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C~ John Hick-nan • Rocky Mountain Rock Products -Deer Creek Canyon Quarry Drainage February 14,1997 Page 2 5. Intercept Channel Location and Description. The intercept channel is planned to divert gulch flows from the south side of the access road to the subject roadside ditch on the north side of the road. The channel itself is planned to cross the road just above (west of) the limit of asphalt pavement, neaz the scale house. The channel will basically act as a driveable low water crossing, with rock armoring to prevent washout of the access road. The routing is that the intercept channel will dischazge to the access road ditch by means of a rock armored rundown/transition section as needed. 6. Sediment Ponds. The "upper pond" is to be a retrofit of an existing culvert intake depression with an overflow spillway for use as a sediment pond. The "scale sediment pond" is not existing, but is proposed as a new improvement to address stormwater runoff water quality at the site. The capacity of the ponds proposed is 0.67 acre-feet based upon water quality capture volumes computed according to guidelines of the Urban Drainage and Flood Control District (iJD). These guidelines aze based upon reseazch and analysis performed or compiled by the District to efficiently address stormwater runoff water quality. The basis of the approach is the "first flush" event of %:" rainfall, which covers more than 90% of precipitation events in the Front Range of the metro Denver azea, and which is felt to be a key threshold for suspension and transport of materials. The Division is correct in its assertion that the indicated volumes are much smaller than that required for full impoundment and slow release of 10-yeaz 24-hour storm event runoff: It should be noted the "first flush" approach is based upon %z" rainfall, with runoff volumes less than this amount dependent primarily upon impetvious azea per the UD procedure. Our basins range from 2% impervious estimated for the upper basin to 5% for the middle basin and 20% for the lower basin due to quarry benches. Critical water quality runoff depths from the UD charts were 0.02" for the upper basin, 0.05" for the middle basin, and 0.12" for the lower basin. 7. Storm Water Management Plan. We understand a key factor in this quarry's condition is that the quarry has been relatively dormant since 1989. We understand through this seven year history that vegetation has substantially taken hold and greatly stabilized the azea at a relatively low level of sediment loss. Given this history, we aze hesitant to recommend extensive disturbance of inactive or even undisturbed azeas for construction of erosion control improvement structures unless the benefit is significant and demonstrable. We find this to be particulazly true in the case of construction of cutoff and diversion ditches for undisturbed area runoff, particulazly in a steep basin such as this where the construction disturbance for the ditches can be substantial.