2012-02-28_REVISION - M1980244 (65)

Home Browse Search

Cripple Creek & Victor Gold Mining Company Squaw Gulch Valley Leach Facility Design The detailed peak flow calculations for the VLF surface water models are included in Appendix F.2. Results from the hydraulic analyses indicate that the operational peak flow for the VLF will be approximately 215 cubic feet per second (cfs) at the headwaters of Squaw Gulch, which occurs during Phase 1 construction activities. The peak flow will reduce over time as the VLF is extended further into Squaw Gulch and the watershed area is reduced. 6.2 Runoff Curve Numbers The composite curve numbers used in the hydrological modeling were calculated from curve numbers for three general land uses inside the sub - watershed areas that contribute to the Squaw Gulch VLF surface -water diversions. These uses are classified as "original ground," "roads and stockpiles," and "fill slopes." As noted above, the values of their Soil Conservation Service (SCS) [now Natural Resource Conservation Service (NRCS)] curve numbers are identical to those submitted by CC &V to the OMLR, as documented in a March 4, 1997, letter from CC &V (1997a) titled, "Cresson Project: Permit M- 1980 -244: Surface Water Drainage Modifications Arequa Gulch Overburden Storage Area," for the Cresson Project. A copy of this letter is presented in Appendix F.1. The three land use categories are described as follows: • Original Ground: SCS Soil Type B (sandy soils and loess) was assumed in these areas and consists primarily of 4 to 12 inches of coarse topsoil covered with grass, brush, or trees and underlain by a loose, rocky sandy soil. The vegetation was viewed as either a thin vegetative stand with limited cover in meadow areas or with good vegetative cover in forested areas. This category also included revegetated ground. o Wooded Curve Number = 66 o Grass Curve Number = 71 • Roads and Stockpile Surfaces: SCS Soil Type D (shallow soils with nearly impermeable sub - horizons) was assumed in these areas due to the expected compaction from heavy mining equipment. The area consists of haul roads and horizontal surfaces of the OSA. These areas are compacted by haul trucks. o Curve Number = 90 • Fill Slopes: SCS Soil Type A was used in these areas due to the coarse nature of rockfill material before revegetation. The area consists of segregated coarse rock overburden faces with a high infiltration rate. Runoff is seldom observed from these areas. o Curve Number = 50 Project No.: 74201125G0 Page 33 1 September 2011 amec0