Probabilistic approach to estimating the effects of channel reaches on flood frequencies

A host of physical parameters and characteristics of catchments and channel reaches are normally needed in watershed planning and stormwater management studies. Some of these are also design variables, such as channel cross‐section size, shape, roughness, and (to a lesser extent) bed slope. Conventional channel routing techniques employ continuity and some form of the momentum equation to determine the downstream impacts of individual flood events. With the introduction of the concept of storage‐induced delay time, a probabilistic approach is developed wherein the role of a given channel reach on the frequency distribution of floods from the catchment upstream can be directly determined. The approach uses the same kinds of channel‐reach parameters as are typically used by many conventional flood routing algorithms. Its physically based nature makes it suitable for watershed planning and stormwater management studies wherein little or no flow data are available for parameter estimation or flow frequency analysis. The validity of this probabilistic approach is demonstrated by comparing its outcomes with the results of a suite of conventional continuous simulations using rainfall data from Halifax, Canada.