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Permanent Urainaaeways Construction and Maintenance <br />• The design of permanent drainageways depicted on 11ap 28-3 follows <br />the outlines of "Procedures for Determining Peak Flows in Colorado" (SCS, <br />1977). A generic design for the drainageways cart not be given because <br />specifications depend on several factors such as drainage area, average <br />slope, and character of the surface (bare soil, disturbed, vegetated, etc.). <br />Further details of the construction guidelines for permanent <br />drainageways are outlined within pages 25-13 through 28-15 of this exhibit. <br />In general, design velocities for the channels will not exceed 3 feet per <br />second (fps) for bare soil or 5 fps for grass lined or rip-rapped <br />channels. Topsoil will be removed from permanent drainage channels prior to <br />construction. As the channels eventually retain some sediment, the bottoms <br />will become vegetated which will further increase stability and reduce <br />erosion potential. In general, channel bottoms will be ten to twelve feet <br />wide. Channel bottoms may be scarified during construction to increase <br />roughness and small check dams will be constructed within the permanent <br />• drainageways. The channel slope will be primarily dictated by the slope of <br />the reclaimed surface which is being drained. An example of channel <br />specifications is presented in Table 28-3, Channel Specifications for <br />Declaimed Area Drainages, page 28-24. The locations of these channels are <br />shown on t4ap 28-3. <br />As indicated by the survey of undisturbed areas, gully downcutting <br />averages approximately three feet. Therefore, Colorado Yampa Coal Company <br />has determined repair of permanent drainageways will begin when average <br />downcutting within the channels averages erecter than three feet. A <br />review of permanent drainageways will be included within the annual rill and <br />gully survey. In order to determine the average downcutting, depth <br />measurements will be taken at intervals not to exceed twenty (20) percent of <br />the total length along the course of the drainageway. The resulting depths <br />will be summed than divided by the number of measurements to determine the <br />average depth of downcutting. <br />. Generally, repair of the channels will include one or a combination of <br />the following: backfilling, scarification, construction of additional <br />28-9 <br />