2012-02-28_REVISION - M1980244 (73)

Home Browse Search

overburden storage area is accomplished upgradient of the southeast side of the Arequa Gulch Overburden storage area. As the oxide overburden volume increases, it is necessary to cover the current diversion and to move diversions farther upslope of the storage area. As the storage area grows, it is necessary to control the runoff from a progressively smaller undisturbed area. As the overburden storage area is progressively reclaimed, the disturbed areas, the potential for erosion, and the sediment control requirements (e.g., detention ponds) will decrease. CC &V notes that the current diversion system has not experienced any continuous flow to the point of diverted flow reaching receiving waters since its construction in 1994. All flows observed to date have been "discontinuous" in that they infiltrate into the channel bottom shortly after interception by the diversion channel, or they accumulate in shallow pools along the thalweg of the channel and seep and evaporate. METHODOLOGY This Plan is comprised of up- gradient diversions that carry water from undisturbed areas around the actively- disturbed areas of the Arequa Gulch Overburden Storage Area, and detention ponds or other temporary containments of runoff from disturbed areas that allow settling of entrained solids. These plans account for the flow velocities and volumes of storm -water runoff that are projected to occur at the site. The storm -water runoff controls are designed to safely pass the flows from the 100 -year, 24 -hour precipitation event (3.5 "), and, with respect to runoff from the disturbed area, to detain at least the runoff from the 10 -year, 24 -hour precipitation event (2.4 "). For purposes of these computations, and consistent with other storm -water runoff planning for the Project, the precipitation events were assumed to follow a Type II distribution, based on the geographical location. The grading designs were developed, and the computations were performed, by CC &V engineering staff. Calculations of runoff quantities were performed using the Softdesk "Hydrology Tools" computer program. This program is based on the graphical methods described in the Soil Conservation Service's (now the "NRCS ") Technical Release Number 55 ( "TR 55 "). Additionally, the Soil Conservation Service's publication "Procedures for Determining Peak Flows in Colorado" (March 1984) was used for technical reference. The software estimates peak flows, peak flow velocities, and cumulative runoff volumes to provide the basis for sizing and protecting drainage channels and impoundments. The area of Arequa Gulch overburden storage was divided into subbasins, each controlled by a specific drainage system and a detention "structure." Flow lengths were measured, slopes were determined, and flow regimes were divided into "Sheet," "Shallow," and "Channel" flow types for each of the subbasins. These flow types were assigned based on field observation of the terrain. Using appropriate descriptors of- channel roughness, the computer routine then developed the total concentration time for the subbasin. A composite runoff coefficient was developed for each basin based on the soil types. The Type II 24 -hour Storm Hydrograph was applied. The flow was routed to the corresponding control structure (detention pond) and the flow rates and volumes used to determine adequacy of the structures. The flow rates computed along the channels provided the basis to evaluate the capacity of a standard slope channel and any road channel ditch planned for use. The soil types in the area are listed, and the assignment of runoff coefficients based on these soils and CC &V observations over the past three years is discussed, in the next section. 2