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Article IV. Flood Hazard Reduction – Specific Standards
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In all areas of special flood hazard where base flood elevation data has been provided pursuant to CMC 16.15.050 or 16.15.140, the following provisions pertaining to residential construction shall be complied with:

(1) New construction and substantial improvement of any residential structure shall have the lowest floor, including any basement, elevated one foot or more above the base flood elevation. Mechanical equipment and utilities shall be waterproof or elevated at least one foot above the base flood elevation; and

(2) Fully enclosed areas below the lowest floor usable solely for parking, access or storage and that are subject to flooding are prohibited, or shall be designed to automatically equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit of floodwater. Designs for meeting this requirement must be certified by a registered professional engineer or architect or must meet or exceed the following minimum criteria:

(a) 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;

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

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

(d) A garage attached to a residential structure, constructed with the garage floor slab below the base flood elevation, must be designed to allow for the automatic entry and exit of floodwaters.

(3) Construction of Below-Grade Crawlspace.

(a) The interior grade of a crawlspace must not be more than two feet below the exterior lowest adjacent grade (LAG).

(b) The height of the below-grade crawlspace, measured from the interior grade of the crawlspace to the top of the crawlspace foundation wall, must not exceed four feet at any point.

(c) There must be an adequate drainage system that removes floodwaters from the interior area of the crawlspace. The enclosed area should be drained within a reasonable time after a flood event.

(d) The velocity of floodwaters at the site should not exceed five feet per second for any crawlspace.

See Technical Bulletin 11 for further information.


Buildings that have below-grade crawlspaces will have higher flood insurance premiums than buildings that have the preferred crawlspace construction, with the interior elevation of the crawlspace soil at or above the base flood elevation (BFE).

(4) All crawlspaces that have enclosed areas or floors below the BFE must be:

(a) Designed and adequately anchored to resist flotation, collapse and lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy;

(b) Have openings that allow for the automatic entry and exit of floodwaters. The bottom of the flood opening can be no more than one foot above the lowest adjacent exterior grade and meet the requirements in subsection (2) of this section;

(c) Constructed with materials resistant to flood damage, including foundation walls, joists, insulation or other materials that will be below BFE;

(d) Located in areas where flood velocities do not exceed five feet per second; and

(e) Any building utility systems within the crawlspace must be elevated above the BFE or designed so that floodwaters cannot enter or accumulate with the system components during flood conditions.

(5) New construction and substantial improvement of any residential structure in an A zone for which a base flood elevation is not available and cannot be reasonably obtained shall be reasonably safe from flooding, but in all cases the lowest floor shall be at least two feet above the highest adjacent grade. (Ord. 12-20 § 2 (Exh. A); Ord. 06-17 § 3 (Exh. A); Ord. 100-98 § 1)