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

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Drainage Design Manual for Maricopa County Hydrology: Introduction criteria for all of Maricopa County, the Policies and Standards manual for each jurisdictional entity should be referenced for specific guidance for incorporated areas. The rainfall-runoff modeling procedures that are contained in the manual are physically based. That is, the procedures are based, to the extent practical, on the physical processes that occur during the generation of storm runoff from rainfall. While the basic procedures are physically based, this does not assure that the rigorous application of the procedures will, in fact, reproduce the actual rainfall-runoff phenomenon of any storm that has occurred or may occur in the future. However, the procedure, when applied with good hydrologic and engineering judgement, should yield consistent results for design purposes. Throughout the development of the Hydrology Manual, three benchmarks were continually applied in judging the applicability of individual procedures and the overall methodologies: accuracy, practicality, and reproducibility. Accuracy is a measure of how well the results of the procedure reproduce the physical process being simulated. Although accuracy is highly desired, it is theoretically impossible to achieve in an earth science such as hydrology, and in a practical sense, accuracy is not feasible to assess except for a few situations where adequate verification data are available. Relative accuracy was assessed throughout the development of the procedures in the manual through testing and verification against recorded data. Practicality is a user’s decision regarding the best and most appropriate level of technology to apply considering the information that is available, the anticipated uses, the consequences of error, and the desired or required output. Whereas both simpler procedures and more sophisticated procedures are available, the adopted methodologies provide a compromise between these two extremes, and the best practical level of technology is judged to be recommended in the manual considering the state of current hydrologic knowledge of arid and semi-arid lands. Reproducibility is a characteristic that provides reasonable confidence that consistent results will be achieved by all qualified users. Reproducibility is highly desirable for a design standard in order to eliminate, to the extent possible, unnecessary conflicts over the interpretation and application of the design method. Reproducibility is achieved through clear and concise manual procedures and user guidance. Every effort has been made toward this end. A brief discussion of the content of each chapter of the Hydrology Manual follows: Chapter 1 Introduction - The introduction states the purpose, scope and limitations, and general use of the manual. Chapter 2 Rainfall - The characteristics of severe storms in Maricopa County are documented as a setting for defining the design rainfall criteria. Procedures and information are provided for the determination of depth-duration-frequency statistics of storms in Maricopa County. These are 1-2 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Introduction derived from NOAA Atlas 14, Arizona, which is currently the most comprehensive and authoritative source of such information. The temporal distribution of rainfall for the majority of design conditions is a 6-hour local storm. The 6-hour storm distribution is based on an analysis by the U.S. Army Corps of Engineers, Los Angeles District, of the 19 August 1954 Queen Creek storm. The Corps’ distribution has been modified somewhat to reflect the design rainfall criteria that are desired for use in Maricopa County, and this modification includes using the hypothetical distribution for drainage areas less than 0.5 square mile. The temporal distribution is a function of drainage area. This reflects the spatial variability of rainfall intensities that are known to exist with severe local storms in Maricopa County. A 2-hour distribution is provided for use in the design of stormwater storage facilities. The reduction of rainfall depth with storm area for the 6-hour rainfall is accounted for by a depth-area reduction curve based on the 1954 Queen Creek storm. In some cases, a general storm may be the accepted design rainfall. In Maricopa County, the general storm to be used is the SCS Type II pattern using areal reductions of point rainfall using NWS HYDRO-40 (Zehr and Myers, 1984). Chapter 3 Rational Method - Use of the Rational Method is to be limited to an area of up to 160 acres. The watershed should be of uniform land use for application of this method. Intensityduration-frequency (I-D-F) statistics are to be obtained from the information contained in Chapter 2. An equation for the estimation of time of concentration is provided that is a partial function of rainfall intensity. Values of the runoff coefficient “C” to be applied to various land uses in Maricopa County are provided in this chapter. Chapter 4 Rainfall Losses - The preferred method for the estimation of rainfall losses is the Green and Ampt infiltration equation with an estimate of surface retention loss. This requires the classification of soil according to soil texture, which is available for most of Maricopa County. Adjustment of the loss rate is available as a function of vegetation cover. Other methods are available to estimate rainfall losses if adequate soils and/or vegetation data are not available. Chapter 5 Unit Hydrograph Procedures - The use of unit hydrographs to route rainfall excess from the land’s surface is recommended, and the procedures recommended to do so are either the Clark unit hydrograph or the application of selected S-graphs. The Clark unit hydrograph is recommended for watersheds or subbasins less than 5 square miles in size with an upper limit of application of 10 square miles. Procedures are provided for the estimation of the two numeric parameters: the time of concentration and the storage coefficient. Two default time-area relations are provided: one for urban watersheds and the other for natural watersheds. Four S- graphs have been selected for use in flood hydrology studies of major watercourses in Maricopa County. The Phoenix Mountain, Phoenix Valley, Desert/Rangeland, and the Agricultural S- graphs are described and guidelines are provided for their selection. A procedure is provided for the estimation of the S-graph parameter, lag. August 15, 2013 1-3
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