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

Drainage Design Manual for Maricopa County
Hydrology: Introduction
• Phase 3 Prepare an in-depth evaluation of regional rainfall data and establish precipitation
design rainfall guidelines and isohyetal maps for Maricopa
County.
As a part of Phase 2, the Drainage Design Manual for Maricopa County, Volume I, Hydrology, will
provide the necessary data for Volume II, Hydraulics.
1.3 SCOPE AND LIMITATION
When using the procedures detailed in this manual, it is important to keep three limitations in
mind. First, this is a hydrologic design manual. The methods, techniques and parameter values
described herein are not necessarily valid for real-time prediction of flow values, nor for recreating
historic events – although some of the methods are physically based and would be amenable
for uses other than design hydrology.
Second, the lack of runoff data for urbanizing areas of the County, for the most part, precludes
the use of flood-frequency analysis for stormwater drainage design. For those watercourses with
sufficient record, flood-frequency analysis may be acceptable. Similarly, for those watercourses
with established regulatory floodplains, the FEMA-accepted flood-frequency curves may be used
for design purposes, unless they are proven inappropriate. The purpose of this manual is to provide
a means of assisting in the prediction of runoff that might result from a design storm of a
given return interval.
Third, the typical design storm normally has no point of reference in terms of a singular historic
event. Rather, it intends to provide the best available information by utilizing historic data as well
as other precipitation design concepts. The design storm provides not only the peak intensities
that would be expected from a storm of a given duration and return interval, but also the volumes
associated with it. The tables describing the temporal distribution of the design storm for use in a
hydrologic model, i.e., HEC-1, are approximately equivalent to the graphs used to determine the
rainfall intensity to be used in the Rational Method. The net effect is that regardless of the size of
the area being investigated or the method of analysis, the same design storm is used as the driving
input.
August 15, 2013 1-5