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Adapting Northwest stormwater designs to climate

February 6, 2023

Hydrology models can no longer assume stationarity

Current drainage systems have been designed based on a traditional assumption of static, unchanging storm events. However, stationarity or the assumption that future events are based on past data can no longer be assumed.

The University of Washington Climate Impacts Group (UW CIG) are not the only scientists to indicate that “storms are expected to become more severe with climate change.” It has made regional climate models available to simulate local-scale changes in precipitation intensity, duration and frequency to assist with stormwater and CSO planning. More accurate future storm scenarios are critical, so stormwater drainage designs are right-sized to effectively protect water quality, communities and habitat from the impact of stormwater runoff.

To accurately capture changes in heavy rainfall events, single-event modelling can be considered to project changes in extreme precipitation. In arid eastern Washington, where less than 10 to 60 inches of rain falls per year and often in less than one-day time frames, single-event modeling is used to size all conveyance systems and erosion and sediment control systems as well as all stormwater quality treatment systems and flow control systems.

In contrast western Washington – west of the Cascade Mountains – receives between 30 and 200 inches of rain per year; urbanization and population density pose greater stormwater management challenges and stricter water quality regulations for receiving waters. To accurately model and compare pre and post development runoff, preceding storm events and longer durations of continuous precipitation are taken into account. Continuous simulation modeling is required to design flow-based stormwater quality treatment systems and flow control systems. Conveyance systems and some erosion and sediment control systems in the region can be sized using either single-event modeling or continuous simulation.

Tools like the Western Washington Hydrology Model (WWHM), a continuous stormwater simulation, may more accurately model future conditions because it incorporates “climate model projections that have been bias-corrected to match the statistics of a particular rain gauge.” The UW CIG said that its heavy precipitation projections can be use in continuous simulation modeling.

In the 1990s Washington state helped jump-start national development of municipal stormwater permit programs; today it continues to investigate modern stormwater management approaches.