Abstract

Channel straightening and dredging were extensively used in the 20th century to enhance agricultural drainage and facilitate crop maintenance and harvest. Although the adverse geomorphological and ecological effects of channelization are widely acknowledged, the use of alternative management strategies remains marginal in Southwestern Québec. Bank stabilisation projects are often carried out to mitigate local erosion problems with little assessment of their effects at the reach and watershed scales and with insufficient guidance on suitable designs. The objective of this research is to assess the impacts of various management strategies by studying a case of straightened agricultural stream. Field measurements in the Richer stream, which drains a small agricultural watershed in the St. Lawrence Lowlands, were used to parameterise a hydro-morphological model at the watershed scale and a 3D computational fluid dynamics model at the reach scale. The increase in stream power associated with the loss in sinuosity since the 1930s has resulted in noteworthy erosion problems in the studied watershed, in particular near residential development where there is limited space available to establish riparian strips. The tested management strategies at the watershed scale are the recreation of meanders and the installation of backwater ponds whereas, at the reach scale, stream barbs and bed weirs are tested. These management strategies are also assessed through a cost-benefit analysis which also takes into account environmental and practical implementation aspects. Results indicate that both the addition of ponds and re-meandering can markedly reduce unit stream power, thus the potential for erosion. Hydraulic structures such as stream barbs and V-shaped bed weirs re-align the flow towards channel centre, thus reducing near-bank velocities. The re-meandering approach involving natural vegetation regeneration has the highest overall effectiveness at the watershed scale whilst V-shaped weirs are found to be moderately effective at the reach scale. The diversification of flow conditions and channel morphologies associated with these approaches were important factors contributing to their higher suitability compared to other potential solutions. The modelling methodology used in this study can help limit the uncertainty surrounding restoration activities by better predicting the efficiency of proposed stabilisation techniques prior to their implementation while considering specific stream and watershed characteristics as well as ecological factors.