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24.50.050 Flood Hazard Areas and Flood Protection Elevations.

City Code Section

(Amended by Ordinances 173979, 178741, 182370 and 191478, effective March 1, 2024.)

  1. Flood hazard areas contain all lands located within the floodway boundary, flood zones within the flood fringe areas, and other identified flood zones.  Identified and unidentified flood hazard areas and flood protection elevations are described in this Section.  When a structure is partially located in a flood hazard area, the entire structure is subject to the requirements for new construction, substantial improvements, and substantial damage.  When a structure is located within multiple flood zones, the more restrictive flood zone elevations apply.
  2. A.  Columbia River FIRM Flood Zone AE.  These flood zones represent areas for which base flood elevations are determined.  The flood protection elevation is the base flood elevation plus 1 foot of freeboard.  The nominal 1-foot increase for freeboard reflects the relatively wide floodplain of the Columbia River.  In the vicinity of the confluence of the Columbia and Willamette Rivers, the Columbia River floodplain is considered to be east of the westerly flood fringe boundary of the Columbia Slough.
  3. B.  Multnomah Drainage District No. 1, Peninsula Drainage District No. 1, and Peninsula Drainage District No. 2 FIRM Flood Zones AH and A.  FIRM Flood Zone AH represents isolated areas of shallow flooding (1 to 3 feet in depth, resulting from upslope runoff) for which base flood elevations are determined.  FIRM Flood Zone A represents areas for which base flood elevations are not determined.  In the case of unidentified watercourses occurring within the boundaries of the Drainage Districts, the base flood elevation will be estimated by procedures described in Subsection I. below.  The flood protection elevation is the base flood elevations plus 1 foot of freeboard.
  4. C.  Columbia River FIRM Flood Zone A.  These flood zones represent areas for which base flood elevations are not determined.  The flood protection elevation is either the grade at the adjacent flood fringe boundary or the crown of the nearest street, whichever is higher, plus 1 foot of freeboard.
  5. D.  Willamette River FIRM Flood Zone AE.  These flood zones represent areas for which the base flood elevations are determined.  The flood protection elevation is the base flood elevation plus 2 feet of freeboard.
  6. E.  Johnson Creek, Fanno Creek and Crystal Springs Creek FIRM Flood Zone AE.  This flood zone represents areas for which the base flood elevations are determined.  The flood protection elevation is the base flood elevation plus 2 feet of freeboard.
  7. F.  Johnson Creek FIRM Flood Zone AH.  This flood zone represents areas of shallow flooding depth (1 to 3 feet) for which base flood elevations are determined.  The flood protection elevation is the base flood elevation plus 2 feet of freeboard.
  8. G.  Johnson Creek FIRM Flood Zone AO.  This flood zone represents areas of shallow flooding depth (1 to 3 feet) for which the depths of flooding are determined.  The flood protection elevation is the depth of flooding shown on the FIRM map plus 2 feet of freeboard above the highest adjacent grade.
  9. H.  Johnson Creek, Fanno Creek, Tryon Creek, and Crystal Springs Creek FIRM Flood Zone A. These flood zones represent areas for which base flood elevations are not determined.  The flood protection elevation is the base flood elevation plus 2 feet of freeboard.  Base flood elevations will be calculated in accordance with Subsection I. below.
  10. I.  Unidentified Watercourse Flood Zones.  These watercourses, generally draining one acre or more, are not identified in a Federal Insurance Study.  The flood protection elevation is the base flood elevation plus 2 feet of freeboard.  The width of the floodway may not be less than 15 feet, unless otherwise approved by the Sewage System Administrator.  The floodway boundary, flood fringe boundary, and flood protection elevation data will be based upon watercourse geometry, slope, channel roughness, effect of obstructions, backwater and other factors that affect flood flow.  The requisite flood hazard data, maps, and sections must be obtained and developed by procedures approved by the Sewage System Administrator.  When appropriate and necessary data are available, the flood protection elevation and floodway and flood fringe boundary data may be provided by the Sewage System Administrator.  If pertinent hydrologic data and topographic data are not available, inaccurate, or outdated, and where substantial alterations or relocations of a watercourse are involved, the Sewage System Administrator may require the permit applicant to secure a registered engineer and surveyor to develop and supply the requisite flood hazard data, maps, and sections.
  11. J.  February 1996 Flood Inundation Areas must have a flood protection elevation that provides a minimum of 2 feet of freeboard above the February 1996 Flood Inundation Elevation.  February 1996 Flood Inundation Areas adjacent to Columbia River FIRM Flood Zone AE, Multnomah Drainage District No. 1, Peninsula Drainage District No. 1, Peninsula Drainage District No. 2 Firm Zone AH, and Columbia River FIRM Flood Zone A must have a minimum freeboard of 1 foot.  The February 1996 Flood Inundation Elevations must be determined using an approved method such as river gage data; high water marks recorded during the flood; data from state, local, or other authorities; inundation area elevation contour interpolation; or a technical analysis stamped by a professional engineer licensed in the State of Oregon and approved by the Sewage System Administrator.