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

Drainage Design Manual for Maricopa County
Hydrology: Multiple Frequency Modeling
For reasons of practicality and to facilitate reproducibility, a single ratio for the 2-, 5- and 10-year
recurrence intervals is provided that represents average conditions in Maricopa County. These
values are listed in Table 6.1 and can be used for both local and general storms for drainage
areas of any size, degree of development or other hydrologic and physiographic conditions.
Table 6.1
RATIOS TO 100-YEAR FLOOD HYDROGRAPHS
FOR THE 2-, 5- AND 10-YEAR RECURRENCE INTERVAL FLOODS
This approach should be used when the results for the 2-, 5- and 10-year flood (peaks and volumes)
using the methods, techniques and parameters in the preceding chapters are unreasonable.
The reasonableness “test” applies to model results (peak discharges and runoff volumes)
as well as to the HEC-1 input parameters, particularly for the unit hydrograph. This alternative
method using the ratios from Table 6.1 does not preclude the use of another method or the use of
different (site specific) ratios with prior approval from the Flood Control District, or local jurisdiction.
6.3 IMPLEMENTATION IN HEC-1
Recurrence Ratio
Interval
%
2 10
5 25
10 35
The ratio for the desired recurrence interval is coded into the 100-year HEC-1 model on field 3 of
the subbasin area (BA) record for each subbasin. Alternatively, for a single storm analysis the
ratio(s) can be coded into the 100-year HEC-1 model on the multiratio (JR) record. In addition to
coding the ratio(s) on this record, the IRTIO variable in field 1 must be set to FLOW to ratio the
runoff, not the precipitation. The JR record cannot be used for a multiple storm analysis due to a
conflict with the JD record used to define the index areas.
6-2 August 15, 2013