Floodplain Management

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Appurtenant Structures and Flood Openings

By John Callen, PE, CFM

From December 2015 Floodplain Management Today

For floodplain managers, compliance for appurtenant structures, sometimes referred to as accessory structures by FEMA, can be a challenge. FEMA has issued several guidance documents on this topic, and some of these guidance documents refer to the ability to put flood vent openings (or wet floodproofing) in appurtenant structures for compliance purposes. However, in Nebraska most local floodplain management ordinances do not allow this to occur unless the local floodplain management ordinance adopts certain language or a variance is issued on a structure by structure basis.

Nebraska minimum standards for floodplain management programs are in the Nebraska Administrative Code Title 455, Chapter 1. These standards, in combination with FEMA’s minimum requirements of ordinances for participation in the National Flood Insurance Program (NFIP), define the minimum Nebraska standards for floodplain management that should be reflected in any Nebraska community local floodplain management ordinance. The minimum standards include a definition of appurtenant structures:

‘002.01 Appurtenant structure. “Appurtenant structure” shall mean a structure on the same parcel of property as the principal structure, the use of which is incidental to the use of the principal structure.’

The minimum standards also define lowest floor:

‘002.12 Lowest floor. “Lowest floor” shall mean the lowest floor of the lowest enclosed area (including basement). An unfinished or flood-resistant enclosure, usable solely for parking of vehicles, building access or storage, in an area other than a basement area, is not considered a building’s lowest floor, provided that such enclosure is not built so as to render the structure in violation of the applicable non-elevation design requirements of this Chapter.’

The usage of flood openings for compliance in appurtenant structures is a special case that allows flooding above the lowest floor as defined in the minimum standards. This is different from flood openings that allow flood waters to enter and exit above-grade crawl spaces or areas below lowest floor. Compliance standards for these scenarios are described in the following two provisions from the minimum standards (emphasis added) and an overview of the two scenarios is shown in figures from FEMA Technical Bulletin 1:

004.09 Appurtenant structures used exclusively for storage of motor vehicles, and storage of other items readily removable in the event of a flood warning may have their lowest floor below one foot above the base flood elevation provided the structure is capable of withstanding hydrostatic and hydrodynamic forces caused by the base flood and provided that no utilities are installed in the structure except elevated or flood proofed electrical fixtures. If the structure is converted to another use, it must be brought into full compliance with the minimum standards governing such use.

004.10 For all new construction and substantial improvements, fully enclosed areas below the lowest floor that are usable solely for parking of vehicles, building access or storage in an area other than a basement and which are subject to flooding shall be designed to automatically equalize hydrostatic flood forces on exterior walls by allowing for the entry and exit of floodwaters. Designs for meeting this requirement must either be certified by a registered professional engineer or architect or meet or exceed the following minimum criteria:

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. The bottom of all openings shall not be higher than one foot above grade. Openings may be equipped with screens, louvers, valves, or other coverings or devices provided that they permit the automatic entry and exit of floodwaters.

The standards of 004.10 should be in all local floodplain management ordinances. The requirements of 004.09, however, are not in most ordinances. If a community wishes to routinely allow appurtenant structures with a lowest floor below the BFE and with flood vent openings installed for compliance purposes, then the definition of appurtenant structure and the provisions of 004.09 must be adopted into the local ordinance. Flood vent openings must follow the placement and sizing standards shown in 004.10.

If 004.09 is not adopted, wet floodproofing is not an option for appurtenant structures and they must be elevated or dry floodproofed or a variance must be issued on a structure by structure basis. For any building with the lowest floor below base flood elevation, applicable flood insurance costs will likely be higher for wet floodproofed appurtenant structures than for elevated or dry floodproofed appurtenant structures.

The provisions of 004.09 in the minimum standards are designed to be in accordance with FEMA Technical Bulletin 7-93, which can be found under Tab 10 in the Nebraska Digital Desk Reference. Additional references of note include FEMA Technical Bulletin 1 (Tab 10), FEMA Region VII guidance on accessory and agricultural structures (Tab 3), and general structure/development flow charts (Tab 5), all in the desk reference. It should be noted that FEMA guidance recommends that wet floodproofing be typically restricted to small appurtenant structures such as detached garages and low cost sheds; the FEMA Region VII guidance recommends 400 square feet or smaller as an allowable size.

If a community wishes to update their ordinance to allow for appurtenant structures to be wet floodproofed, the appropriate definitions and standards must be adopted. In addition, a size standard could be considered by communities; this would further define the building sizes where wet floodproofing is allowed. If you have questions or need further assistance, contact Katie Ringland at Katie.Ringland@nebraska.gov.

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