Rainfall Intensity in Design

An empirical, dimensionless-hyetograph that relates depth and duration, and thus whether a storm is front loaded, back loaded, or uniformly loaded, based on 92 gaging stations for storms known to have produced runoff is available for Texas. Statistical characteristics of storm interevent time, depth, and duration, based on analysis of hourly rainfall data for 533 rain gages are used to "dimensionalize" this hyetograph and produce a set of simulated storms. These simulated storms are analyzed to generate a set of rainfall intensities, and these intensities are compared to global maximum observed rainfalls, intensities estimated using the National Weather Service TP-40, and HY-35 publications, and a current Texas Department of Transportation design equation. The simulated storms agree well with the other methods for rare (i.e. 90-th percentile and above) occurances and lie within the global maxima envelope. The simulated results are quite different for common (i.e. 50-th percentile) events.