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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
5.7 PROCEDURES
Procedures for calculating the unit hydrograph parameters are provided in the following sections.
Notes and general guidance on the application of these procedures and the methodologies presented
in this chapter are provided along with a detailed example in Section 9.4.4.
5.7.1 Clark Unit Hydrograph
1. From an appropriate map of the watershed, measure drainage area (A) and the values
of L and S.
2. If S is greater than 200 ft/mi, adjust the slope using Table 5.2 or Figure 5.4.
3. Using either Figure 5.5 or Table 5.3, select a resistance coefficient (K b ) for the basin
or subbasin based on a resistance classification and the drainage area (in acres). For
a basin or subbasin of mixed classification;
• A representative K b can be interpolated from Figure 5.5, or
• An arithmetically averaged K b can be calculated based on the area of each
unique K b present in the basin or subbasin.
4. Calculate T c as a function of i using Equation (5.5)
a. Enter the following data into an HEC-1 input file:
• Design rainfall per the methodology and procedures in Chapter 2.
• Basin area.
• Rainfall loss data per the methodologies and procedures in Chapter 4.
• Clark unit hydrograph parameters (values set to zero).
b. Run HEC-1 with the input file from Step 5 at an output level of zero for each subbasin.
From the HEC-1 output file, find the rainfall excess at each time interval.
Rank the values from the highest to the lowest. The average rainfall intensity is
found by summing up the first ten highest rainfall excess values and dividing the
result by the length of ten time intervals.
c. Directly solve Equation (5.5) for T c using the computed average rainfall intensity.
5. Calculate R using Equation (5.8).
6. Select the appropriate time-area relation for the basin or subbasin.
5-30 August 15, 2013