Impacts of Hydromodification and Sediment Supply Alterations on Bedload Transport and Bed Morphology in Urbanizing Gravel-bed Rivers

Urbanization is known to change the hydrologic and sediment supply regimes of rivers, causing more frequent, flashier flood events (hydromodification) and a reduction and redistribution of sediment sources. Presently, the impact that these changes have on bedload transport in gravel-bed channels and the resulting impact on bed morphology remains largely unknown due to a lack of process based studies. A better understanding of how riverbed form and processes evolve with urbanization is critical as they are a primary factor in controlling stream stability, providing habitat for aquatic species and influencing flood elevations. Additionally, stream rehabilitation is becoming increasingly common in urban rivers and an understanding of how sediment transport dynamics change with the alterations common to urbanization is critical for a successful design. \n \nThis thesis explores the impact of urbanization, which is the combination of hydromodification and alterations to sediment supply, on the morphodynamics (linkages between channel form and process) of bedforms in gravel-bed rivers. Specific objectives are: 1) to determine if detectable differences in bed morphology exist between rural and urban rivers in the same hydrophysiographic region; 2) to characterize the sediment transport dynamics of a highly urbanized channel; 3) to investigate the differences in geomorphically significant flows and sediment transport characteristics for different levels of watershed urbanization; and 4) to generalize field specific results using a mobile-bed laboratory flume to investigate the sediment transport characteristics for different levels of watershed hydromodification. \n \nLongitudinal profiles of 11 rural and 9 urban watercourses with pool-riffle dominated morphologies in the same hydrophysiographic region (Southern Ontario, Canada) were investigated using three objective bedform identification methods; zero-crossing analysis, bedform differencing technique and residual pool depth analysis, as well as visual field identification. Results indicate that urban rivers possess deeper pools and a more topographically variable bed. \n \nA field investigation was undertaken to characterize event-based sediment transport dynamics of a highly urbanized gravel-bed river over a three year period. Mimico Creek, located in Toronto, Ontario, Canada, is nearly fully urbanized and lacks significant stormwater management controls, therefore making it a representative study reach. Bedload transport was measured using both active and passive sampling methods to characterize the mobility and transport dynamics of the entire range of surface particles. During this time, over 10 floods were sampled ranging from the approximate threshold discharge to well over the bankfull discharge. Coarse particle mobility differed from that previously reported in literature for rivers with more natural flow regimes. A strong link was found between coarse particle mobility and the transport dynamics of finer material which tends to dominate the bedload. \n \nThe measured bedload transport data were also used to calibrate a fractional sediment transport model which was combined with hydrometric data corresponding to different levels of watershed urbanization to perform a geomorphic work analysis. Urbanization is increasing the frequency, volume and time of competent discharge events (capable of performing work on the channel). Greater increases of intermediate discharge events were observed. Less urban streams are more influenced by larger discharge events, while urbanization is shifting the geomorphic significance to lower (but still competent) discharges. \n \nInspired from the field observations, an unsteady flow laboratory experiment was conducted to provide more generalized results. Three land-use scenarios representing different levels of watershed urbanization were developed from measured hydrometric data. Results show that both unsteady bedload transport dynamics and resulting bed morphology change with different levels of urbanization. Shorter duration hydrographs (corresponding to urban conditions) possess higher transport rates, less pronounced bedload hysteresis loops and more topographic variability of the bed. A proposed parameter for evaluating the degree of hysteresis shows sediment transport is closely linked with falling limb dynamics. \n \nThe key conclusion from the field, modeling and laboratory studies is that bedforms in gravel-bed rivers appear to be evolving to a state with more topographic variability. This variability is hypothesized to be additional form roughness to dissipate energy introduced due to hydromodification. These results are unique in literature and further our understanding of urban river processes.

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