2003-11-03_REVISION - M1992069 - Laserfiche WebLink

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~~-oaS DIVISION OF MINERALS AND GEOLOGY Department of Natural Resources 1373 Sherman St., Room 215 Denver, Colorado 80203 Phone: (303) 866-3567 FAX: (303) 832-8106 DATE: October 31, 2003 Greg E. Walther Executive Director TO: Kate Pickford FROM: Allen Sorenson ~S RE: Use of the East Pit as Regional Storm Water Detention Pond, Aggregate Industries- WCR, Inc., 83rd Joint Venture Pit, File No. M-1992-069 The City of Greeley intends to construct a storm water drainage channel, called the North Eagleview Drainage Channel, that will discharge into the reclaimed east pit at the 83`d Joint Venture gravel operation. Based on the drawings provided by Applegate Group with the Applicant's submittal dated October 8, 2003 (received at Division of Minerals and Geology (DMG) on October 14, 2003), the North Eagleview Drainage Channel has been designed, and the designs include details for the outlet works to the east gravel pit. The design drawings show the drainage channel being carried under F Street in concrete box culverts. At the north end of the box culverts, the channel will be lined with a concrete cellulaz mattress that will extend to the bottom of the reclaimed gravel pit. The invert of the channel at the top of the reclaimed east gavel pit slope will be 30 feet wide. The channel will be trapezoidal with 4:1 side slopes and about 6 feet deep, sufficient to pass the peak flows in the channel resulting from the 100-year storm, a storm that has been thoroughly analyzed for the urban drainage basin at issue. Based on the information included in the October 14, 2003, the DMG can approve the outlet works of the proposed North Eagleview Drainage Channel into the east gravel pit as they are depicted in the design drawings. Significant changes to the design drawings would require review and DMG approval. For example, if the Applicant chooses to substitute riprap for the concrete cellulaz mattress, DMG must be provided the gradation and bedding plans for the riprap. STATE OF COLORADO COLORADO DIVISION O F MIN SRALS GEOLOGY 0.EC LAMATION•M IN ING SAFETY•SCIENCE Bill Owens Gavemor Ronald W. Cattany Division Director Natural Resource Trustee There is considerable discussion in the October 14, 2003 submittal about the outfall from the east gravel pit to the historic channel that flows to the river. However, no design plans or drawings are included for the actual spillway. The original technical revision states that outfall flows in the historic channel leading to the river would not be expected to cause significant erosion, and this statement is ratified in the North Eagleview Regional Detention Pond Concept Phase report, which states "It appears that no additional work is required downstream of the railroad bridge..." The Concept Phase report fiuther presents calculations to show that the historic channel would have been subject to 100-year flood flows of up to 434 cfs without the existing and proposed flood control structures up gradient. However, DMG is concerned with the stability of the actual outfall spillway channel that is to be constructed at the top of the gravel pit slope. This outfall was modeled in the Concept Phase report as being 10 feet wide on an invert elevation of 4669 feet. The maximum flow in the channel will be 150 cfs during the 100-yeaz flood when the pond surface elevation is at 4671.5 feet. Assuming that the outfall will be trapezoidal, Office of Mined Land Reclamation Active and Inactive Mines Colorado Geological Survey