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

Drainage Design Manual for Maricopa County
Hydrology: Introduction
1
1 INTRODUCTION
TABLE OF CONTENTS
1 INTRODUCTION
1.1 OVERVIEW . . . . . . . . . . . . . . . . . . . 1-1
1.2 PURPOSE . . . . . . . . . . . . . . . . . . . 1-4
1.3 SCOPE AND LIMITATION . . . . . . . . . . . . . . . 1-5
1.4 USING THIS MANUAL . . . . . . . . . . . . . . . . 1-6
1.5 APPLICATION . . . . . . . . . . . . . . . . . . 1-6
1.1 OVERVIEW
The objective of the Drainage Design Manual for Maricopa County, Hydrology, (hereinafter
referred to as the Hydrology Manual) is to provide technical procedures for the estimation of flood
discharges for the purposes of designing stormwater drainage facilities and regulating watercourses
in Maricopa County. Two methodologies are defined for the development of design discharges:
the Rational Method, and rainfall-runoff modeling using a design storm. For small,
urban watersheds, less than 160 acres and fairly uniform land-use, the Rational Method is
acceptable. Use of this method will only produce peak discharges and runoff volumes. This
method should not be used if a complete runoff hydrograph is needed, such as for routing
through detention facilities. For larger, more complex watersheds or drainage networks, a rainfall-runoff
model should be developed. The Hydrology Manual provides guidance in the development
of such a model and the estimation of the necessary input parameters to the model.
Although not necessarily required, the use of the U.S. Army Corps of Engineers’ HEC-1 Flood
Hydrology Program facilitates the use of the procedures that are contained in the Hydrology
Manual (The Hydrology Manual was written to supplement the HEC-1 User’s Manual.). The manual
also provides indirect methods intended to be used as confidence checks and verification of
the reasonableness of the results obtained from the two methodologies discussed above.
The Hydrology Manual can be used to develop design discharge magnitudes for storms of frequencies
up to and including the 100-year event. The design storm is of 6- or 24-hour duration
and that storm is to be used for the design of all stormwater drainage facilities except stormwater
storage facilities. The criteria to be applied to the 2-hour storm is also provided in the Hydrology
Manual for use in design of stormwater storage facilities, as a minimum recommended criteria for
Maricopa County. The criteria for design of stormwater storage facilities in unincorporated areas
of Maricopa County is the 100-year, 2-hour storm. Although this is the minimum recommended
August 15, 2013 1-1