NUMERICAL SIMULATION OF FLOW AND MORPHOLOGICAL EVALUATIONS IN RIVERS UNDER DAM-BREAK FLOWS

摘要

This paper presents a numerical model to simulate the unsteady water and riverbed profile due to failure of a large dam in a natural river. The mathematical model is built upon the conservative laws of shallow water hydrodynamics. Over the recent decades, there have been continuing efforts to understand dam-break hydraulics and the majority of the studies have been carried out in laboratories where channel bed is assumed to be fixed under such a highly transient flow. Dam-break hydraulics of natural rivers is indeed complicated, and involves not only water flow, but also morphological changes in the riverbed, which in turn has modified the predicted flow profiles. Although very few numerical models are proposed for mobile beds, applications were only in simple laboratory channels. The present investigation carried out in a Himalayan river clearly shows that the free surface profiles and hydrographs are greatly modified by natural riverbed mobility under high supercritical dam-break flows, which is extremely significant for flood predictions. Again as the change in the topography is not confined only to a rise (or a lowering) of the riverbed but also causes consequence in the form of a completely different cross section. Hence in the model another new important point is included to get the ultimate cross-sections of the river channel after the dam-break flood. The role of the form of the governing equations for developing suitable numerical simulation models for real-world complex conditions are also analysed. Numerical models applied earlier, providing comparable solutions, are referred as experimental and real dam break with small dam height is not found to be acceptable in case of large dam like Dibang failure conditions. To fulfil above, a simple, comprehensive model is proposed. The model is tried to be made detailed one with specific features such as consideration of the changes in riverbed elevations and frictions, channel cross sections, sediment entrainment, depositions across the bottom river channel, etc. These indeed are enhanced aspects of this study as compared to most of the past work available in literature where fixed riverbed has been considered.