Urbanization typically leads to erosion and instability in rivers, and many management and restoration strategies have been developed to dampen the worst impacts. Further research should be done to assess stream behaviour close to an identified geologic control point. In this model the coarse lag material from glacial sources provides a natural resilience to the bed and incision occurs only after the increased flows from urbanization are combined with higher slopes as a result of channel straightening or avulsions.
A five stage model is presented to describe channel evolution in the lower reaches. Channel enlargement continues in the contemporary channel at an estimated rate of 0.23 m2/year. When the increase in flow capacity is considered, the enlargement ratio is 1.9 since 1958 and up to 6.0 in comparison with the pre‐urban channel. The enlargement ratio of the channel since 1958 is 2.6, but could be as high 8.2 in comparison with the pre‐urban channel. Channel enlargement is assessed by comparing 50 year old construction surveys, a recent survey of a relic channel, low‐precision surveys of channel change over a 15 year period, and high‐precision surveys over a three year period. The system is 90% developed with an imperviousness of approximately 47%. In this study we i) assess channel enlargement and ii) propose a channel evolution model for an urban semi‐alluvial creek in Toronto, Canada. Semi‐alluvial channels characterized by a relatively thin alluvial layer over clay till and a convex channel profile in a temperate climate are not represented in currently available models. Channel enlargement is a commonly observed effect and channel evolution models can help guide management efforts, but the models must be used in the proper geologic and climatic context. The impact of urbanization on stream channels is of interest due to the growth of cities and the sensitivity of stream morphology and ecology to hydrologic change. Urban channel incision has frequently cut off streams from their floodplains, reducing floodplain sediment retention and water quality functions. Channels draining urban watersheds are still responding to this land use change by downcutting and widening. Urban expansion in the region began in the 1960s, with major urbanization occurring over the last 25 years. Watershed TIA explained 65-72% of channel capacity enlargement. Channels in urban watersheds were incised and had median full-channel capacities approximately 3.4 times greater than channels draining rural watersheds.
Bankfull cross-sectional areas were approximately 1.78 times greater for urban watersheds than for rural watersheds. Urban cross-sectional area, channel incision ratio, and channel grain size (gravel%, D50, and D84) were greater, relative to rural channels. In this study, urban channel response was evaluated in small Inner Coastal Plain watersheds (10% TIA) and 20 rural reaches (<10% TIA), and at 10 stormwater outfall sites (180 cross-sections). Stream channel response to urban land use has not been well documented for southeastern Coastal Plain streams.
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