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

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