Research Reports

Intensity of Extreme Rainfall over Minnesota

Principal Investigator:

Richard H Skaggs

November 1998

Report no. MnDOT 1998-09U

Projects: Intensity of Extreme Rainfall Events Over Minnesota

Topics: Storm water

This study looks at precipitation design values, among the most important and widely used pieces of climatological information. Researchers explored the question of whether a high-density network can result in more realistic time series of annual 24-hour extreme precipitation amounts. They also looked at the possible impact of the variability and fluctuations of climate, since standard sources assume a static climate.

Study areas included Minneapolis and St. Paul, and an area west of Duluth, Minnesota, near Hibbing. The following highlights study conclusions: * The spatial variability of the design values estimated for 20 km by 20 km is far too great to make that approach practical. * Based on the experience with the Hibbing study area, it is likely that the density of observations over large parts of the state would be too small to allow using 10 km by 10 km or 20 km by 20 km areas. * If the purpose of the design values is to provide guidance on extreme precipitation likely to be experienced a point, the current standard sources underestimate the values about one inch for a 24-hour duration and 100-year return period * If the purpose of the design values is to provide guidance on extreme precipitation likely to be experienced at some point over an area, the current standard sources greatly underestimate the values.* There are no long-term trends in the magnitude of extreme precipitation events.

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