Enlargement and evolution of a semi‐alluvial creek in response to urbanization

Abstract

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. 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. 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. In this study we: (i) assess channel enlargement; and (ii) propose a channel evolution model for an urban semi‐alluvial creek in Toronto, Canada. The system is 90% developed with an imperviousness of approximately 47%. 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 enlargement ratio of the channel since 1958 is 2.6, but could be as high 8.2 in comparison with the pre‐urban channel. 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. Channel enlargement continues in the contemporary channel at an estimated rate of 0.23 m2/year. A five stage model is presented to describe channel evolution in the lower reaches. 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. Further research should be done to assess stream behaviour close to an identified geologic control point. Copyright © 2018 John Wiley & Sons, Ltd.

Process‐based assessment of success and failure in a constructed riffle‐pool river restoration project

“…A recent study comparing the mean annual velocity of riverine tracer particles showed that bed particles move farther downstream per year in rivers with urbanized watersheds (Papangelakis et al, 2019). The increase in the virtual velocity of bed particles and the increase in mobility of the coarsest fractions was linked to the channel enlargement observed (Bevan et al 2018;Papangelakis et al, 2019). A reasonable goal of river restoration could, therefore, be to decrease the annual velocity of bed sediment particles to pre-urbanization values.…”

“…The majority of development occurred in the early 1960s, before the appreciation for the need for stormwater management (TRCA, 2015). Consequently, there is no stormwater control infrastructure in the watershed to dampen flood magnitudes (TRCA, 2015), resulting in frequent flooding, increased discharges, and flashy hydrographs (Bevan et al, 2018;Barr, 2017;MacVicar et al, 2015).…”

How Effective Are Retention Ponds at Mitigating the Effects of Urban Runoff?

“…A larger magnitude of shear stress and stream power, driven by the increased volume of runoff being discharged from the retention pond, could have resulted in a greater amount of sediment removal from this portion of the channel (Hammer, 1972;Arnold et al, 1982;Booth, 1990;Booth and Jackson, 1997;Booth et al, 2002;Walsh et al, 2005;Nelson et al, 2006;Hancock et al, 2010;Tillinghast et al, 2011;Mogollón et al, 2016;Bevan et al, 2018). This display of channel down-cutting is a commonly observed phenomenon in channels with urbanized watersheds (Hammer, 1972;Booth, 1990;Booth and Jackson, 1997;Walsh et al, 2005;Nelson et al, 2006;Tillinghast et al, 2011;Bevan et al, 2018). Pogonia Creek exhibits a relatively constant slope along the length of the channel, as this stream's watershed has not been significantly altered by urbanization.…”

“…Walsh et al (2005) coined the phrase 'urban stream syndrome' to describe the ecological degradation that is regularly observed in streams draining urbanized areas. There are many different symptoms of urban stream syndrome, including alteration of channel morphology, increased concentrations of nutrients and pollutants, decreased biotic richness, and increased hydrograph flashiness in streams draining urbanized watersheds (Walsh et al, 2005;Bevan et al, 2018). Due to the increase in semipermeable, impermeable, and effective impervious cover (including land that was compacted during construction) from urbanization, the pathway that water follows through the watershed changes.…”

“…Quasi-equilibrium expansion, on the other hand, entails a more balanced proportion of deepening and widening causing channel expansion. Studies have demonstrated that the magnitude of channel enlargement varies with watershed-specific conditions, such as land use, extent of impervious area, and duration of impervious area's presence in the drainage basin (Hammer, 1972;Bevan et al, 2018). As described by Hancock et al (2010), channel enlargement can significantly impact the surrounding landscape and downstream ecosystem.…”