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

Drainage Design Manual for Maricopa County
Hydrology: Rainfall
although local storm cells (typically of lesser intensity than without frontal activity) can be imbedded
in larger, general storm systems.
General winter storms usually move in from the north Pacific Ocean, and produce light to moderate
precipitation over relatively large areas. These storms occur between late October and May,
producing the heaviest precipitation from December to early March. Such storms could last over
several days with slight breaks between individual storms. Because of orographic effects, the
mountain areas generally receive more precipitation than the lower desert areas. These storms
are characterized by low intensity, long duration, and large areal extent, but on occasion, with an
additional surge of moisture from the southwest, can contribute to substantial runoff volumes and
peak discharge on major river systems.
General summer storms are often associated with tropical storms. The Pacific Ocean north of
the equator and south of Mexico is a breeding ground for such storms. On the average, about
two dozen tropical storms and hurricanes are generated in this area from June through early
October; most move in a northwesterly direction. The remnants of these storms can be caught
up in the large scale circulation around a low pressure center in southern California and therefore
can bring a persistent flow of moist tropical air into Arizona. The storm pattern consists of a band
of locally heavy rain cells within a larger area of light to moderate rainfall. Whereas general winter
storms can cover much of the state, general summer storms are more localized along bands
of rainfall. They are similar to winter storms in that higher elevations receive greater rainfall
because of orographic influences. The period of late September through October may have
storm patterns which are similar to both general summer and winter events.
Local storms consist of scattered heavy downpours of rain over areas of up to about 500 square
miles for a time period of up to 6 hours. Within the storm area, exceptionally heavy rains usually
cover up to 20 square miles and often last for less than 60 minutes. They are typically associated
with lightning and thunder, and are referred to as thunderstorms or cloudbursts. While they
can occur any time during the year, they are more frequent during summer months (July to September)
when tropical moisture pushes into the area from the southeast or southwest. These
storms turn into longer duration events in late summer and may be associated with general summer
storms (see above). Local storms generally produce record peaks for small watersheds.
They can result in flash floods, and, sometimes, loss of life and property damage.
2.1.2 Design Rainfall Criteria for Maricopa County
The critical flood-producing storm for most watersheds in Maricopa County is the local storm.
The limit of such storms is generally less than 500 square miles with durations less than 6 hours.
Local storms are characterized by central storm cells (possibly as large as 100 square miles) that
produce very high intensity rainfalls for relatively short durations. The rainfall intensities diminish
as the distance from the storm cell increases. Therefore, for the majority of watersheds and
2-2 August 15, 2013