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Storm Water Requirements of the McLean County Subdivision Ordinance Part of Chapter 6 of the Manual of Practice 6.01 INTRODUCTION No subdivision plan or plat shall be recommended for approval, which does not make adequate provision for storm or flood water runoff channels or basins. The storm water drainage system shall be separate and independent of any sanitary sewer or collection tile system. Storm sewers, where required, shall be designed by the Rational Method or any other reasonable method as approved by the County Engineer. A copy of all design computations shall be submitted along with the engineering plans. Underground and/or surface storm water drainage systems shall be installed to service the entire subdivision. On site stormwater detention/retention shall be provided unless otherwise approved. 6.02 Design Standards All subdivisions shall include a storm water drainage system designed in such a way to provide that all lots and outlots in the subdivision will be graded and shaped so as to provide an adequate outlet for that property. This drainage system shall provide for any drainage that naturally flows through the development from adjoining property. A. Storm Sewers 1. Design Criteria a. Design Formula - Unless otherwise approved by the County Engineer, formulas to be used in connection with the calculation of run-off reasonably expected from the minimum design storm shall be the Rational Method for total contributing areas of twenty (20) acres (8 hectares) or less and the Soil Conservation Service Method as outlined in their Technical Release No. 55 for areas greater than twenty (20) acres (8 hectares). Calculations are to be submitted substantially in the form provided in Exhibit P of the Appendix. b. Minimum Design Storm - The minimum design storm used in calculating run-off in the Design Formula will be the average rainfall intensity associated with an average recurrence interval of five years for the storm period calculated by the Time of Concentration as outlined by the latest Technical Letters of the Illinois State Water Survey for rainfall frequencies. The corresponding charts and tables have been provided in Exhibit P of the Appendix for time of concentration, run-off factors and coefficients and frequency intensities for use in either method for ---PAGE BREAK--- run-off estimation. c. When changing sewer sizes the sewers shall match at the 9/10 diameter point. 2. The stormwater drainage system shall connect all inlets and catch basins to a storm sewer, pipe or conduit of sufficient size, grade and capacity to carry the run-off reasonably expected from the Minimum Design Storm on the area in the natural drainage area if that area is improved with the type of improvements permitted and to a maximum density authorized by the then-existing zoning ordinances of the County for property within the jurisdiction of the County; however no storm sewer shall be smaller than twelve (12) inches (300mm) in diameter. ---PAGE BREAK--- 3. The stormwater drainage system shall connect all storm sewers to other storm sewers or improved drainageways of sufficient size, grade, and capacity to carry the runoff reasonably expected from the Minimum Design Storm in the natural drainage area if that area was improved with the type of improvements permitted and to the maximum density authorized by the then-existing zoning ordinances of the County for property within the jurisdiction of the County. 4. Manholes a. Public manholes shall be installed at the end of each storm sewer line, at all changes in grade or alignment, at all intersections and at distances not greater than 400 feet (120m) between manholes for sewers of 15 inches (375mm) or less and 500 feet (150m) for sewers of 18 to 30 inches (450mm-750mm). Greater spacing will be permitted in larger sewers and in those carrying a settled effluent. b. Public manholes in improved streets or other hard surfaced public rights-of-way accessible to vehicular traffic shall be not more than 800 feet (360m) apart or at such lesser distances as is required to permit every storm sewer in the proposed development to be inspected, tested and cleaned from two surfaced manholes separated by not more than 1,200 feet (360m) measured in a straight line along the sewer. c. Minimum drop in a manhole shall be 1 inch (25mm) and the maximum drop in a manhole shall be 24 inches (600mm). 5. Inlets Inlets for local streets shall be provided for all low points and the maximum spacing shall not exceed four hundred (400) feet (120m), except that the first inlet shall be spaced approximately four hundred feet from the high point or at no greater distance than six hundred (600) feet (180m) when approved by the County Engineer. Inlets for all other street classifications shall meet IDOT design criteria. B. Drainage Ways 1. All drainage ways through the proposed development shall be improved to a size and in a way adequate to carry the runoff reasonably expected from the Minimum Design Storm in the natural drainage area if that area was improved with the type of improvements permitted and to the maximum density authorized by the then-existing zoning ordinances of the County for property within the unincorporated areas of McLean County and the land use element of the County's Comprehensive Plan. ---PAGE BREAK--- Storm water requirements, page 2 a. Design Formula: Unless otherwise approved by the County Engineer, formulas to be used in connection with the calculation of runoff reasonably expected from the Minimum Design Storm shall be the Rational Method for total contributing areas of 20 acres (8 hectares) or less and the Soil Conservation Service Method as outlined in their Technical release No. 55 for areas greater than 20 acres (8 hectares). b. Minimum Design Storm: The Minimum Design Storm used in calculating runoff in the Design Formula will be the average rainfall intensity associated with an average recurrence interval of twenty-five (25) years for the storm period calculated by the Time of Concentration as outlined by the latest Technical Letters of the Illinois State Water Survey for rainfall frequencies. The corresponding charts and tables have been provided in Exhibit P of the Appendix for Time of Concentration, runoff factors, and coefficients and frequency intensities for use in either method for runoff estimation. 2. Drainage ways shall have a flat bottom, maximum 3:1 side slopes, the top of the bank shall be constructed one foot above computed water surface elevation for the Minimum Design Storm, and have a ten (10) foot (3m) maintenance/access lane on each side of the drainage way. C. Retention and Detention Facilities 1. No development shall be authorized in McLean County unless it has an approved on-site detention or retention facility. Such facility shall be designed based on the Design Formula. a. Design Formula: Unless otherwise approved by the County Engineer, formulas to be used in connection with the calculation of runoff volumes and allowable release rates reasonably expected from the Minimum Design Storm shall be the Rational Method as outlined of the latest Illinois Division of Highway Standards for the Storm Water Runoff and the method outlined by the Metropolitan Sanitary District of Greater Chicago's sewer permit ordinance of 1972 as modified in Exhibit P of the Appendix for Storage for total contributing areas of twenty (20) acres (8 hectares) or less and the Soil Conservation Hydrograph Method for areas greater than twenty (20) acres (8 hectares). The corresponding instructions, charts, tables and forms have been provided in Exhibit P of the Appendix of this Manual for use in either method of calculation. For areas of development up to five acres (2 hectares), the following shall be required. ---PAGE BREAK--- Storm water requirements, page 2 AREA REQUIRED STORAGE RATE MAXIMUM RELEASE RATE ALLOWED up to 1 acre (0.4 hectare) 10700 cu. ft/acre (750 cu. m/hectare) 1.05 cfs/ acre (0.073 cu. m/hectare) up to 2 acres (0.8 hectare) 9100 cu. ft./acre (640 cu. m/hectare) 0.90 cfs/ acre (0.063 cu. m/hectare) up to 3 acres (1.2 hectare) 7800 cu. ft./acre (545 cu. m/hectare) 0.78 cfs/ acre (0.055 cu. m/hectare) up to 4 acres (1.6 hectare) 6900 cu. ft./acre (480 cu. m/hectare) 0.64 cfs/ acre (0.045 cu. m/hectare) up to 5 acres (2.0 hectare) 6200 cu. ft./acre (435 cu. m/hectare) 0.60 cfs/ acre (0.042 cu. m/hectare) b. Minimum Design Storm: i Storage volume will be determined from inflow hydrographs generated by the Design Formula using a minimum design storm with a range of rainfall intensities associated with an average occurrence interval of one hundred (100) years and an assumed coefficient for the post development zoning district as set forth in Exhibit P of the Appendix of this Manual. ii Allowable release rate will be determined by the Design Formula using a minimum design storm with an average rainfall intensity associated with an average recurrence interval of three years for the storm period calculated by the Time of Concentration as outline by the latest Technical Letters of the Illinois State Water Survey for rainfall frequencies and a runoff coefficient of 0.25. 2. Bank Stabilization a. Retention facilities shall be provided with wave shelves along the entire perimeter in accordance with standards in this manual. b. Retention and detention facilities shall have a maximum 4:1 bank slope. c. Shoreline surfaces subject to wave action shall be stabilized with structural material such as riprap, revetment matting, retaining walls, etc. ---PAGE BREAK--- Storm water requirements, page 2 6.03 DESIGN CALCULATION REQUIREMENTS Calculations required to demonstrate compliance with the design standards enumerated in the previous section of this Manual shall be submitted substantially in the form and content as shown and provided in Exhibit P of the Appendix. Calculations submitted with Preliminary Plans are not required to be of greater detail as the calculations required to be submitted with Public Improvement Engineering Plans and Specifications. ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK--- ---PAGE BREAK---