2008-01-28_REVISION - M1997054 (28)

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1 ~~ ~I g. Risk Analysis Due to the extended period of operation for the granite quarry, a concise risk analysis was conducted to provide the quarry operators with the statistical likelihood that a significant storm event would occur during the operational lifespan of the quarry. It was determined that within the proposed 50-year operational lifespan of the quarry, the risk of a storm event corresponding to a 100-year return period occurring is 39.5%. In other words, there is a 39.5% chance that a 100-year storm event will occur during the granite extraction, thereby reaffirming the need for well designed and well maintained Stormwater management techniques and facilities. DRAINAGE DESIGN As stated within the Parkdale Quarry Stormwater Management Plan, all stormwater runoff from disturbed areas and stockpiles associated with the sand and gravel pit will be directed into the mining pit and allowed to inf ltrate. The significantly large hydraulic conductivity associated with sand and gravel deposit dictates the ample capacity of the sand and gravel pit to serve as its own sediment basin during significant storm events. The internal drainage will tend to collect near the northwestern corner of the pit, where the pit floor will be graded to serve as a collection point for stormwater inflows. Sediment contained within the stormwater will then be allowed to settle to the pit floor where it will be retained, ' while the water will become hydraulically connected with the groundwater underlying the pit, and will subside with the phreatic surface as the groundwater level drops following periods of post-storm system- wide infiltration. This multi-faceted Stormwater control approach has worked successfully for many years within the confines of the Parkdale Quarry sand and gravel pit, and thus no evidence is known to exist that would suggest alternative Stormwater management techniques are required. ' Stormwater runoff intercepted by the granite quarry shall follow one of two paths for treatment and release. Until a given area of the land encompassed within the proposed quarry boundary becomes disturbed, the drainage from that area shall remain unchanged, and shall follow historic pathways to the basin outlet along Tallahassee Creek. Once disturbed, the basins will undergo aretain-and-release scenario similar to that employed in the sand and gravel pit. Specifically, stormwater runoff intercepted by Basins B, C, and D will flow directly into the quarry, and will be retained within the pit bottom. The water contained within the quarry bottom will then be pumped to any combination of three discharge points along the Tallahassee Creek floodplain, whereupon it will resume its place m the natural flow ' regime. The pumping rate should be sufficiently slow enough to allow for suspended sediment contained within the Stormwater to settle to the quarry floor prior to its delivery into Tallahassee Creek. Alternatively, the drainageway of Basin A is to be impeded by the proposed location of the quarry processing facility. To account for this blockage, the proposed quarry plan has incorporated the implementation of a six foot corrugated metal pipe (CMP) culvert to convey stormwater to Tallahassee Creek. The culvert will be approximately 540 feet in length, and when installed with a slope as small as 0.8% will be capable of conveying the entirety of the calculated 100-year Basin A storm event. The exact length, depth, and slope of this culvert will be determined in the field as the processing facility is constructed. A sedimentation pond area shall be located upstream of the culvert inlet to allow any sediment laden Stormwater from Basin A to settle prior to entering the culvert, and in turn entering Tallahassee Creek. Parkdale Quarry Drainage Report ~ Fremont County File No. CUP 07-003