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

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Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses in the shallow soil horizon. For design rainfalls in Maricopa County, this represents a significant storage volume for infiltrated rainfall and so when developing loss rate parameters for areas of Maricopa County that contain significant areas classified as hydrologic soil group D, the reason for that classification should be determined. Hydrologic soil group D should be retained only for: • clay soils, • soils with a permanent high water table, and • rock outcrop. Hydrologic soil group D should probably not be retained in all situations where the classification is based on shallow soils over nearly impervious layers, site specific studies and sensitivity analyses should be performed to estimate the loss rates to be used for such soils. 4.4 RECOMMENDED METHODS FOR ESTIMATING RAINFALL LOSSES Many methods have been developed for estimating rainfall losses; five are listed as options in the HEC-1 Flood Hydrology Package. They are: 1. Holtan Infiltration Equation 2. Exponential Loss Rate 3. NRCS Curve Numbers (CN) Loss Rate 4. Green and Ampt Infiltration Equation 5. Initial Loss Plus Uniform Loss Rate (IL+ULR) Of these five, however, only the Green and Ampt and IL+ULR are recommended for estimating rainfall losses in Maricopa County for the reasons discussed below. The Holtan Infiltration Equation is an exponential decay type of equation for which the rainfall loss rate asymptotically diminishes to the minimum infiltration rate (f c ). The Holtan equation is not extensively used and there is no known application of this method in Arizona. Data and procedures to estimate the parameters for use in Maricopa County are not available. Therefore, the Holtan equation is not recommended for general use in Maricopa County. The Exponential Loss Rate Method is a four parameter method that is not extensively used, but it is a method preferred by the U.S. Army Corps of Engineers. Data and procedures are not available to estimate the parameters for this method for all physiographic regions in Maricopa County, but Exponential loss rate parameters have been developed from the reconstitution of flood events for a flood hydrology study in a portion of Maricopa County (U.S. Army Corps of Engi- 4-6 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses neers, 1982a). However, adequate data are not available to estimate the necessary parameters for all soil types and land uses in Maricopa County, and this method is not recommended for general use in Maricopa County. The NRCS CN method previously was (pre-1990) the most extensively used rainfall loss rate method in Maricopa County and Arizona, and it had wide acceptance among many agencies, consulting engineering firms, and individuals throughout the community. However, because of both theoretical concerns and practical limitations, the NRCS CN method is not recommended for general use in Maricopa County. As mentioned previously, the two recommended methods for estimating rainfall losses in Maricopa County are the Green and Ampt infiltration equation and the initial loss and uniform loss rate (IL+ULR) method. Both methods, as programmed into HEC-1, can be used to simulate the rainfall loss model as depicted in Figure 4.2. For a full discussion of these methods, see Section 4.4.1 and Section 4.4.2. The IL+ULR is a simplified model that is used extensively for flood hydrology and data often are available to estimate the two parameters for that method. The Green and Ampt infiltration equation is a physically based model that has been in existence since 1911, and is an option in HEC-1. The preferred method, and the most theoretically accurate, is the Green and Ampt infiltration equation. That method should be used for most studies in Maricopa County where the land surface is soil, the infiltration of water is controlled by soil texture (see APPENDIX C), and the bulk density of the soil is affected by vegetation. Procedures were developed, and are presented, to estimate the three parameters of the Green and Ampt infiltration equation. The alternative method of IL+ULR can be used in situations where the Green and Ampt infiltration method is recommended, but its use in those situations is not encouraged, and, in general, should be avoided. Rather, the IL+ULR method should be used in situations where the Green and Ampt infiltration equation with parameters based on soil texture is not appropriate. Examples of situations where the IL+ULR method is recommended are: large areas of rock outcrop, talus slopes, forests underlain with a thick mantle of duff, land surfaces of volcanic cinder, and surfaces that are predominantly sand and gravel. Because of the diversity of conditions that could exist for which the IL+ULR method is to be used, it is not possible to provide extensive guidance for the selection of the two parameters of the IL+ULR method. Other methods should be used only if there is technical justification for a variance from these recommendations and if adequate information is available to estimate the necessary parameters. Use of rainfall loss methods other than those recommended should not be undertaken unless previously approved by the Flood Control District and/or the local regulatory agency. August 15, 2013 4-7
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