2008-03-05_REVISION - M1997054 (31)

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 decrease due to the reduction of overland surface area available for stormwater conveyance. Basin A- is to be routed under the proposed processing facility via a 72 inch culvert, which will daylight directly onto the Tallahassee Creek Floodplain. Basins B, C, and D flow directly into sediment ponds within the quarry pit, from which the runoff water will be pumped to Tallahassee Creek at the discharge point locations specified on the Drainage Plan map. The following table details the basin areas and 100-year peak stormwater flows associated with each basin. The mining will proceed in a phased approach to limit the area of disturbance at any one time and to facilitate concurrent reclamation activities. Basin Designation Basin Area (acres) Peak Discharge (cfs) A 132.0 182.3 B 109.5 138.6 C 50.7 96.3 D 19.0 45.7 Table : Quarry Drainage Basins Sand and Gravel Pit: The area occupied by the existing sand and gravel pit is located across Tallahassee Creek from the granite quarry, upon the shelf created by the large alluvial deposit that separates Tallahassee Creek and the Arkansas River. The preexisting drainage patterns encountered by the sand and gravel pit exist only on the extreme western edge of the facility, and are considerably smaller than those encountered by the granite quarry, see attached Site Drainage Map within Appendix B. The drainages originate along the easternmost ridge of Cactus Mountain, with the vast majority of the stormwater runoff naturally directed south to the Arkansas River or north into Tallahassee Creek. That said, two minor drainage basins located directly west of the proposed sand and gravel pit expansion have been identified to potentially contribute runoff flows into the operational pit. Both Basins E and F originate as steep, relatively impervious basins, but are small enough in size to generate appreciable runoff only during significant storm events. These basins, like those northeast of the granite quarry, are comprised of loosely weathered aggregate, and tend to convey far more water than they are capable of absorbing. The basins historically emptied onto the perched alluvial deposit that serves as the location of the pit, where the inflowing stormwater runoff was typically infiltrated, or if in sufficient quantity directed north into Tallahassee Creek. All stormwater received by the sand and gravel pit will be directed along the dewatering system already operational along the pit bottom, into the existing network of sedimentation ponds prior to being pumped from the pit into Tallahassee Creek at rates less than or equal to the historic flowrates, per the pit's existing Stormwater Management Plan (SWMP). The following table details the basin areas and 100-year peak stormwater flows associated with each basin. Basin Designation Basin Area (acres) Peak Discharge (cfs) E 21.8 42.2 F 30.4 70.5 Table : Sand and Gravel Pit Drainage Basins Parkdale Quarry Drainage Report I Fremont County File No. CUP 07-003 M