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<br />t" <br />I <br /> <br />the Gumbel and log-normal plots were selected for further analysis. <br />The CUHP program was the most sensitive to variations in the percent <br />imperviousness and the infiltration rate. Unfortunately, these are the very same <br />variables that were contentious in published materials for Harvard Gulch. A <br />number of variations of these parameters were selected based on the published <br />values and realistic estimations. Since Zarriello published that Harvard Gulch <br />consisted of primarily SCS class B soils, a number of iterations were performed <br />using the SCS class B infiltration rate and varying percent imperviousness (30% <br />and 40%). In an area as large as Harvard Gulch (-3 mh it is likely that a variety <br />of soil types exist. In practice it may be assumed that industry would use the <br />'standard' SCS soil values for the metropolitan area (SCS soil types C and D). <br />Therefore, several iterations were performed using the infiltration rate for class C <br />and D soils (C and D types both share the same infiltration rate) and varying <br />percent imperviousness (27%, 30%, and 40%). All of the results are presented in <br />appendix A. In addition, constant infiltration rates of 0.5 and 1.0 inches per hour <br />(in/hr) were input with a 30% imperviousness to compare the effect of constant <br />infiltration versus Horton's infiltration decay equation. <br />Table 5 presents the results of the analysis. Using the equation describing <br />each linear regression, the 'actual' and 'CUHP predicted' 5-year and 100-year <br />peak runoff values were calculated. The percent difference of the predicted from <br />the actual are then tabulated. <br />To better visualize the data, the percent variance was plotted for each <br />regression/parameter type and is presented in graph 4. From the graph and <br />chart it is clear that no parameters and regression type predicted both the 5-year <br />and 1 OO-year peak flow with exceptional accuracy. The relative importance of <br />the 5-year versus the 1 OO-year variance is, of course, dependent upon the storm <br />water structure under consideration. A major detention or retention facility for <br />mitigating a large flood event would obviously be designed to accommodate the <br />50 or 1 OO-year runoff. On the other hand, smaller flood control structures like <br />culverts and storm water sewers would be designed to adequately convey short- <br />term events like the 2-year or 5-year runoff. While both events are important, <br />defining the impact of CUHP use in industry was the goal of this investigation. It <br />is assumed that CUHP is predominantly used in industry for relatively <br />small scale construction (Le. roads, residential development, and small <br />containment for water quality and detention). These types of structures are <br />govemed by regulations that emphasize small retum periods, not major flood <br />events. It is obviously not acceptable for a roadway design to fail every two <br />years. Furthermore, it is assumed that the design of a structure intended to <br />control a 1 OO-year event would be accompanied by a great deal of statistical and <br />real-life data, not assumptions, since considerable loss of property and/or life are <br />likely at risk in the event of a failure. <br />Giving greater consideration to the 'smaller' return period events, only two <br />combinations of parameters did not underestimate the 5-year peak flow, both <br />including an effective imperviousness of 40 percent. Regardless of the <br />regression type, both SCS class B and class CoD soil types returned a positive <br /> <br />Application and Evaluation of CUHP <br /> <br />'Page 13 of 52 <br />