Sediment Transport and Morphological Evolution In Rivers In Response to Catastrophic Dam-Break Floods

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0510830Intellectual Merit: Extensive sediment transport research has been accomplished primarily insteady flow, although significant amounts of sediment movement occur in unsteady flow.Despite some early attempts, sediment transport in unsteady flow remains an open researchquestion in hydrologic sciences, mainly due to the inherent mathematical complexities in thefully coupled flow and sediment equations, and the technical difficulties encountered inexperimental measurements. Availability of a more sophisticated mathematical approach (Tsai2002, 2003, 2005 and Tsai and Yen 2001, 2003; Tsai and Kuai 2004 a,b; Kuai et al. 2004)suggests that it is time to revisit the problem of sediment transport in unsteady flow. Success ofthis pilot study would pave the way for a new approach to investigating the coupling of waterand sediment movement and to experimenting with transport of sediment in highly unsteadyflows such as dam-break induced floods.Research Hypotheses and Scientific Objectives: The major objectives of this basic research areto quantify the time-varying changes in water- and bed-surface elevation and sedimentconcentration associated with the passage of a dam-break induced flood and to examine thevalidity of the quasi-steady uniform flow assumption in unsteady sediment transport modeling.The basic hypotheses guiding the proposed research are that time dependence of inflowhydrograph affects sediment transport so that standard steady flow formulas will not provideaccurate estimates of sediment discharge, and that when analyzing the hydraulic response of abed to rapidly changing inflow hydrographs, the time scale of interest concerning bed elevationmay no longer be long compared to that of the flow so that only fully coupled models of flowand sediment equations would properly characterize the phenomena. The work proposed here isconsidered the first phase of our unsteady sediment transport research. Specifically, this researchaims to address the following issues: (1) For the simulation of a catastrophic flood induced bythe breach of a dam in a natural river, can the decoupled and steady uniform flow modelingapproach still yield satisfactory results? (2) Is the time scale of interest concerning bed elevationchange still long compared to that of the flow in rapidly changing flows such as dam-breakinduced floods? (3) How can the phase difference among sediment concentration, friction factor,flow velocity and depth, as evidenced in laboratory and field observations, be quantified andinterpreted? The proposed research intends to provide an improved physical framework to studysediment transport and morphological evolution in natural rivers in response to a highly unsteadyflow such as dam-break induced floods. The main research tasks involve model development,calibration, verification and comparison, collection and compilation of existing laboratory andfield data, and evaluation of critical flow and sediment parameters involved in thismorphological process.Broader Impacts: The most significant and broadest impacts of this project are twofold. First,research outcomes will provide useful information for policy decision-making by federal andstate agencies, such as evaluation of transport capacities of rivers and streams, estimation ofenvironmental impacts of riverbed and bank erosion and deposition, and assistance in theevaluation of design alternatives. Potential applications of the research results include design ofdams and reservoirs, floodplain analysis and management, and estimation of possible maximumscour during large flood events. Secondly, this proposed project would also have significanteducational impact on both undergraduate and graduate levels. Findings of this fundamentalresearch will be integrated into the teaching of science, math and engineering to benefit a broaderstudent community.