(A) Generally. This section establishes performance standards for development within the regulatory floodway. The only development in a regulatory floodway which will be allowed are appropriate uses which will not cause an increase in flood heights for all flood events up to and including the base flood. Only those appropriate uses listed below will be allowed in the regulatory floodway. Appropriate uses do not include the construction or placement of any new structures, fill, building additions, buildings on stilts, piles, piers, or columns, fencing (including landscaping or planting designed to act as a fence), and storage of materials except as specifically defined above as an appropriate use. If the development is proposed for the regulatory floodway portion of the regulatory floodplain, the standards of this section apply in addition to the performance standards for regulatory floodplain development.
(B) Appropriate uses. Only the construction, modification, repair, or replacement of the following appropriate uses will be allowed in the regulatory floodway:
(1) Public flood control projects and private improvements relating to the control of drainage, flooding of existing buildings, erosion, water quality, or habitat for fish and wildlife;
(2) Structures or facilities relating to functionally water dependent uses such as facilities and improvements relating to recreational boating, and as modifications or additions to existing wastewater treatment facilities;
(3) Storm and sanitary sewer outfalls;
(4) Underground and overhead utilities if sufficiently flood-proofed;
(5) Recreational facilities such as playing fields and trail systems including any related fencing (at least 50% open when viewed from any one direction) built parallel to the direction of flood flows, and including open air pavilions;
(6) Bridges, culverts, and associated roadways, sidewalks, and railways necessary for crossing over the regulatory floodway or for providing access to other appropriate uses in the regulatory floodway and any modification thereto;
(7) Parking lots and any modifications thereto, where the existing depth of flooding for the base flood elevation is less than six inches and aircraft parking aprons built at or below ground elevation (the depth of flooding can be greater than six inches for parking lots used for short term outdoor recreational use facilities where the applicant agrees to restrict parking during overbank flooding events and agrees to accept liability for all damage caused by vehicular access during all overbank flooding events);
(8) Regulatory floodway re-grading, without fill, to create a positive non-erosive slope toward a channel;
(9) Flood-proofing activities to protect previously existing lawful structures including the construction of watertight window wells, elevating structures, or the construction of flood walls around residential, commercial, or industrial principal structures where the outside toe of the floodwall shall be no more than ten feet away from the exterior wall of the existing structure, and which are not considered to be substantial improvements to the structure;
(10) The replacement, reconstruction, or repair of a damaged building, provided that the outside dimensions of the building are not increased, and that the activity is not a substantial improvement. An activity that is a substantial improvement shall conform to § 151.149(H)(2)(b) for residential structures or § 151.149(H)(3)(b) for non-residential structures;
(11) Modifications to an existing building that would not increase the enclosed floor area of the building below the base flood elevation, and which will not block flood flows including but not limited to fireplaces, bay windows, decks, patios and second story additions. No enclosed floor areas may be built on stilts; and
(C) Mitigation. Construction of allowed uses will be considered permissible, provided that the proposed project meets the following engineering and mitigation criteria and the standards of § 151.149 are so stated in writing with supporting plans, calculations, and data prepared by a licensed professional engineer.
(1) All effective regulatory floodway conveyance lost due to the development of appropriate uses, other than bridge or culvert crossings or on-stream structures or dams, shall be replaced for all flood events up to and including the base flood. In calculating effective regulatory floodway conveyance, the following factors shall be taken into consideration.
(a) Regulatory floodway conveyance:
K = 1.4886 AR2/3
n
where “n” is Manning’s roughness factor, “A” is the effective area of the cross-section, and “R” is the ratio of the area to the wetted perimeter.
(b) The same Manning’s “n” value shall be used for both existing and proposed conditions unless a recorded maintenance agreement with a federal, state, or local unit of government can assure the proposed conditions will be maintained or the land cover is changing from a vegetative to a non-vegetative land cover.
(2) Transition sections shall be provided and used in calculations of effective regulatory floodway conveyance, in the design of excavations in the regulatory floodway, between cross-sections with rapid expansions and contractions, and when meeting the regulatory floodway delineation on adjoining properties. The following expansion and contraction ratios shall be used.
(a) Water will expand no faster than at a rate of one foot horizontal for every four feet of the flooded channel’s length.
(b) Water will contract no faster than at a rate of one foot horizontal for every one foot of the flooded channel’s length.
(c) Water will not expand or contract faster than one foot vertical for every ten feet of flooded channel’s length.
(d) All cross-sections used in the calculations shall be located perpendicular to flood flows.
(e) In the design of excavations in the regulatory floodway, erosion protection shall be provided on land upstream and downstream of proposed transition sections.
(3) The development of all appropriate uses shall not result in an increase in the average channel or regulatory floodway velocities or stage, for all flood events up to and including the base flood event. However, in the case of bridges or culverts or on-stream structures built for the purpose of backing up water in the stream during normal or flood flows, velocities may be increased at the structure site if erosion and sedimentation will be avoided by the use of rip-rap or other design measures.
(4) In the case of on-stream structures built for the purpose of backing up water, an increase in upstream stage when compared to existing conditions for all flood events up to and including the base flood event shall be contained within recorded easements. A dam safety permit or letter indicating that a permit is not required must be obtained from the Illinois Department of Natural Resources, Office of Water Resources for any structure built for the purpose of backing up water in the stream during normal or flood flow.
(5) If flood-proof construction is required beyond the outside dimensions of an existing habitable, residential, or commercial building, the outside perimeter of the flood-proofing construction shall be placed no further than ten feet from the outside of the building. Compensation of lost storage and conveyance will not be required for flood-proofing activities.
(6) For public flood control projects, the permitting requirements of this section will be considered met if the applicant can demonstrate to the Illinois Department of Natural Resources, Office of Water Resources, or Lake County Stormwater Management Commission in areas outside of Illinois Department of Natural Resources, Office of Water Resources jurisdiction, through hydraulic and hydrologic calculation that the proposed project will not singularly or cumulatively result in increased flood heights outside the project right-of-way or easements for all flood events up to and including the base flood event. (See § 151.149.)
(7) General criteria for analysis of flood elevations.
(a) The flood profiles, flows, and regulatory floodway data in the regulatory floodway study referenced in § 151.147 must be used for analysis of the base conditions. If the study data appear to be in error or conditions have changed, the Illinois Department of Natural Resources, Office of Water Resources shall be contacted for approval and concurrence on the appropriate base conditions data to use.
(b) If the base flood elevation at the site of the proposed construction is affected by backwater from a downstream receiving channel with a larger drainage area, the proposed construction shall be shown to meet the requirements of this section for the base flood elevation of the regulatory floodway conditions and conditions with the receiving channel stream at normal water elevations. Additional receiving stream elevations may be considered for design if appropriate and approved by the Lake County Stormwater Management Commission or Illinois Department of Natural Resources, Office of Water Resources.
(c) If the applicant is informed, in writing, by the Illinois Department of Natural Resources, Office of Water Resources, the Lake County Stormwater Management Commission or other jurisdictional authority that a downstream or upstream restrictive bridge or culvert is scheduled to be removed, constructed, 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.
(8) If the appropriate use will result in a change in the regulatory floodway location or base flood elevation, the applicant shall submit to the Lake County Stormwater Management Commission the information required to be issued a Conditional Letter of Map Revision from the Illinois Department of Natural Resources, Office of Water Resources and the Federal Emergency Management Agency. The application will not be considered complete until the Conditional Letter of Map Revision is received. No filling, grading, dredging, or excavating shall take place until a conditional approval is issued by the Planning, Building and Development Director. The construction or placement of structures within the currently effective floodway boundary shall not take place until a final Letter of Map Revision is issued by the Illinois Department of Natural Resources, Office of Water Resources and the Federal Emergency Management Agency which revises the floodway boundary.
COMMENTARY: The Illinois Department of Natural Resources, Office of Water Resources has retained permit authority for any Illinois Department of Natural Resources, Office of Water Resources project, dams, and the like and all other state, federal, or Lake County Stormwater Management Commission projects. The Lake County Stormwater Management Commission will issue permits to local units of government for regulatory floodway development. |
(9) For those circumstances listed below and located in a regulatory floodway, the following information shall be submitted to the Illinois Department of Natural Resources, Office of Water Resources or the Lake County Stormwater Management Commission:
(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 or culvert crossing or approach road is not a source of flood damage and the analysis indicating the proposed flood profile; and
(c) Alternative transition sections and hydraulically equivalent storage.
(Ord., § 8.6, passed 10-13-2009; Ord. passed 10-9-2012; Ord. 23-1056, passed 8-8-2023)