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

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Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses 4.4.2 Initial Loss Plus Uniform Loss Rate (IL+ULR) This is a simplified rainfall loss method that is often used, and generally accepted, for flood hydrology. In using this simplified method it is assumed that the rainfall loss process can be simulated as a two-step procedure, as illustrated in Figure 4.5. Initially, all rainfall is prevented from becoming runoff until the accumulated rainfall is equal to the initial loss; and second, after the initial loss is satisfied, a portion of all future rainfall is lost at a uniform rate. All of the rainfall is lost if the rainfall intensity is less than the uniform loss rate. According to HEC-1 nomenclature, two parameters are needed to use this method: the initial loss (STRTL), and the uniform loss rate (CNSTL). Because this method is to be used for special cases where infiltration is not controlled by soil texture, or for drainage areas and subbasins that are predominantly sand, the estimation of the parameters will require model calibration, results of regional studies, or other valid techniques. It is not possible to provide complete guidance in the selection of these parameters; however, some general guidance is provided: A. For special cases of anticipated application, the uniform loss rate (CNSTL) will either be very low for nearly impervious surfaces, or possibly quite high for exceptionally fast-draining (highly pervious) land surfaces. For land surfaces with very low infiltration rates, the value of CNSTL will probably be 0.05 inches per hour or less. For sand, a CNSTL of 0.5 to 1.0 inch per hour or larger may be reasonable. Higher values of CNSTL for sand and other surfaces are possible; however, use of high values of CNSTL would require special studies to substantiate the use of such values. B. Although the IL+ULR method is not recommended for watersheds where the soil textures can be defined and where the Green and Ampt method is encouraged, some general guidance in the selection of the uniform loss rate is shown in Table 4.3 and Table 4.4. Table 4.4 was prepared based on the values in Table 4.3 and the hydraulic conductivities shown in Table 4.1. In Table 4.4, the initial infiltration (II) is an estimate of the infiltration loss that can be expected prior to the generation of surface runoff. The value of initial loss (STRTL) is the sum of initial infiltration (II) of Table 4.4 and surface retention loss (IA) of Table 4.2; STRTL = II + IA. C. The estimation of initial loss (STRTL) can be made on the basis of calibration or special studies at the same time that CNSTL is estimated. Alternatively, since STRTL is equivalent to initial abstraction, STRTL can be estimated by using the NRCS CN equations for estimated initial abstraction, written as: 200 STRTL = − 2 CN (4.5) 4-18 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses Estimates for CN for the drainage area or subbasin should be made referring to various publications of the NRCS, particularly TR-55 (NRCS, 1986). Equation (4.5) should provide a fairly good estimate of STRTL in many cases, however, its use should be judiciously applied and carefully considered in all cases. FIGURE 4.5 REPRESENTATION OF RAINFALL LOSS ACCORDING TO THE INITIAL LOSS PLUS UNIFORM LOSS RATE (IL + ULR) Table 4.3 PUBLISHED VALUES OF UNIFORM LOSS RATES Hydrologic Soil Group (1) Musgrave (1955) (2) Uniform Loss Rate, inches/hour USBR (1975) 1 A 0.30 – 0.45 0.40 0.30 – 0.50 B 0.15 – 0.30 0.24 0.15 – 0.30 C 0.05 – 0.15 0.12 0.05 – 0.15 D 0 – 0.05 0.08 0 – 0.05 Notes: 1. Design of Small Dams, Second Edition, 1975, Appendix A. 2. Design of Small Dams, Third Edition, 1987. (3) USBR (1987) 2 (4) August 15, 2013 4-19
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