Laserfiche WebLink
September 1998 Seneca 1/W South Expansion 'Drainage and Sediment Control Plan ~ 2 <br />2.2 Storm Duration <br />A storm duration of 24 hours waz used for the design and evaluation of the Seneca IIW <br />sedimentation ponds. <br />2.3 Return PeriodlPrecipitation <br />A precipitation amount is required for the appropriate return period. The following return <br />periods and precipitation events were used for the Seneca IIW (South Expansion) mine site: <br />10-year, 24-hour storm event 1.7 in <br />25-year, 24-hour storm event 2.1 in <br />2.4 Hydrograph Response Shape <br />A unit hydrograph shape is chosen for each drainage area or sub-area modeled to predict the <br />run-off response. The hydrograph responses available in the SEDCAD+ model are slow, <br />medium, and fast. A slow response corresponds to a forested area or an area with a number of <br />obstructions. A fast response corresponds to an unvegetated or poorly protected area. Medium <br />and fast hydrograph responses were chosen for the subject site. <br />The internal convolution increment is 0.05 hours, and values are saved at the user specified <br />interval of 0.1 hours or greater. A convolution increment of 0.1 was specified for the subject <br />site. It should be noted that a time of concentration less than 0.125 hours bypasses the unit <br />hydrograph technique and instantaneous run-off is assumed. As explained in the SEDCAD+ <br />User's Manual (Schwab and Warner, 1987), the time of concentration restriction of 0.125 hours <br />is somewhat mandated due to a combination of array size restrictions, minimum internal <br />convolution interval, and the user specified time increment for saving convoluted values. <br />2.5 Drainage Basin Area <br />The drainage basin areas were determined by digitizing directly from scaled drawings. The <br />watersheds were located based on area topography and separated by conditions representing <br />various stages of disturbance. <br />2.6 Time of Concentration, T~ <br />The time of concentration was calculated using the SCS upland method (Soil Conservation <br />Service, 1986). The hydraulic lengths, drainage heights and associated slopes were determined <br />from scaled drawings. The values calculated for each structure are shown on a subwatershed <br />basis in the SEDCAD+ printouts. Overland flow conditions were assumed for a maximum of <br />400 feet.. <br />2.7 Muskingum Routing Parameters, K, X <br />The Muskingum Routing Parameters were also calculated using the SCS upland method. The <br />hydraulic lengths, drainage heights and associated slopes were determined from scaled drawings. <br />The values calculated between each structure and/or subwatershed are shown in the <br />SEDCAD+ printouts. • <br />Montgomery Watson M[mng Group' 163 S. Union Bowleuaid, Sune 460 "lnkevrood, Colorado 80I78'(970J 7633140 <br />