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Agricultural <br />T~ = 7.2 A.~ L.ZS L.zs S-.z <br />Urban <br />T~ = 3.2 A' LZS L~a S-'a RTIMP-36 <br />where T~ = time of concentration, in hours <br />A = area, in square miles <br />S = watercourse slope, in ft/mile <br />L = length of the watercourse to the hydraulically most <br />distant point, in miles <br />l.~ = length measured from the concentration point along L <br />to a point on L that is perpendicular to the watershed <br />centroid, in miles, and <br />RTIMP= effective impervious area, in percent. (Note: RTIMP <br />must be greater than 1 percent.) <br />In using those T~ equations, the following points should be noted and observed: <br />1. The ~rea (A) is determined from the best available map. The delineation of the <br />drainage boundary needs to be carefully performed, and special care must be <br />taken where there is little topographic relief. In urban areas, land grading and <br />road construction can produce drainage boundaries that separate runoff from <br />contributing areas during small and lower intensity storms. However, larger and <br />more intense storms, such as the design storm for an inflow design flood, can <br />produce runoff depths that can cross those intermediate drainage boundaries <br />resulting in a larger total contributing area. For urban watersheds, it is generally <br />prudent to consider the largest reasonable drainage area. <br />2. Determination of the hydraulically most distant point will define both L and S. <br />Often, the hydraulically most distant point is determined as the point along the <br />watershed boundary that has the longest flow path to the watershed outlet (or <br />subbasin concentration point). This is generally true where the topography is <br />relatively uniform throughout the watershed. However, there are situations <br />19 March 2007 $ <br />