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

Drainage Design Manual for Maricopa County
Hydrology: Application
9.4 UNIT HYDROGRAPH
9.4.1 Procedures for the 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 Figure 5.5 and 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)
5. 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; and
• Clark unit hydrograph parameters (values set to zero).
6. Run HEC-1 with the input file from Step 5 at an output level of zero for each subbasin.
Rank the incremental rainfall excess values from smallest to highest for each subbasin
and sum the ten (10) highest values. Compute the average rainfall intensity, i, by
dividing the sum by the total of ten (10) computation time intervals and convert to units
of hours (total of 10 highest rainfall excess values/(10(NMIN/60)).
7. Compute T c using the equation from Step 4 above.
8. Calculate R using Equation (5.8).
9. Select the appropriate time-area relation for the basin or subbasin.
As an alternative to the above procedures, the DDMSW program will compute the rainfall
excess directly and perform the necessary iterations to compute the T c and R parameters.
August 15, 2013 9-43