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24.50.060 Provisions for Flood Hazard Reduction.

(Amended by Ordinance Nos. 165678, 169905, 172209, 173979, 176955, 178741, 182370, 184235, 189338 and 189806, effective December 18, 2019.)  In all flood hazard areas defined in Section 24.50.050, the following provisions are required:

A.  Permits.  All permit applications shall be reviewed to determine whether proposed building sites will be reasonably safe from flooding.  A development or building permit shall be obtained before construction or development begins within any area of flood hazard.  Such applications for permits shall include the following specific information:

1.  Elevation of lowest floor, including basement, for all structures and floodproofed elevations for nonresidential structures.

2.  Elevation of lowest point of bridge structures.

3.  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 shall be plotted within 1 contour interval of the true location.

4.  All necessary permits obtained from the federal and state governmental agencies from which prior approval is required.

5.  Where elevation data is not available either through the Flood Insurance Study or from another authoritative source (Section 24.50.050 G.), applications for building permits shall 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 two feet above grade in these zones may result in higher insurance rates.

B.  Elevation reference.  The survey reference datum for finished lowest floor including basement, floodproofed elevations, and finished site grades shall 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 shall be indicated on all relevant plan and Section drawings, and the certified Flood‑Elevation Certificate.

C.  Certification of elevations and floodproofing.  All finished elevations as specified hereunder shall be certified on a FEMA (FIA) Elevation Certificate by a licensed surveyor secured by the permittee, and made part of the permit records.

1.  As‑built elevation of lowest floor including basement, of all new or substantially improved structures;

2.  As‑built floodproofed elevation of all new or substantially improved nonresidential structures;

3.  As‑graded elevation of lowest grade within 25 feet of structures;

4.  As‑graded elevation of lowest crawl space grade, as applicable.

All floodproofing materials and methods for nonresidential structures shall be certified by a licensed professional engineer or architect as meeting the criteria in Section 24.50.060 F7.

D.  Floodway.  Encroachments into the floodway by development and structures defined in Section 24.50.020 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) shall 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 community.  Technical analysis shall be reviewed and approved by the Sewage System Administrator.  However, the minimum width of the floodway shall not be less than 15 feet.

E.  Alteration of watercourses.  The Bureau of Development Services shall:

1.  Notify adjacent communities and the Department of Land Conservation and Development 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.

2.  Require that maintenance is provided within the altered or relocated portion of said watercourse so that the flood carrying capacity is not diminished.

F.  Flood hazard areas.

1.  General.  All new construction and substantial improvements shall be anchored to prevent flotation, collapse or lateral movement of the structure.

2.  Residential construction.

a.  New construction and substantial improvement of any residential structure shall have the lowest floor, including basement, elevated to or above flood protection elevation.  Floodproofing of “lowest floor” space is not permitted. 

b.  Fully closed areas below the lowest floor that are subject to flooding are prohibited or shall be used solely for parking of vehicles, building access or limited storage and be designed to automatically equalize hydrostatic flood forces on exterior walls by allowing the entry and exit of floodwaters.  Designs for meeting this requirement must either be certified by a registered professional engineer or architect or must meet or exceed the following minimum criteria:

(1)  A minimum of two openings having a total net area of not less than one square inch for every square foot of enclosed area subject to flooding shall be provided;

(2)  The bottom of all openings shall be no higher than one foot above grade;

(3)  Openings may be equipped with screens, louvers, or other coverings or devices provided that they permit the automatic entry and exit of floodwaters.

(a)  Fills required to elevate the lowest floor to the flood protection level shall comply with Chapter 24.70.  Fill selection and placement shall recognize the effects of inundation from floodwaters on slope stability, fill settlement, and scour.  The minimum elevation at the top of the fill slope shall be at the base flood level.  Minimum distance from any point of the building perimeter to the top of the fill slope shall be either 25 feet or twice the depth of fill at that point, whichever is the greater distance.

(b)  Piling foundations required to elevate the lowest habitable floor to the flood protection level shall comply with Section 1809 and 1808 of the Structural Specialty Code.  Pilings shall be spaced no more than 10 feet apart, and reinforcement shall be provided for piling more than 6 feet above the ground level.

3.  Subdivision proposals.

a.  All subdivision proposals shall be consistent with the need to minimize flood damage;

b.  All subdivision proposals shall have public utilities and facilities such as sewer, gas, electrical and water systems located and constructed to minimize flood damage;

c.  All subdivision proposals shall have adequate drainage provided to reduce exposure to flood damage; and,

d.  Where base flood elevation data has not been provided or is not available from another authoritative source, it shall be generated for subdivision proposals and other proposed developments which contain at least 50 lots or 5 acres (whichever is less).

4.  Nonresidential construction.  New construction and substantial improvement of any commercial, industrial, or other nonresidential structure shall either have the lowest floor, including basement, elevated to the level of the flood protection elevation, or, together with attendant utility and sanitary facilities, shall:

a.  Be floodproofed so that below the flood protection elevation the structure is watertight with walls substantially impermeable to the passage of water;

b.  Have structural components capable of resisting hydrostatic and hydrodynamic loads and effects of buoyancy;

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 their development and/or review of the structural design, specifications and plans.  Such certifications shall be provided to the Bureau of Development Services.

d.  Nonresidential structures that are elevated, not floodproofed, must meet the same standards for space below the lowest floor as described for residential structures.

e.  Applicants floodproofing nonresidential buildings shall be notified that flood insurance premiums will be based on rates that are one foot below the floodproofed level (e.g. a building constructed to the base flood level will be rated as one foot below that level).

5.  Manufactured homes.  All manufactured homes to be placed or substantially improved within Zones AO, AH and AE shall be elevated on a permanent foundation such that the lowest floor of the manufactured home is at or above the flood protection elevation and be securely anchored to prevent flotation, collapse or lateral movement and shall be installed using methods and practices that minimize flood damage.  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.  Utilities.  All new and replacement water supply and replacement sanitary sewage systems shall be designed to minimize or eliminate infiltration of flood waters into the systems and discharges from the sanitary sewage systems into flood waters.  On‑site waste disposal systems shall be located to avoid impairment to them or contamination from them during flooding.

7.  Construction materials and methods.  All new construction and substantial improvements shall 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 shall be protected to or above the flood protection elevation.

8.  Balanced Cut and Fill Required.  In all Flood Management Areas of the City not addressed by Section 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 Section 24.50.050 A. through I.

a.  Excavation shall not be counted as compensating for fill if such areas will be filled with water in non-storm winter conditions.

b.  Temporary fills permitted during construction shall be removed.

9.  Tank anchoring.  Tanks containing hazardous materials must be anchored to prevent flotation if they are located in areas of special flood hazard or flood management areas.

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 management areas.

11.  AH/AO Zone Drainage.  Adequate drainage paths shall be provided around structures on slopes to guide floodwaters around and away from proposed structures.

G.  Johnson Creek Flood Zones - Special Provisions.  In addition to other requirements of this chapter the following requirements shall apply within designated portions of the Johnson Creek Flood Zones:

1.  All Johnson Creek Flood Zones

a.  Balanced cut and fill.  Within all areas of the Johnson Creek Flood Zones, all new fills below the base flood elevation shall be accompanied by an equal amount of excavation on the same site so that the storage capacity of the floodway and floodway fringe is retained.

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 shall be determined by the Bureau of Environmental Services.

2.  Johnson Creek Flood Risk Area.  The following provisions shall apply within the Johnson Creek Flood Risk Area, as established in Chapter 33.535 of the City Code:

a.  Balanced cut and fill.  The requirements of subsection G.1. above, shall apply within the Johnson Creek Flood Risk Area.

b.  Reduction in flooding capacity prohibited.  Structures, fill or other development shall only be allowed in the Johnson Creek Flood Risk Area when they are designed so that there is no significant reduction in the storage capacity of the floodway and floodway fringe and there is no significant impediment to the passage of flood waters.

c.  Exceptions to Section 24.50.060 G.2.:

(1)  One story detached accessory buildings used as tool and storage sheds, playhouses and similar uses, provided the floor area does not exceed 120 square feet.

(2)  Parking garages accessory to one and two family structures, provided the floor area does not exceed 300 square feet.

(3)  Fences which do not prevent the flow of water.

d.  Buildings designed to meet all of the following criteria shall be presumed to comply with Section 24.50.060.G.2.:

(1)  At least 50 percent of perimeter walls located at, or below, the base flood elevation shall remain open and unenclosed;

(2)  At least 25 percent of each perimeter wall located at, or below, the base flood elevation shall remain open and unenclosed; and

(3)  The footprint of all portions of the building located at, or below, the base flood elevation shall not exceed 15 percent of the footprint of the building located above the base flood elevation.