In the development of rainfall frequency and volume data for Minnesota, the Minnesota Stormwater Manual Sub-Committee conducted an analysis of rainfall distribution for several locations in Minnesota. The methodology and results of this analysis are described in Issue Paper B. Frequency-volume graphs were developed for several locations in the state and are illustrated below.
In viewing these graphs, note that the volume represents the percent volume captured for a particular size storm and not the cumulative volume. For example, consider a BMP designed to capture the first inch of precipitation. A 1 inch storm captures about 90 percent of all rainfall events at Minneapolis-St. Paul International Airport. Storms up to 1 inch represent about 60 percent of the total volume. However, for storms exceeding 1 inch in depth, the first inch will be captured by the BMP. Thus, the annual volume captured by the BMP is much greater than 60 percent. For a B soil, the annual volume captured by the BMP will be about 90 percent. The table below summarizes annual volume captured by a BMP at the Minneapolis-St. Paul International Airport for different soil types.
|Soil||Water quality volume (VWQ) (inches)|
1Values were determined using the MIDS calculator. BMPs were sized to exactly meet the water quality volume for a 2 acre site with 1 acre of impervious, 1 acre of forested land, and annual rainfall of 31.9 inches.