Laserfiche WebLink
<br />1 <br />1.49 <br />n <br />where: <br />A = area of channel flow at depth of flow <br />n = Manning' s roughness factor <br />R = hv_drau'i c radius of channel in feet <br /> ( determ ined by divi ding the area by the wetted perimeter; <br />S = gradient of channel in decimal <br />The parameters of the bridge crossing a~ the time of the picture <br />are given below. The key determininc 'actor is `low depth, which <br />appears to be approximately 5 inches high. <br />Bottom Sides Flow Channel P1annin9's Velocity Flow <br />Width Slope Depth Slope n ft/sec cfs <br />feet <br />------ ----- feet 3; <br />----- ------- const. <br />--------- <br />-------- ---- <br />;a 2 O.a2 .035 .030 5.0 0'.0 <br />To this flow amount, the amount of f',ow leaving the ditch just <br />upstream from the bridge crossing must be added. Frcm the picture <br />and the above est'mate, it is very reascnable to assume that this <br />amount is approximately 25 cfs. Therefcre, the ccmb'.ned amount of <br />So cfs exceeds one peak flow of 49.- cfs for the .0 year - 24 <br />hour storm. `ui"y realize that these caiculac'ons are not <br />precise, unfor`;,unateiy we don'- trul.e know the exac~ rainfa <br />amount and the exact per;od that .~ fel',. Also, we have nc flow <br />gage or we'r in she drainage ditch. However, the calculations are <br />reasonable and cna~lenge the D',v'sicn tp prove that the storm <br />produced a flow that was under the peak flow rrom the .0 year - <br />2t hour storm. <br />As the calc::lst_cns show, an event cccurred on that day which <br />produced a unc-. sim~l.=_r ~o that -rom a 1C year ~a hcur s~orm. <br />' Ther=fore the discharge from ?cna ~°-?. ~hcu',d also be exempt <br />