Explore lava flow hazard
Where is the lava flow hazard?
When a volcano erupts, how much time do residents have to evacuate? That depends. Lava flows downhill and travels faster over steep ground. In the early 1990s, residents of Kalapana, a town in the southeastern Puna region, had days or even weeks to prepare for a lava flow that eventually covered the town. By contrast, a 1950 lava flow down the western flank of Mauna Loa reached the sea in about four hours. Although scientists monitor ground movement on the island continuously, there is no way to know how much advance notice residents living downhill of an eruption will receive.
What emergency responders and governments can plan for is evacuation scenarios. Knowing where lava flow hazard is greatest and how many people live in these areas can help responders understand where to set up evacuation shelters, send resources, and more. In this section, you'll find areas with high lava flow hazard by filtering the layer.
- Go to the ArcGIS Online group, Analyze Volcano Shelter Access in Hawaii.
- Under the Recently added content section, click the thumbnail of the Shelter Access Analysis map to open it.
The map opens to the island of Hawaii.
As shown in the legend, the island is divided into nine categories of lava flow risk, from extremely high hazard to extremely low hazard. Under the partially transparent zones, census blocks symbolize population density (people per square mile) in shades of gray. The map also shows the locations of volcanoes and emergency shelters.
This tutorial uses Map Viewer.
- If necessary, on the ribbon, click Open in Map Viewer.
- Sign in to your ArcGIS organizational account.
If you don't have an organizational account, see options for software access.
- On the Contents (dark) toolbar, click the Layers button. Click the Lava Flow Hazard Zones layer to select it.
- On the Settings (light) toolbar, click Filter.
- In the Filter pane, click Add expression.
- Build the expression SEVERITYCODE is High Hazard.
- Click Add expression two more times and build the expressions SEVERITYCODE is Very High Hazard and SEVERITYCODE is Extremely High Hazard.
You now have three expressions. The Filter is currently set to show features that match all the expressions, which means that no data is showing because each feature has a single rating. To find hazard zones that meet one of the three criteria, you'll change the filter results to match at least one expression.
- For Filter results, click Match all expressions and choose Match at least one expression.
- In the Filter pane, click Save.
The three highest-hazard lava flow zones are now shown on the map. These are the areas you'll be analyzing.
- In the Layers pane, for Lava Flow Hazard Zones, click Options and choose Rename.
- Rename the layer High Lava Flow Hazard Zones and click Ok.
Where are the people?
Next, you’ll use Census data to understand where on the island people live. Understanding the distribution of people and how many live in areas of high risk is helpful for planning where emergency shelters are needed, and how large they should be.
- In the Layers pane, drag the Census Blocks 2020 layer above the High Lava Flow Hazard Zones layer.
- On the Settings (light) toolbar, click Styles.
The layer is currently symbolized by the Total Population attribute.
- For Pick a style, choose the Dot Density style.
The layer redraws using the dot density style. This style uses the same Census Block polygon areas, with each dot representing a specific number of features. For example, each dot may represent 40 people. The Dot Density style doesn't show where in the polygon the features are actually located, but scatters the dots randomly through the polygon.
Most of the island's population lives around the perimeter of the island, with population centers concentrated in Kailua-Kona to the west and Hilo to the east. Next, find how much of the population is within high-risk areas.
- Click Done.
- On the Contents toolbar, click Save and open. Click Save as.
- In the Save map window, for Title, type Hawaii Shelter Access Analysis and add your initials.
- In the Save map window, click Save.
Analyze shelter access
What areas currently have access to volcano shelters?
Next, you'll analyze accessibility by creating drive-time areas around the emergency shelters. Travel area analysis uses either distance or expected travel time along a street network to show the area within reach of a given location or set of locations. You'll use a drive-time value of 15 minutes to define a shelter as easily accessible. This number was chosen arbitrarily, and you might want to repeat the analysis later with other values to see how the results change.
This analysis does not attempt to model emergency conditions. In a real emergency, all sorts of things may happen. Evacuation routes may be cut off by lava flow, roads may be jammed, emergency shelters themselves may lie in the lava flow path, and so on. Nonetheless, the ability to see which areas of the island have easy access to a shelter, and which do not, is a useful planning aid.
- On the Settings (light) toolbar, click Analysis.
- In the Analysis pane, click Tools and search for Generate Travel Areas and select the Generate Travel Areas tool.
The Generate Travel Areas tool calculates the area that can be reached within a specific travel time or distance along a street network. In this case, you want to calculate a 15-minute drive to shelters.
- For Input layer, click Layer and choose Emergency Shelters.
- For Travel mode, make sure Driving Time is selected.
- For Cutoffs, type 15 and click Add. Make sure that Cutoff units are set to Minutes.
- For Travel direction, click Away from input locations and choose Toward input locations.
Because people will be traveling to the shelters, Leaving the Arrival time parameter set to Time unspecified assumes that traffic flow is unimpeded, and motorists observe the speed limit. These conditions probably wouldn't hold in an emergency, but you don't have a strong reason to base traffic conditions on a specific day or time.
- For Overlap policy, click Overlap and choose Dissolve.
The default Overlap option creates a unique drive-time area for each shelter. Dissolving the areas creates one output feature instead of many. The same total area will be covered.
- Name the result layer Shelter in 15 minutes and add your name or initials to the end to make it unique in the organization.
- Click Estimate credits.
Running an analysis tool consumes credits. Credits are the currency used in ArcGIS Online. They are consumed during specific transactions, such as performing analytics, storing features, and geocoding. Before running an analysis, it's a good idea to check how many credits will be consumed. When running analysis tools, the credit cost is typically calculated by multiplying the number of features by the credit cost of the tool. This analysis will consume 15 credits.
If you don’t have 15 credits or prefer not to spend them, add the Shelter in 15 minutes layer owned by Learn_ArcGIS and skip to step 12.
- Click Run.
- In the Analysis pane, click the History tab.
The tool's status is shown here. When the tool completes, the output appears on the map. It shows the areas on the island that are within a 15-minute drive of a shelter.
Now that you know where people can access shelter within a 15-minute drive, you'll focus on areas outside these shelter areas. To find high-risk areas without shelter within 15 minutes, you'll use the Erase method of the Overlay Layers tool. The Overlay Layers tool combines two layers into a single layer using one of three spatial criteria: intersect, union, and erase.
- Click the Tools tab. In the Manage data category, click Overlay Layers.
- For Input features, choose High Lava Flow Hazard Zones, and for Overlay features, choose Shelter in 15 minutes.
- For Overlay type, choose Erase, and name the output layer High risk areas without shelter.
- Click Run.
- In the Layers pane, hide all layers other than High-risk areas without shelter.
Now that you know what areas are outside of a 15-minute drive to shelters, you'll use the Enrich Layer tool to calculate how many people live in these areas. The Enrich Layer tool uses a data apportionment algorithm to redistribute population and other data from standard census geographies, like block groups, into input polygon features.
Another potential method of estimating at-risk population would be to use the Summarize Within tool to sum the population in each census block by high hazard zones. Some census tracts are split between multiple hazard areas, so in this case the Enrich Layer tool will produce more accurate results.
- In the Analysis pane, click the back button. In the Tools pane, search for and select the Enrich Layer tool.
- For Input features, choose the High-risk areas without shelter layer.
- For Enrichment data, click Variable.
- In the Data Browser window, click At Risk. For At Risk Variables, choose 2022 Total Population (Esri).
The variable is added to the Selected Variables count in the top right corner.
The At Risk category contains many other demographic variables relevant to disaster response and evacuation planning. You can add other indicators as desired, including poverty, disability status, age dependency, access to vehicles, and many other variables that could be important to understanding your population of interest. Note that additional variables will add to the credit cost estimated below.
- In the Data Browser, click Select.
- Name the Result layer Population in high risk areas without shelter access and add your initials.
This analysis will cost 0.11 credits.
If you added additional data variables, check the Estimated credits calculator.
- Click Run.
When the analysis is complete, the layer will be drawn on top of the map.
Symbolize shelter access
Finally, you'll symbolize the data to show where there are large populations without access to volcano shelter within 15 minutes of their home. In the last section, you used the Enrich Layer tool to calculate the population of high-risk areas, but these people aren't evenly distributed. To visualize where people are living within these areas, you'll use the Overlay Layers tool to clip the Census block layer to the High risk without shelter access layer.
- In the Analysis pane, click the back button. In the Manage data category, click Overlay Layers.
- For Input features, choose the Census Blocks 2020 layer, and for Overlay features, choose the Population in high risk areas layer.
- For Overlay type, choose Erase.
- Name the Result layer Population distribution and click Run.
The Population distribution layer is added to the Layers pane. It should draw as a series of blue polygons, the result of combining multiple census tracts. Now that you have all the data on your map, you’ll symbolize it.
- In the Layers pane, turn on and off layers and reorder them so that the following layers are drawn on the map in this order:
- Emergency Shelters
- Population distribution
- Population in high-risk areas without shelter access
It’s usually best practice to draw vector data with point features on top, line features below that, and polygon features at the bottom of the map.
- In the Layers pane, click Population distribution to select it, and open the Styles pane.
You'll symbolize this layer using Dot density like you did with the original Census layer, but using a darker color.
- For Choose attributes, click Field and choose 2020 Total Population. Click Add.
- Under Pick a style, click Dot Density and click Style options.
- Click Symbol style. For Dot color, choose the dark blue color ramp and click Done.
- Close the Symbol style window and click Done twice.
- In the Layers pane, click Population in high-risk areas without shelter access to select it, and open the Styles pane.
- For Choose attributes, click Field and choose SEVERITY Click Add.
- For Types (unique symbols) click Style options.
- In the Style options pane, click the symbol for High Hazard.
- In the Symbol style pane, click Fill color and type or paste the Hex code # ff2e14.
- Change the Very High Hazard and Extremely High Hazard symbols to # ff7d00 and # ffc800 respectively.
The hazard zones should now match the original layer’s symbology, though they’ll be a little brighter because they don’t have the same transparency applied to the layer.
- In the Style options pane, click Done. On the Settings toolbar, click Properties. In the Appearance group, change Transparency to 50 percent.
- Save the map.
In this tutorial, you analyzed lava hazard and population to show where Hawaiian residents can’t drive to volcano shelters within 15 minutes. Based on this analysis, it might be beneficial to identify more shelter locations near populations who are unable to reach existing shelters within 15 minutes. To further hone your analysis, you could also choose to try calculating other drive times, or adding additional population risk factors to help responders identify areas where residents might need assistance with evacuation.
While the conditions you used to analyze the island today may not be the same during a disaster, analyses like these can help local governments and other responders prepare.
You can find more tutorials in the tutorial gallery.