Estimating event-scale scour in gravel-bed rivers: a dimensionless alternative to regime-based equations

Event-scale scour in gravel-bed rivers is important for the design of aquatic habitat and buried infrastructure such as pipelines, yet current engineering practice remains disconnected from modern active bed layer theory. This study considers a dataset of 129 scour observations from 69 individual flood events, including 107 from published studies and 22 derived from pre- and post-flood cross sections in Alberta rivers. The Alberta derived scour dataset as well as the compiled dataset both suggest that average scour depths are broadly consistent with commonly cited grain-size-based benchmarks and extreme event-scale scour can greatly exceed values typically associated with armour-layer thickness. Comparison of the observed dataset with Blench-style regime equations indicates that the Blench equation systematically overpredicts the observed scour depths. The observed dataset was also used to develop a new dimensionless formulation, based on unit discharge and normalized by coarse surface grain size, to offer an alternative and improved methodology to Blench’s equation. The proposed formulation reduces prediction bias while maintaining equivalent design conservatism as compared to Blench’s equation.