(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)