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Manning's equation is A culvert or bridge can act as a reservoir outlet if there is significant storage behind it. The 1.49 r23 s1/2 [Eq. 3-4] procedures in TR-55 can be used to determine the V = peak flow upstream of the culvert. Detailed n storage routing procedures should be used to where determine the outflow through the culvert. V = average velocity (ft/s), r = hydraulic radius (ft) and is equal to alp,, Example 3-1 a = cross sectional flow area (ft2), pw = wetted perimeter (ft), s..'= slope of the hydraulic grade line (channel The sketch below shows a watershed in Dyer slope, ft/ft), and County, northwestern Tennessee. The problem is to j n = Manning's roughness coefficient for open compute Tc at the outlet of the watershed (point D). channel flow. The 2-year 24-hour rainfall depth is 3.6 inches. All three types of flow occur from the hydraulically most Manning's n values for open channel flow can be distant point (A) to the point of interest (D). To J obtained from standard textbooks such as Chow compute Tc, first determine Tt for each segment (1959) or Linsley et al. (1982). After average velocity from the following information: is computed using equation 34, Tt for the channel segment can be estimated using equation 3-1. Segment AB: ; dense gr 0 Sheet ftfloft;w and length (L) sloi ems( _ Segment BC: Shallow concentrated flow; unpaved; Reservoirs or lakes s = 0.01 ft/ft; and L = 1400 ft. Segment CD: Channel flow; Manning's n = .05; Sometimes it is necessary to estimate the velocity of flow area (a) = 27 ft2; wetted flow through a reservoir or lake at the outlet of a perimeter (per.) = 28.2 ft; s = 0.005 watershed. This travel time is normally very small ft/ft; and L = 7300 ft. and can be assumed as zero. See figure 3-2 for the computations made on worksheet 3. Limitations . Manning's kinematic solution should not be used for sheet flow longer than 300 feet. Equation 3.3 was developed for use with the four standard rainfall intensity-duration relationships. 100 ft 1.400 ft 7 300 °t F A B t p • In watersheds with storm sewers, carefully (not to scale) identify the appropriate hydraulic flow path to estimate Tc. Storm sewers generally handle only a small portion of a large event. The rest of the peak flow travels by streets, lawns, and so on, to the outlet. Consult a standard hydraulics textbook to determine average velocity in pipes for either pressure or nonpressure flow. . The minimum Tc used in TR-55 is 0.1 hour. i 34 (210-VI-TR-55, Second Ed., June 1986)