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

Drainage Design Manual for Maricopa County
Hydrology: Multiple Frequency Modeling
6
6 MULTIPLE FREQUENCY
MODELING
TABLE OF CONTENTS
6 MULTIPLE FREQUENCY MODELING
6.1 BACKGROUND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.2 APPROACH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1
6.3 IMPLEMENTATION IN HEC-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2
6.1 BACKGROUND
Originally, the Hydrology Manual was intended to be used for the development of flood discharges
and runoff volumes resulting from infrequent storms, such as the 100-year rainfall. Data
that were collected and used in the selection and development of the methods, techniques and
parameters are representative of infrequent storms. While it was recognized that the application
of the methods, techniques and procedures may not be appropriate for more frequent storms,
this limitation was not perceived as a significant issue at that time.
Recently, there has been an increasing need for runoff magnitudes from more frequent storms,
particularly in regard to the design of storm drains, but also for regulatory and planning purposes.
However, use of the methods, techniques and parameters presented in the preceding chapters
may result in the overestimation of runoff magnitudes for those types of events. The threshold at
which this occurs often is the 10-year recurrence interval. Several different alternative
approaches were considered that could be used in place of or to supplement the methods, techniques
and parameters presented in the preceding chapters. Each alternative method was evaluated
in regard to the three benchmarks (accuracy, practicality and reproducibility) that were
used to evaluate the original methods, techniques and parameters. The alternative approach to
be used in Maricopa County for the estimation of runoff for more frequent storms is a ratio that is
applied to the 100-year runoff hydrographs.
6.2 APPROACH
Ratios for the 2-, 5- and 10-year recurrence intervals are based on analysis of USGS gage data
for watersheds throughout the State of Arizona. That data reflects the wide range of hydrologic
and physiographic characteristics that exist in Arizona. This variability was considered in the
analysis in regard to the conditions that are specific to Maricopa County.
August 15, 2013 6-1