Hydrologic Impact of Planned Unit Developments

In comparison with single family detached dwelling units, planned unit developments are characterized by larger percentages and higher concentrations of impervious area and, thus, larger peak rates of storm water runoff. Using the Land Use Intensity (LUI) scale as a measure of the intensity of development a family of curves are presented that relate peak discharge and the LUI rating for different return periods. Rooftop detention of storm water is a practical urban storm water management alternative. The results of hydrological model analyses suggest that rooftop detention may reduce peak discharges by approximately 4% without requiring significant structural changes; the effect of rooftop detention is greater on the smaller more frequent storms and for developments characterized by lower LUI ratings.