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<br />The long-term storms of record have relatively low peak intensity compared to the <br />theoretical thunderstorm events shown in Tables N-1 and IV-2. The percent <br />imperviousness of a basin is important in the rainfall-runoff relationship because all rainfall <br />except that trapped by surface retention becomes runoff from an impervious surface. The <br />soil infiltration characteristics of a basin are important because if the rate of rainfall is less <br />than the rate of infiltration, no runoff results from the rainfall. <br /> <br />SNOWMELT <br /> <br />A detailed analysis of snowmelt characteristics of the study area is beyond the scope of this <br />document. The likelihood of snowmelt yielding higher peak flows or runoff volumes than <br />the design storm events was calculated. <br /> <br />The volume of runoff on the Rocky Flats Plant site for the 1-,4-, and 10-day storm events <br />was calculated. The following criteria were used for this analysis: (1) soil infiltration rate <br />and percent imperviousness as listed in Table III-I, and (2) rainfall depth as shown in Table <br />IV-12, with a peak intensity of 1.0 inch/hour for one hour and intensities for other time <br />periods at less than 0.5 inch/hour. <br /> <br />There are numerous parameters affecting the rate of runoff from snowmelt. A simple <br />"degree-day" procedure is commonly used (Linsley, 1982). This depends on the following <br />parameters: (1) moisture content of the snowfall, (2) the mean daily temperature, and (3) <br />the degree-day factor. <br /> <br />The peak flow for long-term storms in all cases is less than that for the 6-hour design storm. <br />The runoff volume for long-term storms is less than that for the 6-hour design storm in all <br />Buffer Zone basins. It is higher for the Core Area basins because of the high percentage <br />imperviousness. The total runoff volume to the A- and B-series ponds is therefore higher <br />in some long-term storms. Planning concerning runoff volumes to the A- and B-series ponds <br />should be done using the values provided in Table IV-14. <br /> <br />Conservative values for these parameters that were used in the analysis are (1) I8-inch <br />snowfall with 3-inch moisture content, (2) a mean daily temperature of 47 degrees F <br />following the snowfall, and (3) a degree-day factor of 0.1 inch/F degree day. This is <br />representative of an extreme snowfall event followed by warm days that commonly occur <br />between March and May. <br /> <br /> <br />This analysis yields a melting rate of approximately 0.2 inches per hour assuming eight hours <br />of melting per day. This is less than the soil infiltration rate so there will be minimal runoff <br />from pervious areas. <br /> <br />TABLE IV-14 <br /> <br />Since the rate of melting is less than the peak design-storm intensity, snowmelt will always <br />yield lower peak flow than the 2-year design storm. The runoff volume will depend on the <br />percentage imperviousness of the basin. For basins with very high percentage of <br />imperviousness (Core Area, future development conditions), this snowmelt would yield a <br />volume of runoff equivalent to the 25-year storm event. The volume would be lower for <br />basins with lower percentages of imperviousness. II <br /> <br />TOTAL RUNOFF VOLUME TO THE A- AND B-SERlES PONDS <br />(ACRE-FEET) <br />100-YEAR EVENT <br /> <br /> <br />PondS <br /> <br />240-Hr <br /> <br /> <br />ruM"DeVelllp.nenftliltllliiIlDS <br /> <br />~litDeVd(lp,*eDt Conditions <br /> <br />IJ6..Hr <br /> <br />6-Ht <br /> <br />UH~ <br /> <br />96~Hr 240-Hr <br /> <br />B-Ponds <br /> <br />64 <br />71 <br /> <br />47 <br />67 <br /> <br />55 <br />80 <br /> <br />66 <br />108 <br /> <br />73 <br />80 <br /> <br />63 <br />79 <br /> <br />75 <br />96 <br /> <br />90 <br /> <br />A-Ponds <br /> <br />116 <br />