Chapter 3: Local Floodplain Regulations and NFIP Standards
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. 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 .)
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.
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.
CONSTRUCTION MATERIALS AND METHODS
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 ).
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.
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.
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.
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.
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. 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.
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 , 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 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.
FUEL STORAGE TANKS
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. contains additional details regarding the potential benefits of adopting more stringent standards.
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Page last updated: January 18, 2017