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4. CULVERT DESIGNS <br />Culverts have been designed using the corrugated metal type, with a Manning's n of 0.024, slopes of <br />1.5% and peak flows of either 10, 25 or 100 year return intervals, depending upon the life of the <br />structure. Standard Federal Highway Administration techniques were used to size the culverts in all <br />locations. Culverts mitered to conform to the slope was used with a Ke of 0.70, although culverts <br />projecting from fill for the HW /D ratios used here also produce almost the same results.The data on the <br />following pages shows the designs for 18 ", 24 ", 36" and 42" CMP's. Along the BB Road detour, 13" x 17" <br />CMP's were used as drainage culverts for the road. These do not have any significant drainage basins <br />since Ditch C1 parallels the road on the south side and captures any significant runoff that would <br />normally drain to the road. For this reason, the capacity of these culverts far exceeds any peak flows that <br />could occur in the minor basins and no designs are shown for these culverts (Culverts 119 through 128). <br />The allowed height of water above each culvert is shown on each of the following charts (from FHA), <br />which are used in determining the headwater to diameter ratio (HW /D) and consequently, the capacity of <br />the culvert. <br />Many culverts have the same basin designs as ditches described earlier. If this is the case, no new <br />design is shown, the table simply refers to that ditch design. <br />Given the above parameters, the following pages show that the capacities of the culverts are as follows: <br />18" <br />9.5 cfs <br />24" <br />18.4 cfs <br />30" <br />30.0 cfs <br />36" <br />47.0 cfs <br />42" <br />70.0 cfs <br />48" <br />82.0 cfs <br />Revised August 2009(PB06) Attachment 2.05.3(3) -1 -64 <br />