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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
The selection of a representative watershed resistance coefficient, K b , similar in concept to Manning’s
n in open-channel flow, is very subjective and therefore a high degree of uncertainty is
associated with its use. To diminish this uncertainty and to increase the reproducibility of the procedure,
a graph is provided in Figure 5.5 for the selection on K b based on watershed classification
and watershed size. Interpolation can be used for a given watershed size and mixed
classification. Equations for estimating K b are given in Table 5.2, along with general descriptions
of land forms/use for which the equation applies.
To estimate T c by Equation (5.5), the average rainfall excess intensity must be estimated. The
average rainfall excess intensity can be estimated by the following method: Run an HEC-1 model
using the FCDMC rainfall loss method to estimate the rainfall excess at each computational time
interval (NMIN). Then, rank the rainfall excess values from the highest to the lowest. The average
rainfall excess intensity (inch/hr) is estimated by summing up the first ten highest rainfall
excess values and dividing the result by 10*NMIN/60. Then, T c is obtained by directly solving
Equation (5.5). The “ten” highest values method has been found to yield a reasonable time of
concentration based on research of Maricopa County watersheds by FCDMC staff. An example
of the procedure can be found in Section 9.4.4. Alternatively, the DDMSW program can be used
to automate this process, which will also populate the HEC-1 input file with the required data.
The computation interval (NMIN) on the IT record of HEC-1 must be selected to correspond to
the time of concentration for the unit hydrograph. This requirement is necessary to adequately
define the shape of the unit hydrograph. From Snyder’s unit hydrograph theory, the unit rainfall
duration for a unit hydrograph (computation interval) is equal to lag time divided by 5.5. For the
SCS Dimensionless Unit Hydrograph, the unit rainfall duration is to equal 0.133 T c , and although
small variation in the selection of computation interval is allowed, the SCS recommends that the
duration not exceed 0.25 T c . Although there is not a rigid theoretical limitation to how small the
computation interval can be, from a practical standpoint, too small of a NMIN could result in
excessive computer output. Therefore, as a general rule the computation interval should meet
the following:
NMIN = 0.15 T c (5.6)
Equation (5.6) is preferred; however, as a general requirement, NMIN should fall in the range
indicated in Equation (5.7).
0.10 T c < NMIN < 0.25 T c (5.7)
August 15, 2013 5-15