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