24.50.060 Provisions for Flood Hazard Reduction.

City Code Section

(Amended by Ordinances 165678, 169905, 172209, 173979, 176955, 178741, 182370, 184235, 189338, 189806 and 191478, effective March 1, 2024.)

  1. In all flood hazard areas defined in Section 24.50.050, the following requirements apply:
  2. A.  Permits.  All permit applications will be reviewed to determine whether proposed development and building sites comply with the provisions of this Chapter and will be reasonably safe from flooding.  A development or building permit must be obtained before construction or development begins within any flood hazard area.  The development or building permit is required for all structures, including manufactured homes, and for all other development, as defined in this Chapter, including fill.  Such applications for permits must include the following information:
    1. 1.  Boundary of flood hazard areas.
    2. 2.  The base flood elevation and design flood elevation.
    3. 3.  Elevation of lowest floor, including basement, for all structures and floodproofed elevations for nonresidential structures.
    4. 4.  When required by the Floodplain Administrator, documentation necessary to verify substantial improvement or substantial damage.
    5. 5.  Elevation of lowest point of bridge structures.
    6. 6.  Existing and proposed topography of the site taken at a contour interval (normally 1 foot) sufficiently detailed to define the topography over the entire site and adjacent watercourses subject to flooding.  Ninety percent of the contours must be plotted within 1 contour interval of the true location.
    7. 7.  All necessary permits obtained from the federal and state governmental agencies from which prior approval is required.
    8. 8.  Where elevation data is not available either through the Flood Insurance Study or from another authoritative source (see Subsection 24.50.050 I.), applications for building permits will be reviewed to assure that proposed construction will be reasonably safe from flooding.  The test of reasonableness is a local judgment and includes use of any available hydrological data, drainage basin hydrology, historical data, high water marks, photographs of past flooding, etc., where available.  Failure to elevate at least 2 feet above grade in these zones may result in higher insurance rates.
  3. B.  Elevation reference.  The survey reference datum for finished lowest floor including basement, floodproofed elevations, and finished site grades must be either the North American Vertical Datum of 1988 or City of Portland datum, whichever is appropriate.  When approved by the City Engineer, a local onsite survey reference datum may be adopted for FIRM Zones A and Unidentified Watercourse Flood Zones.  The survey reference datum must be indicated on all relevant plan and section drawings, calculations, and the certified FEMA Elevation Certificate.
  4. C.  Certification of elevations and floodproofing. 
    1. 1.  All elevations specified below must be certified on a FEMA (FIA) Elevation Certificate by a licensed surveyor or engineer secured by the permittee and made part of the permit records.
      1. a.  During construction elevations of the lowest floor, including basement, of all new or substantially improved structures upon placement of the lowest floor but prior to further vertical construction;
      2. b.  As-built finished elevation of lowest floor including basement of all new or substantially improved structures;
      3. c.  As‑built finished floodproofed elevation of all new or substantially improved nonresidential structures;
      4. d.  As‑graded finished elevation of lowest grade within 25 feet of structures; and
      5. e.  As‑graded finished elevation of lowest crawl space grade, as applicable.
    2. 2.  All floodproofing materials and methods for nonresidential structures must be certified by a licensed professional engineer or architect as meeting the criteria in Subsection 24.50.060 F.7.
  5. D.  Floodway.  Encroachments into the floodway by development and structures defined in Section 24.50.030 are prohibited unless it is demonstrated by technical analysis from a registered engineer that the development will result in no increase in the base flood elevation.  In areas where a regulatory floodway has not been designated, no new construction, substantial improvement or other development (including fill) will be permitted within Zone AE, unless it is demonstrated that the cumulative effect of the proposed development, when combined with all other existing and anticipated development, will not increase the water surface elevation of the base flood more than 1 foot at any point within the flood hazard areas regulated by the City.  Technical analysis will be reviewed and approved by the Sewage System Administrator.  However, the minimum width of the floodway shall not be less than 15 feet, unless otherwise approved by the Sewage System Administrator.
  6. E.  New Technical Data and Notifications of Other Entities.  The Bureau of Development Services will:
    1. 1.  City Boundary Alterations.  Notify the Federal Insurance Administration in writing whenever the boundaries of the City have been modified by annexation or the City has otherwise assumed authority or no longer has authority to adopt and enforce floodplain management regulations for a particular area, to ensure that all Flood Hazard Boundary Maps (FHBM) and Flood Insurance Rate Maps (FIRM) accurately represent the City’s boundaries.  The Bureau will include within such notification a copy of a map of the City suitable for reproduction, clearly delineating the new corporate limits or new area for which the City has assumed or relinquished floodplain management regulatory authority.
    2. 2.  Watercourse Alterations.  Notify adjacent communities, the Department of Land Conservation and Development, and other appropriate state and federal agencies, prior to any alteration or relocation of a watercourse as identified in the Flood Insurance Study and Flood Insurance Rate Map, and submit evidence of such notification to the Federal Insurance Administration.  The applicant must provide this notification to the Federal Insurance Administration as a Conditional Letter of Map Revision (CLOMR) along with either:
      1. a.  A proposed maintenance plan to ensure the flood-carrying capacity within the altered or relocated portion of the watercourse will be maintained; or
      2. b.  Certification by a registered professional engineer that the project has been designed to retain its flood carrying capacity without periodic maintenance.
    3. 3.  New Technical Data.  Notify the Federal Insurance Administration of changes in the special flood hazard area based on new or improved scientific data that more accurately reflect the flood hazard boundaries and water surface elevations.  The City will apply to FEMA for a CLOMR prior to permit issuance for:
      1. a.  Proposed floodway encroachments that will increase the base flood elevation; and
      2. b.  Proposed development that will increase the base flood elevation by more than 1 foot in areas where FEMA has provided base flood elevations but no floodway (see Subsection 24.50.060 D.)
    4. 4.  Letter of Map Revision.  Require the applicant to notify FEMA within six months of project completion when FEMA has issued a CLOMR for the project.  This notification to FEMA must be provided as a Letter of Map Revision (LOMR).
  7. F.  Flood hazard areas.
    1. 1.  General.  All new construction and substantial improvements must be anchored to prevent flotation, collapse or lateral movement of the structure resulting from hydrostatic and hydrodynamic loads and effects of buoyancy.
    2. 2.  Residential construction.
      1. a.  New construction and substantial improvement of any residential structure must have the lowest floor, including basement, elevated to or above the flood protection elevation.  Floodproofing of “lowest floor” space is not permitted. 
      2. b.  Enclosed areas below the lowest floor that are subject to flooding are prohibited unless they are used solely for parking of vehicles, building access or limited storage and are designed to automatically equalize hydrostatic flood forces on exterior walls by allowing the entry and exit of floodwaters.  Designs for meeting this requirement must meet or exceed the following minimum criteria:
        1. (1)  There must be a minimum of two openings on different sides of each enclosed area;
        2. (2)  The total net area of all openings for each enclosed area must not be less than one square inch for every square foot of enclosed area subject to flooding or be certified by a registered design professional;
        3. (3)  The bottom of all openings may be no higher than 1 foot above grade;
        4. (4)  Openings may be equipped with screens, louvers, or other coverings or devices provided that they permit the automatic entry and exit of floodwaters; and
        5. (5)  An agreement approved by the Floodplain Administrator not to convert the use of the enclosed area must be recorded against the property deed when required by the Floodplain Administrator.
      3. c.  Fill required to elevate the lowest floor to the flood protection level must comply with Chapter 24.70.  Fill selection and placement must recognize the effects of inundation from floodwaters on slope stability, fill settlement, and scour.  The minimum elevation at the top of the fill slope must be at or above the design flood elevation.  Minimum distance from any point of the building perimeter to the top of the fill slope must be at least 25 feet or twice the depth of fill at that point, whichever is the greater distance.
    3. 3.  Subdivision proposals.
      1. a.  All subdivision proposals must be consistent with the need to minimize flood damage;
      2. b.  All subdivision proposals must have public utilities and facilities such as sewer, gas, electrical and water systems located and constructed to minimize flood damage;
      3. c.  All subdivision proposals must have adequate drainage provided to reduce exposure to flood damage; and,
      4. d.  Where base flood elevation data have not been provided or are not available from another authoritative source, it must be generated for subdivision proposals and other proposed developments that contain at least 50 lots or 5 acres.
    4. 4.  Nonresidential construction.  New construction and substantial improvement of any commercial, industrial, or other nonresidential structure must either have the lowest floor, including basement, elevated to the level of the flood protection elevation, or, together with attendant utility and sanitary facilities, must:
      1. a.  Be floodproofed so that below the flood protection elevation the structure is watertight with walls substantially impermeable to the passage of water;
      2. b.  Have structural components capable of resisting hydrostatic and hydrodynamic loads and effects of buoyancy; and
      3. c.  Be certified by a registered professional engineer or architect that the design and methods of construction are in accordance with accepted standards of practice for meeting provisions of this Subsection based on the professional engineer’s or architect’s development or review of the structural design, specifications and plans.  Such certifications must be provided to the Bureau of Development Services.
      4. d.  Nonresidential structures that are elevated, but not floodproofed, must meet the same standards for space below the lowest floor as described for residential structures.
      5. e.  Applicants floodproofing nonresidential buildings will be notified that flood insurance premiums will be based on rates as if the building is 1 foot below the floodproofed level (i.e., a building constructed to the base flood level will be rated as 1 foot below that level).
    5. 5.  Manufactured homes.  All manufactured homes to be placed or substantially improved within the flood hazard area must be elevated on a permanent foundation such that the lowest floor of the manufactured home is at or above the flood protection elevation; securely anchored to prevent flotation, collapse or lateral movement; and installed using methods and practices that minimize flood damage.  The construction must conform to the requirements of Subsection 24.50.060 F.2.  Anchoring methods may include, but are not limited to, use of over‑the‑top or frame ties to ground anchors (Refer to FEMA’s "Manufactured Home Installation in Flood Hazard Areas” guidebook for additional techniques).
    6. 6.  Utilities.  All new and replacement water supply and sanitary sewage systems must be designed to minimize or eliminate infiltration of floodwaters into the systems and discharges from the sanitary sewage systems into floodwaters.  On‑site waste disposal systems must be located to avoid impairment to them or contamination from them during flooding.
    7. 7.  Construction materials and methods.  All new construction and substantial improvements must be constructed with materials and utility equipment resistant to flood damage, using methods and practices that minimize flood damage.  Electrical, heating, ventilation, plumbing and air‑conditioning equipment and other service facilities must be protected to or above the flood protection elevation.
    8. 8.  Balanced Cut and Fill Required.  In all flood hazard areas of the City not addressed by Subsection 24.50.060 G., balanced cut and fill shall be required.  All fill placed at or below the base flood elevation shall be balanced with at least an equal amount of soil material removal.  Soil material removal shall be within the same flood hazard area identified in Subsections 24.50.050 A. through I.
      1. a.  Excavation shall not be counted as compensating for fill if such areas will be filled with water in non-storm winter conditions.
      2. b.  Temporary fills permitted during construction shall be removed.
    9. 9.  Tanks.   
      1. a.  Underground tanks must be anchored to prevent flotation, collapse, and lateral movement under conditions of the design flood.
      2. b.  Above-ground tanks must be installed at or above the flood protection elevation or be anchored to prevent flotation, collapse, and lateral movement under conditions of the design flood.
    10. 10.  Uncontained hazardous materials as referred to in Section 101 (14) of the Comprehensive Environmental Response, Compensation and Liability Act of 1980 (42 U.S. Section 9601 et seq.) (CERCLA), Section 502 (13) of the Clean Water Act and any other substances so designated by the Director of the Bureau of Development Services are prohibited in flood hazard areas.
    11. 11.  AH/AO Zone Drainage.  Adequate drainage paths shall be provided around structures on slopes to guide floodwaters around and away from proposed structures.
  8. G.  Johnson Creek Flood Zones - Special Provisions.  In addition to other requirements of this Chapter, the following requirements apply within designated portions of the Johnson Creek Flood Zones:
    1. 1.  All Johnson Creek Flood Zones
      1. a.  Balanced cut and fill.  Within all areas of the Johnson Creek Flood Zones, all new fills below the base flood elevation must be accompanied by an equal amount of excavation on the same site so that the storage capacity of the floodway and flood fringe is retained.
      2. b.  Mitigation payment allowed in lieu of balanced cut and fill.  After September 1, 1998 residential properties within the area of the 100 year floodplain, but outside of the floodway and Flood Risk Area, and bounded by I-205 on the west, SE 142nd Avenue on the east, and the Springwater Corridor Trail on the south, may elect to pay into the Johnson Creek Fill Mitigation Bank in lieu of creating a balanced cut and fill.  The amount of the payment will be determined by the Bureau of Environmental Services.
    2. 2.  Johnson Creek Flood Risk Area.  The following provisions apply within the Johnson Creek Flood Risk Area, as established in Chapter 33.537:
      1. a.  Balanced cut and fill.  The requirements of Subsection G.1. above, apply within the Johnson Creek Flood Risk Area.
      2. b.  Reduction in flooding capacity prohibited.  Structures, fill or other development are only allowed in the Johnson Creek Flood Risk Area when they are designed so that there will be no significant reduction in the storage capacity of the floodway and flood fringe and there will be no significant impediment to the passage of floodwaters.
      3. c.  Exceptions to the applicability of Subsection 24.50.060 G.2.:
        1. (1)  One-story, detached accessory buildings used as tool and storage sheds, playhouses or similar uses, provided the floor area does not exceed 120 square feet.
        2. (2)  Parking garages accessory to one- and two-family structures, provided the floor area does not exceed 300 square feet.
        3. (3)  Fences that do not prevent the flow of water.
      4. d.  Buildings designed to meet all of the following criteria will be presumed to comply with Subsection 24.50.060 G.2.:
        1. (1)  At least 50 percent of perimeter walls located at, or below, the base flood elevation will remain open and unenclosed;
        2. (2)  At least 25 percent of each perimeter wall located at, or below, the base flood elevation will remain open and unenclosed; and
        3. (3)  The footprint of all portions of the building located at, or below, the base flood elevation may not exceed 15 percent of the footprint of the building located above the base flood elevation.

See something we could improve on this page? Give website feedback.

Search Code, Charter, Policy