Laserfiche WebLink
2 <br />a Type II Soil Conservation Service storm distribution was used to con- <br />vert total rainfall to design storm rainfall. The Type II storm distribu- <br />tion is shown on LRCidE Table 1 ^esign Rainfall, ~ai~ich also summarizes the <br />10, ES and 100-year design storm rainfall. <br />Runoff Volumes <br />Runoff volumes were computed using equation 10.10 and the procedure outlined <br />in Chapter 10 of the Soil Conservation Service's Yational Engineering Hand- <br />book, Section 4 - Hydrology, dated 1971. Antecenen[ Pfoisture Condition - II <br />(AMC) curve numbers (Cfl) were estimated far the tributary areas of each pond <br />considering the worse case for the land use, and ground cover. AMC-II is <br />considered applicable for average conditions and allows for a total rain- <br />fall of 1.4 to 2.1 inches within the preceeding five days of [he storm. For <br />the 10-year 24-hour precipitation event, the AMC-II curve numbers indicated <br />a relatively low runoff volume (less than 0.2 inches). In discussions with <br />Energy Fuels Corporation personnel, it was learned that for this area, the <br />most critical runoff volumes occur during the snowmelt season, especially if <br />rainfall accompanies the phenomenon. In the definition of precipitation <br />event, the OSM regulations include ". .water emanating from snow cover as <br />snot.melt in a limited period of time." Considering a water equivalent of <br />30%, the AMC-iI curve numbers indicate a surface runoff volume equivalent to <br />approximately one-half inch of snowmelt. One-half inch of snowmelt within <br />a 24-hour period of time is expected to occur more frequently than once in <br />10 years on the average. The state of the art for computing snowmelt runoff <br />hydrographs is not well developed, and snowmelt with rainfall hydrographs <br />are beyond the scope of this investigation. The AP1C-II condition was con- <br />sidered to give runoff volumes too low when snowmelt is considered. Te <br />generate flood hydrographs with more conservative runoff volumes (greater <br />total volumes) the AMC-Ill condition was utilized. The AAIC-IIL assumes a <br />greater antecedent moisture condition which exists if greater than 2.1 inches <br />of rainfall have fallen within the previous five days. This technique is not <br />intended as an accurate representation of the actual snowmelt hydrology. <br />LRCI.'E Table 2, Curve Numbers, is reproduced from Chapter 10 of the Soil Con- <br />• servation Service's national Engineering Handbook, Section 4, Hvdrolog}•. <br />Permit Renewal No. 3 Exhibit Sq I/ I S/00 <br />Page 4 oC?2 <br />