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

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Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses units to obtain subbasin effective impervious area (EFF) in percent. Compute the final composite value of RTIMP using Equation (4.6). EFF RTIMP = RTIMP (4.6) L + ---------- ( RTIMP 100 N ) 7. Estimate the vegetative cover (VC) for the natural portions of the drainage area or subbasin. Select values of VC for each land use using Table 4.2. Arithmetically areaweight the values of VC if the drainage area or subbasin is composed of land use subareas of different VC. Arithmetically average the natural VC and the areaweighted land use VC. 8. Adjust the XKSAT value for VC using Figure 4.4, if appropriate. 9. Arithmetically average DTHETA dry (natural portions of the drainage area or subbasin) and DTHETA normal (Developed portions of the drainage area or subbasin), if appropriate. B. Alternative Methods: As an alternative to the above procedures, Green and Ampt loss rate parameters can be estimated by reconstitution of recorded rainfall-runoff events on the drainage area or hydrologically similar watersheds, or parameters can be estimated by use of rainfall simulators in field experiments. Plans and procedures for estimating Green and Ampt loss rate parameters by either of these procedures should be approved by the Flood Control District and/or the local agency before initiating the procedures. 4.5.2 Initial Loss Plus Uniform Loss Rate Method A. When soils data are available: 1. Prepare a base map of the drainage area delineating modeling subbasins, if used. 2. Delineate subareas of different infiltration rates (uniform loss rates) on the base map. Assign a land-use or surface cover to each subarea. 3. Determine the size of each subbasin and size of each subarea within each subbasin. 4. Estimate the impervious area (RTIMP) for the drainage area or each subarea. 5. Estimate the initial loss (STRTL) for the drainage area or each subarea by regional studies or calibration. Alternatively, Equation (4.5) or Table 4.2 and Table 4.4 can be used to estimate or to check the value of STRTL. 4-22 August 15, 2013
Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses 6. Estimate the uniform loss rate (CNSTL) for the drainage area or each subarea by regional studies or calibration. Table 4.3 can be used, in certain situations, to estimate or to check the values of CNSTL. 7. Calculate the area-weighted values of RTIMP, STRTL, and CNSTL for the drainage area or each subbasin. 8. Enter the area-weighted values of RTIMP, STRTL, and CNSTL for the drainage area or each subbasin on the LU record of the HEC-1 input file. August 15, 2013 4-23
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