Floodplain status determination

Computer generated flood analysis shows chance of flooding over a 30-year period overlaid on aerial photo of a city. A wide area of reds and oranges shading upstream of an encroachment of the valley indicates more flood potential than downstream of the restriction.


Letter of Map Amendmentedit

Insufficient map topographic detail or accuracy can result in the unwarranted determination of Special Flood Hazard Area (SFHA). An application for a Letter of Map Amendment (LOMA) uses an Elevation Certificate (prepared by a Registered Land Surveyor or Registered Professional Engineer) to ask FEMA to remove the flood insurance requirement on individual properties.

Online Letter of map changeedit

FEMA's website "Change a Flood Zone Designation - Online Letter of Map Change" says homeowners and other interested parties may submit an Online Letter of Map Change (LOMC). FEMA says this can be used for property that was incorrectly included in a flood zone or if the addition of fill has elevated the property above the flood zone. Information on the property's location, legal description, and use of fill are required for FEMA to determine if the property is located in a flood zone. FEMA might request additional information.

Letter of Map Revisionedit

For multiple properties or a larger area, an application for a Letter of Map Revision can be submitted when the landscape topography is different from that shown on the floodplain boundary and/or flood heights shown on the FIRM and the Flood Insurance Study. A Letter of Map Revision based on Fill (LOMR-F) is used when landscape topography is altered by humans, usually to increase the land elevation and remove land from the floodplain. A Conditional Letter of Map Revision (CLOMR) and Conditional Letter of Map Revision Based on Fill (CLOMR-F) are strongly advised as a mechanism to obtain FEMA feedback on the project before site changes are made, especially in light of the increasing attention on the nexus between the NFIP and the Endangered Species Act. 44 C.F.R. § 65.6(10) says "A revision of flood plain delineations based on topographic changes must demonstrate that any topographic changes have not resulted in a floodway encroachment."

Proposed encroachmentsedit

FEMA says that justifiable encroachment within the floodplain might include “construction or modification of a bridge, culvert, levee, or similar measure”. The NFIP prohibits communities to issue variances "within any designated regulatory floodway if any increase in flood levels during the base flood discharge would result". However, a community may request FEMA's prior approval for a justifiable encroachment that would increase the base flood level. This community request must include seven required submittals such as a request for conditional approval of map change, an evaluation of alternatives, a request for revision of base flood elevation determination, and a request for floodway revision. All requests for revision of base flood elevations must include supporting data with "all the information FEMA needs to review and evaluate the request. This may involve the requestor’s performing new hydrologic and hydraulic analysis and delineation of new flood plain boundaries and floodways, as necessary.” Details of the required supporting data are listed in 44 C.F.R. § 65.7.

Outdated flood mapsedit

The U.S. Geological Survey says increased runoff resulting from urban growth generally increases flood hazards to communities and that flood hazard maps based on data before urban growth may no longer be accurate.:4 The U.S. Geological Survey gives the example that construction encroaching into the floodplain narrows the floodplain width thereby increasing the resistance to flow. Consequently, the water is at a higher stage as it flows past the construction causing backwater to flood a larger area upstream.:2 FEMA's computer model image at right "represents the percent chance of flooding at least one time during a 30-year period for a given cell, or location, within the mapped floodplain".

Updating flood mapsedit

NFIP regulation "Requirement to submit new technical data" says "A community’s base flood elevations may increase or decrease resulting from physical changes affecting flooding conditions. As soon as practicable, but not later than six months after the date such information becomes available, a community shall notify the Administrator of the changes by submitting technical or scientific data in accordance with this part. Such a submission is necessary so that upon confirmation of those physical changes affecting flooding conditions, risk premium rates and flood plain management requirements will be based upon current data." FEMA requires "Copies of the input and output data from the original and revised hydraulic analyses shall be submitted" with the hydraulic analysis supporting revisions to flood maps. A 2015 FEMA website identifies that the U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center's River Analysis System (HEC-RAS) computer program has been adopted for the preparation of studies and restudies for the NFIP.

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