2008-01-28_REVISION - M1997054 (28)

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1 incorporating wildlife habitat and open space. The proposed permit amendment would increase the size and depth of the quarry. The final land use for the granite quarry will include an empty reservoir below the elevation of 5,$00 feet. The existing permit contains a 3-acre sandstone quarry, located on the north side of Tallahassee Creek. This application does not affect or propose to change any mining or reclamation regarding the sandstone quarry. DRAINAGE BASINS c. Major Basin Description The historic hydrologic regime of the Tallahassee Creek Drainage Basin is typically comprised of seasonal snowmelt runoff earlier in the spring followed by periodic stormwater runoff later in the year. The fluvial geomorphology of the watershed, combined with existing agricultural/irrigation demands, dictates that for much of the year Tallahassee Creek has no surface flow along the reach adjacent to the Parkdale Quarry. The watershed is comprised of approximately 296.4 square miles of contributing area, for which a peak flowrate of approximately 7,100 cubic feet per second is expected at the basin outlet during storm events corresponding to a 100-year statistical return period. The relative steepness of the watershed indicates an increased susceptibility to episodic flooding events, but the expanse of the watershed provides a means of attenuation, which in many cases will result in reduced runoff peak flows at the basin outlet. Many areas within the Parkdale Quarry property boundary will be left undisturbed, and thus will maintain historic drainage patterns. Thus, this drainage study will focus on the existing and proposed disturbance areas within the Parkdale Quarry property boundary, for which the potential for runoff regime alterations exist. The following contains descriptions of the drainage basins related to the Granite Quarry and Sand and Gravel Pit areas. The proposed amendment does not change the Sandstone Quarry.. ' Granite Quarry: Historically, the drainage basins encountered by the existing granite quarry have conveyed stormwater runoff from the western side of Table Mountain to Tallahassee Creek. These drainage basins are comprised of small, ephemeral washes that only exhibit surface flow immediately following localized storm events or during periods of snowmelt. The drainages are relatively steep, and typically contain loose surface layers of weathered rock intermittently breached by rocky outcrops. Thus, the capacity of the quarry drainage basins to infiltrate rainfall is considerably less than the alluvial deposits near the basin outlets. The quarry drainage basins convey the vast majority of precipitation inputs through to their outlets. Currently runoff from these basins is directed through the quarry via undisturbed drainages and/or temporary ditches to the Tallahassee Creek Floodplain, whereupon it flows overland into Tallahassee Creek or coverts to sub-surface flow, depending on the hydrogeologic conditions encountered. The granite quarry permit boundary encompasses four historic basins, designated as Basins A - D (please see the attached Existing Basin Delineation). Each of the four basin delineations contains areas of the drainage network above the quarry boundary in addition to areas within the boundary. The basins roughly correspond to the mining phases. The result of delineating basin boundaries in this way dictates that as mining progresses within the quarry, the drainage basin areas, and thus runoff contributions Parkdale Quarry Drainage Report ~ Fremont County File No. CUP 07-003