Regional Rainfall Frequency Analysis for the State of Michigan

Effective storm water management plans depend on reliable rainfall intensity-duration-frequency (IDF) relationships. Due to the perception of high-intensity rainfall events as occurring more frequently than expected, the Michigan Dept. of Transportation (MDOT) commissioned a study to update rainfall IDF estimates for each of seven durations (1, 2, 3, 6, 12, 18, and 24 h) and six recurrence intervals (2, 5, 10, 25, 50, and 100 years). In contrast to a traditional at-site frequency analysis using method of moments estimators, this study applied a regional frequency analysis approach based on L -moments. Data were compiled from 76 hourly recording stations and 152 daily recording stations, and trend and outlier analyses were conducted on both annual maximum series (AMS) and partial duration series (PDS) data. With the entire state considered a homogeneous region, two regional index flood models were applied: a generalized Pareto distribution fit to PDS data (PDS/GPA model), and a generalized extreme value distribution fit to AMS data (AMS/GEV model). Verification of results indicated that the revised rainfall IDF estimates provide more reliable values than those previously used.