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Hydraulics of East Creek, British Columbia : a headwater stream with high relative roughness Perkins, Tobiah John

Abstract

Headwater streams provide important aquatic habitat, link hillslope sediments with lowland systems, and are an extensive and important component of the fluvial system. Headwater streams are characterised by steep gradients, poorly sorted sediments, large roughness elements and often exhibit diverse morphologies, described as step-pool, rapid or riffle-pool. Despite their importance, headwater streams have received relatively little attention compared to lowland systems and analysis has centred on approaches developed lowland rivers assuming hydrostatic pressure distributions and low channel gradients. In order to investigate the applicability of these methods in a small headwater streams, two hydrodynamic models were applied. The 1-dimensional model, HEC-RAS and 2-dimensional model, River 2D were use to characterise the hydraulics of East Creek, British Columbia. Two reaches, one with rifflepool morphology, the other with rapid morphology were studied. Both reaches exhibit high relative roughness. The hydrodynamic models were calibrated to observed water surface data by adjusting the roughness coefficient employed by the model and the results were compared to observed depths and velocities. Two bedload transport formulae were used in conjunction with the models to provide an indirect test of the hydraulic results. Both hydrodynamic models were able to replicate the observed conditions with a reasonable degree of accuracy, although extensive calibration was required, particularly in HEC-RAS and the roughness coefficient incorporated many resistance components (e.g grain roughness, form roughness). Similar results were found in certain areas with River 2D. When applied predicatively, neither model accurately captured the variation in flow resistance and thus calculated results deviated from the observed conditions. It was also found that River 2D was unable to model very low flows were form resistance from large bed material was high. Results of the sediment transport analysis were also poor, but further consideration of both hydrodynamic and bedload transport model resolution may provide more reliable results.

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