Impacts of climate change on storm event-based flow regime and channel stability of urban headwater streams

Due to the recent improved availability of global and regional climate change (CC) models and associated data, the projected impact of CC on urban stormwater management is well documented. However, most studies are based on simplified design storm analysis and unit-area runoff models; evaluations of the long-term, continuous hydrologic response of extensive stormwater control measures (SCM) implementation under future CC scenarios are limited. Moreover, channel stability in response to CC is seldom evaluated due to the input data required to develop a long-term, continuous sediment transport model. The study objective was to evaluate the impact of CC on storm event-based flow regimes and channel stability in a small, urbanized catchment (0.9 km²) in Montgomery County, Maryland, USA. This study employed a previously developed sequential, hierarchical modeling approach, integrating a watershed-scale Storm Water Management Model (SWMM) with the Hydrologic Engineering Center River Analysis System (HEC-RAS) to achieve the study goal. Ensemble modeling results indicate that conclusions related to impacts on SCM performance drawn from simplified, unit area models are not supported by findings from dynamic, continuous simulations that consider the complexities of real urban catchments and SCM interactions. Despite a general decrease in the total rainfall amount of individual storm events for most storm events, there is a noted increase in intensity for nearly all future storm events compared to current climatic conditions. This change in storm event-based rainfall pattern is expected to drive the catchment-scale hydrology to a flashier regime in the future, which in turn is expected to increase the extent of channel erosion compared to the current climate condition. A multicriteria design approach considering the interplay of multiple SCMs and local sediment transport capacity is thus necessary to ensure channel stability under changing climate.