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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 ADCIRC 1 coastal circulation model in conjunction with the STWAVE 2 model. 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.

Coastal Analysis and Mapping - RiskMAP6 flood elevations (BFEs) will vary in each zone. Changes in flood zone and BFEs can have a significant impact on building requirements and flood

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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