Development of a Duration-based Management Framework for In-stream Construction-induced Suspended Sediment

Globally, environmental regulations related to turbidity and suspended sediment concentration do not provide sufficient guidance to ensure effective environmental management of in-stream construction projects. These projects are commonly implemented out of the public’s best interest yet pose unavoidable risk to sensitive aquatic ecosystems. In-stream construction activities occur within the wetted-perimeter of a river, characterized by short term (commonly several days to several weeks), point source sediment releases. There are no well-established, scientifically studied methods to quantify exposure risk of these temporally- and spatially-complex releases. As a result, many river projects implement inconsistent management frameworks, with limited focus on reducing project-specific environmental risks in an effort to comply with generalized regulations. Many management practices focus strictly on reducing suspended sediment concentrations by constructing sediment control measures, without considering the implications these measures have on the increased duration of aquatic ecosystem interactions and suspended sediment exposures. These concentration-based management strategies have been assumed to be effective based strictly on management precedent, while their ability to reduce ecosystem exposure risk remains unverified through scientific investigation.

The City of Calgary (Alberta, Canada) proposes to conduct a gravel relocation project within the Bow River that is deemed of environmental importance. Unfortunately, under the traditional management framework based on fixed suspended sediment concentration limits, the project cannot be implemented. It is argued that concentration-based criteria do not reflect environmentally acceptable thresholds. A theoretical in-stream construction-induced suspended sediment (ICISS) exposure risk model was used to compare ecosystem stress predictions for varying rates of gravel placement activities. The model combines suspended sediment dosage-effect relationships from the literature and a sediment transport model that predicts plume dispersion in the downstream of the construction activities. An optimum construction approach and ICISS management strategy were devised based on environmental and economic considerations. Results indicate that a duration-based management framework provides greater flexibility and effectiveness, which will enable the project to proceed. The approach will assist in the management of future projects anticipated to create dynamic ICISS releases by providing a framework that supports consistent and effective management strategies for highly variable and complex river construction projects.