Loading...
All developments shall obtain a stormwater management permit, requiring compliance with §§ 151.206 and 151.207 of this subchapter, after the effective date hereof, if one of the following scenarios apply:
(A) Two or more single-family, detached dwellings are to be constructed on a site having five or more acres;
(B) A two-family attached, multi-family attached dwelling, condominium, apartment or other residential building comprised of more than two units is to be constructed on a site one acre or more in size;
(C) Any nonresidential development on a site one acre or more in size, unless the development consists solely of the installation, repair or replacement of the underground or overhead lines of a public utility within a public right-of-way.
(D) Redevelopment on a site one acre or more in size, resulting in 25,000 square feet or more of additional impervious area in aggregate, from the effective date of this subchapter; or
(E) Road development after the effective date hereof in rights-of-way under the ownership or control of a unit of local government, resulting in two acres or more of additional impervious area; or
(F) The repair or replacement of an existing parking lot or privately-owned road within the floodplain results in any additional impervious surface, an increase in peak flows, a change in the location of the stormwater discharge, or an increase in the elevation of the parking or driving surface.
(Ord. 2019-03, passed 4-15-2019)
(A) Stormwater facilities shall be required and designed so that runoff exits the site at the point whereit exited prior to development (unless a change is required and approved in writing by the village), and in a manner so as not to increase flood damage downstream. Concentrated discharges from new developments must enter conveyance systems capable of carrying the design flow rate without increasing flood damage, erosion or maintenance costs downstream.
(B) Minor stormwater systems shall be sized to convey runoff from the tributary watershed under fully developed conditions consistent with the design requirement of the permitting authority.
(C) Major stormwater systems shall be sized to carry the base flood without causing additional flood damage.
(D) Stormwater systems shall properly incorporate and be compatible with existing subsurface and surface discharge systems, including agricultural systems. Designs shall not cause new or increased damage to existing drainage systems, or to existing adjacent or tributary agricultural land uses.
(1) Offsite outfall. Agricultural subsurface and surface drainage systems shall be evaluated with regard to their capacity and capability to properly convey low-flow ground water and site runoff storage facility release without new or increased damage to downstream structures and land uses. If the outfall drain tile and surface drainage systems prove to be inadequate to handle the design release rate, one of the following solutions shall be used:
(a) The site stormwater system and offsite outfall shall be designed so that it does not exceed the existing capacity of the downstream drainage system; or
(b) Modify the existing systems or construct new systems that will not conflict with the existing systems and will not impact existing land uses; or
(c) Alternate solutions may be submitted to the Administrator for approval.
(2) Onsite. Agricultural drainage systems shall be located and evaluated. All existing onsite agricultural drain tiles not serving a beneficial use shall be abandoned by trench removal prior to other development, and recorded on engineering details for the project. If any existing drain tiles continue to drain upland watersheds, the developer must maintain drainage service during construction until new storm sewers can be installed for a permanent connection.
(3) Offsite tributary. Existing drainage systems shall be evaluated with regard to existing capabilities and reasonable future expansion capacities. All existing tributary drain tiles shall be incorporated into the new stormwater system that includes observation structures located at the limits of the site and shall provide a free flow discharge. Agricultural tributary surface conveyance shall be accepted by the new development, with consideration given to the water quality and the ability to filter out sediment.
(4) Preservation of existing systems. New roadway construction shall preserve existing subsurface systems within the right-of-way. Inspection wells shall be placed in the right-of-way. Tiles found not to be flowing between inspection wells at the end of construction shall be replaced.
(E) Design runoff rates shall be calculated using event hydrograph methods. Design runoff rates for minor stormwater systems may be calculated using the rational method if the design watershed is less than 20 acres.
(F) Any design runoff rate calculation method shall use Bulletin 70, “Northeast Sectional Rainfall Statistics”, and shall calculate flow from all tributary areas upstream of the point of design. Peak discharges for conveyance design purposes shall be based on the critical duration considering the appropriate rainfall distribution.
(G) Major and minor stormwater systems shall be located within easements or rights-of-way, explicitly providing for government access or maintenance of such facilities by the grantee.
(H) Maximum flow depths for new transverse stream crossing shall not exceed one foot at the crown of the road during the base flood condition. The maximum longitudinal flow depth on a roadway shall not exceed six inches at the crown for flow depth (in feet) and velocity (in feet per second) shall not exceed four feet per second for the base flood condition.
(I) Transfers of waters between watersheds (diversions) shall be prohibited, except when such transfers will not violate this subchapter and are otherwise lawful.
(J) Developments shall incorporate best management practices in accordance with the Clean Water Act (33 U.S.C. §§ 1251 et seq., as amended), and as required by the IEPA’s National Pollutant Discharge Elimination System (NPDES) permit program.
(Ord. 2019-03, passed 4-15-2019)
(A) The area of hydrological disturbance on the site shall be used to calculate the required site runoff storage volume. The tributary area of the site at the point of discharge shall be used to calculate the allowable release rates of the primary restrictor for the site runoff storage facility.
(B) The site runoff storage volume required shall be calculated based on maximum release rates of 0.15 cubic feet per second per acre for the 100-year, 24-hour storm event, and 0.04 cubic feet per second per acre for the two-year, 24-hour storm event, according to the methods identified in division (C) below.
(C) Event hydrograph routing methods, such as HEC-1, HEC-HMS, and TR-20 or TR-55 tabular method, using SCS curve number methodology, shall be used to calculate design runoff volumes for site runoff storage facilities. Event methods shall incorporate the following assumptions:
(1) Antecedent moisture condition equals two;
(2) Appropriate Huff rainfall distribution, except that SCS type II distribution is acceptable with TR-55 tabular method only; and
(3) Twenty-four-hour duration storm, with a 1% probability (100-year) of occurrence in any one year as specified by Bulletin 70, “Northeast Sectional Rainfall Statistics”.
(D) For sites where the undeveloped release rate is less than the maximum release rate of this section, the developed release rate and corresponding site runoff storage volume shall be based on the existing undeveloped release rate for the development site.
(E) Hydraulic computations for the release structure must assume backwater conditions up to the ten-year flood elevation on the adjacent receiving watercourse.
(F) Retention requirement.
(1) The runoff from a three-quarter-inch rainfall event over the hydraulically connected impervious area of a new industrial, commercial or multi-family development shall be stored below the elevation of the primary gravity outlet (retention) of the site runoff storage facility. The facility may be designed to allow for evapotranspiration or infiltration of this volume into a subsurface drainage system, and shall not be conveyed through a direct positive connection to downstream areas.
(2) The hydraulically connected impervious area used in the calculation of required retention volume may be reduced by the village if the soils are undisturbed in situ or prepared to maximize infiltration and deep-rooted grasses or other plants that assist in the promotion of infiltration and transpiration are planted in areas appropriately dedicated. The reduction in hydraulically connected impervious area used in the calculation shall be equal to the area of the development meeting the above soils/planting requirement.
(3) Subsurface drainage systems may be designed as a component of the retention portion of the site runoff storage basin to assist in infiltration in accordance with the following criteria:
(a) No such subsurface drainage pipe shall be located within ten feet of drainage pipes directly connected to the site runoff storage basin; and
(b) For purposes of meeting the maximum subsurface drainage requirements, flow control orifices and weirs may be incorporated into the design.
(G) Site runoff storage facilities shall be designed and constructed with the following characteristics.
(1) At least one foot of freeboard above the 100-year design water surface elevation shall be required within the storage facility.
(2) Storage facilities shall be accessible and easily maintained.
(3) All design site runoff storage volume shall be provided above the seasonal high ground water table or the invert elevation of the ground water control system.
(4) Storage facilities shall be designed to control sediment and floating debris. Unless specifically approved by the Administrator, concrete-lined, low-flow ditches shall not be used in site runoff storage basins.
(5) Storage facilities shall minimize the negative impacts of stormwater runoff into the waterquality by incorporating best management practices in the design of the facilities.
(6) Storage facilities shall maximize the distance between site runoff storage inlets and outlets to the extent possible.
(7) Storage facilities shall be designed so as not to exceed the existing pre-development peak runoff rate of the 100-year, critical-duration rainfall, when the primary restrictor is assumed to be blocked.
(8) Storage facilities with single-pipe outlets shall have a minimum inside diameter of 12 inches. If design release rates necessitate a smaller outlet to be constructed, perforated risers or flow control orifices shall be used to prevent smaller openings from clogging.
(H) Storage facilities located within the regulatory floodplain shall:
(1) Comply with this subchapter; and
(2) Store the required amount of site runoff to meet the release rate requirement under all stream flow and backwater conditions, up to the ten-year flood elevation on the adjacent receiving watercourse. The village may approve designs that can be substantiated through hydrologic and hydraulic engineering analysis to provide a new watershed benefit not otherwise realized by strict application of the requirements set forth in this division.
(I) Storage facilities located within the regulatory floodway will be allowed only as a variance and shall:
(1) Meet the requirements for locating storage facilities in the regulatory floodplain; and
(2) Be evaluated by performing hydrologic and hydraulic analysis consistent with the standards and requirements for watershed plans; and
(3) Provide a net watershed benefit.
(J) Site run-off facilities may be located offsite if:
(1) The offsite storage facility meets all of the requirements of this article; and
(2) Adequate storage capacity in the offsite facility is dedicated to the development; and
(3) The development includes a means to convey stormwater within an easement to the offsite storage facility.
(K) Site runoff storage volume provided by enlarging existing regulatory floodplain storage (on stream runoff storage) will be allowed only as a variance. The applicant must demonstrate that flood damage will not be increased, and the development will not increase flood flows for both the two-year and 100-year floods.
(L) Structures built across the channel to impound water to meet site runoff storage requirements shall be prohibited on any perennial stream, unless part of a public flood control project with a net watershed benefit. Those streams appearing as blue on a USGS quadrangle map shall be assumed to be perennial, unless better data is provided by the developer. In all cases, it must be demonstrated that all such structures will not cause short-term or long-term instability, and that they are permitted under a joint application to the IDNR, COE and IEPA. Where such facilities are approved, the applicant must also comply with division (F) of this section.
(Ord. 2019-03, passed 4-15-2019)
(A) The BFE shall be delineated on the site topography to establish the regulatory floodplain limits for regulation under this subchapter, using the Flood Insurance Rate Maps (FIRMs) and Flood Insurance Study (FIS) of Grundy County, dated August 2, 2012, by the Federal Emergency Management Agency, in accordance with this subchapter.
(B) The BFE shall be:
(1) In the case of AE Zones on the FIRM, the 1% annual chance flood profile elevation in the FIS; or
(2) In the case of AH Zones on the FIRM, that elevation delineated on the FIRM; or
(3) In the case of a AO Zones on the FIRM, that depth delineated on the FIRM added to the highest adjacent grade or at least two feet above the highest adjacent grade if no depth number is provided; or
(4) For each of the remaining floodplains delineated as A Zones on the FIRM, according to the best data available from federal, state or other sources.
(a) Should no other data exist, the BFE should be determined by an engineering study financed by the applicant, and conducted by a professional engineer using appropriate hydrologic and hydraulic models as follows:
1. Hydrologic models: TR-20, HEC-1, HEC-HMS;
2. Hydraulic models: HEC-2, HEC-RAS, WSP-2; or
3. A technique approved by the Administrator and IDNR-OWR.
(b) Where a channel has a tributary drainage area of 640 acres or more, the above analysis shall be submitted to IDNR-OWR for approval.
(C) Floodway delineation and elevations.
(1) The location of the regulatory floodway shall be as delineated on the FIRM. Where an interpretation is needed to determine the exact location of the regulatory floodway boundary, IDNR-OWR shall be contacted.
(2) General criteria for analysis of flood elevations in the regulatory floodway are as follows:
(a) The flood profiles, flows and data from the current applicable regulatory map must be used for analysis of the base conditions. If the study data appears to be in error or conditions have changed, FEMA and IDNR-OWR shall be contacted for approval and concurrence on the appropriate base conditions data to use. The same Manning’s “n” value shall be used for both existing and proposed conditions, unless a recorded maintenance agreement obligates a public entity to maintain the proposed conditions, or the land cover is changing from vegetative to non-vegetative. The Administrator shall be copied on all related correspondence.
(b) If the BFE at the site is affected by backwater from a downstream receiving stream with a larger drainage area, the proposed development shall be shown to meet the requirements of this section with the receiving stream at both the normal water elevation and the BFE.
(c) If the applicant is informed by IDNR-OWR, a local government or private owner that a downstream or upstream restrictive bridge or culvert is scheduled to be removed, reconstructed or modified, or a regional flood control project is scheduled to be built, removed, constructed or modified within the next five years, the proposed development shall be analyzed and shown to meet the requirements of this section for both the existing conditions and the expected flood profile conditions when the bridge, culvert or flood control project is built, removed or modified.
(d) If the use will result in a change in the location of the regulatory floodway or a change in the BFE, then no development shall take place until a CLOMR has been issued by FEMA. Filling, grading, dredging or excavating may take place upon issuance of a CLOMR by FEMA. No further development activities shall take place or building permit issued in existing or proposed floodplain until a LOMR is issued by FEMA. The Administrator shall be copied on all related correspondence.
(e) In the circumstances listed below and located in a regulatory floodway, at a minimum, the information set forth below shall be submitted to IDNR-OWR for its review and approval.
1. Analysis of the flood profile due to a proposed bridge, culvert crossing or roadway approach.
2. An engineer’s determination that an existing bridge, culvert crossing or roadway approach is not a source of flood damage, and the analysis indicating the proposed flood profile.
3. Alternative transition sections and hydraulically equivalent compensatory storage.
4. Stormwater management permits issued to local units of government for regulatory floodway and floodplain development.
5. IDNR-OWR will issue permits for any IDNR-OWR, state, federal or community projects for activities or developments that are located within the floodway.
(Ord. 2019-03, passed 4-15-2019)
The following general performance standards are applicable to all development in a regulatory floodplain. The standards of this section apply except when superseded by more stringent requirements in the following sections.
(A) All development in the regulatory floodplain shall comply with the compensatory storage volume standards of this subchapter.
(B) For all projects involving a channel modification, fill, stream maintenance or a levee, the flood conveyance and storage capacity of the regulatory floodplain shall not be reduced.
(C) If the proposed development would result in a change in the regulatory floodplain or BFE, the applicant shall obtain a CLOMR from FEMA prior to development, followed by a LOMR from FEMA following development. No buildings may be built in the existing or proposed regulatory floodplain unless the building meets all the building protection standards identified within this subchapter. Proposed changes to the regulatory floodway delineation and the BFE must also be submitted to the IDNR-OWR for its approval.
(D) Prior to the commencement of any construction, modification or removal of a dam, the developer shall obtain an IDNR-OWR dam safety permit or a letter from the IDNR-OWR stating that a permit is not required.
(E) For public flood control projects, the requirements will be deemed met if the applicant demonstrates to IDNR-OWR and the Stormwater Committee:
(1) By hydraulic and hydrologic modeling, that the proposed project will not singularly or cumulatively result in increased flood heights outside the project site, or that any increases will be contained in easements for all flood events, up to and including the base flood event;
(2) That the project will be operated and maintained by a public entity;
(3) That the project will reduce flood damage to an existing building or structure.
(F) Nothing in this section precludes the design, engineering, construction or financing, in whole or in part, of a public flood control project by persons who are not public entities.
(Ord. 2019-03, passed 4-15-2019)
For proposed structures located outside the regulatory floodplain, the lowest opening shall be above the FPE (unless allowed in accordance with the regulations stated in this subchapter), including doors, access ways and window wells, as depicted on the building plans reviewed and enforced by the Building and Zoning Official.
(Ord. 2019-03, passed 4-15-2019)
Loading...