Expansion of the Coastal High Hazard Area in the Florida Keys With a Focus on Islamorada, Village of Islands
The South Florida Regional Planning Council received a Coastal Resilience Award from the Gulf of Mexico Alliance to conduct an innovative surge analysis using new data and integrated methods to identify exposure to a Category 1 hurricane. Modelling was done for the Florida Keys Region at current sea levels, as well as with projecting the Coastal High Hazard Area (CHHA) expansion as the Category 1 hurricane inundation zone grows with two future sea-level rise scenarios.
Explore the tabs below to learn more about the project, including initial maps of the new results for Category 1 Hurricane Maximum of Maximum (MOM) Wave Heights with an emphasis on comparing the old and new basins for the Florida Keys and Islamorada, Village of Islands. The metadata and methodology are briefly described, and and the last tab in an overview of initial results. University of Miami intern Shanna Haley created an ESRI StoryMap as a tool for viewing and interpretting the results at http://arcg.is/2gWNZNM. The results will be used to identify assets under current and future risk within the CHHA and will be presented at a stakeholder engagement meeting in the Spring of 2018.
This map shows the variation in wave height for a Category 1 hurricane at high tide modelled with the new Super Basin
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In May of 2016, National Oceanic and Atmospheric Administration (NOAA) released a significantly updated Sea, Lake, and Overland Surges from Hurricane (SLOSH) basin for South Florida. This new Super Basin surge model has drastically improved vertical and horizontal resolution, and it now accounts for additional hydrological processes such as Kelvin waves (which travel parallel to coastlines), and the robust buffering capacity of mangroves. The mangrove effects were accounted for by introducing friction values into forested cells in the model. The inclusion of these and other hydrodynamic mechanisms into the model has dramatically altered wave heights in all regions along the coasts.
These updates have enabled surge predictions which are considerably more accurate than previous models. Because of these improvements, inundation depths and extents have shifted substantially in some locations, which could lead to shifting evacuation zones and necessitate updates to building codes, comprehensive plans, and CHHA delineation. The increased resolution is also valuable, allowing for more localized recommendations for adaptation and response. As CHHA areas are specified in many of Florida’s Planning Statutes, it is essential to have the most up-to-date surge modeling for computing the area and implementing it in comprehensive plans, particularly with redevelopment.
This map shows the differences of modeled surge heights using the new Super Basin in the Florida Keys (top), and compares it to the old the Florida Bay basin (bottom) for a Category 1 hurricane at high tide. It is of particular importance to note the significantly increased resolution and the greater water heights shown using the new basin model. The same spatial extent goes from containing 16,255 grid cells up to 73,217 grid cells. The new model indicates potentially increased risk as the maximum grid cell value is 11.8ft NAVD88 as compared to 7.9ft NAVD88 from the old model.
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This map shows the differences of modeled surge heights using the new Super Basin zoomed into Islamorada (top), and compares it to the old the Florida Bay basin (bottom) for a Category 1 hurricane at high tide. It is of particular importance to note the significantly increased resolution and the greater water heights shown using the new basin model. The same spatial extent goes from containing 546 grid cells up to 2650 grid cells. The new model indicates potentially increased risk as the maximum grid cell value is 6.8ft NAVD88 as compared to 4.5ft NAVD88 from the old model.
Click image to enlarge
Read more about the new data!
The new SLOSH Super Basin was created by simulating several thousand hypothetical hurricanes under different storm conditions to create a grid of Maximum Envelopes of Water (MEOWs) and the Maximum of MEOWs (MOMs). The NOAA Primer on Mapping Coastal Inundation outlines methods for using the GIS spline tool to interpolate SLOSH grid cells and compare wave height to a LIDAR elevation, thus generating a high-resolution surface for depth of inundation. This procedure can be applied to various sea-level rise scenarios via an additive method within the model.
The SLOSH cells were converted to points by using the feature to point tool. Cells that were assigned a value of 99.9, are “dry” cells, meaning the surge height in this area is zero, and were removed. To smooth out the surface the Spline tool was used to interpolate the points.
After interpolating the points the data was then ready to be compared to LiDAR elevations to determine inundated areas. This was done by using the raster calculator to subtract the LiDAR DEM from the interpolated surface.
This method produced a raster layer representing the inundated areas. Areas containing a positive value (including zero) represented areas inundated by water. Areas containing a negative value, indicate that the land elevation is higher than the surge height, and therefore the land is dry. This “wet” raster layer was then converted to polygons, using the raster to polygon tool, to depict the areas inundated by storm surge. The results also provide a surface for a 10ft horizontal resolution depth of inundation. The model was run again for USACE 2040 and 2060 SLR scenarios at medium and high.
The initial results were uploaded to an ESRI StoryMap, which can be viewed at http://arcg.is/2gWNZNM
The maps below show the CHHA for the Upper and Lower Keys.
Areas shaded yellow represent land inundated by a category 1 storm surge at current sea levels. This indicates the current CHHA for the Keys.
The teal represents the areas inundated by a storm surge for category 1 hurricane with the 2040 high sea-level rise projection of 1 foot. These results indicate the 2040 CHHA.
The red represents areas inundated by the 2060 high sea-level rise projection of 2 feet and the corresponding CHHA.
The Lower Keys have a lower average elevation, with most sitting around 5 feet. The lower elevations make the Lower Keys more vulnerable to storm surge, with most of the islands being inundated from storm surge at current sea levels. However, Key West, with a maximum elevation of 18ft sees little effects are current sea levels, and does not become impacted from storm surge until 2 feet of sea-level rise.
The Upper Keys, having a higher average elevation, feel the effects more gradually. As the sea levels rise the areas inundated slowly grow with a tipping point between 1 and 2 feet of SLR.
These results show that between one and two feet of sea-level rise there is a tipping point. There is minimal change between current sea levels storm surge inundation and 1 foot of SLR inundation. There is a drastic change between 1 foot of SLR and 2 feet of SLR.
The recommendations in the StoryMap focus on how the National Flood Insurance Program (NFIP)’s Community Rating System (CRS) can be utilized to increase resilience and in turn provide flood insurance discounts to residents.