(A) Preparation of stormwater management plan. All computations, plans and specifications related to the implementation of this subchapter must be prepared and sealed by a professional engineer registered in this state. After the development has been completed, a professional engineer registered in this state shall certify to the city that the development was built according to the plans and specifications approved by the city. All exceptions shall be noted with explanations. The developer is subject to corrective actions if changes are made without approval from the City Engineer.
(B) Plan requirements. Unless the City Engineer excludes specific items, the stormwater management plan shall include but not be limited to the following information:
(1) A topographic map of the project site, and a drainage basin limits map and other pertinent data necessary to define flows entering the development from adjacent land. Maps shall be of suitable scale and contour interval, and include the extent of floodplains, calculated high water elevations, the shoreline of existing lakes, ponds, swamps and detention basins as well as their inflow and outflow structures, if any. Maps shall also include the 50- and 100-year floodplain elevations for any streams for which detailed flood studies have been prepared by the State Division of Water Resources or federal agencies. Floodway limits should also be shown as defined by available studies;
(2) The locations and invert elevations of all sanitary and storm sewers in the developing area or in adjacent areas;
(3) Detailed calculations of runoff anticipated for the developed site which indicates design volumes and existing and proposed runoff rates for each portion of the watershed tributary to the storm drainage system. The project engineer shall submit the calculations used to determine the runoff volumes and rates as well as a restatement of the criteria used throughout the calculations. Calculations are to be provided for a five-year storm, 50-year storm and a 100-year storm event;
(4) A site plan of the proposed stormwater management system, including the location and size of all drainage structures, storm sewers, channels and channel sections, detention basins, outlet lines and analyses of the effect of the improvement on the receiving outlet pipe, the associated channel and high water elevations;
(5) The slope, type and size of all existing and proposed storm sewers and other waterways;
(6) A plot or tabulation of storage volumes with corresponding water surface elevations and of the basin outflow rates for five-year, 50-year and 100-year water surface elevations for all detention basins. If development work is to be performed in phases, the tabulations should be performed independently for each phase;
(7) Design hydrographs of inflow and corresponding outflow for both the 50-year and 100-year design runoff events for the site’s projected final state of development and the calculated five-year, 50-year and 100-year peak inflows from the development under natural, existing conditions and under the projected final state of development for all detention basins;
(8) A profile and one or more cross-sections of all existing and proposed channels or other open drainage facilities, showing existing conditions and the proposed changes thereto. In addition, the project engineer will provide high water elevations expected from stormwater runoff under the controlled conditions called for by these regulations and the relationship of structures, streets and other utilities to the channels; and
(9) An engineering cost estimate detailing and explaining all construction costs associated with the stormwater management plan.
(C) Design criteria for stormwater management plan.
(1) Methods of determining stormwater runoff rate and volume. Drainage and storage facilities shall be designed using one of the methods of calculating the runoff discharge rate and total volume given below. The City Engineer may approve other calculation methods.
(a) TR-20;
(b) Rational Method or Modified Rational Method;
(c) TR-55;
(d) Quick TR-55; or
(e) Pond - 2.
(2) Release rate.
(a) The controlled release rate of stormwater runoff from developments subject to this section shall not exceed the subject property’s rate of runoff from a five-year storm prior to the proposed development. The design rate at which stormwater runoff is delivered to a designated stormwater storage area shall be based on a 50-year storm after full development. The storm sewers, however, shall be designed to deliver a runoff resulting from a ten-year storm, flowing under gravity conditions. The balance of runoff is to be delivered by overland flow or in the storm sewers under surcharge conditions.
(b) In the event the natural downstream channel or storm sewer system is inadequate to accommodate the required release rate, the allowable release rate shall be reduced to that rate permitted by the capacity of the downstream channel or storm sewer system without overflowing its banks or surcharging of sewers.
(3) Development design. Streets, blocks, lots, parks and other public grounds shall be located and designed to give a continuous surface relief path avoiding the flooding of buildings and structures, and to preserve and utilize existing and planned streams, channels and detention basins. Whenever possible, streams and floodplains shall be located within parks and other public grounds.
(4) Prohibition. It is unlawful for any person to construct or cause to be constructed any drainage facility for the purpose of the detention or retention of water within a distance of ten feet plus one-and-one half times the depth of any drainage facility adjacent to the right-of-way of any public highway without the written permission of the highway authority having jurisdiction over the public highway. It is also unlawful for any person to construct or cause to be constructed any earthen berm such that the toe of the berm will be nearer than ten feet to the right-of-way of any public highway without the written permission of the highway authority having jurisdiction over the public highway.
(D) Storm sewers. When storm sewers are required, storm sewers and inlets shall be designed with sufficient design capacity to transport surface water falling or draining onto the streets in a minimum ten-year storm reoccurrence period under the projected final state of development. Inlet spacing shall be determined in accordance with the most recent version of the Illinois Department of Transportation Drainage Manual. The developer’s engineer shall submit calculations showing compliance with these regulations.
(1) The City Engineer may approve alternate drainage systems if a detailed engineering study performed by a state registered professional engineer demonstrates that the alternate drainage system provides a level of service equal to the standard required without increasing maintenance and operating costs to the city.
(2) Storm sewer service connections shall be provided for each lot for all developments. If a sump pump is discharged into a storm sewer or storm sewer service connection, an air gap or backflow prevention device shall be installed in the sump pump line to prevent flooding should surcharging of the storm sewer occur.
(3) The developer shall maintain temporary drainage courses and structures that are not in the public right-of-way and/or are not part of the proposed plat which are necessary to protect the public from flood risk until they are replaced with permanent facilities. Maintenance of the facilities shall be assigned according to § 152.352.
(4) When an existing drainage course in the area of the proposed development abuts an existing closed conduit system, the developer shall extend the existing conduit system in full compliance with this subchapter.
(5) No plat of any development shall be approved unless the developer provides for and guarantees the construction of a storm sewer system adequate to serve the needs of the area when it is fully developed as proposed in the general area plan or, if no general area plan is required, the area covered by the development plat. The City Engineer shall approve the location and design of the storm sewer system.
(6) Stormwater management facilities shall be designed to facilitate the effective and efficient operation of stormwater facilities of adjacent areas of the same watershed and to minimize operation and maintenance costs.
(E) Detention facilities. The increased stormwater runoff resulting from a proposed development may be stored in appropriate detention facilities including wet bottom reservoirs, dry bottom reservoirs, parking lots and underground storage systems. The following standards shall govern the design of detention facilities.
(1) Storage volume. The volume of storage capacity provided in detention facilities shall be sufficient to store the excess stormwater runoff resulting from a 50-year rainfall as published by the Illinois Department of Transportation in its Drainage Manual. This volume shall be the required volume necessary to store the 50-year rainfall event for the developed site, minus the released rainfall available for a five-year rainfall event for the predeveloped site as determined in division (C)(2) above.
(2) Release rate. At no time during the design storm shall the stormwater runoff release rate exceed the allowable release rate required in division (C)(2) of this section.
(3) Release velocity. Detention facilities shall release stormwater at a nonerosive velocity. Protected channels receiving detention discharge shall incorporate features to reduce velocity to nonerosive levels where the discharge enters the unprotected channel.
(4) Spillway. An emergency spillway shall be provided to permit the safe passage of runoff generated from a 100-year storm or greater under developed conditions.
(5) Freeboard. Wet and dry bottom reservoirs, greater than four feet in depth or larger than one-tenth acre in size, shall have adequate capacity to contain the storage volume of tributary stormwater runoff in a 50-year storm with at least one foot of freeboard above the water surface. Wet and dry bottom reservoirs less than or equal to four feet in depth and less than one-tenth acre in size shall be designed with freeboard adequate to store the 100-year storm or six inches above the water surface elevation for the 50-year storm, whichever is greater.
(6) Physical aspects of wet and dry bottom reservoirs. To ensure public safety, the protection of the facility, the ease of facility maintenance, facility durability and aesthetics, the following shall be considerations in detention facility design.
(a) The water depth near the perimeter of a storage pool shall be limited. Access restrictions (fence, walls and the like) may be required if land availability dictates greater water depth.
(b) The amount and geometric shape of available land sites and location of the detention facility will influence depth and side slopes specifications and other design factors.
(c) To facilitate maintenance and reduce safety hazards, the side slopes for grassed areas should not be steeper than one foot vertical in four feet horizontal.
(d) To promote surface drainage for grass mowing and multiple purpose use, grass bottoms in detention basins should be designed with minimum slopes of 1%.
(e) Provisions should be made to protect the facility from soil erosion under all probable flow conditions imposed by the design storm.
(f) Control devices should be adequately protected from theft and vandalism.
(g) The facility’s landscaping should harmonize with the surrounding area.
(h) The design shall facilitate control and removal of debris both in the storage structure and in all inlet or outlet devices.
(i) Inflow and outflow structures, pumping stations and other structures should be protected and designed to minimize safety hazards.
(j) To promote the facility’s durability, longevity and physical appearance, construction specifications should require quality materials and workmanship.
(k) The outflow structure shall have an orifice, weir plate or similar control device to contain the basin’s outflow at the same release rate specified in division (C)(2) above.
(l) To minimize debris, a debris catcher chamber shall be installed at the inlet end of reservoirs greater than ten acre-feet. For ponds smaller than ten acre-feet, a manhole constructed two feet deeper than the inlet and outlet pipes may serve as a debris catcher.
(7) Dry bottom reservoir construction detail. A dry bottom reservoir should be designed to drain within 24 hours after a 25-year rain event, and after associated flows have ceased. Further, the following construction details shall apply to the design and construction of a dry bottom reservoir.
(a) Grass and other vegetation used to line the bottom and sides of the reservoir shall be able to sustain a minimum 30-hour period of inundation.
(b) Whenever possible, the dry bottom reservoir should be considered for other land uses during dry periods. Appropriate secondary land uses include: baseball fields, tennis courts, playgrounds and parks.
(c) To expedite drainage, French drains shall be included in any dry bottom reservoir having multiple land uses. Each tile line shall have a cleanout and/or manhole at each end to allow maintenance.
(8) Wet bottom reservoir construction details. Wet bottom reservoirs shall be constructed with a permanent pool that is not considered a part of the storage capacity of the reservoir. Only the volume above the permanent pool of water may be included in the drainage calculations. Considerations to be utilized in design and construction of a wet bottom reservoir are the following.
(a) When possible, facilities should be provided that completely drain the full reservoir, if needed.
(b) The minimum permanent pool depth shall be four feet, except when stocked with fish, in which case a minimum of 25% of the reservoir shall have a normal permanent pool depth of at least eight feet. The desirable edge slope for safety shall be 1:8 with a maximum of 1:4.
(c) If fishing, boating and other recreational activities are to be allowed, supplementary facilities for these activities should be designed and constructed in accordance with appropriate codes and ordinances.
(d) The design shall minimize problems of erosion due to ice, wind or wave action.
(e) The design shall prevent pollution of the reservoir. The operation and maintenance plan shall require the responsible party to test the reservoir’s water quality twice yearly in accordance with Illinois Environmental Protection Agency (IEPA) requirements. Whenever pollutant levels are found to exceed IEPA standards, the plan shall require corrective actions to lower levels in compliance with IEPA standards.
(9) Parking lot detention. Paved parking lots may be utilized for stormwater detention with the following restrictions.
(a) The water depths shall be designed to pond a maximum depth of six inches for a 100-year rainfall event where vehicles will be parked.
(b) The parking lot may not violate appropriate provisions of other ordinances governing their design and operation.
(c) Developer shall erect signs stating “This stall subject to flooding during heavy rainfall events” in each stall where the depth of water in the stall during the 100-year peak event is greater than or equal to three inches. Signs shall be approved by the City Engineer.
(10) Minimal detention excepted. Where the management plan calls for less than 300 cubic feet of detention, the construction requirement is not mandatory.
(11) Other detention facilities. Upon City Engineer approval, other types of detention facilities may be provided to satisfy a part or all of the detention requirements. Alternative facilities will be evaluated on a case-by-case basis.
(Ord. 10-3277, § 5-3.2, passed 1-4-2010) Penalty, see § 152.999