Flood Hazard Mapping

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Coastal Hazard Bulletin

Issue No. 15

The purpose of the Coastal Hazard Mailing List is to provide up-to-date coastal flood hazard information pertinent to the Federal Emergency Management Agency and the National Flood Insurance Program. Subscribers will receive new mailings on a monthly basis.

Data Requirements for Coastal Flood Hazard Analyses

In previous coastal flood hazard Listserv bulletins, the application and use of individual CHAMP modules has been discussed. However, definitions and descriptions of data requirements for coastal flood hazard analyses upon which CHAMP is based have not yet been presented. A brief introduction to some of the main modeling and mapping theoretical components upon which CHAMP is based is provided below. These definitions and descriptions should be considered when developing coastal map revision requests. More detailed information is provided in the Guideline and Specifications for Wave Elevation Determination and V Zone Mapping and the Guidelines and Specifications for Flood Hazard Mapping Partners; specifically, in Appendix D.

This discussion will assist the map revision requester to submit a complete request including providing MT-2 Forms in each individual package. Therefore, please use this bulletin as a resource in addition to the aforementioned documents and forms when submitting a map revision request.

Request type
Coastal V-zone revision requests can be accomplished through the Flood Map Revision Process, for which the Letter of Map Revision Request (LOMR) process is the best avenue. The LOMR application process is accomplished through the completion of MT-2 Forms. In coastal V zones, map revision requests cannot be accomplished through the MT-1 Process (LOMA/LOMR-F). This is because the determination of flood hazards in the coastal zone not only considers ground elevation of the subject area with relation to the Base (1% annual chance [100-year]) Flood Elevation (BFE) as shown on the effective Flood Insurance Rate Map (FIRM), but also considers the inland limit of the Primary Frontal Dune and high velocity wave action which constitutes the Coastal High Hazard Area, or V zone.

Letters of Map Change (LOMRs and LOMR-Fs) typically require user fees which can be found at: http://www.fema.gov/plan/prevent/fhm/frm_fees.shtm.

Transect selection/layout
Computer models used to determine flood hazards are executed along transects, which are cross sections taken perpendicular to the mean shoreline to represent a segment of coast with similar characteristics. The requester should locate transects with careful consideration given to the physical and cultural characteristics of the land so that they will closely represent conditions in their locality. Transects are to be placed closer together in areas of complex topography, dense development, unique flooding, and where computed wave heights and runup may be expected to vary significantly. Wider spacing may be appropriate in areas having more uniform characteristics. For example, a long stretch of undeveloped shoreline with a continuous dune or bluff having a fairly constant height and shape, and similar landward features may require a transect only every 1 to 2 miles, whereas a developed area with various building densities, protective structures, and vegetation cover may require a transect every 1,000 feet.

Topography
The topographic data, usually in the form of maps, associated with a map revision request must have a contour interval no greater than 5 feet or 1.5 meters (revisions that are based solely on more detailed topography than that used in the effective maps are free of charge). FEMA does not require more detailed information such as spot elevations or a smaller contour interval, although they can be useful in the definition of the dune or bluff profile and in the delineation of floodplain boundaries. The topographic data must be recent and reflect current conditions or, at a minimum, conditions at a clearly defined time. Transects need not be specially surveyed unless available topographic data are unsuitable or incomplete. The requester shall examine the topographic data to confirm that the information to be used in the analysis and mapping represents the actual planimetric features that might affect identification of the coastal hazards. The community, county, and state are usually the best sources for topographic data. The requester should consider U.S. Geological Survey (USGS) 7.5-minute series topographic maps.

Datum
The vast majority of products pertinent to FEMA and the NFIP are currently referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29) for vertical control data (ground, structure, and flood elevations). FEMA has converted some of its map products to the new national datum, the North American Vertical Datum of 1988 (NAVD 88). If the datum on the FIRM is NAVD 88 then the map revision request should be submitted in NAVD 88.

Stillwater Elevation
The stillwater elevation is the maximum storm-induced water-surface elevation, primarily a combination of the normal astronomic tide and the storm surge. Stillwater elevations for the open coast are usually determined through analysis of historical gage data or by utilizing a storm surge model. Various one-, two-, and three-dimensional models can also be used to determine stillwater elevations for estuaries, embayments, and flooded inland areas. Typically, for the Gulf of Mexico and the Atlantic Ocean (south of Long Island, NY), older effective coastal Flood Insurance Studies (FISs) were developed using the standard FEMA surge models TTSURGE or FEMA SURGE (FEMA, August 1988) to model hurricane storm surge levels. For the northern Atlantic Ocean region, the Stone & Webster Northeaster Model was applied or the elevations were derived from a tide gage analysis by the U.S. Army Corps of Engineers (USACE), New England District. For the Great Lakes region, the USACE, Detroit District, has established return period lake levels for stillwater elevations used in FISs. However, many of the Great Lakes region FISs do not use the updated 1988 published lake level return period information and need to be updated. Stillwater elevations for many parts of the country have been determined and are published in FIS Reports.

Wave Setup
Nearshore wave action can increase mean water elevation in front of a shore barrier by the phenomenon called wave setup, which is the superelevation of the water surface owing to waves alone and not including storm surge. When applying the storm surge elevation to a request, wave setup should be considered seperately, and should be added to the stillwater elevation if not already done so in the effective FIS. Many of the older effective studies did not consider wave setup; in contrast, most of the new restudies are required to consider the application of wave setup.

The estimation of wave setup should follow the guidance provided in the USACE Shore Protection Manual (1984). There may be some sort of evidence that provides justification for the addition of wave setup to the existing stillwater elevations. The evidence may be found in the form of recent tide gage data analysis at open coast tide stations, or through an assessment of historical high water information from a significant coastal flood event. If effective stillwater elevations have been exceeded by recent flood events of a magnitude less than the those expected for a 1% annual chance flood event, the need for wave setup would be justified. Technical verification for wave setup may also be established from the assessment and comparative analysis of the high watermark information with existing stillwater elevations. Open coast tide stations can be analyzed for the presence of wave setup in the detailed tide measurement records during a significant historical coastal flood and wave event.

Erosion Treatment
The primary factor controlling the basic type of dune erosion is the pre-storm cross section lying above the 1% annual chance Stillwater elevation (SWEL) (frontal dune reservoir). The requester shall determine this area to assess the stability of the dune as a barrier. If the elevated dune cross-sectional area is very large, erosion will result in retreat of the seaward duneface with the dune remnant remaining as a surge and wave barrier. On the other hand, if the dune cross-sectional area is relatively small, erosion will remove the pre-storm dune leaving a low, gently sloping profile. Different treatments for erosion are required for these two distinct situations because no available model of dune erosion suffices for the entire range of coastal situations.

To prevent dune removal during the 1% annual chance storm, the frontal dune reservoir must typically have a cross-sectional area of at least 540 square feet (or 20 cubic yards volume per foot along the shore)(Figure 1)(FEMA, 1986; FEMA, November 1988). For more massive dunes, erosion will result in duneface retreat, with an escarpment formed on the seaward side of the remaining dune (Figure 1). To compute the eroded profile in such cases, FEMA has adopted a simplified version of the dune retreat model developed by Delft Hydraulics Laboratory of the Netherlands. This treatment is also appropriate in cases where sandy bluffs or headlands extend above the 1% annual chance SWEL.

If a dune has a frontal dune reservoir of less than 540 square feet, storm-induced erosion can be expected to obliterate the existing dune with sand transported both landward and seaward. The requester shall estimate the eroded profile using duneface removal procedures (Figure 1). Those procedures provide a realistic eroded profile across the original dune, but do not determine detailed sand redistribution by dune erosion, overwash, and breaching.

Figure 1. Open coast sand dunes must be eroded in accordance with FEMA's 540 square foot rule. Sand dunes with a frontal dune reservoir less than 540 square foot will be treated as dune removal, while dunes with a frontal dune reservoir greater than 540 square foot will be treated as dune retreat.

WHAFIS
Wave height analyses are usually computed with the Wave Height Analysis for Flood Insurance Studies (WHAFIS) model, originally developed by the National Academy of Sciences in 1979, revised in 1984 to include the effects of marsh grass, and revised in 1988 (WHAFIS 3.0) (FEMA, September 1988) to incorporate wave regeneration equations in the 1984 edition of the USACE Shore Protection Manual. WHAFIS calculates wave heights and corresponding wave crest elevations along the transect and requires stillwater elevations (with wave setup added, if appropriate), topographic data, land-cover data, and an available fetch. The input criteria for fetch should be calculated using procedures in the USACE Shore Protection Manual (as incorporated into WHAFIS 3.0), while the topographic data should be taken from 5-foot contour maps or better, and the land-cover data should be taken from aerial photographs or field surveys no more than five years old. The first row of buildings, buildings elevated on pilings, and protective structures in poor condition should not, in most cases, be considered in the analysis as obstructions to wave propagation.

Wave Runup
Runup calculations on beach slopes are performed with the RUNUP 2.0 model. The wave runup model requires medium deep-water wave characteristics and bathymetry in addition to the WHAFIS data, and computes a runup value (depth) and the inland extent of flooding. The wave runup elevation (SWEL plus runup depth) is extended horizontally seaward until it intersects with the wave crest elevation profile to yield the final wave envelope profile for the transect. Runup on sloped and vertical structures should follow guidance in the USACE Shore Protection Manual.

Primary Frontal Dune
The Primary Frontal Dune is a continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms. The landward limit of the primary frontal dune, also known as the toe or heel of the dune, occurs at a point where there is a distinct change from a relatively steep slope to a relatively mild slope. The primary frontal dune toe represents the landward extension of the Zone VE coastal high hazard velocity zone.

Mapping
All submitted mapping should document topographic data sources (USGS quads, ground surveys, aerial photography, etc.) vertical datum, scale, contour interval, transect location(s), inland limit of the V zone, inland limit of the Primary Frontal Dune, zone boundary locations and BFEs for each flood zone. All mapping should be certified by a licensed and registered surveyor or engineer. An annotated portion of the FIRM with proposed flooding boundaries should also be submitted as part of a map revision request.

Coastal High Hazard Area
Coastal high hazard area is an area of special flood hazard extending from offshore to the inland limit of a primary frontal dune along an open coast and any other area subject to high velocity wave action from storms or seismic sources. These areas are typically called V Zones.

Coastal BFEs consist of the 1% annual chance stillwater elevation plus any increases due to the addition of wave setup, and the effects of wave heights and/or wave runup.

References
Federal Emergency Management Agency. (1986). Assessment of Current Procedures Used for the Identification of Coastal High Hazard Areas (V Zones). Washington, D.C.

Federal Emergency Management Agency. (August 1988). Coastal Flooding Hurricane Storm Surge Model, Volumes 1, 2, and 3. Washington, D.C.

Federal Emergency Management Agency. (September 1988). Wave Height Analysis for Flood Insurance Studies (Technical Documentation for WHAFIS Program Version 3.0). Washington, D.C.

Federal Emergency Management Agency. (November 1988). Basis of Erosion Assessment Procedures for Coastal Flood Insurance Studies. Washington, D.C.

U.S. Army Corps of Engineers, Coastal Engineering Research Center. (1984). Shore Protection Manual, Volumes I and II, 4th Edition, Washington, D.C.

Future Coastal Hazard Listserv Bulletin Topics

Future bulletins will consider different map revision request scenarios in order to provide a more detailed description of the application of individual CHAMP modeling techniques. The next two bulletins will address CHAMP modeling techniques for a non-restricted fetch (open-ocean) wave height dominated V-zone case.

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Last Modified: Tuesday, 19-Jun-2007 11:57:20 EDT


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