Laserfiche WebLink
The HEC 1 program used for this report was revised on August 2, 1988. <br /> The newer versions have significant improvements in the methodology used <br /> for flood routing and peak flow determination. One area in particular <br /> ' where substantial improvements have been made is the Kinematic wave <br /> routine. The reason this is mentioned is that identical input parameters <br /> used in the Chen report for Subbasin 20 were used with the newer version <br /> of HEC 1 and the peak flows produced were significantly lower. The Chen <br /> report showed a peak flow of 278 cfs vs 186 cfs using the newer version. <br /> Therefore the peak flows found in this report are substantially lower than <br /> the Chen flows. In addition many of the basin parameters used by Chen <br /> were extremely conservative for a lhr-100 year storm. <br /> The Chen curve numbers in particular were very conservative. Included <br /> in Appendix A-Calculations are copies of published hydrologic soil groups <br /> ' and curve numbers used for this report. Some of this information may not <br /> have been available at the time of the Chen study. The following <br /> assumptions were used in this study. The primary soil group in the area <br /> is a Group A. For conservatism a Group B soil and the Antecedent Moisture <br /> ' Content (AMC) type 3 was used. The AMC III condition basically assumes <br /> that it has been raining for some time and most of the ground is soaked. <br /> It should also be noted that a desert shrub cover condition was used as <br /> opposed to a sagebrush cover which resulted in another built-in element of <br /> conservatism. Table 6.1 shows the peak flow and where appropriate the <br /> istorm volume. For consistency the drainage basins were numbered the same <br /> as in the Chen report. New subbasins are designated with the letters A,B, <br /> etc. The results of the HEC 1 runs and the detailed channel designs are <br /> ' included in Appendix A - Calculations. <br /> ' -11- <br />