(A) Regulatory floodplain. The floodplains are those lands within the jurisdiction of the county that are subject to inundation by the base flood or 100-year frequency flood. The floodplains of the county are generally identified on the countywide flood insurance rate map of the county prepared by FEMA. The BFE shall be delineated onto the site topography to establish the regulatory floodplain area limits for regulation under this chapter. Regulatory floodplains shall be delineated onto the site map from the current FEMA FIRM, FBFM or LOMR and include those areas of the SFHA which are not regulatory floodplains.
(B) Base flood elevation (BFE). The BFE shall be:
(1) The elevation of the 100-year profile corresponding to the location of the development as indicated in the flood profiles in the FEMA flood insurance studies listed in Appendix B (which may be updated from time to time);
(2) In the case of FEMA delineated “AH Zones” the elevation noted on the map shall be the BFE. In the case of FEMA delineated “AO Zones” the BFE shall be the elevation or depth added to the highest adjacent grade (or at least two feet about the highest adjacent grade if no depth number is provided) shown on the countywide flood insurance rate map of the county;
(3) If a base flood elevation or 100-year frequency flood elevation is not available for a particular site, then the protection standard shall be according to the best existing data available from federal, state or other sources. When a party disagrees with the best available data, they shall submit a detailed engineering study needed to replace existing data with better data and submit it to IDNR/OWR and FEMA for review and consideration prior to any development of the site.
(a) When no BFE information exists, the BFE for a riverine SFHA shall be determined using a site-specific floodplain study by a professional engineer using appropriate hydrologic and hydraulic models as follows:
1. The base flood elevation for a riverine SFHA shall be determined from a backwater model, such as HEC-II, HEC-RAS, or a dynamic model such as HIP;
2. The flood flows used in the hydraulic models shall be obtained from a hydrologic model, such as HEC-HMS, HEC-1, TR-20 or HIP or by techniques presented in various publications prepared by the United States Geological Survey for estimating peak flood discharges; or
3. A technique approved by the Chief Subdivision Engineer and the IDNR/OWR.
(b) Where a channel has a tributary drainage area of 640 acres or more, the above analyses shall be submitted to the IDNR/OWR for concurrent approval.
(c) For a non-riverine regulatory floodplain, the historic flood of record plus three feet may be used for the BFE instead of performing a detailed hydrologic and hydraulic study.
(4) For floodplains that are not regulatory, are not draining more than 640 acres, and with no BFE determined, the Chief Subdivision Engineer may require a site-specific floodplain study for the purpose of establishing an FPE for the development.
(C) Regulatory floodplain locations.
(1) The location of the regulatory floodway shall be as delineated on the current effective regulatory maps maintain by the county. The floodways are designated on the countywide flood insurance rate map of the county, prepared by FEMA. When two floodway maps exist for a waterway, the more restrictive floodway limit shall prevail. The location of the regulatory floodway boundary shall be scaled onto the site plan using references common to both the map and the plan (typically the township section lines). Where an interpretation is needed to determine the exact location of the regulatory floodway boundary, IDNR/OWR should be contacted.
(2) Note: If an area of the site is located in the regulatory floodway that is higher than the BFE, that area is subject to the floodway standards of § 164.064, including the appropriate use criteria, until such time as a LOMR is received from FEMA with concurrence by IDNR/OWR.
(3) General criteria for analysis of flood elevations in the regulatory floodway are as follows.
1. The flood profiles, flows and data in the current effective FIS 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 County Executive and the Chief Subdivision Engineer shall be copied on all related correspondence.
2. If the BFE at the site of the proposed development 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 and BFEs.
3. If the applicant is informed by IDNR/OWR, local governments or a private owner that a downstream or upstream restrictive bridge or culvert is scheduled to be removed, reconstructed, 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 as applicable.
4. If the appropriate use will result in a change in the regulatory floodway location or a change in the BFE, the applicant shall submit the information required to be issued a conditional letter of map revision (CLOMR) to IDNR/OWR and FEMA. A public notice inviting public comment on the proposed change in the BFE or location of the regulatory floodway will be issued by IDNR/OWR or its designee before a CLOMR is issued. Filling, grading, dredging or excavating may take place upon issuance of a conditional approval from IDNR/OWR and the Administrator. No further development activities shall take place in the existing or proposed floodplain until a letter of map revision (LOMR) is issued by FEMA unless such activities meet all the requirements of § 164.062. The Director shall be copied on all related correspondence.
5. For those circumstances listed below and located in a regulatory floodway, at a minimum, the following information shall be submitted to IDNR/OWR for their review and concurrence:
a. Analysis of the flood profile due to a proposed bridge, culvert crossings and roadway approaches;
b. An engineer’s determination that an existing bridge, culvert crossing or approach road is not a source of flood damage and the analysis indicating the proposed flood profile;
c. Alternative transition sections and hydraulically equivalent compensatory storage; and
d. Stormwater management permits to local units of government for regulatory floodway and floodplain development.
6. IDNR/OWR will issue permits for any IDNR/OWR projects, dams and the like all other state, federal or local projects.
(Ord. 10-164, passed 6-17-2010)