Drainage Design Manual, Hydrology - Flood Control District of ...

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Drainage Design Manual for Maricopa County Hydrology: Rational Method The Rational Equation is based on the concept that the application of a steady, uniform rainfall intensity will produce a peak discharge at such a time when all points of the watershed are contributing to the outflow at the point of design. Such a condition is met when the elapsed time is equal to the time of concentration, T c , which is defined to be the floodwave travel time from the most remote part of the watershed to the point of design. The time of concentration should be computed by applying the following equation developed by Papadakis and Kazan (1987): T c 11.4L 0.5 0.52 – 0.31 = K b S i – 0.38 (3.2) where: T c = time of concentration, in hours. L = length of the longest flow path, in miles. K b = watershed resistance coefficient (see Table 3.1 or Figure 3.1). S = watercourse slope, in feet/mile. i = rainfall intensity, in inches/hour.* *It should be noted that i is the “rainfall excess intensity” as originally developed. However, when used in the Rational Equation, rainfall intensity and rainfall excess intensity provide similar values because the hydrologic characteristics of small, urban watersheds result in minimal rainfall loss. This is due to the extent of imperviousness associated with urban watersheds and to the fact that the time of concentration is usually very short. Rational Method runoff coefficients for land uses are provided in Table 3.2. 3-2 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rational Method Table 3.1 EQUATION FOR ESTIMATING K B IN THE T C EQUATION K b = m log 10 A + b Where A is drainage area, in acres Equation Parameters Type Description Typical Applications m b A Minimal roughness: Land surfaces that are relatively smooth and/or well graded. Surface runoff is sheet flow. Commercial/industrial areas Residential areas Parks and golf courses -0.00625 0.04 B Moderately low roughness: Land surfaces have irregularly spaced roughness elements that protrude from the surface but are still relatively uniform. Surface runoff is predominately sheet flow around the roughness elements. Agricultural fields Pastures Desert rangelands Undeveloped urban lands -0.01375 0.08 C Moderately high roughness: Land surfaces that have significant large to medium-sized roughness elements and/or poorly graded land surfaces that cause the flow to be diverted around the roughness elements. Surface runoff is sheet flow for short distances draining into meandering drainage paths. Hillslopes Brushy alluvial fans Hilly rangelands Disturbed lands, mining, etc. Forests with underbrush -0.025 0.15 D Maximum roughness: Rough land surfaces with torturous flow paths. Surface runoff is concentrated in numerous short flow paths that are often oblique to the main flow direction. Mountains Some wetlands -0.030 0.20 Note: A is the area of the entire subbasin, not the area of the surface type A, B, C or D within the subbasin. August 15, 2013 3-3
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