A Levee Policy for the National Flood Insurance Program (1982) / Chapter Skim
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'APPENDIX B: FEMA INTERIM POLICY ON LEVEES'
'APPENDIX B: FEMA INTERIM POLICY ON LEVEES'
Pages 80-88
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From page 80... ...
The degree of protection afforded by a levee system is uncertain because of both the uncertainty involved in establishing the 1 percent chance flood elevations and the uncertainty involved in the structural stability of the levee itself. Common engineering practice in the construction of flood protection works, such as dams and levees, is the inclusion of a freeboard allowance above the computed water surface levels, to allow for all of the uncertainties in analysis, design, and construction that cannot be fully or readily considered in an analytical fashion.
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From page 81... ...
Inspection and Evaluation Prior to any levee system receiving credit on any future map issued by FIA, the contractor responsible for the production or revision of that map will be required to determine that the minimum design requirements itemized above have been met. In addition, a field inspection or suitable alternative, will be required to verify that the levee appears structurally sound and adequately maintained.
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From page 82... ...
Mapping of Leveed Areas Levees meeting the design, closure, and inspection requirements listed above will be credited with providing protection from the 1 percent chance flood on FHBMs and FIRMs. However, the protected area will be identified with a note on the maps stating This area protected from the 100-year flood by levee, dike or other structure subject to failure or overtopping during larger floods.
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From page 83... ...
EXCEPTIONS TO LEVEE FREEBOARD REQUIREMENTS (AUGUST 17, 1981) Background The interim levee policy issued on February 10, 198l, provides for exceptions to the minimum freeboard standard provided that the levee owner can demonstrate, to FEMA's satisfaction, that adequate protection is provided against the 1 percent chance (100-year)
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From page 84... ...
Elements Required The purpose of a freeboard allowance is to provide for those factors that cannot be rationally accounted for in design flood profile computations. When exceptions to the freeboard standard are sought, the levee owner must demonstrate that those factors that have not been directly considered in the 100-year profile computations are of insignificant consequences and do not appreciably increase the probability that the levee will overtop or fail during the true 100-year event.
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From page 85... ...
Seepage Levees work primarily on the principle of providing a mass of material large enough to prevent shear failure by the hydrostatic and hydrodynamic forces acting on the levee embankment and foundation. Because of the construction costs involved, levees rarely contain the impervious cores or drainage layers common to earth dams.
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From page 86... ...
Requests for exception from the standard freeboard requirements should demonstrate that no appreciable erosion of the levee embankment can be expected during the 100-year flood, as a result of either stream currents or waves, or that anticipated erosion would not result in failure of the levee embankment or foundation directly or indirectly through reduction of the seepage path and subsequent instability. The following factors should be addressed in these analyses: 1.
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From page 87... ...
, the exception request should address the above 5 factors, the amount of settlement that has occurred since construction, and the amount and rate of expected future settlement. Where additional loss of freeboard due to expected future settlement could contribute significantly to the probability of levee overtopping or structural failure during the 100-year flood, an exception to the standard freeboard requirement will not be granted.
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From page 88... ...
It is not applicable to coastal flooding situations or to structures constructed partially or wholly of concrete, or similar impervious materials. This is because the interim levee policy incorporates certain assumptions regarding the nature of flooding and modes of failure of typical earthen riverine levees.
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