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~' Brian Munson -2- November 17, 1981 In an attempt to verify those estimates, regression equations obtained from Technical Manual 1 (TMI) Manual For Estimating Flood Characteristics of Natural Flow Streams in Colorado, 1976, which was referenced in the application, were used. Using the regression equation for the Mountain Region resulted in values significantly less than those given in the application. Using the regression equation for the Northern Plateau Region resulted in values significantly larger than those given in the application. Rockcastle Company should specify how each of the peak flow estimates were obtained, give the input parameters and equations used. I am willing to allow all of the culvert installations as proposed to be used with the stipulation that, should any damage occur to or because of any culvert installation, this Division retains the right to impose specific culvert sizes to be used to replace any improperly sized culvert. Drainage Ditches: Most of the drainage ditches have velocities under conditions of the 10-year, 24-hour storm event which are erosive. This concern is mitigated somewhat by the fact that Rockcastle states the ditches frequently are in bedrock sandstone. A solution to the concern of channel erosion is to line all sections of ditches which have erosive velocities in erodible material with rock riprap. Many channel sections have slopes far in excess of the listed slopes on page III-18 and thereby also have far greater velocities. The ditch section near the NE corner of Pit 4 has a slope of over 15% and has an estimated velocity of about 9 fps. Rockcastle Company should commit to Lining all ditch sections where the velocities would be erosive in the channel type given. Of particular concern are curves and ditches carrying flow from undisturbed areas. From careful examination of the Mining Plan Map, it appears that several sections of the proposed ditches can be eliminated without significantly reducing erosion control. Much of the upland sections of the ditches at Pit 4, Pit 2 and Pit 3 could be eliminated. This Division does not wish to encourage the construction of unnecessary structures. In areas these ditches serve, a satisfactory erosion control system could be implemented with the use of silt fences, straw bales, or other temporary erosion control devices. Such exemptions from sediment pond requirements are allowed by Section 9.05.2(3) of the Regulations, The Rockcastle Company should consider such measures, as it may reduce construction and maintenance costs. Ponds There are three items of concern in regard to sediment pond designs. They are sediment storage volumes, dewatering devices, and the need for the acquisition of water storage rights.