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ATTAC14MENT <br /> e <br /> Table 2-2a.—Runoff curve numbers for urban areast <br /> Curve numbers for <br /> Cover description hydrologic soil group— <br /> Average percent <br /> Cover type and hydrologic condition impervious areae A B C D <br /> etation established) <br /> developed urban areas (vegetation t Fully p 9 <br /> # Open space OaNvrts, parks, golf courses, cemeteries, <br /> etc.)3: 68 79 86 89 <br /> Poor condition (grass cover < 501c} .............. <br /> Fair condition (grass cover 50% to 75%)........... 49 69 79 84 <br /> t Good condition (grass cover > 75%) .............. 39 61 74 80 <br /> Impervious areas: <br /> Paved parking lots, roofs, driveways, etc. 98 98 98 98 <br /> (excluding right-of-way). ......................... <br /> Streets and roads: <br /> Paved; curbs and storm sewers(excluding <br /> right-of-way).................................. 98 98 98 98 <br /> Paved; open ditches(including right-of-way) ....... 83 89 92 93 <br /> 2 82 87 89 <br /> Gravel (including right-of-way) ................... 7 85 89 91 <br /> Dirt(including right-of-way) ..................... 72 <br /> Western desert urban areas: <br /> Natural desert landscaping(pervious areas only)°... 63 77 85 88 <br /> Artificial desert landscaping(impervious weed <br /> barrier, desert shrub with 1-to 2-inch sand <br /> or gravel mulch and basin borders). .............. 96 96 96 96 <br /> l <br /> Urban districts: <br /> Commercial and business.......................... 85 89 92 94 95 <br /> Industrial........................................ 72 81 88 91 93 <br /> Residential districts by average lot size: 65 ,7,7 Sb 90 92 <br /> 118 acre or less(town houses).. <br /> 1/4 acre ......................................... 38 61 75 83 87 <br /> ......................................... 30 57 72 81 86 <br /> 1/3 acre <br /> 1l2 acre.......................................... ' 25 20 51 70 80 85 <br /> 51 68 79 8 <br /> 1 acre . 2 <br /> 2 acres .......................................... 12 46 65 77 ffi <br /> Developing urban areas <br /> Newly graded areas(pervious areas only, 77 86 91 94 <br /> ! no vegetation)s................................... <br /> i Idle lands(CN's are determined using cover types <br /> similar to those in table 2-2c). <br /> 'Average runoff condition,and 1m =0.2S. <br /> ! The average percent impervious area shown was used to develop the composite CN's.Other assumptions are as follows:impervious areas <br /> are directly connected to the drainage system,impervious areas have a CN of 98,and pervious areas are considered equivalent to open <br /> space in good hydrologic condition.CN's for other combinations of conditions may be computed using figure 2-3 or 2.4. <br /> 3CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. <br /> 4Composite CN's for natural desert landscaping should be computed using figures 2.3 or 2.4 based on the impervious area percentage(C\ <br /> =98)and the pervious area CN.The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition. <br /> sC:omposite CN's to use for the design of temporary measures during grading and construction should be computed using figure 2.3 or 24, <br /> based on the degree of development(impervious area percentage)and the CN's for the newly graded pervious areas. <br /> (210-VI-TR-55, Second Ed., June 1986) 2-5 <br />