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Bill 51 Case Study <br /> (May 31, 2007) <br /> <br /> Determine installation costs and land requirements for a new 20-acre farm in <br /> <br /> Ka'u to comply with the proposed Bill 51 requirements for agricultural <br /> <br /> developments to retain on their property the stormwater generated by a 100-year <br /> storm. <br /> <br /> Assumed: <br /> 1) Farm location is 5 miles north of Pahala. <br /> 2) Soil is Alapai silty clay loam, 10%-20% slope, Hydrologic Soil Group-A <br /> (well-draining) <br /> 3) Drainage from 20 acres of farm site only. No upper drainage area to <br /> the farm. If there is a drainage area above the farm, it will be <br /> conveyed through the farm with a newly enlarged drainageway with a <br /> 100-year stormwater capacity. <br /> 4) Farm is 10 acres of row crop on contour and 10 acres fallow with good <br /> cover, <br /> Computed average Runoff Curve Number, 70. <br /> 5) Rainfall intensities are obtained from National Weather Bureau, <br /> Technical Paper No. 43. The 100-year, 24-hour rainfall is 21 inches <br /> 6) The farm is laid out as a square with a 1,000-ft flow path to calculate <br /> time of concentration. Time of concentration is 0.1 hours. <br /> Methodology: <br /> 1) WinTR-55 computer program used to calculate total runoff. WinTR-55 <br /> output in separate hydrology file. <br /> 2) Calculate total volume of runoff by multiplying inches of runoff by the <br /> 20 acre area. <br /> 3) Determine the volume and total dimensions of a basin to retain the <br /> storm runoff. <br /> 4) Determine basin costs and land requirement. <br /> Results: <br /> 1) Runoff from 100-year storm = 16.6 inches = 1.38 ft (4.4 inches lost to <br /> infiltration). <br /> 2) Runoff volume from 20-acre farm = 1.38 ft. X 20 ac. = 27.7 ac-ft. or 9.0 <br /> million gallons. <br /> 3) With a 12-ft deep pool requiring a 15-ft high embankment along lower <br /> boundary of property, a basin to contain the runoff will be <br /> approximately 750 ft. by 130 ft at the design pool level. Additional <br /> storage volume will be needed if sediment storage is required. <br /> 4) The embankment and design pool will require 4.7 acres of land. An <br /> 18-ft high cut slope behind the pool will require an additional 1.1 acres. <br /> 5) The volume of excavation is approximately 130,000 cy. At $10/cy the <br /> excavation cost is $1.3 million. <br /> <br />