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DRAINAGE CRITERIA MANUAL RUNOFF . runoff coefficients recommended in this manual. As a result these recommendations need to be used with a great deal of caution whenever working outside the Denver region. For urban areas, the time of concentration consists of an inlet time or overland flow time (ti) pl us the travel time (tt) in the storm sewer, paved gutter, roadside drainage ditch, or drainage channel. For non-urban areas, the time of concentration consists of an overland flow time (t.) plus the time - , of travel in a combined form, such as a small swale, channel, or drainageway. The travel portion (tt) of the time of concentration can be estimated from the hydraulic properties of the storm sewer, gutter, swale, ditch, or drainageway. Inlet time, on the other hand, will vary with surface slope, depression storage, surface cover, antecedent rainfall, and infiltration capacity of the soil, as well as distrance of surface flow. The time of concentration can be represented by Equation 3-2 for both urban and non-urban areas: tc = ti + tt (3-2) . In which tc = t. = , tt = time of concentration (minutes) initial, inlet, or overland flow time (minutes) travel time in the ditch, channel, gutter, storm, etc. (minutes) 3.4.1 Time of Concentration In Non-Urbanized Basins The initial or overland flow time {t.) in non-urbanized watersheds may be , calculated using Equation 3-3: 1.8 (1.1 - c5h[T (3-3) t. = , J..rS In which t. = initial or overland flow time (minutes) , C5 = runoff coefficient for 5-year frequency (from Tabl e 3-1) L = length of overland flow, (feet. , 500 I maximum) S = average basin slope (percent) 4It Equation 3-3 is considered adequate for distances up to 500 feet and has been reduced to a graph in Figure 3-1. For longer basin lengths, the time of 5-1-84