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Several factors contribute to these stable ditches: flow quantities <br />• are relatively small, even for the 10-year design storm, the <br />natural soil materials are well graded and non-erosive, the steep <br />slopes result in shallow flow depths which impose less shear stress <br />on the soil particles than the condition of deeper-flows with the <br />same velocity. <br />However, NCIG commits to the following condition related to the <br />drainage ditches: All ditches will be inspected after each runoff <br />event. Any channel erosion will be immediately repaired as <br />follows. The channel will be reshaped to its original design cross <br />section. The flow area within the channel plus 0.5 feet above the <br />water depth will be lined with an erosion control blanket such as <br />a long lasting coconut fiber with nonphotodegradable netting. The <br />blanket will be installed according to the manufactures <br />recommendations (stapled in place, edges buried). The type of <br />blanket will be one that is certified for channel linings, high <br />velocity runoff, and steep slopes. In the event of a subsequent <br />failure in a section of ditch that has been lined with an erosion <br />• control blanket, the repair will be made using grouted riprap. <br />The eroded channel would be reshaped, graded bedding (100a finer <br />than 3", 20~ finer than 3/9") material added, riprap stone with a <br />median diameter of 6" which meets Colorado Dept. of Highways <br />Specification 506 placed on the bedding. The grout will consist of <br />2400 psi concrete and have a high slump to allow for penetration <br />through the riprap layer. <br />Culverts have been installed within the site at locations where the <br />ditches cross roads. These culverts are all designed to pass the <br />10-year, 24-hour storm with headwater to diameter ratio of 1.0 and <br />at least 1 foot of freeboard between the design water surface and <br />the roadway. The sizes of the as-built corrugated metal pipe (CMP) <br />culverts are listed in Table 9.2-3 and the locations are provided <br />in Figure 9.2-3. These culverts have been inspected and have <br />. NCIG - Coal Ridge #1 4-41 ABC/1145/910509 <br />ir~-7 <br />