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Surface Water Responses to Comments by CMLRD of December 13, 1988 on "Summit Pit" January 19, 1989 rr:ised Feb. 10, 1989 These n~sponsrs .ire ~n.~~l^ Co ce~nnv~nts a:rl uuestions rats~d in the u^_c~~~ubor 13, 1988 i~~tl~•r Lrom C. M. 1•trt~~LL, CMLRU Reclnination S,~eciaLisC, L'o h1. U. St•utton, permit conr.dinator for Rocky ?fountain Asphalt, Int.'s Summit Pit. 'Chose repsonses are made nf.ter consull:ation wiC!:,7ohn Doerfer, CMLRD hydrologist, regarding clarific•ztion of comments anti questions. The following section numbers are tae same as the December letter. v. a. Settling Basi^ Thie settling basin will be located approximately 500 feet above the entrance to the sit? from Highway 24. This is in the lJa~ vicinity where the narrow 'r;aldo Canyon begins to widen but is ~ downstream of the location of aggregate extraction from the pit. The settling basin will provide detention ponding to enhance deposition of suspended sediments that may result from land disturbance caused by operation of the pit. Sizing of the basin is based upon a water detention volume difference for the 10- year, 24-hour design storm between the pre-pit and fully' developed pit configuration. The runoff volume difference was determined using the HEC1 hydrologic model of the basin for the pre-pit and the condition where the fully developed pit contributes 100 per cent runoff from ttte disturbed area. The pit is planned in two phases (reference Exhibit D - Mining Plan), Phase I is for 11.4 + acres which will have final reclamation performed upon it at ttte time Phase II is under way. Phase II will be on 16.7 + acres. ,~;•.;;~ J For the Phase I, the detention volume required is 0.8 acre-feet (AP) of storage. Tlie sediment storaye volume for Phase I is 0.02 ~ AF for a total pond volume of '0.82 nF. This volume would be `~ o provided by constructing a 8 feet high rock dam across the main `+ Waldo Canyon channel. The dam will be filled with a core of ~ relatively low permeability material. If onsite material is not ,$ ~ suitable, this small quantity of material will be imported. The objective of the detention/sediment dam is not to be impermeable, .. ~ a but rather to slow the runoff in order that precipitation of the suspended sediment occurs. The dam would extend approximately c ? / 100 feet across the canyon. The length of the pond would be '; ~ about 300 feet. The Phase II settling basin volume would be ' ~ incresed to 1.63 AF. A low level outlet from the pond would consist of a 36 inch diameter corrugated metal pipe (CMP) attached to a vertical riser that is a CMP slotted above the sediment storage volume. Since the detention pond is on the main Waldo Canyon channel and runoff from all the upland area would pass through it, there is a need to provide a substantial emergency spillway. It is planned to construct an emerg?ncy spillway in a rock cut around one of the dam abutments. The granite will provide a ~cc. l • :.~Rf