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

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Drainage Design Manual for Maricopa County Hydrology: Rainfall Losses Selection of IA, RTIMP, and Percent Vegetation Cover for Urban Areas Table 4.2 contains suggested values for IA, RTIMP, and percent vegetation cover for various natural conditions and urban land use types. The values in Table 4.2 are meant as guidelines and are not to be taken as prescribed values for these parameters. Note that the values for RTIMP reflect effective impervious areas not total impervious areas. Also, note that the values for percent vegetation cover are for pervious areas only. These three parameter values are used in the calculation of average subbasin parameters for the Green and Ampt loss method as described above. Sound engineering judgement and experience should always be used when selecting rainfall loss parameters and assigning land use categories for any given watershed. 4-16 August 15, 2013
Drainage Design Manual for Maricopa County Table 4.2 IA, RTIMP, AND VEGETATIVE CANOPY COVER FOR REPRESENTATIVE LAND USES IN MARICOPA COUNTY Hydrology: Rainfall Losses Land Use 1 Code Land Use Category Description IA 2 inches RTIMP 2,3 % Vegetation Cover 2,4 % VLDR Very Low Density Residential 3 40,000 sq. feet and greater lot size 0.30 5 30 LDR Low Density Residential 3 12,000 – 40,000 sq. feet lot size 0.30 15 50 MDR Medium Density Residential 3 6,000 – 12,000 sq. feet lot size 0.25 30 50 MFR Multiple Family Residential 3 1,000 – 6,000 sq. feet lot size (# du/ac) 0.25 45 50 I1 Industrial 1 3 Light and General 0.15 55 60 I2 Industrial 2 3 General and Heavy 0.15 55 60 C1 Commercial 1 3 Light, Neighborhood, Residential 0.10 80 75 C2 Commercial 2 3 Central, General, Office, Intermediate 0.10 80 75 P Pavement and Rooftops Asphalt and Concrete, Sloped Rooftops 0.05 95 0 GR Gravel Roadways & Shoulders Graded and Compacted, Treated and Untreated 0.10 5 0 AG Agricultural Tilled Fields, Irrigated Pastures, slopes < 1% 0.50 0 85 LPC Lawns/Parks/Cemeteries Over 80% maintained lawn 0.20 Varies 5 80 DL1 Desert Landscaping 1 Landscaping with impervious under treatment 0.10 95 30 DL2 Desert Landscaping 2 Landscaping without impervious under treatment 0.20 0 30 NDR Undeveloped Desert Rangeland Little topographic relief, slopes < 5% 0.35 Varies 5 Varies 6 NHS Hillslopes, Sonoran Desert Moderate topographic relief, slopes > 5% 0.15 Varies 5 Varies 6 NMT Mountain Terrain High topographic relief, slopes > 10% 0.25 Varies 5 Varies 6 Notes: 1. Other land use or zoning classifications, such as Planned Area Development and Schools must be evaluated on a case by case basis. 2. These values have been selected to fit many typical settings in Maricopa County; however, the engineer/hydrologist should always evaluate the specific circumstances in any particular watershed for hydrologic variations from these typical values. 3. RTIMP = Percent Effective Impervious Area, including right-of-way. Effective means that all impervious areas are assumed to be hydraulically connected. The RTIMP values may need to be adjusted based on an evaluation of hydraulic connectivity. 4. Vegetation Cover = Percent vegetation cover for pervious areas only. 5. RTIMP values must be estimated on a case by case basis. 6. Vegetation Cover values must be estimated on a case by case basis. August 15, 2013 4-17
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