Conservative Design Rainfall Distribution

To design urban storm water infrastructures, hydrologists apply the SCS Types I, II, and III 24-h rainfall distribution curves to simulate the design rainfall hyetographs by which storm runoff can be predicted accordingly. Although SCS 24-h rainfall curves are recommended by the Natural Resources Conservation Service and have been widely adopted for many cosmopolitan areas in the United States, it is not enough to understand how these curves compare with observed data. In this study, a 57-year hourly rainfall data recorded in Denver was analyzed and then compared with the SCS 24-h rainfall curves. It is concluded that these SCS 24-h Types I and II curves do not represent the statistical average values. Rather, they represent the worst time distribution to form a severe storm. Using the concept of enveloping curves, conservative rainfall distributions can be derived using a similar approach as revealed by the SCS 24-h curves. The approach suggested in this study requires much less rainfall data than the statistical average.