9. An urban watershed has 120 acre area. The land use type of this basin is mostly covered by concrete and roof. Calculate the peak flow rate at this basin's outlet under the condition of 50-year frequency 1-hour rainfall. Use the rational formula: Q =CIA. Rainfall intensity (in./hr) or (mm/hr) 2 1 100-yr frequency 50-yr frequency J 0 10 20 30 40 50 60 70 80 90 100 110 120 Duration (min) Character of surface Developed Asphaltic 20-yr frequency 10-yr frequency 5-yr frequency Runoff coefficients for use in the rational method Return Period (years) 10 2 5 0.77 0.80 0.81 0.83 25 0.86 0.88 0.73 Concrete/roof 0.75 Grass areas (lawns, parks, etc.) Poor condition (grass cover less than 50% of the area) Flat, 0-2% 0.32 0.34 Average, 2-7% 0.37 0.40 Steep, over 7% 0.40 0.43 0.43 0.45 0.37 0.40 0.46 0.49 50 100 500 0.90 0.95 1.00 0.92 0.97 1.00 0.44 0.47 0.58 0.49 0.53 0.61 0.52 0.55 0.62