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

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Drainage Design Manual for Maricopa County Hydrology: Rainfall 2.2 RAINFALL DEPTH The most commonly used descriptor of rainfall is the rainfall depth; however, for modeling purposes, two other rainfall descriptors must be defined. First, the rainfall duration and frequency of occurrence of rainfall depth for that duration must be assigned. Second, since the rainfall depth is a descriptor of the rainfall occurrence at a point in space, both the spatial and the temporal distribution of the rainfall depth must be defined. In this section, the rainfall depth-duration-frequency statistics for use in Maricopa County are described. Subsequent sections describe the spatial and temporal distributions that are to be applied for the 6-hour local storm, the 24-hour general storm, and the temporal distribution for the 100-year, 2-hour storm. 2.2.1 Data Source The most comprehensive and available source of rainfall data analysis for Maricopa County is the NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 1: Semiarid Southwest (NOAA Atlas 14) (Arizona, Southeast California, Nevada, New Mexico, Utah) (Bonnin et al, 2004). The NOAA Atlas 14 is to be used for all drainage design purposes in Maricopa County. The District has elected to use the mean partial duration time series point precipitation values from NOAA Atlas 14 rather than the values for the upper or lower bound of the 90 percent confidence intervals. For critical projects that can significantly affect public safety, health and welfare, including floodplain delineation and dam safety studies, the engineer/hydrologist should check model results against indirect methods as defined in Chapter 8. These analyses should include performance of parameter sensitivity analyses, including the use of the upper bound of the 90 percent confidence interval point precipitation data, to ensure the model results are reasonable in comparison with available historic gage data for the watershed or hydrologically similar watersheds. As a result of such analyses, the engineer/hydrologist may elect to use the point precipitation values from the upper bound of the 90 percent confidence interval instead of the mean values, in order to better conform with available appropriate gage data. This application will be acceptable to the District. Use of the values for lower bound of the 90 percent confidence interval is not recommended. The NOAA Atlas 14 data available through the NOAA Atlas 14 web site are not to be used for studies in Maricopa County. Instead, the NOAA Atlas 14 maps in Appendix A.1, the ESRI ASCII Grid data files available on the District’s web site, or the data supplied with the District’s DDMSW computer program are to be used. This data was taken from NOAA Atlas 14, Volume 1, Version 4.0, dated June 19, 2006. This is the version the District has reviewed and accepted for use in Maricopa County. Subsequent versions published by NOAA shall not be used until the District has reviewed the data, formally adopted its use by revising this document, or issued an addendum to this document, posted the new version on the District web site, and updated DDMSW. 2-4 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rainfall As a historical study reference, point precipitation isopluvial maps generated using the mean NOAA Atlas 2 data are included in Appendix A.2 for reference when utilizing historical studies done using this data. 2.2.2 Depth-Duration-Frequency Statistics The depth-duration-frequency (D-D-F) statistics in the NOAA Atlas 14 are shown as a series of isopluvial maps of Arizona for specified durations and return periods (frequencies). Selected isopluvial maps for Maricopa County have been reproduced from the NOAA Atlas 14 and these are contained in the Hydrology Manual (Figure A.1 through Figure A.60 of Appendix A.1). Areas immediately adjacent to Maricopa County are provided in the isopluvial maps; however, flood studies of certain large watersheds may require reference to ESRI ASCII Grid data available on the District’s web site. 2.2.3 Rainfall Statistics for Special Purposes There may arise situations for special purposes where it is necessary to define rainfall D-D-F statistics other than those provided in Figure A.1 through Figure A.60. In those situations, the ESRI ASCII Grid data available on the District’s web site should be used. Users of this manual who may also be interested in defining general storm criteria for large watersheds, should note that it may be necessary to consider storms of durations longer than 24-hours. Provision of the 24-hour rainfall statistics does not preclude the use of a longer duration rainfall if deemed appropriate for a particular watershed or study. The 24-hour isopluvial maps are provided in this manual for the user’s convenience because this is the rainfall depth often specified for general storms. If rainfall depths are needed for a duration longer than 24-hours, the District’s Engineering Division should be consulted. 2.3 DEPTH-AREA RELATION The NOAA Atlas 14 rainfall depths from the isopluvial maps in Figure A.1 through Figure A.60 of Appendix A.1, are point rainfalls for specified frequencies and durations. This is the depth of rainfall that is expected to occur at a point or points in a watershed for the specified frequency and duration. However, this depth is not the areally-averaged rainfall over the basin that would occur during a storm. A reduction factor is used to convert the point rainfall to an equivalent uniform depth of rainfall over the entire watershed. As the watershed area increases, the reduction factor decreases which has the effect of reducing the point rainfall value. The reduction reflects the greater non-homogeneity of rainfall for storms of larger areas. Regional research by the Agricultural Research Service, U.S. Department of Agriculture, for the Walnut Gulch Experimental Watershed near Tombstone, Arizona, indicated that local storms are August 15, 2013 2-5
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