sediment that moves in a shallow layer close to the bed surface) and suspended load (fine sediment that moves through the entire flow cross-section). The effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity. In particular, a correction factor is proposed to modify van Rijn’s formulas of equilibrium bed-load transport rate and near-bed suspended-load concentration for the simulation of sediment transport under high-shear flow conditions. The governing equations are solved by an explicit finite-volume method with a first-order scheme for intercell fluxes. The model has been tested for two experimental cases that yielded fairly good agreement between simulations and measurements. The sensitivities of model results to parameters such as the sediment nonequilibrium adaptation length, Manning’s roughness coefficient and the proposed correction factor were verified. The proposed model has also been compared to an existing model, and the results indicate that the new model is more reliable.

Technical Abstract: A 1-D finite-volume model has been developed to simulate sediment movement in flow over a movable bed following a dam-break. The effects of sediment concentration and bed change on the flow are considered by using the continuity and momentum equations. Sediment transport modules simulate the non-equilibrium transport of bed load and suspended load. The bed-load transport capacity and suspended-load near-bed equilibrium concentration are determined by using the van Rijn’s formulas which are modified to include the effect of sediment concentration on sediment entrainment. An explicit algorithm is adopted to solve the flow, sediment transport, and bed change equations. The model has been tested against results of two experiments involving cases of small-scale dam-breaks. The simulated water and bed surface profiles are in fairly good agreement with the measured data. The results show that the flow and sediment discharges rapidly increase to their maximum values and then gradually decrease. Intense erosion occurs near the dam-break wave front. Sediment moves mainly in bed load with small-scale dam-breaks, but in suspension in the large-scale case tested here. The sensitivity of the model results to the sediment non-equilibrium adaptation length, Manning’s roughness coefficient and the proposed correction factor k sub-t has been analyzed. It has been shown that the influence of the bed-load nonequilibrium adaptation length L sub-b and suspended-load adaptation coefficient alfa on the simulation results may exist at limited levels, and the influence of Manning’s roughness coefficient is very small. The influence of the correction factor k sub-t is somehow important to the simulated erosion magnitude. The present model has been compared with the existing movable-bed dam-break flow model by Cao et al. (2004). The present model yielded much better results. In the hypothetical case of large-scale dam-break flows over a movable bed, both models predict a hydraulic jump that initially appears near the dam site, and then propagates upstream and gradually reduces in height. However, a separate bore upstream of the wave front is predicted by Cao et al.’s (2004) model but not by the present model. The model of Cao et al. (2004) exaggerates the separate bore because of unreasonable prediction of sediment entrainment by the original Cao’s (1999) formula. This can be avoided by imposing an upper bound to the prescribed sediment entrainment rate.