Coastal Mapping Basics

The effects of tides, combined with large amounts of water and waves from storms make coastal areas some of the most at-risk areas when it comes to flooding.  To identify and map coastal flood hazards accurately, many factors must be taken into account, such as the amount of development in the area, the types and strength of storms that historically have affected the area, and the onshore and offshore topography.  This section provides an overview of the steps involved to identify and map the coastal flood hazards shown on the FEMA Flood Insurance Rate Map (FIRM).  The FIRM is used to determine flood insurance requirements for residents and where floodplain development regulations apply in a community. 

  

Each FIRM shows Special Flood Hazard Areas (SFHAs), or the areas subject to flooding by a storm or flood event that has a one percent chance of being equaled or exceeded in any given year.  The SFHA includes all ‘A’ and ‘V’ FIRM zones, the most common being:  A, AO, AH, AE, VE, and V.  For a full listing of flood zones with detailed descriptions, visit FEMA’s Flood Zones webpage.  Insurance rates and building requirements are tied to SFHA boundaries and the Base Flood Elevations (BFEs) that are established for these areas and shown on the FIRM.  A BFE is the water level expected in the SFHA for a one-percent-annual-chance flood event.  To accurately model and map coastal SFHAs and BFEs, study teams made up of engineers, coastal scientists, surveyors, and mapping specialists conduct coastal flood studies that include storm surge analysis and overland wave modeling.  The process is described below.  

 

Coastal flooding is usually caused by coastal storms, including tropical storms, hurricanes, and nor’easters.  Storm surge is the amount of water, combined with the effect of normal tides that is pushed towards the shore during a storm.  The height of the storm surge is driven by many variables, such as the strength and size of the storm, as well as how quickly and the direction in which the storm moves.  Using information from historical storms that have affected the study area, a representative set of storm events are determined and then modeled.  The storm surge elevations generated from this model are then used to determine the probability of a storm surge event of a given magnitude (for example, a storm that has a one percent chance of occurring in a given year). 

Storm surge analyses for FEMA coastal flood studies are often performed using the ADCIRC (ADvanced CIRCulation) coastal circulation and storm surge computer model in conjunction with the Simulating Waves Nearshore (SWAN) computer model.  Once complete, the storm surge analysis will result in the development of stillwater elevations for the one-percent-annual-chance flood event.   A stillwater elevation does not take into account all effects from waves coming ashore during a storm event.  Therefore, a separate overland wave modeling analysis is needed in addition to the storm surge analysis to accurately determine coastal SFHAs and BFEs.

 

In addition to storm surge, waves play an important role in coastal flooding.  An overland wave modeling analysis is required to fully evaluate the effect that waves have on coastal areas during a flood event.  FEMA typically uses a computer simulation model called the Wave Height Analysis for Flood Insurance Studies (WHAFIS) to perform overland wave modeling.  This model takes into account water depth, wind speed, vegetative cover, building density, and other factors to predict the heights of waves, which plays an important role in determining coastal BFEs and flood zones.  Wave runup and wave setup (discussed below) are also important factors in overland wave modeling.

   

Wave runup. The rush of water that extends inland when waves come ashore is called ‘wave runup.’  The overland wave analysis will determine the elevations to which wave runup extends during a storm event.  These elevations may be higher than the stillwater elevations computed as part of the storm surge analysis. 

Wave setup. In addition to the effects of wave runup and the computed storm surge stillwater elevation, there is also an increase in the water level caused by waves breaking ashore during a storm event.  This increase in the water level is called ‘wave setup’, which can be a significant factor in determining coastal BFEs.  Wave setup is affected by the height of the waves, the speed at which waves approach the shore, and the slope of the ground near the shore.

Erosion (or, the removal of soil) due to storms and the presence of coastal structures, such as seawalls are also taken into account when overland wave modeling is performed.

  

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Mapping Coastal Hazards

Once the coastal storm surge analysis and overland wave modeling are complete, the most up-to-date topographic information available will be used to map the coastal SFHA boundaries based on the results of these analyses.  BFEs will also be added to the FIRM at this point in the mapping process. The different flood risk zones that comprise the coastal SFHA - ‘V’ zones and ‘A’ zones (described below), will also be delineated.  

Coastal High Hazard Areas (V Zones)

Coastal high hazard areas, designated as V zones on the FIRM, are the areas of the coastal floodplain subject to the highest risk.  Typically, this is the area where the computed wave heights for the base (one-percent-annual-chance) flood are 3 feet or more.  V zones are subject to more stringent building requirements and different flood insurance rates than other zones shown on the FIRM because these areas are exposed to a higher level of risk than other coastal flooding areas.  

Portions of the SFHA landward of a V zone (i.e., areas where wave heights are computed as less than 3 feet) are mapped as ‘A’ zones on the FIRM.  While the wave forces in coastal A zones are not as severe as those in V zones, the capacity for the damage or destruction of buildings is still present.

  

   

 

An additional area that may be shown on coastal FIRMs for informational purposes is the Limit of Moderate Wave Action (LiMWA).  The LiMWA identifies areas that will be affected by waves with a 1.5 foot wave height or greater within the coastal A zone.  While FEMA currently does not require special floodplain management standards or flood insurance purchase requirements based on LiMWA delineations, it is likely that properties and structures within the LiMWA will receive substantial damage from wave action during a one-percent-annual-chance flood event.  As a result, communities are encouraged to adopt the more stringent building construction standards applicable for V Zones in these areas.

 

 

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