Stream instability risk assessment in an urban area as a management tool

English Abstract:

Stream stability in urbanized watersheds is an increasingly critical issue for planners and managers. Unfortunately, due to lenient legislation and a lack of awareness of the impact of changes in land use, urban development in the province of Quebec has heavily impacted streams and rivers. It has also resulted in the construction of roads and houses within the migration channel of these water courses.

The St-Regis creek drains a 93 km² watershed located in the Saint-Lawrence lowlands, an area known for its fertile soils. Only 3 km² of forest cover remains in the watershed which is now predominantly occupied (82 %) by the culture of major crops. The remaining area (15 %) is relatively dense and continuous suburban development located in the downstream area of the St-Regis watershed. Many of the watercourse’s meanders have been straightened during the urban development and subsequent and extensive bank stabilization was added to protect roads and houses. Yet, the channel continues to migrate. A problematic section of approximately 3.5 km is the focus of this study. A risk assessment approach has been proposed to the city managers to advise the planning of stream interventions. The risk assessment approach is seen as a compromise between the need for stable banks and allowing for the natural stream functions of flooding and migration.

Future channel trajectories can be predicted by considering stream history and hydraulics, in combination with RVR meander software output, to classify the fluvial plain according to the risk of channel migration on a short, mid and long term basis. Based on these data and long term land use plans, stream interventions are devised focusing on minimal impact to the stream and cost-efficient methods. The integration of geomorphological concepts and the consideration of stream history and trajectory, is a new approach in Quebec and a potential sustainable replacement to the usual interventionist approach.