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

Drainage Design Manual for Maricopa County
Hydrology: Rainfall
2
2 RAINFALL
TABLE OF CONTENTS
2 RAINFALL
2.1 GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1
2.1.1 Storm and Flood Occurrence in Maricopa County ...................................... 2-1
2.1.2 Design Rainfall Criteria for Maricopa County .............................................. 2-2
2.2 RAINFALL DEPTH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-4
2.2.1 Data Source ................................................................................................ 2-4
2.2.2 Depth-Duration-Frequency Statistics .......................................................... 2-5
2.2.3 Rainfall Statistics for Special Purposes....................................................... 2-5
2.3 DEPTH-AREA RELATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5
2.4 DESIGN STORM DISTRIBUTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-10
2.4.1 2-hour Storm Distribution .......................................................................... 2-10
2.4.2 6-hour Storm Distribution .......................................................................... 2-12
2.4.3 24-hour Storm Distribution ........................................................................ 2-16
2.5 PROCEDURE FOR THE DEVELOPMENT OF THE DESIGN RAINFALL . . . . . . . 2-19
2.1 GENERAL
Precipitation in Maricopa County is strongly influenced by variation in climate, changing from a
warm and semi-arid desert environment at lower elevations to a seasonally cool and moderately
humid mountain environment. Mean annual precipitation ranges from about 7 inches in the
Phoenix vicinity to more than 25 inches in the mountain regions of northern Maricopa County.
Precipitation is typically divided into two seasons of comparative rainfall depths: summer (July
through September) and winter (December through March). Warm, moist tropical air can move
into Arizona at any time of the year, but most often does so in the summer months, resulting in
severe storms and local flooding. Storms of large areal extent are usually associated with frontal
or convergence storm activity that may result in long duration rainfall and flooding of major drainage
watercourses. These types of storms and flooding usually occur in the winter, but occasionally
occur in the summer.
2.1.1 Storm and Flood Occurrence in Maricopa County
Storms in Maricopa County are often classified as general winter, general summer, and local
storms. General storms are usually frontal or convergence type that cover large areas and have
traditionally contributed to flooding of the major drainage watercourses in the County. Local
storms are usually associated with convective activity and hence normally occur in the summer,
August 15, 2013 2-1