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1 <br /> Also the peak rainfall intensity using the Corps' rainfall modeling would be 2.7 <br /> inches per hour for 30 minutes where Boulder City, County, and UDFCD criteria can <br /> have five minute peak duration intensities over 8.0 inches per hour. While these <br /> peak intensities are not likely to increase major floodplain flows significantly they <br /> certainly would increase local basin runoff and possibly West Valley Overflows. <br /> The Corps' rainfall modeling applies different rainfall over five zones to reflect the <br /> ' wide range of elevations which we generally agree with. We would question the <br /> raw NOAA data used at the highest elevation zone, but any error here would be of <br /> low significance because of the routing effect of Gross. <br /> tAlso, a realistic analysis of operating levels and flood hydrology effects in Gross <br /> may reveal that Gross is even more effective in reducing the overall flood runoff <br /> t volume downstream than as modeled by the Corps. On the other hand, peak <br /> discharges below Gross, particularly in the urban areas below Highway 93 are not <br /> significantly affected by runoff upstream of Gross. <br /> ' The Corps reduces the rainfall for the magnitude of the basin area by approximately <br /> 12.7%. This correction accounts for the phenomena that the rainfall gauge data <br /> ' analyzed reports point values, and such peak point rainfall will not occur uniformly <br /> over a watershed. We generally agree with the concept of point rainfall reduction <br /> ' for the area below Gross Reservoir where modeling the effects of South Boulder <br /> Creek spills, however, local area runoff modeling also needs to be simulated <br /> without major area reduction effects. <br /> The Corps also uses and Expected Probability Correction of about 4.3 to 4.7%. <br /> This correction increases the base rainfall. In simplistic terms, probability theory <br /> indicates that raw gauge data will tend to miss or misreport the greatest events. <br /> We understand the Corps currently does not often use Expected Probability <br /> Corrections. Few local jurisdictions do. <br /> ' We believe it is important in the case of South Boulder Creek and the West Valley <br /> Overflow Area to simulate both a realistic regional storm that has the correct NOAA <br /> ' statistics, and a local storm, without large area correction factors. This procedure <br /> will identify which type of event creates the worst situation at each location. <br /> Comparisons made in Section IV indicate that different types of events control at <br /> various locations in the lower watershed. <br /> ' The current UDFCD and Boulder manuals have specific procedures for the rainfall <br /> arrangement in the peak 2 hour period, and where this peak period should be placed <br /> in a longer duration event. This procedure should be reviewed in future work. <br /> ' III - 4 <br />