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Runoff curve numbers for urban areasl Curve numbers for Cover description hydrologic soil group— Average percent Cover type and hydrologic condition impervious area' Fully developed urban areas (vegetation established( Open space pawns, parks, golf courses, cemeteries, etc.)3: Poor condition (grass cover < 50%) ............... Fair condition (grass cover 500% to 75%).......... . Good condition (grass cover > 75%) .............. Impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way) .......................... Streets and roads: Paved; curbs and storm sewers (excluding right-of-way) .................................. Paved; open ditches (including right-of-way) ....... Gravel (including right-of-way) ................... Dirt (including right-of-way) ..................... Western desert urban areas: Natural desert landscaping (pervious areas only)+... Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2 -inch sand or gravel mulch and basin borders) ............... Urban districts: Commercialand business .......................... Industrial........................................ Residential districts by average lot size: 1/8 acre orless (town houses) ...................... 1/4 acre ......................................... 1/3 acre ......................................... 1/2 acre ......................................... 1 acre ........................................... 2 acres .......................................... Developing urban areas Newly graded areas (pervious areas only, novegetation)s................................... Idle lands (CN's are determined using cover types similar to those in table 2-20. A B C D 68 79 86 89 49 69 79 84 39 61 4 80 98 98 98 98 77 86 91 94 'Average runoff condition, and I. - 02S. Me average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CN's for other combinations of conditions may be computed using figure 23 or 2.4. 3CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. 4Camposite CN's for natural desert landscaping should be computed using figures 23 or 2.4 based on the impervious area percentage (CN 98) and the pervious area CN. The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition. 'Composite CN's to use for the design of temporary measures during grading and construction should be computed using figure 23 or 2.4. based on the degree of development (impervious area percentage) and the CN's for the nemiy graded pervious areas. (210 -VI -TR -55, Second Ed., June 1986) 2-5 So,rce- t (AS PA ` SCS / lq& �F�cVCe �j(y} 9 - `t� 98 98 98 98 83 89 92 93 76 85 89 91 72 82 87 89 63 77 85 88 96 96 96 96 85 89 92 94 95 72 81 88 91 93 65 77 85 90 92 38 61 75 83 87 30 57 _ 72 81 86 25 54 70 80 85 20 51 68 79 84 12 46 65 77 82 77 86 91 94 'Average runoff condition, and I. - 02S. Me average percent impervious area shown was used to develop the composite CN's. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. CN's for other combinations of conditions may be computed using figure 23 or 2.4. 3CN's shown are equivalent to those of pasture. Composite CN's may be computed for other combinations of open space cover type. 4Camposite CN's for natural desert landscaping should be computed using figures 23 or 2.4 based on the impervious area percentage (CN 98) and the pervious area CN. The pervious area CN's are assumed equivalent to desert shrub in poor hydrologic condition. 'Composite CN's to use for the design of temporary measures during grading and construction should be computed using figure 23 or 2.4. based on the degree of development (impervious area percentage) and the CN's for the nemiy graded pervious areas. (210 -VI -TR -55, Second Ed., June 1986) 2-5 So,rce- t (AS PA ` SCS / lq& �F�cVCe �j(y} 9 - `t�