(A) Permitted uses.
(1) The uses listed in § 151.03.32 Floodway Overlay District are allowed with a permit.
(B) Procedures for determining floodway boundaries and base flood elevations.
(1) Requirements for detailed studies. Developments greater than 50 lots or five acres, or as requested by the Zoning Administrator, shall be subject to a detailed study to determine the regulatory flood protection elevation and the limits of the Floodway District. The determination of the floodway and flood fringe must be consistent with accepted hydrological and hydraulic engineering standards, and must include the following components, as applicable:
(a) Estimate the peak discharge of the regional flood.
(b) Calculate the water surface profile of the regional flood based upon a hydraulic analysis of the stream channel and overbank areas.
(c) Compute the floodway necessary to convey or store the base flood without increasing flood stages more than one-half foot. A lesser stage increase than one-half foot is required if, as a result of the stage increase, increased flood damages would result. An equal degree of encroachment on both sides of the stream within the reach must be assumed in computing floodway boundaries, unless development or geographic features warrant other analysis, as approved by the Department of Natural Resources.
(2) Alternative methods. For areas where a detailed study is not available or required, the regional flood elevation must be determined using best available data. Until a floodway determination can be completed, the entire floodplain must be treated as floodway, with allowable activities restricted to those identified in § 151.03.32.
(a) Development allowed in floodways (e.g. bridges, culverts, grading, filling, stabilization projects) must not cumulatively increase flood stages more than one-half foot during a base flood event, as determined by a professional engineer or by using accepted engineering practices approved by the Department of Natural Resources. A stage increase less than one-half foot must be used if increased flood damages would result.
(b) Development prohibited in floodways (e.g. buildings) require a floodway/flood fringe determination to verify the development is within the flood fringe. The floodway/flood fringe determination must be done by a professional engineer or utilize other accepted engineering practices approved by the Department of Natural Resources. Any such proposal must assume a one-half foot stage increase for the purposes of determining the regulatory flood protection elevation to accommodate for future cumulative impacts.
(3)
For areas in and along lakes, wetlands, and other basins that are not affected by velocities, where the floodway has not been determined, an alternative to subsections (B)(1) and (B)(2) of this section is:
(a) All areas that are at or below the ordinary high water level (OHW) will be considered floodway, and all areas below the base flood elevation but above the OHW will be considered flood fringe, provided that within 25 feet of the OHW, or within the Shore Impact Zone as identified in the community’s shoreland ordinance, whichever distance is greater, land alterations shall be restricted to:
1. The minimum required to accommodate beach and access areas, and accessory structures as permitted, not to exceed a volume greater than ten cubic yards; projects involving volumes exceeding ten cubic yards require engineering analysis as provided in subsections (B)(1) and (B)(2) of this section, whichever is applicable; and
2. The minimum required to accommodate shoreline stabilization projects to correct an identified erosion problem as identified by the Chisago Soil & Water Conservation District.
(C) The Zoning Administrator will review the submitted information and assess the technical evaluation and the recommended Floodway and/or Flood Fringe District boundary. The assessment must include the cumulative effects of previous floodway encroachments. The Zoning Administrator may seek technical assistance from an engineer or other expert person, or agency, including the Department of Natural Resources.
(D) Once the Floodway and Flood Fringe District boundaries have been determined, the Zoning Administrator must process the permit application consistent with the applicable provisions of this chapter.
(Ord. 20220120-01, passed 1-20-22)