Design Rainfall Curve

Hydrologic Engineering, April __________________________________________________________________________________ DESIGN RAINFALL CURVE James C.Y. Guo, Professor, Civil Engineering, U. of Colorado at Denver. __________________________________________________________________________ Abstract To design urban storm water infrastructures, hydrologists apply the SCS Type I and II 24-hr rainfall distribution curves to create various rainfall hyetographs by which storm runoff can be predicted accordingly. Although SCS Type I and II 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 understanding how these curves compare with the observed. In this study, a 57-year hourly rainfall data recorded in Denver, Colorado was analyzed and then compared with the SCS 24-hr rainfall curves. It is concluded that these SCS 24-hr Type I and II curves are not the statistically averaged, rather, they represent the worst time distribution to form a sever storm. Using the concept of enveloping curves, conservative rainfall distributions can be derived using the similar approach as revealed by the SCS 24-hr curves. This approach requires much less rainfall data than the statistical average.