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

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Drainage Design Manual for Maricopa County Hydrology: Unit Hydrograph Procedures 5.5 ESTIMATION OF PARAMETERS The following procedures are recommended for the calculation of the Clark Unit Hydrograph parameters for use in Maricopa County. Other general procedures, as previously discussed, can be used; however, those should be approved by the jurisdictional agency prior to undertaking such procedures. 5.5.1 Time of Concentration Time of concentration is defined as the travel time, during the corresponding period of most intense rainfall excess, for a floodwave to travel from the hydraulically most distant point in the watershed to the point of interest (concentration point). Note especially that T c is not the travel time taken for a particle of water to move down the catchment, as is often cited in engineering texts. The catchment is in equilibrium when T c is reached because the outlet then “feels” the inflow from every portion of the catchment (Bedient and Huber, 1988). Since a wave moves faster than a particle of water, the time of concentration (and catchment equilibrium) occurs sooner than if based on overland flow or channel water velocities. An empirical equation for time of concentration, T c has been adopted with some procedural modifications from Papadakis and Kazan (1987). T c 11.4L 0.5 0.52 – 0.31 = K b S i – 0.38 (5.5) where: T c = time of concentration, in hours. L = length of the hydraulically longest flow path, in miles. K b = watershed resistance coefficient (see Figure 5.5, or Table 5.3). S = watercourse slope, in feet/mile. i = the average rainfall excess intensity, in inches/hour. L is the length of the flow path from the basin outlet to the hydraulically most distant point in the watershed. The hydraulically most distant point is not necessarily the longest path, but may be a shorter length with an appreciably flatter slope. Watercourse slope S is the average slope of the flow path for the same watercourse that is used to define L. The magnitude of S can be calculated as the difference in elevation between the two points used to define L divided by the length, L. Watersheds in mountains can result in large values for S, which may result in an underestimation of T c . This is because as slope increases in natural watersheds the runoff velocity does not usually increase in a corresponding manner. The slope of steep natural watercourses is often adjusted to reduce the slope, and the reduced slope of steep natural watercourses should be adjusted by using Table 5.2 or Figure 5.4. 5-12 August 15, 2013
Drainage Design Manual for Maricopa County Figure 5.4 SLOPE ADJUSTMENT FOR STEEP WATERCOURSES IN NATURAL WATERSHEDS (SOURCE: DRAINAGE CRITERIA MANUAL, URBAN DRAINAGE AND FLOOD CONTROL DISTRICT, COLORADO, MAY 1984.) 350 Hydrology: Unit Hydrograph Procedures 300 Adjusted Slope for Use in T c Calculations, in ft/mile 250 200 150 100 50 0 0 100 200 300 400 500 600 Measured Watercourse Slope, in ft/mile 5-13 August 15, 2013
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