Abstract

Extreme precipitation events are a concern for stormwater management. Many jurisdictions in Puget Sound are experiencing more frequent stormwater challenges, and are exploring solutions to reduce stormwater risks and increase long-term resilience. In Anacortes, current stormwater issues are isolated to winter storm events – summer events, such as thunderstorms, have not been an issue. Staff at the City cited specific management issues associated with particular flow levels within the stormwater system. The amount of water entering the stormwater system is related to both instantaneous and antecedent precipitation totals, over durations ranging from 15 minutes to 5 days. In general, the city has only experienced Combined Sewer Overflow (CSO) events when daily total rainfall exceeds 0.5 in., and even for higher precipitation totals CSO events have only happened when the preceding days have also been wet. Because antecedent conditions are so important, it is difficult to identify a specific precipitation threshold related to flow exceedances in the stormwater system. Instead, the city has hired HDR Inc. to conduct continuous simulations of the system, which can account for the combined effects of antecedent and instantaneous precipitation.