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DRAINAGE CRITERIA MANUAL RUNOFF tc recommended for urbanized area should not be less than 5 minutes and if . calculations indicate a lesser value, use 5-minutes instead. The inlet time can be estimated by calculating the various overland distances and flow velocities taken from the most remote point. A common mistake in urbanized areas is to assume travel velocities that are too slow. Another common error is to not check the runoff peak resulting from only a part of the basin. Sometimes a lower portion of the basin produces a larger peak than that computed for the whole basin. This error is most often encountered in long basins, or in basins where the upper portion contains grassy park land and the lower portion is developed urban land. When studying a tract of land proposed for subdivision, do not necessarily take the overland flow path perpendicular to the contours since the 1 and wi 11 be graded and swa 1 es will often intercept the natural contour and conduct the water to the streets, thus cutting down on the time of concentration. Example. Time of Concentration In Urbanized Basin Given: Find: Solution: Step 1: 5-1-84 A 100 acre single family residential development. Watershed has a length of 3000 feet. The upper overland flow portion is from back of a lot and is 100 feet in length at an average slope of 2 percent. The overland flow is mostly over grass and a short driveway, having a composite 5-year runoff coefficient of 0.35. The upper 1000 feet of the gutter in the watershed has an average slope of 1.0 percent. . Time of concentration at the first storm sewer inlet located 1000 feet from the top of the watershed. Find the initial (overland) flow time for the uppermost 100 feet. Using Figure 3-1, with a 2.0 percent slope and C5 = 0.35 obtain, t. = 11 minutes 1 .