Integrating river ecohydraulics in urban stormwater management

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Preserving urban river ecosystems while providing access to riverbank parks is a goal shared by many Canadian municipalities for improving the quality of life of their citizens. Home to local fauna and flora, urban rivers and their riverbanks offer great recreational opportunities for city dwellers. It is easy to forget that they constitute a major part of the municipal drainage system, as they evacuate stormwater in one case, and are sites to implement stormwater ponds in the other. While vegetation on riverbanks plays an important role in decreasing velocities at which runoff reaches the river, the storm sewer network is the greatest accelerator, discharging stormwater more rapidly compared to natural runoff. This has a direct impact on river hydraulics through point discharges at relatively high velocities during rainstorms, which tend to disturb river ecosystems in the vicinity, as well as downstream, of the storm outlets. Furthermore, stormwater also carries potential contaminants from runoff on the urban landscape that can negatively impact the local ecosystems. The flow regime of urban rivers differs most from the regime in natural rivers in the lower range of the rainfall intensity spectrum. This is also the range that offers the best possibilities for restoring adequate water quality and controlling velocities before release into the receiving waters. With rainfall events in Eastern Quebec on average every three days in summertime, an integrated stormwater management system for preserving ecosystems along an urban river reach must aim at providing the proper range of eco-hydraulic characteristics (velocities and pollutant loads) to sustain the biodiversity in local habitats for frequent rainfall events. As urban pressure on streams goes stronger, there is less space for traditional solutions, costs increase, and optimization becomes a necessity.