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Hydraulic and design aspects of natural and constructed riffles in gravel-cobble bed rivers Walker, Daniel Richard

Abstract

The reintroduction of riffle-pool sequences has increasingly been promoted as an appropriate rehabilitation alternative for the re-naturalization of modified gravel-cobble bed channels. However, in the absence of hydraulic design guidelines, enhancement efforts often fail to evaluate the subsequent impacts to flood levels and sediment transport capacity. This has led to increased flooding and channel instability concerns. Accordingly, the intent of this thesis was to develop a hydraulic design procedure for the evaluation of the potential long-term hydraulic effects of riffle rehabilitation in uniform and degraded gravel-cobble bed rivers. Accordingly, a hydraulic analysis of natural and constructed riffle-pool sequences in four gravel-cobble bed streams was performed to investigate the variation in energy across constructed riffles at varying discharges up to and including bankfull. Significantly, riffle energy loss is shown to be substantial ranging between 50 and 100% of the total mini-reach loss. This indicates that the sampled rock-riffles are not being completely "drowned out" at higher stages, and that riffle-pool reconstruction may influence channel flow resistance, flood levels and sediment transporting capacity. Therefore, a comprehensive hydraulic analysis should be considered an essential component of rigorous riffle-pool rehabilitation design. A result of particular significance to the hydraulic design and analysis of rock-riffles is the strong relationship observed between riffle energy loss and amplitude for the rock-riffles sampled. This suggests the resultant rock-riffle energy loss in similar channels can be effectively described using a project-specific design variable. Drawing on this observation, an iterative procedure for the hydraulic design and analysis of rock-riffles in uniform or channelized gravel-cobble bed rivers is developed. Specifically, the procedure, which includes quantitative guidelines for the design of rock-riffle amplitude, spacing and stability, considers both the expectant energy loss and the potential effects on channel sediment transport capacity. This enables the creation of design channel profiles that better reflect the individual catchment and reach characteristics, the governing fluvial processes, and the specific rehabilitation goals and objectives. When used in association with appropriate geomorphic, hydraulic, hydrologic and ecologic appraisal, this new hydraulic analysis procedure should therefore provide a sound basis for the effective design and evaluation of riffle-pool rehabilitation projects in relatively steep gravel-cobble bed rivers

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