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The expected flood discharge from the PMP event has been a combined effort of <br />AWA and Wheeler. The PMF values for the various storage impoundments at the <br />Henderson Mill were evaluated using the U.S. Army Corps of Engineers' HEC-1 computer <br />model. The various input parameters required for the model are described in the subsequent <br />sections. <br />Precipitation <br />PROBABLE MAXIMUM FLOOD DISCHARGES <br />AWA has performed a site - specific evaluation of hydrometeorological parameters and <br />has provided precipitation depth - duration values for the PMP applicable to the Henderson Mill <br />drainage basins. The report describing the results of this analysis is included herein as <br />Appendix A. Based on the results of the AWA analysis, a design storm depth of 5.6 inches of <br />rainfall in a 3-hour period was used in this analysis. This is a local storm event that provided <br />the highest intensity rainfall and largest total storm precipitation depth. This design storm is <br />based on the maximized and transposed Opal, Wyoming storm that occurred on August 16, <br />1990 and represents a local PMP event. <br />Unit Hydrograph <br />The unit hydrograph is a key component of the precipitation -runoff model. In this <br />analysis, the dimensionless unit hydrograph technique was used to develop a synthetic unit <br />hydrograph for each subbasin tributary to the Henderson Mill facilities. This procedure is <br />documented in Chapter 4 of the USBR Flood Hydrology Manual and recommended by the <br />SEO Dam Safety Project Review Guide. A unit hydrograph is defined by the USBR as "the <br />discharge hydrograph resulting from one inch of direct runoff generated uniformly over the <br />tributary area at a uniform rate during a specified time period ". <br />This unit hydrograph procedure is based on the observation that the unit <br />hydrograph lag time is a function of measurable basin parameters. For the <br />dimensionless unit hydrograph, the lag time is defined as the time from the center of unit <br />6 <br />