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April 2011 www.txchart.com
Coastal Analysis and Mapping
Integrating Modern Technology with the Latest Data
Started as part of the Federal Emergency Management Agency (FEMA) Map
Modernization program (Map MOD) and continuing under the Risk MAP
(Mapping, Assessment, Planning) effort, FEMA is undertaking a comprehensive
analysis of the Texas coastal flood hazards. Integrating the latest topographic data sets with state-of-the art modeling techniques, FEMA will provide citizens
and community officials in the Gulf Coastal counties of Texas with up-to-date
flood hazard information, replacing maps and studies that are based on data
and modeling technology as far back as the 1970s.
Comprised of several components, the FEMA coastal flood study process
examines surge height, wave height and how far the surge with wave action
will travel inland. This fact sheet provides an overview of the science behind a coastal flood risk study, which is composed of two basic parts: coastal storm
surge analysis and coastal hazard analysis and mapping. A more detailed
explanation of the components of the study can be found at www.txchart.com.
Coastal Storm Surge Analysis
Simply stated, storm surge is water, combined with normal tides, that is
pushed toward the shore by strong winds. Determining the height of the
storm surge is complicated because it is affected by many variables, including
storm intensity, storm track and speed, wave height and period, offshore depths, and shoreline configuration. Sophisticated models and computing
capabilities are needed to handle the complexities of integrating the large
quantities of data and simulations. The U.S. Army Corps of Engineers and
project partners, together with FEMA are conducting the surge analysis. This
analysis includes a state-of-the-art elevation model that was created at a 10-
meter resolution covering the entire storm surge model area for the Texas Gulf Coast counties. The elevation model is produced by merging the latest coastal
LiDAR (Light Detection and Ranging), which is a modern mapping technique
for acquiring topographic information from aircraft, and other topographic
survey datasets with the most reliable bathymetric datasets (underwater
depth measurements) for the region.
Storm surge simulations are performed using the ADCIRC1 coastal circulation
model in conjunction with the STWAVE 2model. Model calculations are performed on the elevation model covering the area from the 60 degree west
meridian to the U.S. mainland. FEMA will use bathymetric and topographic
elevations interpolated from the elevation model and extend sufficiently inland
to allow for inland storm surge flooding.
1 Advanced CIRCulation for Model of Oceanic, Coastal, and Estuarine Waters; www.adcirc.org 2 STWAVE - STeady State spectral WAVE; chl.erdc.usace.army.mil
Coastal Flood Zones
Within the coastal Special Flood
Hazard Area (SFHA), there are two primary zones: Zone AE and Zone VE. Zone VE has a
wave component that is greater than 3 feet in height. Coastal
Zone AE has a wave component of 0-3 feet in height.
Base flood elevations (BFEs) will vary in each zone. Changes in
flood zone and BFEs can have a significant impact on building
requirements and flood insurance costs. Because waves
can diminish in size in a short distance, particularly where the
ground beneath the surge elevation is steep, BFEs can
change dramatically.
LiMWAs
Recent post-disaster assessment
and wave tank research have shown that waves as small as
1.5 feet can cause significant structural damage. Since 2009,
all coastal studies must map the limit of the 1.5-foot wave as an
informational layer; this is known as the Limit of Moderate
Wave Action (LiMWA).
The NFIP Community Rating
System (CRS) provides credits for communities requiring VE
zone construction standards in areas defined by LiMWA or
areas subject to waves greater than 1.5 feet.
June 2011 www.txchart.com
Because of the limited record of hurricanes and tropical
storms for the study area, a synthetic set of tropical
storms was created to estimate the representative 100-
year and 500-year return period surge elevations. There are approximately 450 synthetic tropical storm
simulations to run; these simulated storms are based
on historical storm parameters and compared to
historical events to help establish validity.
The results from the storm surge analysis are new
stillwater elevations (SWELs) combined with setup (the additional elevation of water due to nearshore wave
action) for 17 coastal counties. The SWELs with setup
provides the starting shoreline conditions for the next
component of the coastal study.
Coastal Hazard Analysis and Mapping
Using the surge data involves several steps, the results
of which are used to produce the final products: a
Digital Flood Insurance Rate Map (DFIRM) and Flood Insurance Study (FIS). This part is guided by the
procedures and includes:
� Defining a base topography data set sufficient for
analysis and mapping; high resolution elevation
models are used.
� Defining cross-shore transects to represent terrain
and variability of shoreline features; this incorporates characterization of land cover from
field reconnaissance and overland features such as
dunes, buildings and plant types.
� Conducting analyses to characterize storm-induced
erosion (e.g., dunes) which may occur within
coastal areas; this also includes identifying the Primary Frontal Dune (PFD), if present, and
incorporating the expected performance of coastal
structures; such as seawalls, dikes, levees and
jetties.
� Conducting overland wave modeling to define
coastal hazard areas and establish base flood
elevations (BFEs); the modeling consists of
incorporating wave setup, conducting wave height analysis using Wave Height Analysis for Flood
Insurance Studies (WHAFIS), and performing wave
run-up and over-topping analysis.
� The mapping process will also include defining the
Limit of Moderate Wave Action (LiMWA). The
LiMWA is a boundary that identifies the location of the 1.5-foot wave height within the Coastal AE Zone
where wave damage is substantial and
communities should consider requiring VE Zone
building criteria. The location of the 1.5-foot wave
height determined during the WHAFIS modeling of
the one-percent- annual-chance condition is
extracted and the LiMWA is mapped accordingly. Mapping of coastal hazard areas utilizing the
results of modeling and analysis; in accordance
with FEMA’s Procedure Memorandum 50 (2008).
Once mapping is complete, the preliminary DFIRM and
FIS are provided to community officials and citizens for
review and use. The typical mapping lifecycle will then continue through to its effective date. As a result, the
coastal communities will have the best available coastal
flood risk information, based on the most current data
and state-of-the-art technology, and are therefore able
to make better informed decisions about what actions
are needed to reduce that risk.
Components of a Coastal Flood Risk Study