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Oklahoma's Floodplain Management 101

Chapter 3: Local Floodplain Regulations and NFIP Standards

Aerial view of flooded farmhouse

Chapter 3 Appendix

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The participation of a community in the NFIP is made possible by its adoption of floodplain management regulations. These regulations must meet the revised standards of the National Flood Insurance Program and the Oklahoma Floodplain Management Act. Check with FEMA or the OWRB before adopting an ordinance to ensure that the most recent and effective requirements are adopted. A community may comply with this requirement by adopting appropriate changes in existing zoning, subdivision, and building ordinances or by adopting a specific floodplain ordinance. Appendix 3-1 contains a copy of the most current Model D ordinance effective December 8, 2004. This example only contains the minimum NFIP standards.

This chapter describes the method of adopting a specific floodplain ordinance. Communities that amend existing ordinances must be sure that all the standards detailed in the second part of this chapter are incorporated into the amendments. (See Appendix 3-2.)

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Floodplain Regulation

Communities that wish to make flood insurance available to their residents must regulate development in floodplains. This can be done by adopting land use regulations (ordinances) that set forth construction standards and establish a permit system that allows the community to enforce those standards. Local regulations for flood loss reduction receive their authority from the police power granted to municipalities by the States. Inherent in the right to enact such regulations is the duty and authority to administer and enforce them.

Floodplain regulations (ordinances) are the foundation of all efforts to prevent flood damage and minimize the impact floods cause within a community. Floodplain regulations are land use controls, the results of which can be measured over the long term. Through the permitting system set forth by ordinance, community development can be compatible with the identified flood hazard.

The standards of the NFIP are the minimum floodplain management efforts required for participation in the NFIP. These standards can differ since they are based on the degree of information provided to the community by FEMA. The comprehensiveness of a community's floodplain ordinance is directly related to information known about the flood hazard. NFIP regulations on which local ordinances are based are progressive, again related to FEMA's published floodplain information. If necessary, communities may strengthen their floodplain management ordinances to be more restrictive than NFIP standards. A floodplain ordinance is designed to be used with a community map identifying flood-prone areas. When the mapping of the flood hazard is absent or deficient, the ordinance becomes more reliant on community judgment and local flooding conditions.

Appendix 3-3 contains an overview of local floodplain management program guidelines. The floodplain ordinance is comprehensive in what it regulates:

  1. Placement of structures, methods of construction, types of structures, and alterations to structures (including manufactured homes);
  2. Subdivisions (no structures in the floodway);
  3. Installations of water and sewer utilities;
  4. Fence construction;
  5. Filling, grading, chanalization, and excavating within the floodplain;
  6. Installation and replacement of roads and bridges;
  7. Storage of materials and equipment; and,
  8. Any related activities that may affect the level of the 100-year flood event.

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General Standards

To reduce potential flood damages effectively, the NFIP has established standards for new or substantially improved construction projects and other developments in identified Special Flood Hazard Areas. Under NFIP Regulations, Part 44 C.F.R. 61.13, the following general standards are outlined which are required of all communities participating in the NFIP. In a community with a Flood Insurance Study or where Base Flood Elevations have been established, specific standards apply along with the general standards. The more specific the flood information provided, the more stringent the standards.

Regulations require that all structures be properly anchored to prevent hydrodynamic and hydrostatic loads from moving them from their foundations during a flood. If a structure is elevated on compacted fill above the known or projected Base Flood Elevation, the anchoring requirement is satisfied. In Oklahoma, anchoring requirements are generally met for most permanent structures through common construction practices. Mobile homes, now termed "manufactured homes," are treated like conventional homes. Manufactured homes placed within Zone A on a community's FHBM or FIRM must be installed using methods and practices that minimize flood damage, which means they must have their lowest floor elevated to or above the BFE on a permanent foundation. The manufactured home must be anchored by the use of over the top or frame ties to ground anchors connecting to the permanent foundation. Manufactured homes are best protected by elevating their compacted fill pads to or above the BFE, which is the preferred method in satisfying anchoring requirements. Specific anchoring standards and compaction standards are set forth in the building codes adopted by each community. These codes are either the UBC (Uniform Building Code), the International Building Code (IBC) or the BOCA (Building Officials Code of America). These anchoring standards must be applied to buildings, manufactured homes, storage sheds, accessory buildings and fuel storage tanks. For manufactured homes in Oklahoma a licensed installer is required.

Buildings can suffer damages in many ways during a flood: hydrostatic pressure can push in foundation walls, hydrodynamic pressure from waves can destroy walls, uplift can cause structure buoyancy problems, and contact with water can warp or damage walls and floors. Because of this susceptibility, the NFIP requires new buildings in flood hazard areas be constructed with materials and by methods to resist or minimize flood damage.

The best and most common method for the reduction of flood damage in Oklahoma is to elevate the structure on compacted earthen fill. Since BFE information is not always available, the structure should have its lowest floor elevated above the historical high water mark. If that level is not known, then the building should be elevated so that sufficient drainage is provided or at least three feet above the highest adjacent grade of the construction site. The fill must be placed in layers and compacted to provide the necessary permeability and resistance to erosion, scouring, and settling, as set forth in NFIP regulations and documentation accordingly. Where feasible, the fill should extend at least 15 feet on all sides of the building and should be no steeper than one foot vertical to 2 feet horizontal. This provides a buffer zone to reduce the effects of flooding.

Structures can also be elevated by means of columns, posts, piers, or foundation walls. With proper design, buildings will have adequate support with minimum obstruction to the floodwaters. Flood damage to a structure can be minimized if the structure is built by a method that creates the least amount of obstruction to flood flows. An example would be to align a house parallel to flood flows. At the building site, consideration should be given to the additional hazards of water borne debris.

When a residential structure is constructed, it must be elevated so its lowest floor is above known flood levels. The FEMA Elevation Certificate is recommended to certify the lowest floor elevation (see Appendix 3-4). Basements are considered the lowest floor and are not allowed in the floodplain where they are subject to the direct and indirect effects of flooding. Prohibiting basements is a prime example of a way to reduce flood damages to residential structures.

Allowable construction methods differ between residential and nonresidential structures. NFIP regulations are more stringent with residential structures. Floodproofing is routinely allowed for non-residential structures and not for residential structures. If the lowest floor is constructed below the BFE, a floodproofing certificate is required (see Appendix 3-5).

Utilities servicing flood-prone structures should also be floodproofed and secured to prevent damage. Control panels located above flood levels will allow for access during periods of flooding. Controls for lower floors and basements could be installed separately to allow them to be disconnected independently. Heating, air conditioning, and ventilating equipment should be placed above the BFE.

NFIP regulations require that new and replacement water supply systems, sanitary sewer systems, and on-site waste disposal systems be designed to minimize or eliminate infiltration of floodwaters into the systems. To meet this requirement, local officials must be confident that community systems are designed to preclude infiltration. For example, manhole covers should be located above the Base Flood Elevation or otherwise designed to minimize flood damage. Waste treatment facilities, including pumping stations, lift stations, lagoons, and treatment plants must be floodproofed. Ring levees may have to be used to protect facilities located below the BFE. At a minimum, it is recommended that new water supply and sanitary sewers be constructed so they remain fully accessible and operational during a 25-year flood event, and that they suffer no physical damage from the 1% chance flood event.

On site waste disposal and treatment systems such as septic tanks must also be designed to minimize flood damage. This requirement may be especially difficult to attain since on site facilities may be located substantially below the first floor level of the structure that they serve. Generally, any inlets to the septic tank or outlets from the tank should be equipped with automatic and/or manual check valves to prevent floodwaters from returning through the pipes. A mound system of waste disposal may have to be used to provide adequate sub surface drainage during flooding.

The NFIP requires new subdivisions to be designed to minimize flood damage. Specifically, the regulations address the need to protect utilities and the need to ensure adequate drainage. For example, electrical, gas, water and sewer facilities should be protected from flood damage. Electrical facilities should be located above the BFE. Gas, water and sewer systems should be designed to withstand infiltration or rupture during flooding. To provide adequate drainage, building sites should be located at least two feet above the street elevation, and streets should drain promptly without ponding unless designed to temporarily hold stormwater surges.

For large subdivisions (50 lots or 5 acres or more), it is the responsibility of the developer to produce the BFE and delineate the boundary of the floodplain on the subdivision proposal. A grading plan showing the proposed elevation of streets and building sites should be included in the proposal. Portions of the grading plan located below the BFE may be used for streets, recreation, and other uses that are least harmed by temporary flooding. All structures must be located above the BFE.

All development permits must be reviewed to see if the proposed action will significantly obstruct floodwaters, thereby increasing flood stages. For communities without BFE data, if development is suspected of increasing flood height, additional justification is needed and the developer should detail to the community how his project will minimize adverse impacts.

For communities with BFEs but without a designated floodway, proposed actions (when combined with other existing and anticipated development) may not increase base flood heights more than one foot anywhere in the identified floodplain. The community must safeguard existing development from possible increased flood heights. To meet this standard, local administrators must, on a case-by-case basis, ensure developers provide them with a hydraulic analysis of the project's impact on flood heights. Before the development can go forward, the analysis must indicate that the project will not significantly increase (more than one foot) the Base Flood Elevation. The one-foot limitation is required by the NFIP and the Oklahoma Floodplain Management Act. Communities can adopt a more stringent requirement. For example, the State of Minnesota restricts any development that will cause more than a one-half foot six-inch rise in the BFE.

Communities that have BFEs established and a floodway delineated should not have to worry about this encroachment section because the FBFM or FIRM (if printed after January 1986) shows the area of the floodplain (the flood fringe) that can be fully developed without causing more than a one-foot increase in the BFE. This has been factored in the computer program that modeled and helped designate the community floodway boundaries during the Flood Insurance Study. Communities must, however, be aware of the regulations governing encroachments into the floodway.

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Specific Standards

Specific standards are required by the NFIP in addition to the general standards in communities where the BFE has been established. Allowable construction methods differ between residential and nonresidential structures. NFIP regulations are more restrictive in dealing with residential than nonresidential structures.

For new or substantially improved homes, apartments and other residential structures, the lowest floor, including the basement, must be elevated to or above the BFE. The lowest floor concept is illustrated in Figure 3-1.

In Oklahoma, the most common method of elevating structures is to build upon fill. However, under specific situations, it is possible to elevate structures by increasing the height of the foundation and making it floodproof. In all cases, the lowest floor must be above the BFE. Requests for letter of Map Amendment (LOMA) must demonstrate for structures that both the lowest floor (including basement) and the lowest adjacent grade to the structure have been elevated by fill to or above the BFE. Appendix 3-6 explains what the lowest floor is.

Recently revised NFIP regulations require new construction or substantial improvements that have fully enclosed areas below the lowest floor (e.g., a single story house on a foundation with no basement) be designed to allow the automatic entry and exit of floodwaters. This is to equalize hydrostatic and hydrodynamic pressure exerted by floodwaters on the exterior walls. Since this practice has many disadvantages in Oklahoma, building on compacted earthen fill is a more desirable alternative.

For all nonresidential structures, such as office buildings or stores, the lowest floor, including the basement, must be either elevated or floodproofed to or above the BFE. The floodproofing method used must ensure the structure is watertight and can resist water pressure in flooding situations. The community must have the assurances of a registered professional engineer that the floodproofing method is effective. Wet floodproofing (allowing water to enter and exit, and not causing damage) for nonresidential structures is not recognized for reduced flood insurance premiums under the NFIP.

Manufactured homes are now treated like any other residence. The lowest floor must be elevated on fill or a permanent foundation so the lowest floor is at or above the BFE. The only exception to this is in pre-FIRM manufactured home parks where substantial flood damage has not occurred.

Many communities in Oklahoma have adopted a Federal Emergency Management Agency model ordinance. The Model D ordinance gives communities an option of placing the manufactured home in the regulatory floodplain either at or above the base flood elevation or 36 inches above the ground. Community officials should select one or the other in this model ordinance or adopt more stringent requirements.

Each model ordinance requires the new placement of the manufactured homes in SFHA to be placed on a permanent foundation so the structure is securely anchored to resist flotation, collapse and lateral movement. Wind forces also need to be considered in the anchoring. In non-hurricane areas the lateral design load is 15 pounds per square foot and the net uplift design load is nine psf. It is important to consider these loads when the manufactured home is installed. Other factors that may affect the installation procedure includes the nature of the soil and the wind load and the specific state regulations. A soils engineer may need to be consulted.

Each manufactured home siting situation will be unique and needs to be evaluated as such. In general, spacing of supporting members should not exceed 10 feet and additional intermediate supports may be required. If a manufactured home is placed parallel to the flow, as shown in Figure 3-2, the drag forces are reduced due to the smaller area being exposed to flow, thereby reducing the tendency for the manufactured home to overturn. Compare this concept to Figure 3-3 where the home is placed perpendicular to the flow, thus increasing the tendency for the manufactured home to overturn. Also, when skirting is installed around the bottom of a manufactured home, construction standards for an enclosure should be followed.

Remember that in Oklahoma it is a state law that a licensed manufactured home installer install manufactured homes. For a list of licensed installers contact the Manufactured Housing Association of Oklahoma.

An enclosure is an area below the lowest floor. Enclosures can only be used for storage, parking and building access. An enclosure is required to have a minimum of two openings having a total net area of not less than one square inch for every square foot of the enclosed space. Also, the bottom of these openings shall be no higher than one foot above grade. The openings can have screens or other coverings placed over them provided they allow for the entrance and exit of flood waters.

Fuel storage tanks are now considered to be a structure, so elevating and anchoring on a permanent foundation are required (see Figure 3-4). In Oklahoma, especially in rural areas, typical fuel for a residential structure and manufactured home is propane. Propane is typically supplied from a fuel storage tank. Placement of these tanks in SFHAs requires a floodplain development permit and they should comply with anchoring and elevation requirements. Underground fuel storage tanks such as gasoline or diesel need to be floodproofed and anchored accordingly. A professional engineer may need to be consulted and a floodproofing certificate obtained ensuring such requirements have been met. When in doubt about the placement of these items contact the NFIP State Coordinator at the OWRB.

In Oklahoma, local floodplain administrators no doubt will have to deal with the construction of storm cellars in the SFHA. The construction of a storm cellar does not qualify for a variance. However, if constructed properly, there should be no problems. Storm cellars constructed in a SFHA should be required to be anchored sufficiently so that flood waters do not buoy up the structure. They should also be constructed so the entrance is constructed so it is at a minimum of one foot above the BFE, as well as any ventilation ducts. If the storm cellar is equipped with electricity it should be floodproofed, as well as any attendant utilities such as sanitary sewer or plumbed for drinking water. When the permit is issued a floodproofing certificate should be required.

Recreational vehicles are allowed on flood-prone sites without full elevation if licensed, highway ready, and in no place for more than 180 days. Recreational vehicles that are placed on a site for more than 180 days are not "ready for highway use" must meet the same flood protection requirements as manufactured homes and/or buildings. A recreational vehicle is ready for highway use if it fully licensed, 400 square feet or less, self propelled or can be towed by a light duty truck, sitting on its wheels or jacking system, attached to the sites only by quick disconnect type utilities and security devices, and is not used as a permanent dwelling.

Communities that have a designated floodway (on a FBFM or a post-1986 FIRM) have additional floodplain management responsibilities. The floodway is the conveyance area within the floodplain in which most development is to be avoided if at all possible. Consequently, NFIP regulations prohibit encroachments, including fill, new construction or other development in the floodways, unless it can be shown that the development would result in no increase in flood heights. Floodways are a compromise of community desires to develop the floodplain. Limited development may be permitted in the flood fringe based upon hydrologic and hydraulic data and computer modeling to determine the amount of encroachment allowed below the lowest floor (i.e., as a single story house on a foundation with no basement) be designed to allow the automatic entry and exit of floodwaters. This is to equalize hydrostatic and hydrodynamic pressures exerted by flood waters on the exterior walls. Since this practice has many disadvantages in Oklahoma, building on compacted earthen fill is a more desirable alternative.

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More Stringent Standards

Communities can and have adopted more stringent floodplain ordinance requirements than the basic minimum NFIP standards. Some communities have joined the Community Rating System. Some communities simply do not allow any development in the regulatory floodway. Many Oklahoma communities have adopted freeboard requirements. Chapter 6 contains additional details regarding the potential benefits of adopting more stringent standards.

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