Calculate landslide potential for communities affected by wildfires

Wildfires make the landscape more susceptible to landslides when rainstorms pass through an area after wildfires. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, and can strip vegetation, block drainage ways, damage structures, and endanger human life. Often there is not enough time between a fire and a rainstorm to implement an effective emergency response plan. However, various post-fire debris-flow hazard assessment models have been developed to estimate the probability and volume of debris flows that may occur in response to a storm.

For more detailed information on hazard assessment, review the following resources:

• USGS Post-Fire Debris Flow Hazard Assessment Viewer
• Post-fire debris-flow models
• USGS site for burn area emergency response

In this tutorial, you'll perform raster analysis in ArcGIS Pro to create a landslide risk map for the Santa Rosa, California, region, which was devastated by a wildfire in 2017. Creating the landslide risk map uses raster function chains to derive a burn severity map, topographic slope map, and a land-cover index map, which are combined into one processing chain. Once you have the result, you will summarize the landslide risk by subbasin and share a tile layer to ArcGIS Online for use in a web map or shared app.

This tutorial was last tested on June 3, 2024, using ArcGIS Pro 3.3. If you're using a different version of ArcGIS Pro, you may encounter different functionality and results.

View final result

Requirements

• ArcGIS Pro (see options for software access)

Create a burn severity map

Download data and open a project template

First, you will prepare for your analysis by downloading the necessary data and opening an ArcGIS Pro project template.

1. Download the LandslideData.zip file.
2. On your C drive, create a folder named LandslideData.
3. Go to the Downloads folder and extract the contents of the .ZIP file you downloaded into the LandslideData folder.
4. Start ArcGIS Pro. If prompted, sign in using your licensed ArcGIS organizational account.

   Note:

   If you don't have access to ArcGIS Pro or an ArcGIS organizational account, see options for software access.

5. Click Start with another template.

   

6. Browse to C:\LandslideData, choose the project template file, Landslide_Risk_Project.aptx, and click OK.

   

7. In the New Project window, name your project Landslide_Risk and save it to the default location (in the Documents folder) on your computer. Click OK.

   

   The new project opens with a map centered on Santa Rosa, California.

   

   Next, you will make a folder connection to the LandslideData folder so you can quickly access the data.

8. On the ribbon, click the Insert tab. In the Project group, click Add Folder.

   

   The Add Folder Connection window appears.

9. Browse to the LandslideData folder you created. Select it, and click OK.

   

   Now that you have the project open and a folder connection to the data, you are ready to perform analysis.

Analyze burn severity

Different types of vegetation and materials burn with different intensity depending on their composition, density, topography, wind, soil moisture, and other factors. Generally, higher burn intensities result in higher water repellence and higher potential for erosion from rainfall. Burn intensity, or severity, can be derived from multispectral imagery with a near-infrared and shortwave infrared band, such as Landsat 8 imagery. Using raster functions in a processing chain, you’ll compute a burn ratio using Landsat 8 imagery acquired before and after the wildfire that took place around Santa Rosa, California, in 2017.

To get started, you'll add Landsat 8 imagery from before and after the fire, a digital elevation model, and land-cover raster datasets.

1. On the ribbon, click the View tab. In the Windows group, choose Catalog Pane.

   

   The Catalog pane appears.

2. In the Catalog pane, on the Project tab, expand Folders, and expand the LandslideData folder. Select all the datasets inside except Basins.tif and drag them onto the map.

   Tip:

   You can select multiple items at the same time by pressing the Ctrl key and clicking the files.

   

   The data appears in theContents pane and on the map.

   

   The data includes Landsat 8 imagery for before (Before_L8.tif) and after (After_L8.tif) the October 2017 wildfires in Napa and Sonoma counties. There are also two layers you'll use as inputs for your risk map. The DEM_30m.tif layer is a digital elevation model showing the terrain's elevation. The Sonoma_NLCD2011.tif layer is a portion of the National Landcover Dataset, which shows land use and predominant vegetation type.

3. In the Contents pane, drag the After_L8.tif layer to the top of the list, and drag Before_L8.tif just below it. Turn off and collapse the Sonoma_NLCD2011.tif and DEM_30m.tif layers.

   

   Currently, the Landsat imagery is displayed in the map using the red, green, and blue bands. To compare the burn scars, you’ll display some of the imagery's multispectral bands.

   

4. Under After_L8.tif, right-click the red color chip and choose sr_band5.

   

5. For green, choose sr_band4, and for blue, choose or verify sr_band3.

   

   This band combination displays the Landsat 8 imagery bands in color infrared mode. Vegetation is shown in bright red. Nonvegetated features such as bare and urban areas are displayed in various shades of gray and blue.

   

6. For the Before_L8.tif layer, set the following:

   • Change the red color chip to sr_band5.
   • Change the green color chip to sr_band4.
   • Change the blue color chip to sr_band3 or verify that it is already selected.

   

   Next, you'll use the Swipe tool to compare the before and after imagery of the wildfire.

7. In the Contents pane, click After_L8.tif to select it.

   

8. On the ribbon, click the Raster Layer tab. In the Compare group, click the Swipe tool.

   

   The pointer displays as an arrow in the map display.

9. Click while swiping across the image in the map to compare the imagery before and after the wildfire.

   

   You can swipe vertically or horizontally. Notice that many areas that are red in Before_L8.tif are gray or green in After_L8.tif, indicating lost vegetation.

10. On the ribbon, click the Map tab. In the Navigate group, click Explore.

    

    The pointer returns to normal. Now that all the data is on the map, you’ll use raster functions to calculate the burn severity.

11. On the ribbon, click the Imagery tab. In the Analysis group, choose Raster Functions.

    

    The Raster Functions pane appears. On the System tab are categories of functions available for raster analysis. For this tutorial, two raster function templates, or RFTs, have been created for you. These custom function templates are listed under the Project tab.

12. In the Raster Functions pane, click the Project tab.

    

    Two raster function templates are included in this project: Landcover_Remap and Burn_Severity.

    Note:

    If you do not see the raster function templates, do the following:

    On the Raster Functions pane, click the Custom tab. Next to Landslide Grenada, click the Import functions button. In the Select Processing Templates window, browse the Folders > LandslideData and click Landcover_Remap.rft.xml, and click OK. Repeat these steps and add the Burn_Severity.rft.xml from the same folder.

13. Right-click Landcover_Remap, point to Move to, then point to Custom and click Custom1.

    

    By moving these functions to a custom category, any edits you make to the RFTs will be saved if they are saved in the Function Editor. Changes made in the Project category will be lost if the project is not saved.

14. Click the Project tab. Right-click Burn_Severity and move it to Custom1.

    Next, you'll open and explore the Burn_Severity RFT.

15. On the Custom tab, right-click the Burn_Severity template and choose Edit.

    

    The Function Editor opens and displays the processing chain.

    

    The Band Arithmetic functions turn the pixels of the imagery into expressions. The postfire imagery is subtracted from the prefire imagery and run through a remap function. The remap function categorizes the pixel values into five categories of burn severity. The breakpoints of the five burn severity values are obtained from a landscape assessment study (Key and Benson, 2005). The Attribute Table function in the processing chain assigns a color ramp to the burn severity map. This has already been created for you.

16. Close the Burn_Severity RFT in the Function Editor pane.

    

    Now that you understand how this raster function works, you'll use it to calculate the burn severity of the project area.

17. In the Raster Functions pane, click the Burn_Severity template.

    The Burn_Severity Properties raster function appears.

18. For Pre-Fire Imagery, choose Before_L8.tif, and for Post-Fire Imagery, choose After_L8.tif.
19. Ensure Output Layer Type is set to Raster Layer and click Create new layer.

    

    The processing may take a few minutes to complete. When finished, the resulting layer is displayed in the map and listed in the Contents pane. Raster functions are temporary in nature—calculations are performed on the fly, or in real time as you move on the map, and are not saved automatically. Burn severity is computed dynamically in the display as you navigate around the layer.

    

    You can scroll through the map and locate the areas that were affected the most. As stated in the legend, those areas are displayed in orange and red on the map.

20. On the Quick Access Toolbar, click the Save button.

    

    You've created and run a raster function template to create a burn severity layer.

Now that you've seen what a raster function can do, you'll learn how to build a raster function template in the next section to create a slope index map.

Create a slope index map

Build the slope raster function template

The slope map is a critical layer in determining slope stability. Slope steepness is derived from a digital elevation model (DEM). The steeper the slope, the more prone it is to slipping, especially during rainfall after stabilizing vegetation has been burned away. Next, you’ll build and save a raster function template to calculate percent slope and use it to create a steepness index.

1. On the ribbon, click the Imagery tab. In the Analysis group, click Function Editor.

   

   The Function Editor pane docks at the bottom of the map window.

2. In the Raster Functions pane, click the System tab. Expand the Surface group.

   

3. Drag the Slope function into the Function Editor pane.

   

4. In the Function Editor pane, on the ribbon, click Add Raster Variable.

   

   A green box titled Raster is added to the Function Editor pane.

   

5. Select the Raster box and position it to the left of the Slope function box.

   

   The Raster box will define the input dataset for the Slope function.

6. Point to the Raster box to see the Out parameter displayed. Click Out and drag to connect the raster element (Out parameter) to the Slope function (DEM parameter).

   

7. Right-click Raster and click Rename. Type Input DEM and press Enter.

   

8. Double-click the Slope function. In the Slope Properties window, click the Variables tab. For DEM, check the IsPublic field.

   

   The IsPublic option allows you to alter the input data in a later tool process.

9. Click OK.

   Your raster function will take an input digital elevation model and calculate its slope. The next function, Remap, will classify the slope into five categories of steepness.

10. In the Raster Functions pane, on the System tab, search for Remap.

    

11. Drag the Remap function into the Function Editor pane to the right of the Slope function.

    

12. Connect Slope to Remap by setting the Out of Slope to the Raster input for Remap.

    

    Next, you’ll set the inputs for the remap to index the slopes (in units of degrees) into five categories.

13. Double-click the Remap function. In the Remap Properties window, click the first box under the Minimum heading and type 0. For Maximum, type 5, and for Output, type 1.

    

14. Set additional categories with the following values:

    	Minimum	Maximum	Output
    2	5	15	2
    3	15	25	3
    4	25	35	4
    5	35	91	5

    

15. Click the General tab. For Output Pixel Type, in the drop-down menu, choose 8 Bit Signed.

    

    The Attribute Table tool, which will be the next tool you add, can only take input rasters that are 8 bit. Hence, you set the Output Pixel Type parameter to 8 Bit Signed.

16. Click OK.
17. Right-click the Remap function and choose Rename. Type Slope Remap and press Enter.

    This will help distinguish between functions later in this tutorial when you string several raster function chains together.

18. In the Raster Functions pane, search for the Attribute Table function and drag it into the Function Editor pane to the right of Slope Remap.

    

19. Connect the output from the Slope Remap function to the input (Raster) of the Attribute Table function.

    

20. Double-click the Attribute Table function and set Table Type to Manual.

    

21. Under the blank table, click the Generate button.

    

22. For Maximum Value, choose 5. Click OK.

    Five rows are added with values 1 to 5 and a default green-to-red color scheme.

    

23. Click Class Name for each row and assign the slope classes as follows:

    Value	Class Name
    1	Flat
    2	Low
    3	Moderate
    4	Steep
    5	Very Steep

    

24. Click OK.

    Now that your raster function template is complete, you'll save it.

Save and run the slope function

Save the raster function template to your project and run it to create the slope index layer.

1. In the Function Editor pane, click Save As.

   

   The Save As window appears.

2. In the Save As window, for Name, type Slope_Index.
3. Ensure Category is set to Custom and Sub-Category is set to Custom1.
4. For Description, copy and paste A raster function template to derive slope from an input DEM.

   

5. Click OK and clear the Raster Functions pane's search bar.

   Your Slope_Index raster function template (RFT) now appears in the Custom category in the Raster Functions pane.

   

6. In the Raster Functions pane, click Slope_Index.

   

7. For Input DEM, choose DEM_30m.tif. Ensure Output Layer Type is set to Raster Layer.

   

8. Click Create new layer.

   When it is finished processing, the layer displays on the map and is listed in the Contents pane with the name Slope_Index_DEM_30m.tif.

   

9. Close the Slope_Index RFT in the Function Editor pane and save your project.

   Now that you have a function to classify slope, you'll combine a number of RFTs together to create a landslide risk map.

Create a landslide risk map

Build the landslide risk raster function

The landslide risk calculation combines the two variables you just worked with: burn severity and slope. It also includes land cover, which is also important to landslide risk. Vegetation stabilizes slopes through root systems. Wildfire can wipe out much of the stabilizing vegetation. Some types of vegetation, especially species of chaparral, have adapted to wildfire, and the root systems are especially deep underground to survive fires. Land cover has already been indexed into five categories for you depending on its stabilizing effect on slopes. To perform the landslide risk calculation, you’ll add the three raster function templates into a chain and process them.

1. On the ribbon, on the Imagery tab, click Function Editor.

   The Function Editor pane appears.

2. In the Raster Functions pane, press the Ctrl key and select the Landcover_Remap, Burn_Severity, and Slope_Index RFTs. Drag them onto the Function Editor canvas.

   

   By default, the RFTs cluster together. You’ll separate them so that you can more easily connect their outputs.

   

3. Click and hold to draw a box around the Slope_Index functions, and drag the entire group so that it’s below the Burn_Severity RFT.

   

   Tip:

   The green input boxes mark the beginning of each function chain. The Slope_Index RFT starts with the green Input DEM box.

   

4. Drag the Landcover_Remap RFT below the Slope_Index RFT.

   

5. Right-click each Attribute Table function and rename it to match the corresponding Remap function.

   Their new names should be Burn Severity Attribute Table, Slope Attribute Table, and Landcover Attribute Table.

   

6. In the Raster Functions pane, click the System tab and search for the Weighted Overlay function. Drag it into the Function Editor pane to the right of the other three RFTs.

   

7. Connect the three Attribute Table outputs to the input parameter of the Weighted Overlay function.

   

8. In the Function Editor pane, click the Auto Layout button.

   

   The RFTs are arranged compactly.

   

9. Double-click the Weighted Overlay function.

   The Weighted Overlay Properties window appears. Inside the Weighted Overlay Table, you can assign weights in percentages to each raster.

10. In the Weighted Overlay Table, in the cell next to <Burn Severity Attribute Table.OutputRaster>, type 30. For the <Slope Remap Attribute Table.OutputRaster> layer, assign 55 percent. For the <Landcover Remap Attribute Table.OutputRaster> layer, assign 15 percent.

    

    These hazard weights are loosely based on research done by the USGS for its National Landslide Hazards Program.

    The Remap Table is still empty. Since all three layers have the same number of index categories, you'll map each of them one-to-one.

11. In the Weighted Overlay Table, click the <Burn Severity Attribute Table.OutputRaster> layer. In the Remap Table, under Value, click NODATA to edit the attribute field and type 1. Under Scale, click NODATA and choose 1.

    

12. In the Remap Table, double-click the empty row at the bottom of the Value column and type 2. For Scale, choose 2.

    

13. Repeat the previous step to add rows 1 through 5 for all three rasters in the Weighted Overlay Table.

    

14. Click OK.
15. In the Raster Functions pane, on the System tab, search for the Attribute Table function and drag it into the Function Editor pane to the right of the Weighted Overlay function.

    

16. Connect the output from the Weighted Overlay function to the input of the Attribute Table function.
17. Double-click the Attribute Table function. In the Attribute Table Properties window, for Table Type, choose Manual.
18. Under the blank table, click the Generate button and set Maximum Value to 5. Click OK.

    Five rows are added with values 1 through 5 and a default green to red color scheme.

19. Click Class Name for each row and assign them the values as follows:

    Value	Class Name
    1	Low
    2	Medium
    3	High
    4	Very High
    5	Extreme

    

20. Click OK to close the Attribute Table Properties window.

Save and run the landslide risk raster function

Next, you will save the raster function template and run it to create the landslide risk layer. This template considers slope, burn severity, and land cover as the primary criteria to determine landslide risk.

1. In the Function Editor pane, click Save As.
2. In the Save As window, for Name, type Landslide_Risk. Ensure Category is set to Custom and Sub-Category is set to Custom1.
3. For Description, copy and paste Raster Function Template to calculate landslide risk based on wildfire burn severity, slope, and landcover.

   

4. Click OK and close the Function Editor pane.

   Your raster function is complete. You'll now run it using distributed raster analytics.

5. In the Raster Functions pane, clear your search.

   

6. If necessary, click the Custom tab. Click Landslide_Risk.

   The Landslide_Risk raster function appears.

7. Fill in the input parameters as follows:

   • For Pre-Fire Imagery, choose Before_L8.tif.
   • For Post-Fire Imagery, choose After_L8.tif.
   • For Slope Input DEM, choose DEM_30m.tif.
   • For Landcover Remap Raster, choose Sonoma_NLCD2011.tif.

   

8. For Output Layer Type, choose Raster Layer and click Create new layer.

   

9. Close the Slope_Index RFT in the Function Editor pane and save your project.

   You have created the landslide risk layer for the area by adding several criteria and functions to one function. The landslide risk layer is useful on its own to identify at-risk areas, but there are always ways to enhance an analysis.

You will further your analysis by considering watershed basins within the study area, as these have an impact on landslide risk as well.

Summarize landslide risk by watershed subbasin

Perform raster analysis

Although the landslide risk map is useful, you want to go further to break down the areas that are most at risk. Because landslide risk is impacted by precipitation patterns and watershed characteristics, you'll summarize risk by watershed basins within the study area using a raster function. Then, you’ll share the resulting layer to ArcGIS Online, where you can add it to a web map or app and share it.

1. In the Catalog pane, click the Project tab and browse to the LandslideData folder connection.
2. Add the Basins.tif raster to the current map.

   

   This is the layer that you will use to summarize landslide risk. You will use a raster function called Zonal Statistics to perform the analysis.

3. In the Raster Functions pane, click the System tab. In the search box, type zonal and open the Zonal Statistics function.
4. In the Zonal Statistics Properties pane, fill in the following parameters:

   • For Zone Raster, choose Basins.tif.
   • For Value Raster, choose Landslide_Risk.
   • Accept the remaining defaults.

   

5. Click Create new raster.
6. In the Contents pane, rename the zonal statistics output layer to Landslide Risk per Basin (Sonoma and Napa). Turn off all layers except the result of the Landslide Risk per Basin (Sonoma and Napa) layer and the basemap.

   When the analysis finishes, a dataset that shows the average risk per watershed is added to the map.

   

   The areas in white and light gray have higher landslide risk values. This is based on the risk you calculated using the raster function template with the weighted inputs of slope, burn severity, and land cover.

Symbolize and share your analysis results

You have located areas with higher landslide risk based on the basins. However, the current results are difficult to see with this symbology, so you'll update it to make it easier to understand the results from your analysis.

1. In the Contents pane, right-click the Landslide Risk per Basin (Sonoma and Napa) layer and choose Symbology.

   The Symbology pane appears.

2. For Color scheme, click the current black to white scheme.

   

3. In the menu that appears, click Show names and choose the Slope color scheme.

   Now the color scheme matches the output of the other layers you created. You will blend the layer to enhance its display.

4. In the Contents pane, ensure that the Landslide Risk per Basin (Sonoma and Napa) layer is selected.
5. On the Raster Layer tab, in the Effects group, for Layer Blend, click the drop-down menu and choose Multiply.

   

   The layer darkens and has a transparency applied to it.

6. Zoom in to the layer to view it up close.

   

   The layer looks better and you can see the underlying basemap. Now the layer is ready to share.

7. In the Contents pane, right-click the Landslide Risk per Basin (Sonoma and Napa) layer, point to Sharing, and choose Share As Web Layer.
8. For Name, if there are underscores present, remove them. For Layer Type, choose Tile.

   

9. Click the Configuration tab.

   

   You will modify some settings to control the output projection and how to handle caching. You want the output projection to be Web Mercator, so you will choose the ArcGIS Online tiling scheme.

   Note:

   If you don't set the tiling scheme and caching options, your layer will publish, but it won't be useable in ArcGIS Online.

10. For Tiling Scheme, click the drop-down menu and choose ArcGIS Online / Bing Maps / Google Maps.
11. Expand Options and click Cache locally.

    

12. Click Publish.
13. At the bottom of the Share As Web Layer pane, click the Manage the web layer link.

    

    The item details page for the tile layer appears.

    

14. Click Open in Map Viewer

    The layer displays, but in the sharing process, the blending is lost. You can quickly blend the layer in the web map to enhance its appearance.

15. In the Properties pane, in the Appearance section, for Blending, click the drop-down menu and choose Multiply.

    

16. Close the Blending window and the Properties pane.

    

    Now the landslide by subbasin layer is symbolized the same way it was in ArcGIS Pro. Next, you'll save the web map so you can share it or embed it in other apps, such as a story or instant app.

17. On the Contents (dark) toolbar, click Save and open.

    

18. In the Save map window, for Name, type Landslide Risk by Subbasin - Napa & Sonoma and click Save.

    Now the web map is part of your content and you can share it with your organization or the public as it is or as part of another app.

    Note:

    To learn more about creating apps, see this tutorial.

In this tutorial, you created raster function templates to process criteria layers, summarized risk by watershed subbasin using a raster analysis tool, and have shared the resulting layer. You now have a final result layer that illustrates areas with the highest landslide risk that you can include in other web maps or apps and share them across your organization.

Acknowledgements

• World Topographic Map sources: Esri, HERE, Garmin, FAO, NOAA, USGS, © OpenStreetMap contributors, and the GIS User Community.
• Digital Elevation Model (DEM) courtesy of U.S. Geological Survey, downloaded from TNM Download (v2.0).
• Landsat scenes (dated: 9/25/2017, 12/13/2017, 12/22/2017, and 12/23/2017) courtesy of U.S. Geological Survey, downloaded from USGS Earth Explorer.
• U.S. Geological Survey, 2014, National Land Cover Database (NLCD) 2011 Land Cover Conterminous United States: U.S. Geological Survey data release, https://doi.org/10.5066/P97S2IID.
• The basins dataset was derived from the Digital Elevation Model, courtesy of U.S. Geological Survey, and processed using the ArcGIS Spatial Analyst extension.