How do Design Storms Compare to Real Events in the Upper Midwest?

Abstract Submission: The temporal distribution (i.e., intensity) of large rainfall events plays a key role in runoff generation and watershed scale stormwater management. Infiltration practices, like rain gardens and swales, and all impervious surfaces are particularly sensitive to intensity, so selecting a realistic hyetograph is vital in accurately forecasting performance for extreme storms. Current design guidelines for the upper Midwest recommend engineers use hyetographs like the MSE 3 distribution, which was created by nesting NOAA’s Atlas 14 estimates for 25-year storm depths with durations between 5 minutes and 24 hours. This distribution has high peak rainfall intensities where 50% of total rainfall occurs in only 4.2% of the total storm duration (1 hr). We analyzed decades of sub-daily precipitation values for real storms throughout an upper Midwest state and characterized the intensity of these events. The presentation will include comparison of real storm intensities to a synthetic hyetograph like MSE 3 and answer the question, “How conservative is the MSE 3 distribution?” Audience members can expect to better understand the strengths and limitations of current design storms and learn about an alternative hyetograph for designing and evaluating stormwater infrastructure performance.

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