Sediment transport and channel morphology of small alluvial channels

Abstract

A number of geomorphological and hydrological factors interact at multiple scales to control sediment transport processes and the resulting morphology of river channels. Predicting how these channels will adjust in response to climate and land-use change requires a thorough understanding of these processes at multiple spatiotemporal scales. Researchers’ ability to investigate these processes at multiple scales has been limited by the dearth of comprehensive long-term sediment monitoring programs. To address this gap, this thesis examines the spatial and time dependence of sediment transport and channel morphodynamics using a 20-year detailed dataset from Goodwin Creek, Mississippi (USA). To examine the timescale dependence of fine sediment transport, we apply wavelet transforms to time series of streamflow, suspended sediment concentration, and sediment load for 11 monitoring stations at Goodwin Creek, with each sub-watershed representing a different land-use and channel boundary condition. We find that although reduced cultivation has caused a decline in fine sediment transport across all time scales, the degree of this decline is considerably influenced by the composition of streambed, and the presence of physiographic features (e.g., gullies, ponds). Over longer timescales, the wavelet transforms suggest that sediment supply shifts from readily accessible to more distant sources. To examine the dependence of spatial scale on sediment transport, we construct sediment budgets for fines, sand, and gravel for two adjacent reaches with contrasting morphological settings. We demonstrate that along-channel variations in gravel and sand storage are controlled by sediment availability within the channel. Substantial changes in fine sediment storage, however, occur during medium magnitude, long-duration flood events. The effectiveness of these events was determined by riverbank stability and near-bank sediment storage inherited from historical flows. Finally, we examine the spatial dependence of bedload transport at the sub-reach scale using a 2D hydraulic model to estimate spatial distributions of shear stress and calculate fractional sediment mobility at East Creek, British Columbia. Results from this model indicate that sediment transport is highly localized and sporadic during low to moderate flows due to the influence of channel morphology.