Sediment Transport and Morphological Evolution In Rivers In Response to Catastrophic Dam-Break Floods
Christina Tsai Principal Investigator
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0510830<br/>Intellectual Merit: Extensive sediment transport research has been accomplished primarily in<br/>steady flow, although significant amounts of sediment movement occur in unsteady flow.<br/>Despite some early attempts, sediment transport in unsteady flow remains an open research<br/>question in hydrologic sciences, mainly due to the inherent mathematical complexities in the<br/>fully coupled flow and sediment equations, and the technical difficulties encountered in<br/>experimental measurements. Availability of a more sophisticated mathematical approach (Tsai<br/>2002, 2003, 2005 and Tsai and Yen 2001, 2003; Tsai and Kuai 2004 a,b; Kuai et al. 2004)<br/>suggests that it is time to revisit the problem of sediment transport in unsteady flow. Success of<br/>this pilot study would pave the way for a new approach to investigating the coupling of water<br/>and sediment movement and to experimenting with transport of sediment in highly unsteady<br/>flows such as dam-break induced floods.<br/>Research Hypotheses and Scientific Objectives: The major objectives of this basic research are<br/>to quantify the time-varying changes in water- and bed-surface elevation and sediment<br/>concentration associated with the passage of a dam-break induced flood and to examine the<br/>validity of the quasi-steady uniform flow assumption in unsteady sediment transport modeling.<br/>The basic hypotheses guiding the proposed research are that time dependence of inflow<br/>hydrograph affects sediment transport so that standard steady flow formulas will not provide<br/>accurate estimates of sediment discharge, and that when analyzing the hydraulic response of a<br/>bed to rapidly changing inflow hydrographs, the time scale of interest concerning bed elevation<br/>may no longer be long compared to that of the flow so that only fully coupled models of flow<br/>and sediment equations would properly characterize the phenomena. The work proposed here is<br/>considered the first phase of our unsteady sediment transport research. Specifically, this research<br/>aims to address the following issues: (1) For the simulation of a catastrophic flood induced by<br/>the breach of a dam in a natural river, can the decoupled and steady uniform flow modeling<br/>approach still yield satisfactory results? (2) Is the time scale of interest concerning bed elevation<br/>change still long compared to that of the flow in rapidly changing flows such as dam-break<br/>induced floods? (3) How can the phase difference among sediment concentration, friction factor,<br/>flow velocity and depth, as evidenced in laboratory and field observations, be quantified and<br/>interpreted? The proposed research intends to provide an improved physical framework to study<br/>sediment transport and morphological evolution in natural rivers in response to a highly unsteady<br/>flow such as dam-break induced floods. The main research tasks involve model development,<br/>calibration, verification and comparison, collection and compilation of existing laboratory and<br/>field data, and evaluation of critical flow and sediment parameters involved in this<br/>morphological process.<br/>Broader Impacts: The most significant and broadest impacts of this project are twofold. First,<br/>research outcomes will provide useful information for policy decision-making by federal and<br/>state agencies, such as evaluation of transport capacities of rivers and streams, estimation of<br/>environmental impacts of riverbed and bank erosion and deposition, and assistance in the<br/>evaluation of design alternatives. Potential applications of the research results include design of<br/>dams and reservoirs, floodplain analysis and management, and estimation of possible maximum<br/>scour during large flood events. Secondly, this proposed project would also have significant<br/>educational impact on both undergraduate and graduate levels. Findings of this fundamental<br/>research will be integrated into the teaching of science, math and engineering to benefit a broader<br/>student community.