Storm Surge Mapping

Best Practices for Mapping Storm Surge Inundation

Introduction

Many coastal areas in the United States, including the East Coast and the Gulf of Mexico, are vulnerable to storm surge flooding. Storm surge driven by hurricanes can result in loss of life, economic and environmental damages, and disrupt infrastructure and livelihoods of coastal communities. The graphic below shows what flood inundation looks like.

Courtesy of National Weather Service - Weather Ready Nation Program.

Developing visualizations of the storm surge hazard that are intuitive and easy to comprehend are vital for understanding the risk of flooding and for decision making when individuals are preparing, mitigating, and responding to this type of weather hazard. The simulation below is an illustration of a hypothetical storm. It shows how quickly a storm surge can inundate an area and how high the flood water depth can be.

The storm surge animation was created by the NCAR GIS Program and supported by the National Science Foundation grant (award No. 1331490).

In this Story Map, we consider different cartographic principals when creating storm surge maps and discuss how to best communicate the information about potential storm surge flooding. We explore different ways to map coastal flooding due to hurricane storm surge using data from NOAA's National Weather Service National Hurricane Center (NHC). For demonstration purposes, we used the NHC's storm surge advisory #25 that was issued for Hurricane Ian which was valid for 09/28/2022 1600 UTC.

Accessing storm surge forecast data

Storm surge models are a useful scientific tool for forecasting hurricane storm surge. These models provide information on potential inundation for areas under storm conditions. NOAA uses the Sea, Lake, and Overland Surges from Hurricane (SLOSH) model to estimate storm surge water heights.

To download SLOSH storm surge forecast data in a GIS (shapefile) format from NOAA NWS NHC's website, click the button below.

The NOAA NWS NHC distribute a dataset called Potential Storm Surge Flooding (Inundation) which maps storm surge values that have a 1 in 10 chance of being exceeded. This is an operational data product that shows the risk associated with coastal flooding from storm surge associated with tropical cyclones. The Potential Storm Surge Flooding data are based on the SLOSH model and take into account forecast uncertainty.

Screenshot of the NOAA NHC website that distributes storm surge advisory data in GIS format.

For every storm, NOAA's NHC provides a number of potential storm surge flooding advisories that are valid for different times. Every advisory starts with an "al" for Atlantic storms or "ep" for East Pacific storms. The files are numbered sequentially beginning with the first storm each season, followed by the storms advisory number.  Lower numbers indicate earlier advisories. Storm surge forecast will differ based on how the storm characteristics change over time. 

All examples in this Story Map are using the advisory #25 for Hurricane Ian, which made landfall off the West coast of Florida on September 28th, 2022. This advisory was valid for Sept. 28th, 2022 for the time 1600 UTC. The file name - AL0922_25 - means that the storm was in the Atlantic Ocean, was the 9th storm of the 2022 season, and it is advisory #25.Every storm surge forecast has an inundation and a tidal mask files. These two files contain the same information about the potential storm surge flooding, but the tidal mask layer include areas (generally symbolized in gray) that are above water at low tide and under water during high tide. These areas also include estuaries, wetlands and other areas that are regularly under water due to natural tidal cycle. 

The maps below illustrate this information and can be downloaded from the NHC website. The tidal mask (map on left) symbolizes areas that regularly flood during high tide in gray. These areas are generally not inhabited such as wetlands. Using the tidal mask help to draw the map readers' attention to the flooded areas that normally are dry, and which will see the most impact due to storm surge. The inundation file (map on the right) maps the full extent of the area impacted and does not differentiate between areas that are often underwater and those areas that are normally dry.

Potential storm surge flooding is shown with (left) and without (right) a tidal mask. Note: Colors used in this color scheme (unique color classification) do not follow the official NOAA NHC potential storm surge flooding color scheme.

Mapping storm surge forecast

Storm surge information is quantitative data, communicating water heights above ground level. Usually the data is available as raster data stored in netCDF or TIF file formats. When working with quantitative data there are several decisions that need to be made concerning the way the data is mapped.

Color selection

One decision to make when mapping storm surge information is which color scheme to use. Using distinct colors (qualitative color scheme) to highlight the differences between the flood levels is a common way to map storm surge. Using a qualitative color scheme will not illustrate order in the data (high to low) but will emphasis differences.

Monochromatic or sequential color scheme uses a single color with varying shades (light to dark). Since storm surge data are quantitative and the values are flooding from low to high, monochromatic schemes work well to illustrate the flood levels and show magnitude. A good online tool to help select colors for cartographic mapping is  ColorBrewer  website.

Looking at the maps below, the one on the left is symbolized using Sequential / Single Hue Blue while the map on the right is using a qualitative color scheme, both from ColorBrewer.

Which method do you think is more effective?

Symbolizing flood levels using monochromatic (left) and qualitative (right) color scheme.

Classification method

Mapping storm surge flooding information using unique flood values (examples above) or using a classification method is another decision to make when mapping storm surge. In the maps above each flood level is mapped using a unique color, however, if there are many different flood levels the map may be very busy and hard to read.

The map below shows flood levels using 5 classes for flood levels using the blue sequential color scheme recommended from ColorBrewer. In the example below flood is being grouped by less than 1 foot, 1 to 3 feet, 3 to 6 feet, 6 to 9 feet, and over 9 feet.

Symbolizing flood levels using a classification method and a sequential blue color scheme.

Scale

When mapping storm surge, scale is an important aspect to consider. Do you want to map storm surge at a regional scale, to see the extent of the surge, or do you want to map storm surge at a local scale to see more details?

The map on left is an example of mapping storm surge flooding at the regional scale. This scale is helpful to understand the extent of the flooding and to assist in making decisions about evacuation. The map on the right is an example of mapping storm surge flooding at the local scale (Naples). This map gives a better understand of flooding impacts for a city.

The map on the left is mapped at a regional scale while the map on the right is mapped at a local scale.

Labels

Labels, or the act of adding text to a map, is extremely important for map comprehension. Labels function in not only naming geographic features but also to show relationship between features.

Visual Hierarchy

Decisions about font style, color and size is a key act in visual hierarchy, which helps readers to understand the map more quickly and easily. 

The label for states may be considered more important than the label for cities, which may be considered more important than street names.

In the example below, larger cities have larger font sizes on the map to create a higher conceptual ranking in the visual hierarchy.  Highways are also labeled with a different font and are placed along the road.

Visual hierarchy for the city labels, illustrate bigger cities will larger text, and smaller cities with smaller text.

Which features should be labeled?

Labeling cities and roads on a map of storm surge is a good way to help map readers to orient themselves and to understand the location of flooding. However, adding locations of places that are important to people, such as schools, churches, landmarks, may also help flooding comprehension for map readers.

People feel connection  to different place in their community and it is important to think of your audience when selecting which features to add to the map. Understanding ones personal risk may increase when seeing that a local church or school is in the flooded area. 

Images in maps

Adding images of flooded places can also help map readers understand their risk. In the map below flooded images are added to the map legend to illustrate what the different flood levels look like to help map readers understand what the depth of flooding would look like on a typical house.

Flooding house to illustrate flood depth levels to map legend.

Flooded images can also be helpful to illustrate what flooding would be on common features such as a local church or grocery store. These images can be added to maps or be used as stand alone imaged.

When generating these images it is important to think of your intended audience and use familiar architecture and iconic buildings that map readers will recognize. Using a stormy sky and dark murky water are good ideas when generating these images. 

The images above were created using Photoshop and a plugin called Flaming Pear - Flood 2. You can learn how to create flooded images using the  Flaming Pear - Flood YouTube  tutorial.

Interactive Maps

Another consideration when it comes to creating and delivering storm surge maps is whether or not to use static or interactive maps. Each have their advantages and disadvantages. Static maps are controlled solely by the map creator, and thus map characteristics such as scale, layer visibility and transparency, and in some cases color, can't be changed by the user. They are often easier and quicker to create, and can be delivered in multiple formats including JPEG and PDF However, users of static maps are not able to interact with the map, control what layers are displayed, or alter the map in any way. Sometimes maps display dense information and there is not an option to zoom in on the map to see more detail.

Interactive maps, on the other hand, give much more control of the map to the user. For instance, users can control the scale and zoom level of the map, allowing them to focus on their area of interest. They have the ability to zoom in and see more of the detail that the data may contain, or then can zoom out to see a broader geographic context. Interactive maps also allow the user to query features on the map. By clicking on a map feature of interest, such as a road, building, or flood hazard layer, users can access information about the feature. This might include a road name, building height, or storm surge category and flood depth. These feature attributes are not accessible through static maps and can provide valuable information to decision makers and other map users.

Interactive web map of southeast Florida displaying data from Hurricane Ian. In the bottom left of the map, click the "Open layer list" button to toggle the layers on and off, or click the "Open legend" button to display the symbology of each layer that is turned on.

3D Visualization

Community decision makers, including forecasters, emergency managers, and private citizens, need meaningful and easily interpreted data to increase their situational awareness and understanding of the hazard. Viewing data such as storm surge scenarios in 3D provides an added sense of realism that traditional 2D data displays can’t achieve. This may include the ability to more quickly recognize landmarks or places of interest, as well as being able to better visualize the impacts being communicated, such as water depths on streets or in buildings.

Key components of a 3D GIS visualization include high-resolution elevation and imagery data, infrastructure data such as buildings or vegetation that contain height information, and visual overlays that highlight the communicated threat or hazard, including storm surge, sea-level rise, and other coastal flood-related scenarios. Additional elements can be included such as weather affects, moving cars and people. Common platforms for both creating and disseminating 3D GIS visualizations include Esri’s ArcGIS (ArcGIS Pro and 3D Web Scenes) and Google Earth. Each of these platforms have their advantages and disadvantages in terms of ease of use, functionality, detail, and data integration.

3D visualizations can also be static maps. The image below shows flooding of Naples that was created in ArcGIS Pro 3D Scene. The buildings symbolized in darker red reflect buildings that may experience more flooding due to storm surge.

3D Scene of Storm Surge in Naples, FL

Summary

It is crucial to create visual representations of storm surge threat that are simple and easy to understand so that readers of the map can make informed decisions while preparing, mitigating, and responding to this weather hazard. Designing a storm surge map that effectively communicates the associated risks involves integrating several cartographic elements, such as color, classification, scale, and labels. Considerations such as geographic scale, dimensions, and realism can enhance or detract from the maps effectiveness. With the aid of GIS technology, adding supplementary information such as images and text can enhance the reader's comprehension and understanding of their vulnerability to the hazard. Ultimately, knowing the map's intended audience and purpose is essential to improving awareness of the risk to storm surge.

This research is supported by the National Science Foundation Award 1853699. The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Contact : The GIS Program  gis.ucar.edu 

Courtesy of National Weather Service - Weather Ready Nation Program.

Screenshot of the NOAA NHC website that distributes storm surge advisory data in GIS format.

Symbolizing flood levels using a classification method and a sequential blue color scheme.

Visual hierarchy for the city labels, illustrate bigger cities will larger text, and smaller cities with smaller text.

Flooding house to illustrate flood depth levels to map legend.

3D Scene of Storm Surge in Naples, FL