A. General Requirements: The planning and zoning commission shall not recommend approval of any plat of subdivision that does not make adequate provision for storm and flood water runoff channels or basins. The stormwater drainage system shall be separate and independent of any wastewater sewer system.
B. Nature Of Stormwater Facilities:
1. Location: The planning and zoning commission may recommend and the board of trustees may require the applicant to carry away by pipe or open ditch any spring, surface water or stormwater runoff that may exist either previously to, or as a result of, the subdivision to protect the public health, safety, and general welfare. Such drainage facilities shall be located in the road right-of-way where feasible, or in perpetual unobstructed easements of appropriate width, and shall be constructed in accordance with approved construction plans.
2. Accessibility To Public Storm Sewers: The applicant shall be responsible for constructing adequate facilities for the control, collection, conveyance, acceptable discharge of stormwater, other surface water and subsurface water which may be detrimental to the safe and convenient use of any portion of the area. The storm drainage system shall provide for runoff from the entire area of the subdivision. It shall take into account land outside the subdivision limits, which normally drains across the area of the subdivision as well as the effects of the subdivision upon downstream drainage systems. As primary focus, the drainage system for the subdivision shall make use of, protect, and improve, as needed, the natural drainage system. Drainage facilities shall be designed and constructed in accordance with the standards and procedures specified in these regulations. (Ord. 2015-18, 4-28-2015)
3. Accommodation Of Upstream Drainage Areas: A culvert or other drainage facility shall, in each case, be large enough to accommodate potential runoff from its entire upstream drainage area, whether inside or outside the subdivision. The subdivider's engineer shall calculate the necessary size of the facility, based on current engineering standards assuming conditions of maximum potential watershed development permitted by the comprehensive plan or zoning ordinance. Such calculations shall be submitted to the Public Works Director or designee for approval. (Ord. 2015-18, 4-28-2015; amd. Ord. 2018-07, 4-24-2018)
4. Effect On Downstream Drainage Areas: The flow rate and velocity of postdevelopment stormwater runoff from the site shall not exceed the flow rate and velocity of predevelopment runoff from the site. Stormwater detention, retention, or other BMP structures shall be designed, constructed, and maintained to achieve this. (Ord. 2015-18, 4-28-2015)
5. Areas Of Poor Drainage: Whenever a plat is submitted for an area that is subject to flooding, the Board of Trustees may approve such subdivision; provided the area is located outside of floodway limits, and that the applicant fills the affected area of the subdivision to an elevation sufficient to place the elevation of streets and lots at a minimum of twelve inches (12") above the elevation of the 100-year floodplain, as determined by the Public Works Director or designee. (Ord. 2015-18, 4-28-2015; amd. Ord. 2018-07, 4-24-2018)
6. Floodplain Area: The Board of Trustees may, when it deems it necessary for the health, safety, or welfare of the present and future population of the area and necessary to the conservation of water, drainage, and wastewater facilities, prohibit the development of a subdivision of any portion of the property that lies within the 100-year floodplain of any stream or drainage course, unless the land elevations are changed to remove or elevate the land above the 100-year floodplain elevation. These floodplain areas shall be preserved from any and all destruction or damage resulting from clearing, grading, or dumping of earth, waste material, or stumps, except at the discretion of the Board of Trustees.
7. Drainage Improvement Plans: Plans must include drainage, streets, and utility locations to be installed in or in conjunction with a subdivision. Overall stormwater drainage/management plan and an erosion control plan and its impact on contiguous land and source of effluent or discharge are also required. The developer shall furnish two (2) copies of the stormwater computations sealed by an Illinois licensed professional engineer. The plans and specifications shall be signed and sealed by the Illinois licensed professional engineer responsible for their preparation. The stormwater drainage/management plan, associated stormwater system design calculations and the soil erosion control plans shall be submitted to the Madison County Soil and Water Conservation District for their review. If a development exceeds one acre, a stormwater permit for the proposed development is required from the Illinois Environmental Protection Agency. The plans must be designed to comply with floodplain regulations, critical infrastructure requirements and hazardous mitigation regulations.
C. Dedication Of Drainage Easements:
1. General Requirements: When a subdivision is traversed by a watercourse, drainageway, channel, or stream, there shall be provided a stormwater easement or drainage right-of-way conforming substantially to the lines of such watercourse, and of such width and construction as will be adequate for the purpose. Wherever possible, it is desirable that the drainage be maintained by an open channel with landscaped banks and adequate width for maximum potential volume of flow.
2. Drainage Easements:
a. Perpetual, unobstructed easements at least twenty feet (20') in width for drainage facilities shall be provided across property outside road lines and with satisfactory access to the road. Easements shall be indicated on the plat. Drainage easements shall extend from the road to a natural watercourse or to other drainage facilities.
b. When a proposed drainage system will carry water across private land outside the subdivision, appropriate drainage rights must be secured and indicated on the plat.
c. Easements shall also be provided for storm drainage facilities that may need to be installed in the future to serve underdeveloped land within the watershed that normally drains across the area of a proposed development.
d. The applicant shall dedicate, either in fee or by a drainage or conservation easement, land on both sides of an existing defined watercourse having a bed and top of bank to a distance of sixty feet (60') back from the top of bank to the lot line.
e. Low lying lands along watercourses subject to flooding or overflowing during storm periods, whether or not included in areas for dedication, shall be preserved and retained in their natural state as drainageways. Such land or lands subject to periodic flooding shall not be utilized for average density procedures or for computing the area requirement of any lot.
D. Design Criteria: A stormwater management plan shall be required for any new residential, commercial, industrial, institutional or utility subdivision having a gross aggregate area of one acre or more. The following rules shall govern the design of improvements with respect to managing stormwater runoff:
1. Design Storm: All storm drainage facilities shall be designed with sufficient capacity and freeboard where necessary to convey the peak rates of runoff from storms with the average return frequencies listed below:
a. 2-year: All detention/retention basins (in addition to 100-year).
b. 10-year: All drainage facilities within road and easements from catch basins grates to outlet structures. Pavement encroachment, or maximum in-street spread, for drainage should be designed according to section 1-304.01 of the Illinois department of transportation drainage manual.
c. 25-year: All cross culverts under drives, minor and local streets, as well as improved streams, swales and ditches. Pavement encroachment, or maximum in-street spread, for drainage should be designed according to section 1-304.01 of the Illinois department of transportation drainage manual.
d. 50-year: All cross culverts and bridges constructed under residential collector and arterial streets.
e. 100-year: All detention/retention basins and bridges.
(1) The lowest opening elevation in all habitable buildings shall be the 100-year hydraulic grade line (HGL) elevation plus one foot (1').
(2) Drainage ditches, open channels, and natural drainageways shall have zero created head compared with predevelopment conditions at the upstream property line along said drainageway up to the 100-year flood frequency if any increase in flooding up to the 100-year frequency storm affects upstream structures or collector roads (or higher category roads), unless the developer acquires permanent flood easements to cover the area with increased flooding levels up to the 100-year flood event from the parties upstream whose structures and/or roads (collector level or higher) will experience an increase of flooding.
(3) Drainage ditches, open channels, and natural drainageways which have no potential to increase flooding upstream of structures or collector roads (or higher category roads) up to the 100-year event, shall not have a created head greater than 0.1 foot at the upstream property line of the project property along said drainageway unless easements are obtained from the upstream property owner to cover the increases of flooding up to the 100-year event, or unless there is a storm sewer, culvert, bridge, dam or other drainage structure within two hundred feet (200') downstream of the upstream property line on said drainageway in which case the created head shall not be greater than 0.5 foot at the upstream property line of the project property along said drainageway unless easements are obtained from the upstream property owner to cover the increases of flooding up to the 100-year event.
2. Storm Sewers:
a. Storm sewer systems shall be designed according to chapter 8 of the Illinois department of transportation drainage manual (latest edition). In the event there is contradictory language between this chapter and the Illinois department of transportation drainage manual (latest edition), the language of this chapter shall govern.
b. The hydrologic methodology for the storm sewer shall be in accordance with table 4-002 of the Illinois department of transportation drainage manual (2011 edition).
c. All culverts shall extend for the length of the property served and shall not terminate in the middle of a lot.
d. Storm sewer materials and construction shall conform to the applicable sections of the Illinois department of transportation's "Standard Specifications For Road And Bridge Construction", latest edition. The minimum diameter for across road culverts shall be fifteen inches (15"). The minimum diameter for storm sewers shall be fifteen inches (15"). The minimum ditch gradient shall be three-tenths percent (0.3%).
e. All storm sewer pipes shall be reinforced concrete pipe, class II minimum, conforming to applicable sections of the Illinois department of transportation "Standards Specifications For Road And Bridge Construction", latest edition.
f. Storm sewer outlets designed with greater than ten percent (10%) slope or with discharge velocities greater than five (5) fps, shall have approximately twenty feet (20') of the pipe constructed at 0.5 percent slope at the outlet and shall have an erosion control system provided.
g. Curb inlet and catch basin spacing shall not exceed a maximum gutter drainage flow distance of two hundred fifty feet (250'). Inlet and basin spacing shall also be provided as needed to conform IDOT gutter spread requirements.
3. Culverts:
a. Culverts shall be designed according to chapter 6 of the Illinois department of transportation drainage manual (latest edition). In the event there is contradictory language between this chapter and the Illinois department of transportation drainage manual (latest edition), the language of this chapter shall govern.
b. The hydrologic methodology for the culverts shall be in accordance with table 4-002 of the Illinois department of transportation drainage manual (2011 edition).
4. Bridges:
a. Bridges shall be designed according to chapter 7 of the Illinois department of transportation drainage manual (latest edition). In the event there is contradictory language between this chapter and the Illinois department of transportation drainage manual (latest edition), the language of this chapter shall govern.
b. The hydrologic methodology for the bridges shall be in accordance with table 4-002 of the Illinois department of transportation drainage manual (2011 edition).
5. Stormwater Detention: If detention basins are necessary, they must be designed in accordance with the requirements of this chapter and currently accepted engineering design practices. The design submission must include the following:
a. Map showing site, its location within the watershed, and the watershed limits. All detention facilities shall be analyzed with hydrograph and storage routing techniques.
b. Examination must be made of the routing of stormwater through the site and its flow path as it is discharged from the site. Subdrainage basin limits within the watershed must be delineated.
c. Designation of the location(s) of the proposed detention basin(s). Provide analysis for the stormwater runoff from those areas tributary to the proposed detention facility. Existing and proposed conditions must be evaluated. Peak storms with return frequencies for the 2-, 10-, 25-, and 100-year events shall be analyzed as follows:
(1) For detention basins with drainage areas less than one hundred (100) acres, the engineer shall use the rational method for detention storage as described in the Illinois department of transportation drainage manual under chapter 4, hydrology and chapter 12, detention storage and as further defined below:
(A) Existing and proposed "C" factors for agricultural, pastures, or wooded areas shall be 0.20 or less unless an extensive survey of soil types and land slopes is presented to verify that a higher value is justified.
(B) Existing and proposed "C" factors for paved and building areas shall be 0.90 or more. This excludes aggregate surfaces which should be evaluated based on the condition and slope of the aggregate surface.
(C) Existing and proposed "C" factors for lawns shall be 0.20 or more unless an extensive survey of soil types and land slopes is presented to verify that a lower value is justified.
(D) For commercial developments, "C" factors for the proposed development shall be calculated by a full tabulation of the proposed surface areas in the development when known, or by calculating the road "C" factor within the right of way and by calculating a composite "C" factor based on the maximum allowed coverage (pavement and building area) per lot for the development.
(E) For residential developments, "C" factors for the proposed development shall be calculated by a composite section of the typical road and lot configuration showing the average amount of paved and building areas per lot, and a full tabulation of the park, other green spaces, and lakes within the development.
(F) Please note that the engineer may also use one of the unit hydrograph methods as described below for drainage areas under two hundred (200) acres if they can demonstrate that subwatersheds have significantly nonhomogeneous land use or significantly different subwatershed timing that effect the proper design of said detention facilities.
(2) For detention basins with drainage areas greater than one hundred (100) acres, the engineer shall use a unit hydrograph routing method that is generally accepted to the Illinois department of natural resources/office of water resources such as the Clark or TR-20 unit hydrograph methodologies inside computer programs such as HEC-1 or HEC-HMS or other computer programs accepted by the Illinois department of natural resources/office of water resources, and as further defined below:
(A) Rainfall data and distributions should be utilized as described in bulletin 70 and circular 173 as published by the Illinois state water survey. Enough durations of each required frequency must be run to determine the peak storm duration for each frequency.
(B) Time of concentrations may be determined as described in either TR-55 or the IDOT drainage manual. Lag time if utilized may be assumed to be equal to 0.6 times the time of concentration.
(C) The selected curve numbers or other runoff coefficients must be calibrated to surveys of existing flood frequencies in other local watersheds which have already been modeled. This information shall be provided to the village. If the engineer does not have calibrated curve numbers for a similar watershed, the village will provide curve numbers with such a basis.
d. The detention basin may be designed to drain dry or to retain water for extended periods after rain events. All detention/retention basins must be properly maintained and kept free of algae growth, trash, and other debris.
e. The depth-area-volume relationship of the basin is dependent on the storage volume needed to reduce the peak inflow rate to some desired peak outflow rate. As the natural contours in the site dictate, ponding depths should be kept as shallow as possible. However, where stormwater storage depths exceed four feet (4'), a bench width of six feet (6') should be provided around the entire perimeter of the basin (exclusive of bermed areas). Outlets from normally dry detention basins can take on a number of forms, limited only by safety and maintenance considerations. Outlet pipe must be at least twelve inches (12") in diameter to facilitate maintenance. If less capacity is required than a twelve inch (12") pipe would provide, flow should be throttled at the pipe entrance. Trash racks or perforated risers are required at inlets to help guard against plugging of the pipe.
f. The side slope of the basin should be no steeper than four horizontal to one vertical (4:1) for the safety of mechanical mowing equipment and the safety of people during those times when water is being stored. All grades on the bottom of this basin should be at least two percent (2%) so that the bottom will drain quickly and leave no wet spots. A low flow concrete channel shall be provided in the basin.
g. An emergency spillway outlet shall be provided for discharge of flows in the event the storage capacity is exceeded or the primary outlet is nonfunctional. The emergency spillway shall be designed for the 100-year storm frequency event assuming the basin is dry, or at normal pool for a wet basin, and the primary outlet is nonfunctional (plugged) at the start of the rainfall event.
h. Easements should be included if necessary and access roads provided to allow vehicles and other equipment the access needed for maintenance of the basin.
i. If detention is to be of the wet basin (retention type), the volume needed for temporary storage must be provided above the normal pond elevation.
j. The outlet structure of wet basins (retention type) must be constructed such that the pond level is maintained. The side slopes beneath the water surface of the pond should be two horizontal to one vertical (2:1), to a depth of three (3) or four feet (4') to discourage the growth of aquatic plants unless a wetland system is developed along the shoreline of the pond. If fish are to be stocked in the pond, at least twenty five percent (25%) of the pond area should be at least ten feet (10') deep.
k. An as built survey of detention basin grades and outlets with an accompanying storage calculation will be required upon completion of the proposed detention basin. Any completed basin below ninety five percent (95%) of the design storage shall be regraded and reseeded to obtain the original design storage.
l. Detention dams which meet the criteria as a regulated dam as described in the most recent version of "Rules For Construction And Maintenance Of Dams" by the Illinois department of natural resources/office of water resources require an IDNR/office of water resources permit prior to village approval for construction.
E. Other Requirements:
1. Unless otherwise waived, a drainage analysis map shall be submitted showing the tributary watershed area, subdrainage basins, and the downstream area affected by runoff. Drainage computations shall consider the entire tributary area (on site and off site) of those drainage basins contributing runoff to all design points.
2. Existing wetlands/watercourses proposed to receive storm drainage discharge shall be analyzed to determine the downstream effects on any watercourse or existing storm drainage system for its adequacy to receive the proposed drainage discharge. The extent to which downstream studies are conducted shall be commensurate with the probable impact of the proposed development. Where it is anticipated that the additional discharge resulting from the proposed subdivision will overload the existing downstream drainage system, the planning and zoning commission may deny the subdivision until the applicant has adequately provided for improvements to the drainage system.
3. Each lot on any final plat or final development plan shall have identified on the plat of record an elevation height that shall be the minimum elevation of the lowest opening (first floor, walkout basement, or basement window) to adjacent grade. This elevation shall be determined and established by the elevation of the 100-year storm in any drainageway or structure adjacent to that lot or surrounding area that may subject that lot to potential flooding from any of those drainageways. That elevation will then be required to have one foot (1') of freeboard. All lots shall be noted on the plat of record as either suitable or unsuitable for walkout basements.
4. All developments must be provided an overland flow path that will pass the 100-year, 24-hour event flow at a stage at least one foot (1') below the lowest grade, adjacent to a structure, in the vicinity of the flow path. Street ponding and flow depths shall not exceed curb heights.
5. When the street side of any principal structure will be constructed below roadway elevation, the applicant shall submit documentation from an engineer showing how they propose to protect the structures from stormwater runoff in excess of the design capacity of the roadway.
6. The hydraulic capacity and the required size and slope of storm sewer pipes and channels shall be established by using Manning's equation. The hydraulic capacity of driveway and roadway cross culverts shall be established only after considering both the inlet and outlet control conditions. The lower of the two (2) flow rates obtained shall be the actual rated capacity. The upstream backwater shall not encroach onto adjacent properties unless backwater is contained within existing watercourse or wetland limits, does not encroach upon roadway beyond existing IDOT standards or driveway areas, and the necessary drainage easements are secured from those affected property owners.
7. Roadway underdrains shall be required where a soils report states this is needed, and their installation shall be performed by the applicant and as directed by the Village or its duly authorized agent, to protect the stability of the roadway. (Ord. 2015-18, 4-28-2015)
8. Suitable head wall or precast end sections shall be provided at the open end of any pipe. Culverts under streets shall have a minimum cover of thirty inches (30") and shall be extended to a minimum of ten feet (10') from the edge of pavement, unless otherwise approved by the Public Works Director. (Ord. 2015-18, 4-28-2015; amd. Ord. 2018-07, 4-24-2018)
9. All new or improved stormwater drainage swales created in new developments shall not allow overland drainage to exceed two hundred fifty feet (250') without being captured by a storm sewer system. The velocity of flow in these drainage swales shall not exceed five (5) fps unless measures are taken to avoid erosion.
10. Nonpaved surface overland flow grades and/or slopes shall be greater than 0.8 percent. Paved surface overland flow grades and/or slopes for swales and ditches shall not have a slope less than 0.5 percent.
11. Reinforced concrete paved swales and ditches a minimum of four feet (4') wide are required to be designed and constructed for any drainage swale with a slope less than 0.8 percent.
12. Streets, blocks, lots, parks, and other public grounds shall be located and laid out in such a manner as to minimize the velocity of overland flow and allow maximum opportunity for infiltration of stormwater into the ground, and to preserve and utilize existing and planned streams, channels, and detention basins. (Ord. 2015-18, 4-28-2015)
13. Additional design standards, as appropriate, may be required by the Public Works Director. (Ord. 2015-18, 4-28-2015; amd. Ord. 2018-07, 4-24-2018)
14. All pipes and drainage structures shall be thoroughly cleaned by the applicant before acceptance by the Village and again following final construction of homes or structures within the subdivision. The applicant shall supply a letter of credit to secure such cleaning.
15. Detention basins shall be located on outlots maintained by the homeowners' association. A special service area may be created for potential maintenance by the Village should the homeowners' association become defunct, unable or unwilling to provide sufficient maintenance of the detention basins.
16. The maintenance and repair of all surface drainage systems either paved or unpaved are the responsibility of the property owner.
F. Dams And Impoundment Structures: Dams and berms for water impoundments, pools, ponds, reservoirs, and small lakes shall be planned, designed and constructed under the supervision of a licensed professional engineer and shall meet the approval of the Illinois Department of Natural Resources/Office of Water Resources, Division of Water Resource Management. An engineering report shall be submitted to the Building and Zoning Administrator on all existing dams within a proposed subdivision. Any dam found to be structurally unsafe shall be reconstructed or reinforced in accordance with the Division of Water Resource Management standards. (Ord. 2015-18, 4-28-2015)
Notes
| 1 | 1. See also title 12 of this code. |