(B) Modification of standards. The Director, after consultation with relevant county staff, shall be authorized to modify the standards of this section provided such modification is not in conflict with other provisions of this chapter.
(C) Outside the right-of-way.
(1) Storm sewer facilities.
(a) General. Storm sewer facilities shall be used for stormwater management facilities only after conducting an analysis of the runoff reduction hierarchy (see § 151.146(D)) and upon determination by the Multi-Disciplinary Team that topography, poor drainage, or development intensity necessitate the use of storm sewers.
(b) Specifications. Storm sewers shall be designed and constructed in accordance with the Standard Specifications, the Illinois Department of Transportation Highway Standards and the Illinois Department of Transportation Bureau of Design and Environmental Manual (Design Manual).
1. The storm sewer system shall be of sufficient capacity to carry the discharge for a storm of ten-year frequency. Depending on site characteristics, a greater frequency design may be required. Storm sewer capacity calculations shall be required.
2. The engineer shall select coefficients of runoff based on the types of development anticipated in all parts of the watershed affecting the drainage structure. Information regarding the future development of the watershed shall be obtained from the Multi-Disciplinary Team.
3. The system shall include all necessary appurtenances such as inlets, catch basins, and manholes as may be required for proper operations and maintenance.
4. In those cases where a subdivision is subject to stormwater flow from adjoining land for a storm of 100-year frequency, the developer may be permitted to substitute an open ditch for storm sewer. The ditch shall comply with grading, seeding, sodding, and easement specifications of this chapter. Ditch capacity calculations and typical ditch cross-sections shall be required.
5. All storm sewers shall be at least 12 inches in diameter. The material can be reinforced concrete pipe or plastic. If plastic is used, manufacturer’s specifications (e.g., installation, depth of coverage) shall be submitted for review and approval by the Multi-Disciplinary Team. All storm sewer outfalls shall contain a concrete headwall or prefabricated end section. A rip-rap-lined apron shall be provided. Rip-rap calculations shall be required to determine the appropriate size. Special considerations shall be given to the avoidance of problems which may arise from the concentration of stormwater runoff onto adjacent property. All storm drainage shall be conducted and/or connected to an approved outfall and shall be included in the developer’s plans for stormwater management. The outfall shall be located at least ten feet from the boundary of an adjacent property. The concentrated flow shall be decentralized as sheet flow to adjacent properties unless connection approval is obtained from appropriate authorities.
6. All stormwater detention basin outlet pipes shall be equipped with anti-seep collars.
(c) Related structures. All manholes and catch basins (and any other special juncture boxes) shall be constructed with concrete material only and shall conform to the Illinois Department of Transportation Highway Standards and Standard Specifications.
(2) Subsurface drain tiles. The following design and construction guidelines shall be met:
(a) The main trunk line shall be a minimum six-inch diameter perforated plastic pipe;
(b) The drain tile shall connect directly to the site’s storm sewer system or discharge into the site’s proposed detention basin. The invert of the outfall shall be above the normal water level;
(c) A minimum of four inches of crushed stone (CA-6 or equivalent) shall be required around the outside diameter of the pipe. Silt sock or filter fabric shall be provided within the trench or around the pipe to prevent siltation;
(d) A four-inch diameter perforated plastic pipe shall be stubbed for each proposed lot within the subdivision. The developer shall tie in sump pumps to the storm sewer system prior to the issuance of a certificate of occupancy. If no storm sewer exists, sump pump discharges shall be at least ten feet from any property line. If discharges are located along side property lines, they shall be designed to enter swales, not abutting property. A minimum of 12 inches of fill shall be placed over the pipe;
(e) Drainage calculations shall be required for the sizing of the trunk line; and
(f) The trunk line shall be contained within a drainage easement with a minimum width of 15 feet.
(3) Swales and overland flow paths.
(a) Suitable means of soil erosion and sedimentation control shall be provided if calculations indicate erosive velocities will be reached.
(b) All swales shall have a minimum slope of 2%.
(c) Subsurface drain tiles described above may be required for all swales that traverse more than one lot and all overland flow paths.
(4) Field tiles. The following design and construction guidelines shall be met.
(a) All on-site field tiles shall be properly abandoned prior to mass grading of the site.
(b) A manhole shall be placed at a point where a field tile enters the proposed development.
(c) When possible, flows entering the development via a field tile shall be conveyed to its original point of discharge in a maintainable stormwater conveyance system. The flows should be routed around the proposed detention system as tile flow is not required to be detained.
(d) Standard engineering practices and the standards stated in this chapter shall be used in designing the conveyance system.
(e) The developer may be required to conduct an off-site field tile investigation to determine impacts on adjacent properties.
(f) Field tile systems disturbed during the site development process must be reconnected by those responsible for their disturbance, unless the approved drainage plan allows their relocation.
(5) Culverts. Material used for culverts that exceed 100 feet in length shall be limited to those permitted for storm sewers as specified in the Illinois Department of Transportation Standard Specifications, as amended. All culverts shall have end protection in the form of either concrete headwalls or prefabricated end sections. Flared end sections shall be required on all culverts and at the ends of all storm sewer pipes. Grates shall be required on all flared end sections.
(D) Inside the right-of-way.
(1) Storm sewer facilities.
(a) When required. Storm sewer facilities shall be required in all subdivisions except single family residential subdivisions containing lots with an average area of 40,000 square feet or more.
(b) Specifications. The design and construction of storm sewers shall be in conformance with the Standard Specifications, the Illinois Department of Transportation Highway Standards and the Illinois Department of Transportation Bureau of Design and Environmental Manual. The storm sewer system shall be designed by the “Rational Method” or another approved hydrograph-providing method, with self-cleaning velocities in conformance with the following requirements.
1. That part of the system which serves as the main or terminal collector shall be of sufficient capacity to carry the maximum discharge for a storm of ten-year frequency. Depending on the site characteristics, a greater frequency design may be required.
2. The engineer shall select coefficients of runoff based on the types of development anticipated in all parts of the watershed affecting the drainage structure. Information regarding future development of the watershed shall be obtained from the Multi-Disciplinary Team.
3. The system shall include all necessary appurtenances such as inlets, catch basins, and manholes as may be required for proper operations and maintenance.
4. In those cases where a subdivision is subject to stormwater flow from adjoining land for a storm of 100-year frequency, the developer may be permitted to substitute an open ditch for storm sewer. The ditch shall comply with grading, seeding, sodding, and easement specifications of this chapter. Ditch capacity calculations and typical ditch cross-sections shall be required.
5. All storm sewers shall be at least 12 inches in diameter. All storm sewers beneath the profile of the proposed street shall be load bearing pipe as determined in the Illinois Department of Transportation Standard Specifications. If a storm sewer composed of material other than reinforced concrete pipe is being proposed, the developer shall submit the manufacturer’s specifications for review. The alternate storm sewer material shall be approved by the County Engineer and the Township Highway Commissioner.
6. Special considerations shall be given to the avoidance of problems which may arise from the concentration of the stormwater runoff onto adjacent property. All storm drainage shall be conducted and/or connected to an approved outfall and shall be included in the developer’s plans for stormwater management. The outfall shall be located at least ten feet from the boundary of an adjacent property.
7. Pipe flowing full shall have a minimum velocity of three feet per second and a maximum velocity of ten feet per second.
8. If sump pumps and downspouts are designed to outlet into the right-of-way, they shall be connected directly to the storm sewer system or into a subsurface drain tile system, with highway authority approval, that discharges directly into the site’s stormwater management system.
9. Sump pumps and downspouts shall be regulated in accordance with § 151.146(H)(2).
(c) Related structures.
1. Catch basins, manholes, inlets, frames, and grates shall have bicycle-safe grates and conform to the Illinois Department of Transportation Highway Standards and Illinois Department of Transportation Standard Specifications.
2. The spacing of pavement drainage structures shall be at maximum intervals of 350 feet. When a crest lies between the drainage structures, the maximum interval may be 700 feet. When a sag lies between drainage structures at a maximum of 700 feet, double structures shall be added at the lowest point in the sag. Structures without sumps shall not be connected directly into the storm sewer.
(2) Subsurface drain tiles. The following design and construction guidelines shall be met:
(a) The main trunk line shall be a minimum six-inch diameter perforated plastic pipe.
(b) The drain tile shall directly connect to the site’s storm sewer system or discharge into the site’s proposed detention basin (the invert of the outfall shall be above the normal water level).
(c) A minimum of four inches of crushed stone (CA-6 or equivalent) shall be required around the outside diameter of the pipe.
(d) A four-inch diameter perforated plastic pipe shall be stubbed for each proposed lot within the subdivision.
(e) Drainage calculations shall be required for the sizing of the trunk line.
(f) The trunk line shall be contained within a drainage easement with a minimum width of 15 feet.
(3) Culverts. The size and capacity of all drainage structures shall be computed using the “Rational Method” or another approved hydrograph-providing method for tributary areas of 100 acres or less. For tributary areas of over 100 acres, an approved hydrologic and hydraulic analysis model shall be used. No driveway culvert shall have a diameter of less than 12 inches, and no cross-street culvert shall have a diameter of less than 15 inches. All culverts shall have a minimum clearance of six inches between the top of the pipe and the subgrade. In no case shall a driveway culvert be less than 20 feet in length. Material used for culverts which exceed 100 feet in length shall be limited to those permitted for storm sewers as specified in the Illinois Department of Transportation Standard Specifications. Inspection and cleanout manholes shall be required every 100 feet for culverts 100 feet or greater in length. All culverts shall have end protection in the form of either concrete headwalls or prefabricated end sections. Flared end sections shall be required on all driveway culverts and at the ends of all storm sewer pipe. Grates shall be required on all flared end sections 24 inches or larger located within the right-of-way.
(4) Ditches.
(a) Drainage ditches shall be designed based on a minimum ten-year storm frequency.
(b) Suitable means of soil erosion and sedimentation control shall be provided if calculations indicate erosive velocities will be reached.
(c) All drainage ditches with a grade of 4% or greater shall be sodded along the two-foot flat bottom ditch and for a distance of two feet up each bank unless other approved means of soil erosion and sedimentation control are provided.
(d) All ditches shall have a minimum slope of 2.0%.
(e) Subsurface drain tiles shall be required for all ditches where a slope of 2.0% cannot be achieved.
(5) Field tiles. See § 151.193(E)(4)(f).
(6) Bridges. All bridges and culverts having a span of more than 20 feet shall be approved by a bridge consultant retained by the Lake County Division of Transportation. The review period shall be 30 days. The developer shall reimburse the Lake County Division of Transportation for all consultant costs, plus an additional 5% for contract administration.
(Ord., § 10.13, passed 10-13-2009; Ord. passed - -)