SHORT-DURATION-RAINFALL INTENSITY EQUATIONS FOR DRAINAGE DESIGN

A procedure for developing rainfall intensity-duration equations for short durations (less than 1 hr) for any location in the United States is described. The procedure uses readily available isopluvial maps developed by the National Weather Service for four regions of the United States: (1) The 37 eastern and central states; (2) the 11 conterminous western states; (3) Alaska; and (4) Hawaii. Rainfall depths for five durations less than or equal to 1 hr obtained for each of the regions were used to determine optimal values of parameters for four empirical forms of rainfall intensity-duration equations. An intensive pattern search over the potential range of parameters was used to obtain the optimal values. For the central and eastern states, optimal equation parameters depend on the ratios of 5- and 15-min rainfall depths to the 60-min rainfall depth at a site. These relations are displayed graphically. For each of the three other regions of the United States, a single set of parameters provides the best equation fit to the available rainfall data. The procedure will be useful in designing minor drainage structures using the well-known rational formula, which requires an estimate of rainfall intensity for a duration equal to the time of concentration of a catchment.