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Preliminary crossing culvert design was performed using a design velocity criterion of 10 <br />feet per second (fps). This indicated that about 96 square feet of culvert azea would be <br />required, or roughly two (2) eight foot (96") diameter culverts. The final crossing design <br />will detail exactly the most efficient and economical culvert crossing, approach and <br />outfall for this application. It is our understanding acceptance of the final crossing <br />design would be made a condition of the UBSR approval. <br />The proposed crossing is located approximately 200 feet downstream of the south <br />property line, at about 6660 feet mean sea level (msl) channel elevation per USGS topo. <br />The required culvert 100-year flood headwater elevation would be approximately 6670 <br />feet msl. The nearest man-made buildings (including a house) are located about li4 mile <br />upstream of the crossing, at about 6710 feet msl elevation per USGS topo. This indicates <br />the culvert headwater and upstream flood backwater should have adequate channel <br />distance and elevation to return to pre-crossing levels, and should not threaten the <br />buildings in any case. <br />1 Included with this report is a conceptual drawing of the road crossing as requested by the <br />DCPWD (Figure E). Final designs will be submitted to the DCPWD prior to <br />construction of the crossing. The crossing will have a 30 foot travel width with side <br />slopes no steeper than 3 feet horizontal to 1 feet vertical (3H:1 V or 3:1). The crossing <br />will be stabilized with riprap erosion protection to prevent damage to the roadway <br />structure and Willow Creek channel. <br />E. Variance From Criteria <br />The DCSDDTC manual was designed more for urban development than mining <br />operations. Consequently, some minor variances from the standard manual criteria were <br />appropriate and necessary. Below is a brief discussion of the variances County staff has <br />allowed for this drainage study: <br />1. The use of the Sedcad runoff prediction method instead of the CUHP runoff <br />prediction method. The Sedcad program has more applications suited for mining <br />' operations. <br />2. Since the Castle Rock Quarry site covers a large area and relatively large basins, <br />' 1" = 400' scale plan maps were more appropriate than larger scale maps. The <br />scale utilized affords a much more useable and understandable view of the <br />property, and drainage basins and features. <br />3. The Douglas County Flood Plain Development permit (required far the Willow <br />Creek access road crossing) and a final grading plan for the access road will be <br />required as a condition of the Board of County Commissioners final UBSR <br />approval. <br /> <br />I JI <br /> <br />