(A) This section shall apply to all developments in which:
(1) More than two single-family structures or more than one two-family structure are to be constructed on a site more than two acres in size.
(2) Multi-family or non-residential land use is to be constructed on a site more than one acre in size.
(3) Existing multi-family or non-residential land uses on a site more than one acre in size, on which new development after March 25, 2010, in the aggregate exceeds 25,000 square feet.
(4) Roadway developments in rights-of-way under the ownership or control of a unit of local government when the contiguous area of new roadway construction (excluding previously paved areas) exceeds two acres.
(5) Exemptions from site runoff storage requirements (stormwater detention) shall be granted under the circumstances listed in Article 200.3 of the Will County Stormwater Management Ordinance, latest edition.
(B) Drainage plan submittal requirements.
Each applicant shall submit the following information, depending on development size, to ensure that the provisions of this ordinance are met. The submittal shall include sufficient information to evaluate the environmental characteristics of the property, the potential adverse impacts of the development on water resources both on-site and downstream, and the effectiveness of the proposed drainage plan in managing stormwater runoff. The applicant shall certify on the drawings that all clearing, grading, drainage, and construction shall be accomplished in strict conformance with the drainage plan. The following information shall be submitted for both existing and proposed property conditions.
Properties smaller than ten acres shall be required to submit only the basic drainage plan. Properties which are ten acres or larger shall comply with the submittal requirements of both the basic drainage plan and the advanced drainage plan.
(1) Basic drainage plan shall include:
(a) Topographic map. A topographic survey of the property, with one-foot elevation contours under existing and proposed conditions, including areas upstream and downstream of the subdivision or development, necessary to determine off-site impacts of the proposed drainage plan. The map datum shall be NAVD 88, correlated to a FEMA reference mark by differential leveling or GPS.
(b) Drainage system. Mapping and descriptions, where relevant, of existing and proposed drainage system features of the property and immediate vicinity including:
1. The banks and centerline of streams and channels;
2. Shoreline of lakes, ponds, and detention basins;
3. Farm drains and tiles;
4. Sub-watershed boundaries within the property;
5. Watershed soils classifications;
6. The property's location within the larger watershed;
7. Location, size and slope of stormwater conduits and drainage swales;
8. Sanitary or combined sewers;
9. Depressional storage areas;
10. Delineation of upstream and downstream drainage features and watersheds which might be affected by the development;
11. Stormwater detention facilities;
12. Roads and streets and associated stormwater inlets;
13. Base flood elevation, regulatory floodway, and base floodplain limit delineation based on topography and the FEMA Flood Insurance Rate Map; and
14. The basis of design for the final drainage network components.
(c) Environmental features.
1. A depiction of environmental features of the property and immediate vicinity including the following:
A. The limits of wetland areas;
B. Any designated natural areas; and
C. Any proposed environmental mitigation features.
2. Advanced drainage plan. In addition to the requirements for the basic drainage plan, the advanced drainage plan shall also include:
A. Elevations and maps of 100-year flooding;
B. Cross-section data for open channel flow paths and designated overland flow paths;
C. Direction of overland stormwater runoff;
D. Flow rates and velocities at representative points in the drainage system; and
E. A statement by the design engineer of the proposed drainage system's adequacy for safely handling runoff from storm events greater than the 1% annual probability (100-year) design storm.
F. The city reserves the right to request that the IDNR-OWR review all drainage plan submittals.
(C) Minimizing runoff volumes and runoff rates. In the selection of a drainage plan for a development, the applicant shall evaluate and implement, where practicable, site design features which minimize the increase in runoff volumes and rates from the site. The applicant's drainage plan submittal shall include evaluations of site design features which are consistent with the following hierarchy:
(1) Minimize impervious surfaces on the property, consistent with the needs of the project;
(2) Attenuate flows by use of open vegetated swales and natural depressions and preserve existing natural stream channels;
(3) Infiltrate runoff on-site;
(4) Provide stormwater retention structures;
(5) Provide stormwater detention structures; and
(6) Construct storm sewers. In developments where the city requires the installation of curb and gutter, storm sewers are preferred. The city may also modify the above hierarchy as required based on an evaluation of the site conditions.
(D) Water quality and alternate uses. The drainage system shall be designed to minimize adverse water quality impacts downstream and on the property itself. Detention basins shall incorporate design features to capture stormwater runoff pollutants. In particular, designers shall give preference to wet bottom and wetland designs in locations adjacent or near existing wetlands or in other areas where they are suitable and acceptable to the city and all flows from the development shall be routed through the basin (i.e., low flows shall not be bypassed). Dry basins with low flow bypasses may be preferred in certain developments to enhance multiple uses where suitable and acceptable to the city. Retention and infiltration of stormwater shall be promoted throughout the property's drainage system to reduce the volume of stormwater runoff and to reduce the quantity of runoff pollutants.
The drainage system shall incorporate multiple uses where practicable. Uses considered compatible with stormwater management include open space, aesthetics, aquatic habitat, recreation (boating, trails, playing fields), wetlands and water quality mitigation. The applicant shall avoid using portions of the property exclusively for stormwater management wherever practicable.
(E) Design criteria, standards and methods. Site runoff storage facilities (stormwater detention basins) shall be designed and constructed in accordance with Section 203 of the Will County Stormwater Management Ordinance, latest edition, and as specified herein.
(1) Detention release rates.
If a release rate is not specified in an adopted watershed plan, and if a development is not exempted from storage requirements, then sufficient flood storage shall be provided so that the maximum site discharge will not exceed 0.15 cfs per acre of development for the one percent annual probability (100-year) storm; and will not exceed 0.04 cfs per acre of development for the fifty percent annual probability (two-year) storm.
For sites where the undeveloped release rate is less than the maximum release rate described above, the developed release rate and corresponding site runoff storage volume shall be based on the existing undeveloped release rate for the development.
(2) Detention basin outlet design. All hydrologic and hydraulic computations shall utilize appropriate assumptions for downstream water surface elevations, from low flow through the base flood, considering the likelihood of concurrent flood events.
(3) Detention storage requirements. The minimum site runoff storage volume to be provided in a detention basin shall be based on the runoff from the 100-year, critical duration event. The design of all detention basins shall be by Runoff Hydrograph Methods described in Section 153.50.040(E)(5); or where development sites are less than five acres, the unit area site runoff storage nomograph from the "Investigation of Hydrologic Design Methods for Urban Development in Northeastern Illinois," dated December 1991 and prepared by the Northeastern Illinois Plan and Zoning Commission (now known as CMAP) may be used in lieu of modeling.
(4) Drainage system design and evaluation. The following criteria shall be used in evaluating and designing the drainage system. The underlying objective is to provide capacity to pass the 10% annual probability (ten-year) peak flow in the minor drainage system, and to provide an overland flow path for the one percent annual probability (100-year) peak flow.
(a) Design methodologies. Major and minor conveyance systems for areas up to ten acres may be designed using the rational formula. The rational formula may also be used in sizing the minor drainage system for larger sites. Runoff Hydrograph methods as described in § 153.50.040(E)(5) must be used for major drainage system design for all systems with greater than ten acres of drainage area. All drainage system design shall be in accordance with Part 708 Rules.
(b) Positive drainage. Whenever practicable, all areas of the property shall be provided an overland flow path that will pass the peak 100-year flow at a stage at least one foot below the lowest foundation grade in the vicinity of the flow path. Overland flow paths designed to handle flows in excess of the minor drainage system capacity shall be provided in drainage easements, which shall not be obstructed. Street ponding and flow depths shall not exceed six (6) inches at the crown. For flow over a new roadway or parallel to a new roadway, the product of the flow depth (in feet) and velocity (in feet per second) shall not exceed four for the base flood condition.
(5) Methods for generating runoff hydrographs. Runoff hydrographs for detention pond design shall be developed using event hydrograph routing methods. Acceptable and preferred programs are TR-20 and HEC-HMS. If an existing regulatory model uses HEC-1, then HEC-1 may be used for the proposed facility. Continuous simulation models are only acceptable for very complex projects and their use must be approved by the Director of Public Works and Engineering or authorized designee. No other models, including the modified rational method, are acceptable.
Unless a continuous simulation model is used, all design rainfall events shall be based on the requirements of the Will County Stormwater Ordinance, latest edition, or Illinois State Water Survey Bulletin 75, whichever is most recent. A critical duration analysis for the design storm of one, two, three, six, 12, 18, 24 and 48 hours duration shall be prepared in accordance with generally accepted engineering principles. The first quartile Huff rainfall distribution shall be used for storms with durations of one, two or three hours. The second quartile Huff rainfall distribution shall be used for storms with durations of six or 12 hours. The third quartile Huff rainfall distribution shall be used for storms with durations of 18, 24 or 48 hours. Computations of runoff hydrographs which do not rely on a continuous accounting of antecedent moisture conditions shall assume an antecedent moisture condition of two.
(6) Wet-bottom detention basin design. Wet detention basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing, and where practicable, to be available for recreational use. Permanent ponds shall be designed in accordance with the City of Lockport standard drawings. Ponds shall have a two-foot horizontal to one-foot vertical side slope from a point one and one-half feet above normal water level, down to a point three feet below normal water level. This two to one sloped area shall be blanketed with geotechnical fabric to prevent weeds. The fabric shall then be covered with a layer of natural stones, having a minimum diameter of eighteen inches. The surface of the stones on the two to one slope shall be arranged in such a manner so as to enable a person to climb out of the pond over the stones.
The area between the top of the two to one slope and the high water level shall be protected by sodding in order to prevent soil erosion. The first twelve feet of the sodded area at the top of the two to one slope shall be sloped at 2% towards the pond. At the toe of the two to one slope, a twelve foot wide safety ledge sloped toward the shore at 4% shall be provided. Beyond the safety ledge, the pond bottom shall slope at two horizontal to one vertical down to the pond bottom elevation.
Points of inflow to the ponds shall be accessible to construction equipment for dredging as necessary. If retention facilities are designated for recreational purposes, appropriate consideration shall be reflected in the design for maintenance of fish life, boating, and safety.
(a) Aeration. All wet-bottom ponds shall be provided with aeration equipment.
(b) Wet-bottom detention basin depth. Wet basins shall be at least three feet deep, excluding nearshore banks and safety ledges. If fish habitat is to be provided, they shall be at least ten feet deep over 25% of the bottom area to prevent winter freeze-out.
(c) Permanent pool volume. The permanent pool volume in a wet-bottom detention basin at normal water level (NWL) shall be equal to the runoff volume from its watershed for the two-year event.
(d) Inlet and outlet orientation. The distance between detention pond inlet and outlet pipes shall be as large as practicable, but shall be at least 50 feet.
(e) Overflow weirs. Detention basins shall provide at least twelve inches of freeboard between the design High Water level (HWL) and the top of berm. Detention basins shall be provided with an emergency overflow weir at or above the HWL near the outlet pipe. The weir shall be designed to have the capacity to convey a flow of one cfs per tributary acre with a flow depth no greater than six inches, and shall be armored from the overflow point down to the outside toe of slope to prevent soil erosion.
(7) Dry-bottom detention basin design. In addition to the other requirements of this ordinance, dry-bottom basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing and to be available for multiple uses wherever practicable.
(a) Dry-bottom detention basin design. In order to prevent soil erosion and invasive weeds, dry-bottom detention basins shall be landscaped, including the establishment of a ground cover over all unpaved areas, through sodding or other means which result in a quality of ground cover comparable to that obtained through sodding. Dry-bottom detention facilities shall be usable as active recreational areas during dry weather conditions. Dry-bottom detention facilities shall be designed so that the bottom has a slope of at least 2%. The bottom of the facility shall be provided with an underdrain (minimum six-inch diameter perforated PVC drain tile) covered on all sides with a minimum of six inches of crushed stone conforming to IDOT Gradation CA-7. Dry-bottom detention facilities shall have side slopes not steeper than four feet horizontal to one foot vertical.
(b) Overflow weirs. Detention basins shall provide at least 12 inches of freeboard between the design high water level (HWL) and the top of berm. Detention basins shall be provided with an emergency overflow weir at or above the HWL near the outlet pipe. The weir shall be designed to have the capacity to convey a flow of one cfs per tributary acre with a flow depth no greater than six inches, and shall be armored from the overflow point down to the outside toe of slope to prevent soil erosion.
(c) Velocity dissipation. Velocity dissipation measures shall be incorporated into dry-bottom basin designs to minimize soil erosion at inlets and outlets and to minimize the resuspension of pollutants.
(d) Stilling/sedimentation basins. A stilling/sedimentation basin shall be constructed at each major inlet to a wetland-bottom or dry-bottom basin. The volume of the basins shall be at least 500 cubic feet per acre of impervious surface in the tributary watershed. Side slopes of the sedimentation basins shall be no steeper than three horizontal to one vertical and basin depths should be at least three feet to minimize resuspension of accumulated sediment.
(8) Minimum detention pond outlet size. Where a single pipe outlet or orifice plate is to be used to control discharge, the minimum desirable diameter is four inches. If this minimum orifice size permits release rates greater than those allowed, and regional detention is not a practical alternative, outlet structures resistant to clogging such as perforated risers or flow control orifices shall be used to protect the smaller restrictor.
(9) Detention in flood plains. The placement of stormwater detention facilities in the regulatory flood plain is strongly discouraged due to questions regarding their operation during major floods. Site runoff storage facilities in the flood plain shall meet all of the requirements of Sections 203.7 and 203.8 of the Will County Stormwater Management Ordinance, latest edition.
The placement of a detention basin in a flood plain shall require compensatory storage for 1.5 times the volume below the base flood elevation occupied by the detention basin, including any berms. All floodplain storage lost below the ten-year flood elevation shall be replaced below the ten-year flood elevation. All floodplain storage lost above the existing ten-year flood elevation shall be replaced above the proposed ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
Detention volume provided by enlarging existing regulatory flood plain storage without providing a structure controlling discharge (on-stream detention) will be allowed only as a variance, requiring a public hearing. The variance will be considered only after applicant demonstrates that flood damages are not increased and the development will not increase flood flows for both the two-year and 100-year floods on the stream with the site fully developed.
City approval of the variance request will be contingent on the applicant showing by detailed hydrologic and hydraulic analysis that the proposed design will provide a net watershed benefit not otherwise realized by strict application of the above design standards.
(10) Protection of wetlands and depressional storage areas. Wetlands and other depressional storage areas shall be protected from damaging modifications and adverse changes in runoff quality and quantity associated with land developments. In addition to the other requirements of this chapter, the following requirements shall be met for all developments whose drainage flows into wetlands and depressional storage areas.
(a) Army Corps of Engineers coordination. Any applicant proposing a subdivision or development which will impact an existing wetland or waters of the U.S. shall obtain a permit or a letter of no objection from the Army Corps of Engineers.
(b) Detention in wetlands and depressional storage areas. Existing wetlands shall not be modified for the purposes of stormwater detention unless it is demonstrated that the existing wetland is low in quality and the proposed modifications will maintain or improve its habitat and ability to perform beneficial functions. Existing depressional storage and release rate characteristics of wetlands and other depressional storage areas shall be maintained and the volume of detention storage provided to meet the requirements of this section shall be in addition to this existing storage.
(c) Sediment control. The existing wetland shall be protected during construction by appropriate soil erosion and sediment control measures and shall not be filled. All runoff from the development shall be routed through a preliminary detention/sedimentation basin designed to capture the two-year, 24-hour event and hold it for at least 24 hours, before being discharged to the wetland. The sedimentation basin shall be constructed before site grading commences. In addition, the drainage hierarchy defined in § 153.50.040(C) shall be utilized to minimize runoff volumes and rates being discharged to the wetland.
(d) Alteration of drainage patterns. Site drainage patterns shall not be altered to substantially decrease or increase the existing area tributary to the wetland.
(e) Vegetated buffer strip. A buffer strip of at least 75 feet in width, vegetated with plant species native to Illinois, shall be maintained or restored around the periphery of the wetland.
(11) Ponding on streets and parking lots.
(a) Streets. If streets are to be used as part of the minor or major drainage system, ponding depths shall not exceed six inches on the pavement at the crown and shall not remain flooded for more than eight hours for any event less than or equal to the 100-year critical duration event.
(b) Parking lots. The maximum stormwater ponding depth in any parking area shall not exceed nine inches, and the parking lot shall not remain flooded for more than eight hours for any event less than or equal to the 100-year critical duration event.
(12) Infiltration best management practices.
To effectively reduce runoff volumes, infiltration practices including basins, trenches, and porous pavement should be located on soils in Hydrologic Soil Groups "A" or "B" as designated by the U.S. Soil Conservation Service. Infiltration basins and trenches designed to recharge groundwater shall not be located within seventy-five feet of a water supply well or a building foundation. A sediment settling basin shall be provided to remove coarse sediment from stormwater flows before they reach infiltration basins or trenches. Stormwater shall not be allowed to stand more than 72 hours over eighty percent of a dry basin's bottom area for the maximum design event to be ex-filtrated. The bottom of infiltration facilities shall be a minimum of four feet above seasonally high groundwater and/or bedrock.
To effectively filter stormwater pollutants and promote infiltration of runoff, sites should be designed to maximize the use of vegetated filter strips and swales. Wherever practicable, runoff from impervious surfaces shall be directed onto filter strips and swales before being routed to a storm sewer or detention basin.
(13) Safety considerations. The drainage system components, especially all detention basins, shall be designed to protect the safety of children and adults coming in contact with the system during runoff events.
(a) Side slopes. The side slopes of all detention basins at the HWL shall be as level as practicable to prevent accidental falls into the basin and for stability and ease of maintenance.
(b) Safety ledge. All wet detention basins shall have a flat safety ledge at least 12 feet in width three feet below the normal water level, sloping towards the shore at a 4% gradient.
(c) Velocity. Velocities throughout the surface drainage system shall be controlled to safe levels taking into consideration flow rates and depths of flow.
(d) Overflow structures. All stormwater detention basins shall be provided with an overflow structure capable of safely passing excess flows at a stage at least one foot below the lowest foundation grade in the vicinity of the detention basin. The design flow rate of the overflow structure shall be equivalent to one cfs per acre of all tributary areas.
(14) Maintenance considerations. The stormwater drainage system shall be designed to minimize and facilitate maintenance. Turf grass side slopes shall be designed to allow mowing equipment to easily negotiate the grade. Pre-sedimentation basins shall be included where feasible, for localizing sediment deposition and removal. Access for heavy maintenance equipment shall be provided.
(F) Accommodating runoff from upstream tributary areas.
(1) Upstream areas not meeting storage requirements. When there are areas not meeting the storage and release rate requirements of this chapter that are tributary to the applicant's property, regionalized stormwater detention on the applicant's property shall be considered by the applicant and the city. The following steps shall be followed:
(a) The applicant shall compute the storage volume needed for his property alone.
(b) Areas tributary to the applicant's property that do not meet the storage and release rate requirements of this ordinance shall be identified.
(c) Using the off-site areas determined above plus the applicant's property area, total storage needed for the combined properties shall be computed. Allowable release rates shall be computed using the combined property areas. If off-site tributary areas are not developed, a reasonable fully-developed land cover, based on local zoning, shall be assumed for the purposes of computing storage requirements.
(d) Once the necessary combined storage is computed, the city may choose to require oversizing the applicant's detention basin to accommodate the regional flows or minimize downstream impacts. Additional costs shall be subject to recapture by the applicant in accordance with § 153.50.030. If the applicant is the owner of portions of upstream areas, recapture will not be allowed for his share of the upstream area. The applicant's responsibility will be limited to the storage for his property as computed in division (a) above plus any upstream property owned by the applicant. If regional storage is selected by the city, then the design produced in division (c) above shall be implemented. If regional storage is not selected by the city, the applicant shall bypass all tributary area flows around the applicant's basin, in pipe or open channel, whenever practicable. If the applicant must route upstream flows through his basin, the applicant must demonstrate that sufficient additional storage volume has been added for conveyance through the basin so that the maximum release rate from the facility does not exceed the allowable release rate for the site plus the bypass flow.
(2) Upstream areas meeting storage requirements. When there are areas which meet the storage and release rate requirements of this ordinance, tributary to the applicant's property, the upstream flows shall be bypassed around the applicant's detention basin, or be routed through the applicant's detention basin if this is the only practicable alternative. Storage needed for the applicant's property shall still be computed. However, if the city decides to route tributary area flows through an applicant's basin, the final design stormwater release rates shall be based on the combined total of the applicant's property plus the tributary areas. Supporting calculations must demonstrate that at no time will the runoff rate from the applicant's property exceed the allowable release rate for his/her property alone.
(G) Construction staging of storage facilities. Where detention, retention, or depressional storage areas are to be used as part of the drainage system for a property, they shall be constructed as the first element of the initial site grading. Any eroded sediment captured in these facilities shall be removed by the applicant before project completion in order to maintain the design volume of the facilities. Pre-sedimentation basins shall be included, where feasible, for localizing sediment deposition and removal. Access for heavy equipment shall be provided. Stormwater facilities shall be functional prior to the issuance of building permits.
(H) Fee in lieu of detention. In instances where regional benefits and economics of scale can be achieved, it will be permissible for adjacent properties to utilize a common regional detention basin. The city shall have the option to require paying a fee calculated based on the documented costs for each acre foot of detention required so that the City of Lockport can build regional facilities or the applicants can jointly build the necessary facilities themselves.
(I) Maintenance responsibilities. Maintenance of stormwater drainage facilities located on private property shall be the responsibility of the owner of the property, or a designated Home Owners Association (HOA). Before a building permit is obtained from the City of Lockport, the applicant shall execute a maintenance agreement with the City of Lockport guaranteeing that the applicant and all future owners of the property will maintain the portions of the stormwater management system located on private property. The maintenance agreement shall also specifically authorize representatives of the city to enter onto the property for the purposes of inspections and maintenance of the stormwater management system. Such agreement shall be recorded with the Will County Recorder of Deeds. The maintenance agreement shall include a schedule for regular maintenance of each aspect of the property's stormwater drainage system and shall provide for access to the system for inspection by authorized personnel of the City of Lockport. The maintenance agreement shall also stipulate that if the city notifies the property owner or HOA in writing of maintenance problems which require correction, the property owner or HOA shall make such corrections within 30 (thirty) calendar days of such notification. If the corrections are not made within this time period, the city may have the necessary work completed and assess the cost of such work to the property owner or HOA.
The City of Lockport has the option of requiring a bond to be filed by the property owner or HOA for maintenance of the stormwater management system.
(J) Construction supervision requirements.
(1) Registered professional engineers. The applicant shall provide an Illinois registered professional engineer, or person working under his direct control and authority, to provide construction stake out and construction observation periodically as required when work is being performed. The applicant's registered professional engineer shall provide weekly construction progress reports to the Director of Public Works and Engineering or authorized designee, if requested.
(2) Professional landscape architect or environmental scientist/ecologist. The applicant shall provide a professional landscape architect or environmental scientist/ecologist, or persons working under their direct control and authority, to provide construction stake out and construction observation periodically as required when landscaping work is being performed. The applicant's professional landscape architect or environmental scientist/ecologist shall provide detailed weekly reports to the Director of Public Works and Engineering or authorized designee, if requested.
(K) Conflicts. In the event that there is a conflict in the requirements of this section and other sections of the development code, the most restrictive interpretation shall prevail.
(Ord. 18-008, passed 2-7-18)