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The purpose of this chapter is to eliminate the transportation or movement of excess storm water and reduce the damage to property and injury to persons occasioned thereby. Further, it is intended to require that each development shall provide for the on-site or off-site detention of excess storm water runoff resulting from that development. For the purpose of this chapter, “excessive storm water runoff” shall include all increases in storm water resulting from: an increase in the impervious surface of the site, including all additions of buildings, roads and parking lots; changes in soil absorption caused by compaction during development; modifications in contours, including the filling or draining of small depressional areas, alterations of drainageways, or regrading of slopes; destruction of forest, alteration of drainageways or installation of collection systems to intercept street flows or to replace swales or other drainageways; or, the alteration of subsurface flows, including any groundwater dewatering or diversion practices such as curtain drains, compared with the site in its natural state.
('69 Code, § 5-18(b)(1)) (Ord. O-83-1, passed 5-9-83)
No development shall cause downstream property owners, watercourses, channels or conduits to receive storm water runoff from proposed developments at a higher peak flow rate than would have resulted from the same storm event occurring over the site of the proposed development with the land in its natural, undeveloped condition. For the purposes of this chapter, “undeveloped condition” shall mean that all the natural retention areas and drainageways plus existing farm drainage tiles and highway drainage structures shall be included in the flow calculations. For the purposes of the following calculation, all ground covers shall be considered to be a meadow or grassland, with the exception that forested areas shall be treated as woodlands. “Channel” or “drainage channel” shall mean the channels used to convey the 100-year drainage flows between successive retention facilities or to retention facilities or from the property.
('69 Code, § 5-18(b)(2)) (Ord. O-83-1, passed 5-9-83)
All storm water storage facilities shall be designed with sufficient capacity to accommodate all runoff caused by the development in excess of the runoff which would have resulted from the site if left in its natural, undeveloped condition. The storage capacity of all storage facilities shall be sufficient to store 115% of the excess flow, in each water shed, which would result from the 100-year storm of 24-hour duration.
('69 Code, § 5-18(b)(3)) (Ord. O-83-1, passed 5-9-83)
The following formula shall be used to calculate the volume of detention required by this chapter:
V = Rd - Rn - Ros - S - GW
V = the change in volume (in cubic feet per second) from the site. This is the base volume of excess storm water flows that would result from the development.
Rd = the volume of storm water runoff (in cubic feet per second) flowing from the site after its development. This determination shall include runoff from pervious and impervious surfaces, changes in areas of forest, changes in soils due to compaction, and changes in the time of concentration, for a 100-year storm of 24-hour duration.
Rn = the volume of storm water runoff (in cubic feet per second) flowing from the site in its natural state. This determination shall include runoff from the site with its natural cover, grassland or woodland for a 100-year storm of 24-hour duration. Farm fields shall be calculated as grassland.
Ros = the volume of storm water runoff (in cubic feet per second) flowing onto the site from upstream properties in their present state of development for a 100-year flood of 24-hour duration.
S = the change in capacity (in cubic feet per second) of the natural on-site detention areas of the site. This indicates any drainageways, small depressional areas or other areas which would naturally retain water during a 100-year storm of 24-hour duration. (This calculation may carry either a plus or minus value.)
GW = the change in subsurface flows due to dewatering techniques which would add to the total surface water runoff during a 100-year storm of 24-hour duration. Included here are dewatering devices such as drain tiles, curtain drains or sump pumps. (This calculation may carry either a plus or minus value.)
('69 Code, § 5-18(b)(4)) (Ord. O-83-1, passed 5-9-83)
All detention facilities and improvements required by this chapter shall comply with the following regulations:
(A) Storage volumes. Storage may be provided by wet or dry bottom basins or reservoirs or roof top storage facilities.
(B) Maximum depth. The maximum planned depth of storm water stored shall not exceed five feet unless natural ground conditions lend themselves to greater depth.
(C) Outlet control structures. Outlet control structures shall be designed as simply as possible and shall operate automatically. They will be designed to limit discharges into existing or planned downstream channels or conduits so as not to exceed existing flow of the site in its natural condition. Perforated risers of concrete, heavy gauge plastic, or other approved materials shall be incorporated on inlet piping for the discharge from all storm water detention basins to allow more gradual discharge of storm water from detention structures. Stone rip-rap shall be placed at the outlet discharge to break the force and dissipate the flow. Perforated risers and rip-rap flow reducing structures shall be designed in accordance with the Illinois Urban Design Manual.
(D) (1) Spillway. Emergency overflow facilities shall be provided unless inflow is controlled to divert flows when the basin is at capacity.
(2) Buffer space. All storm water detention basins shall be designed to provide area downstream of the outlet and alongside the structure to allow for proper maintenance of the sides and bottom of the berm. There shall be at least 15 feet of space around the basin to allow a vehicle to reach all parts of the structure, including the outlet discharge, without encroaching on the neighboring property.
(E) Dry bottom basins. For basins designed without permanent pools.
(1) Interior drainage. Provisions must be made to facilitate interior drainage, to include the provisions of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, or the installation of subsurface drains.
(2) Multi-purpose features. These may be designed to serve secondary purposes for recreation, open space or other types of use which will not be adversely affected by occasional or intermittent flooding.
(3) Cleaning. The basins shall be designed for periodic cleaning and removal of sediments, which shall be removed from the site or otherwise disposed of in an appropriate manner.
(F) Wet basins. For basins designed with permanent pools.
(1) Depth for fish. If fish are used to help keep the basin clean, at least one quarter of the area of the permanent pool must have a minimum depth of ten feet.
(2) Facilities for emptying. For emergency purposes, cleaning or shoreline maintenance, facilities shall be provided or plans prepared for the use of auxiliary equipment to permit emptying and drainage.
(3) Pollution abatement. Aeration facilities may be required when the quality of the influent and detention time would result in a lowering of dissolved oxygen content in the basin.
(4) Slopes. Approach slopes shall be at least 6:1, but not more than 3:1 and shall be at least four feet to six feet wide and slope gently toward the basin. The side slopes shall be of nonerosive material with a slope of 1:1 or flatter. The ledge shall be four feet to six feet wide and slope gently toward the shore to prevent people or objects from sliding into deep water. There shall be a freeboard of 12 inches to 18 inches above the high water elevation on all retention basins. Alternate designs for side slopes may be considered under special circumstances where good engineering practice is demonstrated.
(5) Cleaning. The basins shall be designed to include sediment traps in all inlets. Sediment traps shall be designed to permit periodic cleaning and maintenance. A basin maintenance plan shall be developed to insure that the design depths of the basin will remain over time.
(G) Building regulations.
(1) Roof top storage. Detention storage requirements may be met either in total or in part by detention on flat roofs. Design specifications of such detention shall be a part of the application for a building permit. These specifications shall include the depth and volume of storage, design of outlet devices and down drains, elevation of over-flow scuppers, design loadings for the roof structure, and emergency over-flow provisions. Roof top storage shall not be permitted to drain directly into sanitary sewers or streets.
(2) Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of storm water on a portion of their surfaces not to exceed 25%. Outlets shall be designed to empty the stored waters slowly and depths of storage must be limited so as to prevent damage to parked vehicles. Storage areas shall be posted with warning signs and shall be designed to fill to maximum depth in not less than two hours.
(3) Detention storage. All or a portion of the detention storage may also be provided in underground detention facilities.
(H) Any development which is adjacent to a ravine or ravine buffer and which has in excess of 3,200 square feet of impervious surface shall provide evaporative storage to control for a one-inch rainfall on those surfaces.
(I) Any development which is adjacent to a ravine or a ravine buffer and which has an impervious surface ratio in excess of one half shall provide evaporative or other storage to control for a two-inch rainfall on those surfaces.
(J) Retention in flood plains shall be permitted only in depressional flood plain areas. Retention will not be permitted in riverine flood plains.
('69 Code, § 5-18(b)(5)) (Ord. O-83-1, passed 5-9-83; Am. Ord. O-03-43, passed 12-8-03)
(A) The facilities for the control of storm water runoff shall be constructed prior to, if possible, the start of any construction or during the earliest possible stage of construction on the site of the project. All costs of construction, including the restoring, temporary seeding, and permanent erosion control measures, shall be borne by the person/persons responsible therefor. The Building Official shall approve the erosion control measures and the timing of their installation prior to their accomplishment.
(B) The entire storm water storage area shall be designed and constructed to fully protect the public health, safety and welfare. If a condition occurs in the storm water storage area which is hazardous to the public health, safety or welfare, the persons responsible for the condition will be required to provide approved corrective measures. In the event these measures are not provided, the city may eliminate the hazard at the expense of the person responsible.
('69 Code, § 5-18(b)(6),(7)) (Ord. O-83-1, passed 5-9-83)
(A) The construction of the storm water storage area and/or storm water passageway shall be under the supervision of an Illinois registered professional engineer. He shall be responsible for all construction in accordance with approved plans.
(B) In addition, a registered engineer employed by the city may inspect all drainage facilities while under construction. When facilities are not constructed according to approved plans, the city has the explicit authority to compel compliance and require correction of any situations which are not according to the approved plans. A set of as-built plans shall be submitted to the Building Official for his approval upon satisfactory completion of the construction of the storm water storage area and/or excess storm water passageway. In addition to the as-built plans, the design engineer shall provide written verification that, as constructed, the basin meets the requirements of the city's storm water detention ordinance.
('69 Code, § 5-18(b)(8)) (Ord. O-83-1, passed 5-9-83; Am. Ord. O-03-43, passed 12-8-03)
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