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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
The second parameter is the storage coefficient, R, which has the dimension of time. This
parameter is used to account for the effect that temporary storage in the watershed has on the
hydrograph. Several methods are available to estimate R from recorded hydrographs for a
basin. As originally proposed by Clark (1945), this parameter can be estimated by dividing the
discharge at the point of inflection of the surface runoff hydrograph by the rate of change of discharge
(slope of the hydrograph) at the inflection point as shown in Figure 5.2.
Another technique for estimating R is to compute the volume remaining under the recession limb
of the surface runoff hydrograph following the point of inflection and to divide the volume by the
discharge at the point of inflection. Both of these methods require the ability to identify the inflection
point on the recession limb of the runoff hydrograph. This is difficult if not impossible for
complex hydrographs and hydrographs with steep rising and recession limbs such as occur from
urban basins and natural watersheds in the Southwest. A method to estimate R by a graphical
recession analysis of the hydrograph has been proposed (Sabol, 1988) and this method provides
much more consistent results than do the previously described methods. The parameter, R,
should be estimated by the analysis of several recorded events; however, in most cases
recorded discharge hydrographs are not available and R must be estimated by empirical equations.
A graphical parameter called the time area relation is necessary to compute the translation
hydrograph. The time-area relation specifies the accumulated area of the watershed that is contributing
runoff to the outlet of the watershed at any point in time. Procedures to develop a timearea
relation for a watershed are discussed in a later section of this manual.
5-4 August 15, 2013