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SEDIMENT CONTROL 1. Bush Draw (2.05.3(3)(c)) Wyoming Fuel Company (WF) has provided a design for anew culvert to be in- stalled at the Bush Draw crossing. The design technique uses the Manning's equation to determine an acceptable culvert diameter. Manning's equation was developed for steady, uniform, open channel flow. while flow through culverts sometimes approximates open channel flow, the contraction of flow at the inlet makes use of Manning's equation inappropriate for design of culverts. Charts for determining culvert headwater are available from several sources. Enclosed is a copy of Chart 5 from Hydraulic Charts for the Selection of High_ w~ Culverts, U.S. Department of Transportation, December, 1965. This chart is for inlet controlled culverts which is generally the case with short highway culvert installations. Entering this chart using the peak flow estimate generated by WF of 70 cfs results in a NW/D ratio of approximately 2.5. This indicates that an embankment height from the base of the culvert inlet to the road surface would need to be about eight feet. The estimated peak flow (70 cfs) appears to be excessive. While the range condition of Bush Draw is very likely "poor", the predominant sage-grass vegetative community presents a fairly dense protective cover and hydrologically probably qualifies as "good". Estimating the percent ground cover at 300, a CN of 74 is generated by Figure S-3, page 23 of Peak Flows in Colorado, SCS 1980. Using a CN of 74 results in a peak flow estimate of approximately 2I cfs. Entering Chart 5 with 21 cfs shows no head above the culvert inlet in a 36" culvert and a headwater of approximately 1. 7D or 3.4' of headwater in the existing installation. Wyoming Fuel Company must use some form of closed conduit hydraulics to design culverts. The proferred chart is a simple and acceptable method. Wyoming Fuel may wish to consider revising their peak flow estimate using a more realistic curve number. 2. Mine Onerati ons Area The response by WF is not complete: The purpose and design of the ditch which runs on the West side of the coal stockpile is not given. The Division realizes that water will continue to reach pond b1 throughout reclamation efforts in the pit 1 area. However, it is important that a ditch be maintained in order to prevent the collected eaater from coursing through the reclaimed surface, causing serious erosion problems. 2B. Culvert Crossing Haul Road Near the Sho Using a peak flow of 3.9 cfs and entering the culvert headwater chart, Chart 5, a substantial headwater is needed to pass the flow through the culvert. WF may either provide a riprap protected embankment 2' in height or choose a larger culvert. As the road is a haul road, a foot of fill above the culvert would appear to be reasonable and would provide sufficient depth for the development of a 2' headwater.