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curve-number-modification-tables_1Sheet3 Chart1 CN=60 CN=65 CN=70 CN=75 CN=80 CN=85 CN=90 CN=95 CN=98 Blank Modified CN 1-year 2-year 100-year dQ S NA Modified Curve Number (for Impervious Areas) Rainfall Reduction (in.)* Modified Curve Number (HSG D Soils, Turf Areas in Good Condition) Q-1 yr (in.) Q-2 yr (in.) Q-100 yr (in.) CN Modified Curve Number (CN=95) Modified Curve Number (CN=90) Modified Curve Number (CN=85) Modified Curve Number (CN=75) Modified Curve Number (CN=65) Modified Curve Number (CN=60) Modified Curve Number TR-55 Curve Number Rainfall Reduction (in.)* = Volume Reduction (cf) / Tributary Drainage Area (sf) x 12 in/ft Enter Unmodified CN for Tributary Area Here 2. Determine amount of rainfall reduction due to BMP (see MSD MEP Calculation Tool). 4. For CPv and Qp routings, apply modified CNs to drainage areas of volume reducing BMPs. Instructions for CN Modification Tables: 1. Determine BMP drainage areas's unmodified CN from TR-55 Table 2-2. Enter in in yellow cell (below). 5. Drainage areas where rainfall reduction capacity exceeds runoff (i.e., no runoff), drainage area may be subtracted from storm routing. No Runoff 3. Using below table, determine modified CNs for each 24-hr storm. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 TR-55 Lookup Table CN Modified CN Curve Number Modification 98.00 95.00 90.00 85.00 80.00 75.00 70.00 65.00 60.00 97.16 96.32 95.47 94.62 93.77 92.92 92.06 91.20 90.34 89.48 88.61 94.15 93.30 92.44 91.59 90.73 89.86 89.00 88.13 87.26 86.38 85.50 89.13 88.27 87.39 86.52 85.64 84.76 83.88 82.99 82.10 81.21 80.31 84.12 83.23 82.34 81.45 80.55 79.65 78.75 77.84 76.93 76.02 75.10 79.10 78.19 77.28 76.37 75.45 74.53 73.61 72.68 71.74 70.80 69.86 74.08 73.15 72.22 71.28 70.34 69.39 68.44 67.49 66.53 65.56 64.59 69.05 68.10 67.14 66.18 65.21 64.24 63.25 62.27 61.27 60.27 59.26 64.02 63.04 62.05 61.06 60.06 59.04 58.03 57.00 55.97 54.92 53.87 58.99 57.97 56.94 55.91 54.86 53.81 52.74 51.66 50.57 49.47 48.35 0.00 0.17 60.00 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.37 60.00 200.00 3.10 2.00 2.00 5.00 3.10 4.00 2.00 2.68 60.00 0.10 0.07 54.41 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.27 56.75 200.00 3.10 2.00 2.00 5.00 3.10 4.00 2.00 2.58 58.99 0.20 -0.03 0.17 52.89 200.00 3.10 2.00 2.00 5.00 3.10 4.00 2.00 2.48 57.97 0.30 -0.13 0.07 47.75 200.00 3.10 2.00 2.00 5.00 3.10 4.00 2.00 2.38 56.94 0.40 -0.23 -0.03 2.28 55.91 0.50 -0.33 -0.13 2.18 54.86 0.60 -0.43 -0.23 2.08 53.81 0.70 -0.53 -0.33 1.98 52.74 0.80 -0.63 -0.43 1.88 51.66 0.90 -0.73 -0.53 1.78 50.57 1.00 -0.83 -0.63 1.68 49.47 1.10 -0.93 1.58 48.35 1.20 -1.03 1.48 47.21 1.30 1.38 46.06 1.40 1.28 44.89 1.50 1.18 43.69 1.60 1.08 42.47 60.00 6.67 1000.00 10.00 0.17 2.50 0.20 2.00 2.50 0.80 0.37 3.10 0.20 2.00 3.10 0.80 2.68 7.10 0.20 2.00 7.10 0.80 0.00 0.30 65.00 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.55 65.00 3.18 65.00 0.10 0.20 61.07 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.45 62.38 3.08 64.02 0.20 0.10 55.96 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.35 59.46 2.98 63.04 0.30 -2.52E-03 0.25 56.13 2.88 62.05 0.40 -0.10 0.15 52.12 2.78 61.06 0.50 -0.20 0.05 46.55 2.68 60.06 0.60 -0.30 -0.05 2.58 59.04 0.70 -0.40 -0.15 2.48 58.03 0.80 -0.50 -0.25 2.38 57.00 0.90 -0.60 -0.35 2.28 55.97 1.00 -0.70 -0.45 2.18 54.92 1.10 -0.80 2.08 53.87 1.20 -0.90 1.98 52.80 1.30 -1.00 1.88 51.72 1.40 -1.10 1.78 50.63 1.50 -1.20 1.68 49.53 1.60 -1.30 1.58 48.41 65.00 5.39 1000.00 10.00 0.30 2.50 0.20 2.00 2.50 0.80 0.55 3.10 0.20 2.00 3.10 0.80 3.18 7.10 0.20 2.00 7.10 0.80 0.00 0.46 70.00 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.77 70.00 3.70 70.00 0.10 0.36 66.96 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.67 67.81 3.60 69.05 0.20 0.26 63.44 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.57 65.45 3.50 68.10 0.30 0.16 59.11 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.47 62.87 3.40 67.14 0.40 0.06 53.02 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.37 60.01 3.30 66.18 0.50 -0.04 0.27 56.77 3.20 65.21 0.60 -0.14 0.17 52.91 3.10 64.24 0.70 -0.24 0.07 47.78 3.00 63.25 0.80 -0.34 2.90 62.27 0.90 -0.44 2.80 61.27 1.00 -0.54 2.70 60.27 1.10 -0.64 2.60 59.26 1.20 -0.74 2.50 58.25 1.30 -0.84 2.40 57.22 1.40 -0.94 2.30 56.19 1.50 -1.05 2.20 55.15 1.60 -1.15 2.10 54.09 70.00 4.29 1000.00 10.00 0.46 2.50 0.20 2.00 2.50 0.80 0.77 3.10 0.20 2.00 3.10 0.80 3.70 7.10 0.20 2.00 7.10 0.80 0.00 0.65 75.00 1.03 75.00 4.24 75.00 0.10 0.55 72.57 0.93 73.14 4.14 74.08 0.20 0.45 69.87 0.83 71.17 4.04 73.15 0.30 0.35 66.81 0.73 69.06 3.94 72.22 0.40 0.25 63.25 0.63 66.80 3.84 71.28 0.50 0.15 58.88 0.53 64.35 3.74 70.34 0.60 0.05 52.63 0.43 61.66 3.64 69.39 0.70 -0.05 0.33 58.65 3.54 68.44 0.80 -0.15 0.23 55.18 3.44 67.49 0.90 -0.25 0.13 50.90 3.34 66.53 1.00 -0.35 0.03 44.38 3.24 65.56 1.10 -0.45 3.14 64.59 1.20 -0.55 3.04 63.61 1.30 -0.65 2.94 62.62 1.40 -0.75 2.84 61.63 1.50 -0.85 2.74 60.63 1.60 -0.95 2.64 59.63 200.00 7.10 2.00 2.00 5.00 7.10 4.00 2.00 75.00 3.33 1000.00 10.00 0.65 2.50 0.20 2.00 2.50 0.80 1.03 3.10 0.20 2.00 3.10 0.80 4.24 7.10 0.20 2.00 7.10 0.80 0.00 0.89 80.00 1.33 80.00 4.79 80.00 0.10 0.79 78.02 1.23 78.41 4.69 79.10 0.20 0.69 75.87 1.13 76.74 4.59 78.19 0.30 0.59 73.53 1.03 74.98 4.49 77.28 0.40 0.49 70.94 0.93 73.12 4.39 76.37 0.50 0.39 68.03 0.83 71.14 4.29 75.45 0.60 0.29 64.69 0.73 69.03 4.19 74.53 0.70 0.19 60.69 0.63 66.77 4.09 73.61 0.80 0.09 55.42 0.53 64.32 3.99 72.68 0.90 -0.01 0.43 61.62 3.89 71.74 1.00 -0.11 0.33 58.61 3.79 70.80 1.10 -0.21 0.23 55.13 3.69 69.86 1.20 -0.31 0.13 50.84 3.59 68.91 1.30 -0.41 0.03 44.24 3.49 67.96 1.40 -0.51 3.39 67.00 1.50 -0.61 3.29 66.04 1.60 -0.71 3.19 65.07 80.00 2.50 1000.00 10.00 0.89 2.50 0.20 2.00 2.50 0.80 1.33 3.10 0.20 2.00 3.10 0.80 4.79 7.10 0.20 2.00 7.10 0.80 0.00 1.18 85.00 1.67 85.00 5.35 85.00 0.10 1.08 83.38 1.57 83.63 5.25 84.12 0.20 0.98 81.65 1.47 82.21 5.15 83.23 0.30 0.88 79.80 1.37 80.72 5.05 82.34 0.40 0.78 77.80 1.27 79.17 4.95 81.45 0.50 0.68 75.64 1.17 77.53 4.85 80.55 0.60 0.58 73.27 1.07 75.82 4.75 79.65 0.70 0.48 70.65 0.97 74.01 4.65 78.75 0.80 0.38 67.71 0.87 72.09 4.55 77.84 0.90 0.28 64.31 0.77 70.04 4.45 76.93 1.00 0.18 60.22 0.67 67.86 4.35 76.02 1.10 0.08 54.73 0.57 65.50 4.25 75.10 1.20 -0.02 0.47 62.93 4.15 74.17 1.30 -0.12 0.37 60.08 4.05 73.25 1.40 -0.22 0.27 56.84 3.95 72.32 1.50 -0.32 0.17 53.00 3.85 71.38 1.60 -0.42 0.07 47.91 3.75 70.44 85.00 1.77 1000.00 10.00 1.18 2.50 0.20 2.00 2.50 0.80 1.67 3.10 0.20 2.00 3.10 0.80 5.35 7.10 0.20 2.00 7.10 0.80 0.00 1.53 90.00 2.08 90.00 5.92 90.00 0.10 1.43 88.68 1.98 88.83 5.82 89.13 0.20 1.33 87.29 1.88 87.62 5.72 88.27 0.30 1.23 85.81 1.78 86.36 5.62 87.39 0.40 1.13 84.24 1.68 85.05 5.52 86.52 0.50 1.03 82.57 1.58 83.68 5.42 85.64 0.60 0.93 80.79 1.48 82.26 5.32 84.76 0.70 0.83 78.87 1.38 80.78 5.22 83.88 0.80 0.73 76.80 1.28 79.22 5.12 82.99 0.90 0.63 74.54 1.18 77.59 5.02 82.10 1.00 0.53 72.06 1.08 75.88 4.92 81.21 1.10 0.43 69.30 0.98 74.07 4.82 80.31 1.20 0.33 66.16 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 0.88 72.16 4.72 79.41 1.30 0.23 62.48 0.78 70.12 4.62 78.50 1.40 0.13 57.87 0.68 67.94 4.52 77.60 1.50 0.03 50.80 0.58 65.59 4.42 76.68 1.60 -0.07 0.48 63.02 4.32 75.77 90.00 1.11 1000.00 10.00 1.53 2.50 0.20 2.00 2.50 0.80 2.08 3.10 0.20 2.00 3.10 0.80 5.92 7.10 0.20 2.00 7.10 0.80 0.00 1.96 95.00 2.55 95.00 6.51 95.00 0.10 1.86 93.93 2.45 94.00 6.41 94.15 0.20 1.76 92.82 2.35 92.97 6.31 93.30 0.30 1.66 91.64 2.25 91.91 6.21 92.44 0.40 1.56 90.41 2.15 90.80 6.11 91.59 0.50 1.46 89.11 2.05 89.66 6.01 90.73 0.60 1.36 87.74 1.95 88.48 5.91 89.86 0.70 1.26 86.30 1.85 87.26 5.81 89.00 0.80 1.16 84.76 1.75 85.98 5.71 88.13 0.90 1.06 83.13 1.65 84.66 5.61 87.26 1.00 0.96 81.38 1.55 83.28 5.51 86.38 1.10 0.86 79.51 1.45 81.84 5.41 85.50 1.20 0.76 77.49 1.35 80.33 5.31 84.62 1.30 0.66 75.29 1.25 78.76 5.21 83.74 1.40 0.56 72.89 1.15 77.11 5.11 82.85 1.50 0.46 70.23 1.05 75.37 5.01 81.96 1.60 0.36 67.22 0.95 73.53 4.91 81.06 95.00 0.53 1000.00 10.00 1.96 2.50 0.20 2.00 2.50 0.80 2.55 3.10 0.20 2.00 3.10 0.80 6.51 7.10 0.20 2.00 7.10 0.80 0.00 2.27 98.00 2.87 98.00 6.86 98.00 0.10 2.17 97.07 2.77 97.10 6.76 97.16 0.20 2.07 96.09 2.67 96.16 6.66 96.32 0.30 1.97 95.08 2.57 95.20 6.56 95.47 0.40 1.87 94.01 2.47 94.21 6.46 94.62 0.50 1.77 92.90 2.37 93.19 6.36 93.77 0.60 1.67 91.73 2.27 92.13 6.26 92.92 0.70 1.57 90.51 2.17 91.04 6.16 92.06 0.80 1.47 89.21 2.07 89.90 6.06 91.20 0.90 1.37 87.85 1.97 88.73 5.96 90.34 1.00 1.27 86.41 1.87 87.51 5.86 89.48 1.10 1.17 84.88 1.77 86.25 5.76 88.61 1.20 1.07 83.25 200.00 2.50 2.00 2.00 5.00 2.50 4.00 2.00 1.67 84.94 5.66 87.74 1.30 0.97 81.51 1.57 83.57 5.56 86.87 1.40 0.87 79.65 1.47 82.14 5.46 85.99 1.50 0.77 77.64 1.37 80.65 5.36 85.11 1.60 0.67 75.46 1.27 79.09 5.26 84.23 98.00 0.20 1000.00 10.00 2.27 2.50 0.20 2.00 2.50 0.80 2.87 3.10 0.20 2.00 3.10 0.80 6.86 7.10 0.20 2.00 7.10 0.80 0.00 1.87 94.00 94.00 0.00 No Runoff 2.45 94.00 94.00 0.00 No Runoff 6.39 94.00 94.00 0.00 No Runoff 0.10 1.77 92.89 92.89 2.35 92.97 92.97 6.29 93.15 93.15 0.20 1.67 91.72 91.72 2.25 91.90 91.90 6.19 92.29 92.29 0.30 1.57 90.49 90.49 2.15 90.80 90.80 6.09 91.43 91.43 0.40 1.47 89.20 89.20 2.05 89.66 89.66 5.99 90.57 90.57 0.50 1.37 87.83 87.83 1.95 88.48 88.48 5.89 89.71 89.71 0.60 1.27 86.39 86.39 1.85 87.25 87.25 5.79 88.84 88.84 0.70 1.17 84.86 84.86 1.75 85.98 85.98 5.69 87.97 87.97 0.80 1.07 83.23 83.23 1.65 84.65 84.65 5.59 87.10 87.10 0.90 0.97 81.49 81.49 1.55 83.27 83.27 5.49 86.23 86.23 1.00 0.87 79.63 79.63 1.45 81.84 81.84 5.39 85.35 85.35 1.10 0.77 77.61 77.61 1.35 80.33 80.33 5.29 84.46 84.46 1.20 0.67 75.43 75.43 1.25 78.76 78.76 5.19 83.58 83.58 1.30 0.57 73.04 73.04 1.15 77.10 77.10 5.09 82.69 82.69 1.40 0.47 70.40 70.40 1.05 75.36 75.36 4.99 81.80 81.80 1.50 0.37 67.42 67.42 0.95 73.52 73.52 4.89 80.91 80.91 1.60 0.27 63.97 63.97 0.85 71.57 71.57 4.79 80.01 80.01 1.70 0.17 59.79 59.79 0.00 No Runoff 0.75 69.50 69.50 0.00 No Runoff 4.69 79.10 79.10 0.00 No Runoff 1.80 0.07 54.09 54.09 0.00 No Runoff 0.65 67.27 67.27 0.00 No Runoff 4.59 78.20 78.20 0.00 No Runoff 1.90 -0.03 0.00 No Runoff No Runoff 0.55 64.86 64.86 0.00 No Runoff 4.49 77.29 77.29 0.00 No Runoff 2.00 -0.13 0.00 No Runoff No Runoff 0.45 62.22 62.22 0.00 No Runoff 4.39 76.38 76.38 0.00 No Runoff 2.10 -0.23 0.00 No Runoff No Runoff 0.35 59.29 59.29 0.00 No Runoff 4.29 75.46 75.46 0.00 No Runoff 94.00 0.64 1000.00 10.00 1.87 2.50 0.20 2.00 2.50 0.80 2.45 3.10 0.20 2.00 3.10 0.80 6.39 7.10 0.20 2.00 7.10 0.80 98.00 95.00 90.00 85.00 80.00 75.00 70.00 65.00 60.00 0.00 98.00 95.00 90.00 85.00 80.00 75.00 70.00 65.00 60.00 0.10 97.16 94.15 89.13 84.12 79.10 74.08 69.05 64.02 58.99 0.20 96.32 93.30 88.27 83.23 78.19 73.15 68.10 63.04 57.97 0.30 95.47 92.44 87.39 82.34 77.28 72.22 67.14 62.05 56.94 0.40 94.62 91.59 86.52 81.45 76.37 71.28 66.18 61.06 55.91 0.50 93.77 90.73 85.64 80.55 75.45 70.34 65.21 60.06 54.86 0.60 92.92 89.86 84.76 79.65 74.53 69.39 64.24 59.04 53.81 0.70 92.06 89.00 83.88 78.75 73.61 68.44 63.25 58.03 52.74 0.80 91.20 88.13 82.99 77.84 72.68 67.49 62.27 57.00 51.66 0.90 90.34 87.26 82.10 76.93 71.74 66.53 61.27 55.97 50.57 1.00 89.48 86.38 81.21 76.02 70.80 65.56 60.27 54.92 49.47 1.10 88.61 85.50 80.31 75.10 69.86 64.59 59.26 53.87 48.35