Chapter 21.80 - Floodplain Management

Floodplain

• Anchoring requirement

  All new construction and substantial improvements, including manufactured homes, shall be adequately anchored to prevent flotation, collapse or lateral movement.

• Maximum time to drain floodwaters from crawlspace72 hours

  There must be an adequate drainage system that removes floodwaters from the interior area of the crawl space within a reasonable period of time after a flood event, not to exceed seventy-two hours.

• Maximum crawlspace depth below grade2 ft

  The interior grade of a crawl space below the BFE must not be more than two feet below the lowest adjacent exterior grade.

• Maximum crawlspace heightMaximum heightThe tallest a building may be, given in feet, in storeys, or both — whichever binds first.4 ft

  The height of the below-grade crawl space, measured from the interior grade of the crawl space to the top of the crawl space foundation wall must not exceed four feet.

• Flood damage-resistant materials required below BFE

  All new construction and substantial improvement of structures, including manufactured homes, shall be constructed with flood damage-resistant materials, and utility equipment resistant to flood damage for areas below the base flood elevation.

• Minimum flood opening area1 sqin/sqftsq ftSquare feet. An acre is 43,560 sq ft; a typical suburban lot is 7,000–10,000.

  Have a minimum of two openings on different sides having a total net area of not less than one square inch for every square foot of enclosed area subject to flooding.

• Maximum heightMaximum heightThe tallest a building may be, given in feet, in storeys, or both — whichever binds first. of flood opening bottom above grade1 ft

  The bottom of all openings shall be no higher than one foot above grade.

• Maximum increase in base flood elevation in floodway1 ft

  No new construction, substantial development, or other development (including fill) shall be allowed within zones A1-30 and AE, unless it is demonstrated that the cumulative effect of the proposed development, when combined with all other development, will not increase the water surface elevation of the base flood more than one foot.

• Manufactured homeManufactured homeA dwelling built in a factory to a federal standard and transported to the site. lowest floor elevation above base flood elevation2 ft

  All manufactured homes ... shall be elevated on a permanent foundation such that the lowest floor of the manufactured home is elevated two feet above the base flood elevation.

• Minimum manufactured homeManufactured homeA dwelling built in a factory to a federal standard and transported to the site. pier heightMaximum heightThe tallest a building may be, given in feet, in storeys, or both — whichever binds first.36 in

  Manufactured home chassis is supported by reinforced piers or other foundation elements of at least equivalent strength that are no less than thirty-six inches in height above grade.

• Maximum flood velocity for crawlspace construction5 ft/s

  The velocity of floodwaters at the site should not exceed five feet per second for any crawl space.

• Maximum consecutive days for recreational vehicle on site180 days

  Be on the site for fewer than one hundred eighty consecutive days.

• Residential lowest floor elevation above base flood elevation2 ft

  All new construction or substantial improvements of residential structures shall have the lowest floor, including basement, elevated two feet above the base flood elevation.

• Maximum lot size for varianceVariancePermission to depart from a specific dimensional rule, granted on a showing of hardship. eligibility0.5 acresacre43,560 square feet — about the size of an American football field without the end zones.

  Variances may be issued for new construction, substantial improvement, and other proposed new development to be erected on a lot of 0.5 acre or less in size contiguous to and surrounded by lots with existing structures constructed below the base flood level.