INSTRUCTIONS FOR COMPLETING
THE FLOOD HAZARD MAPPING NEEDS ASSESSMENT
Introduction
As of September 2000, the State of North Carolina, through the North Carolina Floodplain Mapping Program (NCFMP), has been designated by the Federal Emergency Management Agency (FEMA) as a Cooperating Technical State. This designation transfers the primary custodian responsibility for all flood insurance rate maps (FIRMs) to North Carolina. Based on this designation and partnership with FEMA, the state has embarked in an endeavor to remap all FIRMs in North Carolina. This will include the collection of new, accurate elevation data, new flood studies, and updated FIRM panels. With this in mind, responses to the attached questionnaire will help the state identify the appropriate streams that need to be studied. FEMA administers the National Flood Insurance Program (NFIP) by identifying Special Flood Hazard Areas (SFHAs) – areas expected to have a 1% or greater chance of flooding in any given year – on FIRMs. As development takes place and floodplain characteristics change, these maps require updating to ensure that communities have a useful tool for floodplain management. Some of the methods of evaluating mapping needs as described below may not apply to each community. However, any pertinent data that are submitted will assist the NCFMP in conducting the studies.
One of the best sources of information regarding the community’s map update needs is the floodplain manager for the community. It is highly recommended that the community floodplain manager by consulted for any of the information requested in the questionnaire, as well as any other general FIRM update needs. More information on the NCFMP is available at
- General Community Information
Part A of the questionnaire contains demographic and general questions about your community.
- Flood data update information
Revisions to your community’s existing FIRMs may be necessary due to increased urbanization or other physical changes in the watershed that alter the floodplain and its delineation on a FIRM. Please provide the following information for those flooding sources in your community, or portions of flooding sources, for which you believe changes in the flood hazard data on the FIRM are warranted. The following sections should help you determine whether an update is needed. For multiple flood sources, please copy pages 2 through 4 of the questionnaire and complete Part B for each source.
Flooding Source Information
Please provide the name of each flooding source that requires updated flood hazard data. Also, list the panel number or numbers of community’s FIRM on which each flooding source is shown. Provide the current flood zone designation(s) shown on the FIRM (i.e., Zone A, A1-30, AE, B, C V, V1-30, VE, etc.). Measure the approximate length of each flooding source and provide it on the questionnaire.
The Base Flood Elevation (BFE) shown on FIRMs is the elevation of the floodwater during a 1% annual chance (100-year) flood. If changes that have occurred in the hydraulic or hydrologic conditions of the flooding source are likely to result in a higher or lower BFE than currently shown on the FIRM, please indicated this on the questionnaire. Also, please estimate the likely change in the BFE (less than one foot, one to five feet, or greater than five feet).
Changes in Flood Data
Six categories of flood data updates are defined in the questionnaire. Please check those that apply to each flooding source and complete the appropriate subsection.
- Zone A Development Near Zone A and/or Unstudied Floodprone Areas
Areas designated on FIRMs as Zone A are areas where approximate methods were used to delineate the SFHA. Because detailed hydraulic modeling was not conducted in these areas, 1% annual chance (100-year) flood elevations are typically not computed for Zone A areas, and the floodplain delineation is only approximate. Consequently, if there has been development near Zone A areas, a detailed study may be warranted to determine flood elevations and boundaries more precisely. Similarly, if there are known floodprone areas that are subject to proposed development, these areas would likely benefit from updated flood hazard data. The density of development (low, moderate, or high) near each Zone A or unstudied floodprone area in the community should be evaluated when determining whether the FIRMS should be updated.
- Recent Flooding Event Contradicts Maps
If a State or Federal water resource agency has made estimates of the average return period of a flood in your community and it is approximately equal to the 1% annual chance (100-year) flood, comparisons with the mapped SFHA should be made to note any differences. However, care must be taken not to assume that a mapping error exists on the FIRM on the basis of historical flood events. The return frequency of flooding can vary greatly from stream to stream depending on the distribution of rainfall over the drainage basins. For example, if precipitation is localized, flooding on a small tributary may approach the 1% annual chance (100-year) event, but not much more than the mean annual flood may occur on the larger receiving stream. Conversely, the receiving stream could be experiencing flooding from rainfall in the upper basins that did not occur in the lower tributary basins, causing less severe flooding on the tributaries than the main stream.
Documentation of observed rainfall amounts and high-water marks, along with any photographs of flooding events within the community, is useful information to review when comparing historical floods with FIRMs. Please note whether such data is available for any recent major flood events. However, anecdotal information on flooding is not considered reliable unless it is combined with surveyed high-water mark locations and includes the date and time of the high-water observation. Information about the performance of bridges and culverts during the flood event is also useful. For example, did any bridge or culvert opening become clogged with debris? Photographs of water levels at bridges and culverts during flooding are also useful.
- Significant Development Throughout the Watershed
Hydrologic conditions determine the quantity and rate of runoff generated by a given rainfall event. This can be affected by various factors, such as the vegetation and land cover in the watershed, the infiltration of the solids, the slope of the land, the amount of impervious land, and the size of the watershed. The methods of hydrologic analysis and discharge date used to develop the effective FIRM are published in the community’s Flood Insurance Study (FIS) report, Section 3.1, Hydraulic Analyses. This section of the FIS report will indicate how the effective discharges were determined, and what information and methodologies were used.
When evaluating hydrologic conditions, consider each watershed or flooding source separately. The hydrologic issues that should be considered in evaluating the community’s need for a flood data update are significant changes in land use and new hydrologic data. Significant development or other changes in land use in the watershed (both within the community and any upstream communities) can significantly change the flood discharges. Often, the increase in impervious areas associated with urbanization causes increases in stream discharges. The amount of development in a community can be evaluated by reviewing a variety of information, including the community’s comprehensive plan, zoning maps, site plans for large project, and stormwater utility plans.
Two kinds of new technical data may significantly affect the base discharge estimation: new regional regression equations, and, in some circumstances, increases in the length of stream gage records. The base discharges computed for the effective FIS report can be compared to the base discharges computed using the most up-to-date U.S. Geological Survey (USGS) regional regression equations. A significant difference would indicate a need for a flood study update. An increase in the length of a stream gage record may also affect the flood discharge estimate. If the effective discharge was estimated by conducting a frequency analysis of a relatively short record of stream gate data, the baseflow estimate may be sensitive to newly added data. If stream gage data with a relatively long record (50 years or more) were used, however, a few additional years usually will not cause significant changes in the base flood estimation.
- New or Modified Flood Control Structures
Certain flood control structures (reservoirs, for example) are designed to reduce the peak discharges downstream. Therefore, any flood control structures constructed since the effective FIRM was published should be carefully evaluated to determine whether they have a significant effect on the base flood discharge. Note, however, that not all reservoirs or ponds provide storage capacity to reduce the peak of the base flood. It is necessary to evaluate the function(s) of a reservoir to determine whether a new structure would impact a community’s flood mapping. In addition to evaluation newly constructed flood control structures, previously existing flood control structures need to be evaluated to determine whether they continue to operated in the same manner they did when the effective FIS and FIRM were developed. For example, changes in the operating flood stages of a reservoir may impact how floodwaters are routed through the reservoir.
- New or Replacement Bridges, Culverts, and Road Crossings
If a bridge opening or culvert is not sized properly, it can cause floodwaters to back up, which increases flood levels upstream. Although most bridge openings and culverts are designed to allow stream flows associated with frequent storm events to pass without such backwater effects, they may still cause increases in 1% annual chance (100-year) flood elevations. Therefore, any bridges, culverts, or other road crossings that have been constructed since the analyses for the effective FIRM and FIS were completed should be evaluated for their potential effect on the 1% annual chance flood.
- Changes in Stream Location or Size
Any significant changes in the stream channel or floodplain geometry could affect the floodplain and floodway. Has any portion of the floodplain been filled? Has the stream channel moved or changed location as a result of significant erosion and/or deposition? Have any portions of the stream been channelized, widened, or dredged? Have there been significant changes in the vegetation in the floodplain? Aerial photographs are a useful tool in evaluating changes in stream channels and floodplains.
Community Recommendation
The last section of the attached questionnaire provides you the opportunity to recommend the mapping option for each flooding source when your community’s FIRM is updated. The mapping options include:
No change
Redelineated 1% annual chance (100-year) and 0.2% (500-year) floodplain boundaries based on current BFEs and new topographic data
Limited detailed analysis and mapping based on new BFEs and topographic data (1% annual chance floodplain boundaries only)
Detailed analyses and mapping based on new BFEs and topographic data (1% and 0.2% floodplains boundaries)
These options are listed in order of increasing level of effort and associated cost. For example, detailed analyses are approximately five times more expensive that limited detailed analyses. Therefore, this questionnaire and your recommendations will be used to assess flood-mapping needs and determine restudy priorities.
Data Submissions
Feel free to include additional comments or description on this document. Also, please attach any applicable explanatory or technical data, maps, photographs, etc. Please include information on anything that the NC FMP can do for you related to problems with your community’s FIS or FIRM. All submissions should be sent via surface mail, fax, or e-mail to the following address:
N.C. Floodplain Mapping Program
4719 Mail Service Center
Raleigh, NC 27699-4719
Attn: Scoping
fax: (919) 715-4080
e-mail:
Thank you for completing the Flood Hazard Mapping Needs Assessment questionnaire.
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