(A) General requirements.
(2) For all regulated activities, pre- and post-construction erosion and sediment control measures and stormwater management BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter. Various BMPs and their design standards are listed in the Illinois Urban Manual. See divisions (B), (C), (D), (E), and (F) of this section and Chapter 170 for further detail requirements.
(3) Unless prohibited by the city zoning ordinance or any ordinance which regulates construction and development within the areas of the city, stormwater management facilities located in the floodplain are permitted when designed and constructed in accordance with the floodplain management ordinance and the requirements of this chapter.
(4) Impervious areas.
(a) The measurement of an impervious area shall include all of the impervious areas in the total proposed development, even if development is to take place in stages or phases.
(b) For development taking place in stages or phases, the entire development plan must be used in determining conformance with this chapter.
(c) Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious and compacted to support vehicular traffic.
(d) For new permeable pavement, designers should use one-half of the measured infiltration rate during design to approximate long-term infiltration rates or similar design standards based on the specifications of pavement used in the design.
(5) A planting plan is required for all vegetated stormwater BMPs. Refer to division (E)(6) for further information.
(6) Prior to construction, a tree protection zone shall be delineated at the dripline of the tree canopy. All trees scheduled to remain during construction shall be marked; however, where groups of trees exist, only the trees on the outside edge need to be marked. A 48-inch high snow fence or 48-inch high construction fence mounted on steel posts located eight feet on center shall be placed along the tree protection boundary. No construction, storage of material, temporary parking, pollution of soil, or regrading shall occur within the tree protection zone.
(7) Drainage easements shall be provided where the conveyance, treatment, or storage of stormwater, either existing or proposed, is identified on the SWM permit. Drainage easements shall be provided to contain and convey the 100-year frequency flood.
(8) A plan for the ongoing maintenance of all stormwater management system components, including wetlands and buffer areas, is required prior to plan approval. See § 162.07 for specific requirements.
(B) Water quality and volume control.
(1) The standards of this division (B) shall apply to all development meeting the conditions for water quality and volume control, as stated in § 162.04(A).
(2) The development shall provide water quality treatment for runoff from impervious areas to minimize impacts of post-development stormwater runoff on water quality. The development shall provide volume control for runoff to meet the requirements of division (C) below. The SWM plan shall include a description of the water quality protection and volume control measures incorporated into the site design. Volume control practices may be incorporated into the water quality control practices.
(3) The first inch of runoff from the new impervious area and the first 0.5 inch of runoff from the redeveloped impervious area of development on the site shall be the water quality control storage.
(4) Volume control practices shall provide water quality treatment of the water quality control storage. Water quality and volume control practices shall be designed according to the following hierarchy. Appendix A outlines design specifications for the following control practices for water quality.
(a) Preservation of natural resource features of the development site (e.g., floodplains, wetlands, prairies, and woodlands);
(b) Preservation of the existing natural streams, channels, and drainageways;
(c) Minimizing impervious surfaces (e.g., narrowing road width, minimizing driveway length and width, clustering homes and shared driveways) created at the site, while maintaining compliancy with other community ordinances (i.e., fire vehicle access, etc.);
(d) The use of native, deep-rooted landscaping as an alternative to turf grass;
(e) The use of open vegetated channels, filter strips, and infiltration (basins, trenches, floodplain restoration, etc.) to convey, filter, and infiltrate stormwater runoff and minimize the usage of minor stormwater systems. Design requirements for non-structural BMPs are found in division (E) below;
(f) Preservation of the natural infiltration and storage characteristics of the site (e.g., disconnection of impervious cover, on-lot bio-retention facilities, rooftop detention, parking lot detention);
(g) Structural measures that provide water quality and volume control (stormwater wetlands, wet detention facilities, sedimentation traps, etc.);
(h) Structural measures that provide only quantity control and conveyance;
(i) Other methods as may be found in the Illinois Urban Manual.
(5) All volume reductions plus volume control practices from proposed BMPs shall equal or exceed the required control volume (one-inch x new impervious area).
(C) Site runoff controls. Site runoff control for large storms, up to the 100-year event, is essential to protect against immediate downstream impacts, flooding, and erosion.
(1) General design methods and standards.
(a) Calculation of required storage. The volume of required stormwater storage shall be calculated on the basis of the maximum value achieved from the runoff of a design event less the volume of water released through the outlet structure. The following standards shall apply to watersheds of various sizes:
1. Development watershed area less than or equal to ten acres. The Modified Rational Method shall be acceptable for development watersheds equal to or less than ten acres in area.
A. In determining the volume of storage required when using the Modified Rational Method, the release rate of the outlet structure shall be assumed to be constant and equal to the release rate through the outlet structure, when one-half of the storage volume is filled.
B. In determining the maximum allowable release rate for the post-development event a runoff coefficient (C value) of 0.25 shall be used for assumed land cover conditions.
C. Roughness coefficients most closely matching those of the TR-55 method shall be used to determine time of concentration. When applying Soil Conservation Service (SCS) methods an SCS Type II rainfall distribution shall be assumed.
2. Development watershed area less than or equal to 2,000 acres. The method utilized for calculation of required volume of storage shall be the “Soil Conversation Service TR-55" methodology for development watersheds less than or equal to 2,000 acres in area.
A. In determining the maximum allowable release rate for the post-development event a curve number shall be used corresponding to the actual soil types found on the development site provided, however, that the land cover row crops, “SR + CR” in “Good” hydrologic conditions are assumed.
B. A roughness coefficient of 0.17 and a ponding adjustment factor of 0.87 shall also be assumed in calculating the maximum allowable release rate.
3. Development watershed area greater than 2,000 acres. Developments and drainage designs for development watersheds larger than 2,000 acres shall use the Soil Conservation Service TR-20 methodology.
A. Other routing techniques may be used in determining required storage volume upon the approval of the Administrator.
B. When applying Soil Conservation Service (SCS) methods an SCS Type II rainfall distribution shall be assumed
(2) Design storm event.
(a) Precipitation values for all return period storms shall be determined utilizing the Illinois State Water Bulletin 70.
1. A 100-year return period storm with a 24-hour duration shall be used for detention design.
A. When using the Modified Rational Method the critical storm duration (that requiring the largest detention volume) for any design event shall be identified and used in determining storage volume.
(3) Release rates.
(a) Release rate shall be determined using one of the following methods:
1. Method 1.
A. Release rate for 100-year, 24-hour design event shall not exceed 0.20 cfs/acre
B. Release rate for the two-year, 24-hour design event shall not exceed 0.04 cfs/acre.
2. Method 2.
A. For the 100-year precipitation, the rate shall not exceed the rate of discharge from the development area for the five-year return frequency precipitation event with the conditions defined in division (C)(1)(a) above.
B. Effective discharge for frequent storm events. The outlet structure maximum discharge for each of the one-year, two-year, and five-year precipitation events shall be no greater than the rate of discharge from the development area assuming row crop agricultural land cover with the required assumptions described in division (A)(1) above.
(b) Emergency overflow. Each stormwater storage facility shall be provided with a means of overflow during any event greater than the 100-year design event.
(c) Flood elevations. The entire stormwater storage facility shall be designed and constructed to fully protect the health, safety, and welfare of the public. The minimum building site elevation adjacent to wet or dry basins shall be set at a minimum of one foot above the maximum created head. The maximum created head will include the energy head at the emergency overflow structure.
(d) Off-site tributary areas.
1. Stormwater storage facilities shall not receive runoff from tributary areas outside the development site unless the administrator determines that runoff from such areas can be accommodated in the storage area in a manner that will protect immediate downstream properties and unless required by other state or federal regulations.
2. When stormwater runoff from tributary areas outside of the development cannot reasonably be directed around the stormwater storage area, the Administrator may allow use of staged release outlet structures, which allow stormwater runoff from off-site areas to pass through the stormwater storage area undetained, while simultaneously detaining and providing controlled release for the volume of excess stormwater runoff from the site.
(e) Compensatory storage. Where portions of the owner’s land are tributary to the same drain for an outlet, but which are within two or more tributary areas to that drain, the owner may construct, upon site-specific approval by the Administrator, compensatory stormwater detention facilities within one tributary area which offset the lack of construction of stormwater detention facilities in another tributary area. Such compensatory storage shall be designed and constructed so that runoff is released into the drain to that rate which would have occurred had stormwater detention facilities been constructed for all the tributary areas. Any site developed using the provisions of this division (C)(3) must also conform to Illinois Department of Transportation (IDOT) regulations where applicable.
(f) Storage duration. The storage of excess stormwater runoff from a 100-year return period storm having a duration of 48 hours, released at the allowable rate, shall not result in a storage duration in excess of 48 hours. Storage duration may be longer due to unique site conditions upon site-specific approval of the Administrator.
(D) Detention facilities. All regulated development shall provide a detention facility unless the overall site is less than one acre.
(1) Design requirements.
(a) Side slopes.
1. Maximum slope of 4:1.
2. Side slopes between 3:1 and 4:1 may be allowed with approval by the Administrator.
(b) Outlet control structure.
1. Debris catcher required for pipes 12 inches or greater.
2. Minimum design and construction standards:
A. Catcher shall minimize soil erosion.
B. Catcher shall not require manual adjustments for its proper operation.
C. Catcher shall operate properly with minimal maintenance or attention.
D. Catcher shall be provided with safety screens for any pipe or opening, other than a weir.
E. Catcher shall be accessible at all times, including times of flood flow.
(c) Emergency overflow.
1. Each stormwater storage facility shall be provided with a means of overflow.
2. This overflow structure shall be constructed to function without special maintenance attention and can become a part of the excess stormwater passageway for the entire development.
(d) Pipe outlets.
1. Minimum size of 6-inch diameter.
2. Multiple outlet pipes of 12 inches or less are to be avoided.
(e) Adequate impact stilling basins shall be provided at the downstream side of any outlet structure to ensure that downstream soil erosion is mitigated as much as practicable.
(f) Warning signs shall be placed at appropriate locations to warn of deep water, possible flood conditions during storm periods, and of dangers that exist to pedestrian and vehicular traffic.
(2) Dry bottom stormwater storage areas.
(a) Where possible, dry detention ponds shall serve a secondary purpose for recreation, open space, or similar types of uses which will not be adversely affected by occasional intermittent flooding and will not interfere with stormwater management.
(b) The maximum planned depth stored stormwater shall be six feet.
(c) Minimum basin bottom slopes.
1. Two percent for turf grass.
2. Between 0.5% and 2% if tile underdrains are provided.
3. Alternative designs using green infrastructure will be considered by the Administrator.
(d) The required freeboard for detention facilities shall be one foot or one-half the depth of the required volume storage, whichever is less.
(e) Paved low flow conduits or channels shall be provided when bottom grade is less than 0.5%.
1. Channels shall be constructed so that they:
A. Will not unnecessarily interfere with any secondary use of the storage area.
B. Reduce the frequency of time that storage area will be covered with water and facilitate dewatering of the soils in the stormwater storage area to avoid saturated soil conditions.
C. Low flow conduits shall facilitate complete interior drainage of the stormwater storage area.
2. Tile underdrain systems may be required in combination with the low flow conduits or channel systems.
3. Alternative designs using green infrastructure will be considered by the Administrator.
(f) Tile underdrains shall be constructed such that they:
1. Will not interfere with any secondary usage of the storage area.
2. Facilitate dewatering of the soils in the stormwater storage area to avoid marshy or saturated
soil conditions.
(g) Permanent erosion control measures shall be utilized to control soil movement and erosion within the storage area and excess stormwater passageway. These measures shall meet or exceed the standards established in the most current edition of the Illinois Urban Manual. The installation of these permanent measures shall take place after silt and sediment producing activities have been substantially complete and any sediment from construction activities has been removed from the stormwater facility.
(h) Temporary erosion and sediment control measures shall meet the requirements of Chapter 170 of the Code of Ordinances.
(3) Wet bottom stormwater storage areas.
(a) Minimum normal water depth (excluding safety ledges and side slopes).
1. Six feet.
2. At least one-quarter of the pond area shall be a minimum of ten feet deep if fish are to be maintained in the pond.
(b) Side slope.
1. Safety ledge: from the normal water elevation to at a minimum five feet below the normal water elevation the side slope shall not exceed 6:1.
2. Below a normal depth of five-foot, side slopes shall not exceed the stable angle of repose under saturated conditions of the soil material of the basin.
(c) Measures shall be included in the design to minimize pond stagnation and to help ensure adequate aerobic pond conditions.
1. Facilities shall be provided to lower the pond elevation for cleaning purposes and shoreline maintenance.
(d) Permanent erosion control measures shall be utilized to protect the shoreline and control soil movement and erosion within the storage area and excess stormwater passageway. These measures shall meet or exceed the standards established in the most current edition of the Illinois Urban Manual. The installation of these permanent measures shall take placed after silt and sediment producing activities have been substantially complete and any sediment from construction activities has been removed from the stormwater facility.
(e) Erosion and sediment control shall meet the requirements of Chapter 170.
(4) Alternative stormwater storage areas. With approval from the Administrator, the following storage facilities may be used.
(a) Paved stormwater storage.
1. Design and construction of the pavement base must ensure that there is minimal pavement damage due to flooding.
2. Control structures in paved areas must be readily accessible for maintenance and cleaning.
3. Flow control devices will be required unless otherwise approved by the Administrator.
(b) Rooftop stormwater storage.
1. Rooftop storage of excess stormwater shall be designed and constructed to provide permanent control inlets and parapet walls to contain excess stormwater.
2. Adequate structural roof design must be provided to ensure that roof deflection does not occur which could cause the roofing material to fail and result in leakage.
3. Overflow areas must be provided to ensure that the weight of stormwater will never exceed the structural capacity of the roof.
4. Any rooftop storage of excess stormwater shall be approved only upon submission of building plans signed and sealed by a licensed structural engineer or architect attesting to the structural adequacy of the design.
(c) Automobile parking lot storage areas.
1. Depth of stored stormwater shall be 0.6 feet.
2. Storage areas shall be located in the most remote, least used areas of the parking facility.
3. Design and construction of automobile parking in stormwater areas must ensure that there is minimal damage to the parking facility due to flooding, including minimal damage to the subbase.
4. Warning signs shall be mounted at appropriate locations to warn of possible flood conditions during storm periods.
(d) Underground stormwater storage.
1. Shall be designed for easy access in order to remove accumulated sediment and debris.
2. Shall be provided with a positive gravity outlet unless otherwise approved by the Administrator.
(5) Joint construction. Stormwater storage areas may be planned and constructed jointly by two or more landowners provided the provisions of this chapter are met.
(6) Early completion of detention facilities.
(a) 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 earthwork program. This shall not prohibit an applicant from proceeding with footings and foundations also, once the necessary stormwater control facilities are functional. 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.
(b) As-built drawings must be prepared and submitted by a registered professional engineer stating conformance with the design plans before final approval of the constructed improvements by the Administrator.
(E) Non-structural BMP design requirements.
(1) Infiltration BMPs shall be spread out, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.
(2) Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and development site conditions and shall be constructed on soils that have the following characteristics:
(a) A minimum depth of 24 inches between the bottom of the facility and the infiltration horizon, unless it is demonstrated to the satisfaction of the Administrator that the selected BMP has design criteria which allow for a smaller separation.
(b) A stabilized infiltration rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the applicant’s professional designer. The stabilized infiltration rate is to be determined in the same location and within the same soil horizon as the bottom of the infiltration facility.
(3) Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity. Staging of earthmoving activities and selection of construction equipment should consider this protection.
(4) Infiltration BMPs shall not be constructed nor receive runoff from disturbed areas until the entire contributory drainage area to the infiltration BMP has achieved final stabilization.
(5) Roof drains and sump pumps shall be tributary to infiltration or vegetative BMPs. Use of catchment facilities for the purpose of reuse is also permitted.
(6) Infiltration practices should not be implemented in any of the following circumstances:
(a) Areas/sites where vehicle fueling and/or maintenance occurs;
(b) Areas/sites with shallow bedrock which allow movement of pollutants into the groundwater;
(c) Areas/sites where contaminants in soil or groundwater could be mobilized by infiltration of storm water;
(d) Areas/sites within a delineated source water protection area for a public drinking water supply where the potential for an introduction of pollutants into the groundwater exists. Information on groundwater protection may be found at http://www.epa.state.il.us/water/groundwater/index.html.
(e) Areas/sites within 400 feet of a community water supply well if there is not a wellhead protection delineation area, or within 200 feet of a private water supply well. Information on wellhead protection may be found at http://www.epa.state.il.us/water/groundwater/index.html.
(7) A planting plan is required for all vegetated stormwater BMPs.
(a) Native or naturalized/non-invasive vegetation suitable to the soil and hydrologic conditions of the development site shall be used.
(b) Invasive vegetation may not be included in any planting schedule.
(c) Prior to construction, a tree protection zone shall be delineated at the dripline of the tree canopy. All trees scheduled to remain during construction shall be marked; however, where groups of trees exist, only the trees on the outside edge need to be marked. A 48-inch high snow fence or 48-inch high construction fence mounted on steel posts located eight feet on center shall be placed along the tree protection boundary. No construction, storage of material, temporary parking, pollution of soil, or regrading shall occur within the tree protection zone.
(d) All planting shall be performed in conformance with good nursery and landscape practice. Plant materials shall conform to the standards recommended by the American Association of Nurseryman, Inc. in the American Standard of Nursery Stock.
(F) Stormwater conveyance system design requirements.
(1) Private storm sewers and swales.
(a) The five-year critical duration storm shall be used as a minimum for the design of storm sewers, storm inlets, and minor swales from site runoff controls. Storm sewer design shall be sized on the assumption that they will flow full or practically full under the design discharge but will not be placed under the pressure head. Hydraulic grade line calculations shall be performed that demonstrate that sewer rims are not inundated at the design storm.
(b) Sites shall be designed to allow for flow from storms greater than five-year critical duration storm to minimally pond water entering any stormwater management facilities.
(c) Storm sewers and swales shall not connect to sanitary sewers.
(d) Storm sewers and swales may connect to existing drain tiles or storm sewers only if the applicant submits a maintenance agreement, recorded easements, and a report that indicates the existing system from the connection to the discharge point in an open channel has adequate hydraulic capacity and structural integrity. The recorded easement and maintenance agreement must extend from the connection to the discharge point in an open channel. The recorded easement and maintenance agreement must be approved by the (Administrator) prior to issuance of a stormwater management permit.
(e) Field tile systems disturbed during development must be reconnected by those responsible for their disturbance unless the approved drainage plan includes provisions for the system. All abandoned field tiles on the site shall be removed in their entirety.
(f) All storm sewers and minor swales shall be located in a public road right-of-way, a maintenance easement, or a covenant running with the land of sufficient size to maintain and reconstruct the conveyance system.
(g) Design practices intended to minimize erosion shall be provided at the inlets and outlets for all pipes, transitions, and channels.
(h) The minimum storm sewer size shall be eight inches for the first pipe reach (except when using pipe as a releasing control device for upstream pipe detention) and greater than or equal to the preceding reach for all subsequent reaches unless approved by the (Administrator).
(i) The minimum design velocity for a storm sewer shall be 2.5 feet per second. The maximum design velocity for a storm sewer shall be 8.0 feet per second.
(2) Overland flow paths.
(a) All areas of development must provide an overland flow path that will pass the 100-year flood flow (including offsite tributary flow) without damage to structures or property. If the drainage area is less than 20 acres, the storm sewer pipe and inlet may be sized for the 100-year flow instead of providing an overland flow path.
(b) The overland flow path shall be protected from any development, such as fencing, landscaping, storage sheds, or other obstructions which could impair its function by impeding flow. This protection shall be established through a properly recorded covenant running with the land, restricting the use of the overland flow path area.
(c) The overland flow path shall be placed over natural grade, not overtop a pipe.
(d) Structures adjacent to an overland flow path shall have the following lowest opening elevation for the following tributary areas:
1. One-half foot above the BFE for tributary areas of 20 acres or less.
2. One foot above the BFE for tributary areas of 20 acres or greater.
(3) Streams and channels.
(a) If the proposed activity involves a channel modification, it shall be demonstrated that the proposed modifications meet all design requirements of this chapter with respect to stormwater runoff flow, volume, and quantity.
(b) All proposed modifications shall meet the requirements of all state and federal design requirements.
(c) A copy of all required state and federal permits shall be furnished to the City of Danville prior to any construction occurring within the channel.
(Ord. 9097, passed 5-16-17) Penalty, see § 162.99