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River Sedimentation Pond design is set forth since they relate to emergency spillway design. • Drainage Area =1.29 acres Curve Number BO Hydrologic Group D Precipitation 10 year 24 hour event 1.6" 25 year 24 hour event 2.0" 100 year 24 hour event 2.6" Peak Flow Calculations Unadjusted peak runoff factor Peak flow = 3.2cfs a10 a25 0100 010 025 x100 .34" .56" .96" 1.12 1.85 3.16 Total Flow Calculations Drainage Area, Da = 1.29 acres Total O10 (.34) 1.29 acres x. 34"/12in/ft = .04ac/ft Total 025 (.56) 1.29 acres x .56"/12in/ft = .06ac/ft Total 0100(.96) 1.29 acres x .96"/12in/ft = .l0ac/ft Design sedimentation pond to contain 10 year 24 hour storm, include sediment storage = .Olac/ft Construct impoundment total volume .O5ac/ft: 5 feet deep; 3 horizontal: 1 vertical sideslopes: triangular shape, 3o feet wide x 30 feet long; dewatering device must convey 0100 = 3.16 cfs. Install 18 inch capable to convey 5.Scfs with CMP Project-no head. • A review of the as built Yampa River Sedimentation Pond computations in my lOJuly90 report are as follows: Total volume =0.537acft Computations: Area of embankment break in slope (measured) 57 x 103 = 5871 5871 x 0.95 =5577sgf~ Area at top of dewatering device (calculated) 51.4 x 95 = 4883 4883 x 0.95 =4639sgft Area at waterline (measured) 38.5 x 82 = 3157 3157 x 0.95 =2999sgft Area at pond bottom (calculated) 20.4 x 67.3 = 1373 1373 x 0.95 =1304sgft Volume from embankment break-in-slope to waterline 0.5(2999+5577) x 3.95 = 16938cuft =0.389acft Volume of water 0.5( 1304+2999) x 3(estimated)=6455cuft or =0.148acft Total =0.537acft The accompanying graph, Yampa River Sedimentation Pond Storage Capacity, utilizes three control points for the graph plot: zero storage; depth and volume of water on 4June90; and depth and total volume to embankment break-in- slope. The line is curvilinear due to the irregular • trapezoidal pond shape, and estimates regarding water depth and projections of bank inclinations. None the less, plots ~~~ I