PERMFILE103025 - Laserfiche WebLink

Home Browse Search

• Design Storm Precipitation. The design storm used by SEDCAD is the NRCS (formerly SCS) Type II 24-hour distribution. This stonn is distributed based on tota124-hour point rainfall values, which were obtained from the NOAA Atlas 2, Volume III for Colorado. These data were then combined with the shorter duration information in the Pueblo drainage criteria (City of Pueblo 1997) to generate the precipitation depth-duration-frequency curves for the project (Figure 2). Storm runoff hydrographs and sediment-graphs were developed for the 10-, 25-, and 100-yeaz events. The shorter intensity storms were used to size the plant azea storm sewer system. Soil Parameters. General soil information was obtained from the SCS (USDA 1979) and from Contour Engineering (Contour Engineering, 2002). The dominant surface-soil types on the Project aze the Manvel and the Penros series. The Manvel series (SCS Type C) consists of deep, well-drained soils formed on alluvial fans in silty colluvial and alluvial material weathered mainly from limestone. The Penrose series (SCS Type D) consists of shallow, excessively drained soils formed on upland hilts and ridges in loamy residuum weathered from limestone. The dominant land use is rangeland with "fair" vegetation coverage, resulting in a Runoff Curve Number of 81. The annual rainfall runoff factor, R, used in the RUSLE analysis was determined to be 40 for the Pueblo area (CDPHE 2002). To develop sediment-graphs, a particle size distribution and erosivity factor (k-value in the RUSLE equation) was determined. Using the grain size distribution for these soils (Contour Engineering 2002), sand, silt and clay were estimated to be 20, 60, and 20 percent, respectively, • resulting in a bulk submerged density of 65 pounds per cubic foot (pcf). Erosivity factors, k, were obtained from the NRCS (USDA 2002), and a constant value of 0.43 was used for both soil series. The soil cover factor C, used in the RUSLE analysis, was determined to be 0.04 (Warner 1998) for existing conditions. However, for the mining scenario, the value was varied for each panel year from a high of 1.0 (recently backfilled overburden) to a low of 0.04 (fully reclaimed at yeaz 6). The representative slope and length LS, parameters, used in the RUSLE analysis for existing conditions, were determined based on a sampling of the surface topographic conditions within each sub-basin using the more conservative (higher) values. For the worst case mining scenario, the LS factors are lower and representative of a flatter, more uniform surface. The SEDCAD program also calculates the R storm factor based on the selected storm distribution (that is, SCS Type II). The R-storm factor represents the percentage of annual sediment loads that the specific storm generates. For instance, sediment loads were calculated for the 10-year event, which has an R-storm factor of 82 percent. Therefore, the annual sediment loads can be estimated by dividing the 10-yeaz sediment loads by 0.82. • swnaP sc m~e za zooz.ao~, aensioz 4