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

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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
Drainage Design Manual for Maricopa County Hydrology: Rainfall drainage areas in Maricopa County, the local storm will produce both the largest flood peak discharge and the greatest runoff volume. Based on a review of meteorologic studies for Arizona (U.S. Army Corps of Engineers, 1974 and 1982a) and a consideration of severe storms in Maricopa County, it was determined that the 6-hour local storm should be used as the design storm criteria for watersheds in Maricopa County with drainage areas of 20 square miles and less. The 6-hour local storm for watersheds between 20 and 100 square miles may be the required design storm criteria, as discussed below. The general design storm for watershed areas between 20 and 500 square miles is the 24-hour storm. For drainage areas between the critical flood-producing upper limit for local storms (100 square miles) and the lower limit for general storms (20 square miles), it cannot be determined whether a local storm or a general storm will produce the greatest flood peak discharges or the maximum flood volumes. For such drainage areas, generally between 20 and 100 square miles, it is necessary to consider both general storms and local storms. This may require that site-specific general storm criteria be developed for the watershed and that various local storms with critical storm centering assumptions be developed using the criteria in this manual. Both of these storm types would be modeled and executed in the watershed model to estimate flood discharges and runoff volumes. It is possible, in certain situations, that the local storm could result in the largest peak discharge and that the general storm could result in the largest runoff volume. The Drainage Policies and Standards for Maricopa County stipulates that the 100-year, 2-hour rainfall be used for the design of stormwater storage facilities. As such, criteria are provided in this manual to define the 100-year, 2-hour rainfall for use in Maricopa County. Record floods for large drainage areas, similar to the Salt River Watershed near Phoenix, were produced by large-scale general storms of multiple-day duration and relatively low rainfall intensities. Therefore, based on that observation, for drainage areas larger than 500 square miles it was determined that the general storm should be used as the design storm criteria. Because of the complexity of design criteria for such large areas as well as other considerations, design rainfall criteria are not defined in this manual. General storm criteria are to be defined for such large, regional flood studies on a case-by-case basis so that the most appropriate meteorologic and hydrologic factors (possibly also including snowmelt for stream baseflow and watershed antecedent moisture conditions) can be properly considered in the flood analysis. The design rainfall criteria to be used in the unincorporated areas of Maricopa County are summarized in the Drainage Policies and Standards for Maricopa County. The specific procedures that are needed to define the design rainfall for the 100-year, 2-hour storm, the 6-hour local storm and the 24-hour general storm are provided in the following sections. Refer to the Policies and Standards manual of the municipality for design rainfall criteria in the incorporated areas of Maricopa County. August 15, 2013 2-3
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