Hazard and exposure exercise

This is an exercise!

Keep in mind that all hazard, exposure and vulnerability data provided in this exercise are fictitious and only designed for an educational purpose.

Following the unrest along the Cumbre Vieja ridge over the past weeks and the subsequent raise to Yellow alert, the international multidisciplinary scientific committee assembled to support PEVOLCA (i.e., you!) has now completed the field investigations required to conduct the hazard assessment¹. Over the past two days in the field, you have gained an understanding of the geological and geographical setting of the island, learning about recent eruptions, their associated hazards and impacts. Specifically, you have gained an overview of possible impacts by observing the consequences of past eruptions at the island scale and on the exposed systems and their interconnectivity. Based on this knowledge, you will today present the result of your hazard assessment for the two main hazards associated with your attributed eruption scenario and provide a first estimate of the nature and the number of assets exposed to various hazard intensities.

You are divided into five groups, each group forming a different interdisciplinary team and focusing on an hypothetical eruption from a different source along the Cumbre Vieja ridge. From an impact perspective, all groups will consider buildings and population and one additional system:

Group	System
Group 1	Electrical Power system
Group 2	Terrestrial and maritime transportation system
Group 3	Water supply system
Group 4	Terrestrial and aerial transportation system
Group 5	Terrestrial transportation system

Available material

Clear some data

Each group is provided with a GIS project (LaPalma_basemap_exercise.qgz) containing the hazard and exposure data for all groups. To avoid running into problems, please remove the data associated with other groups and keep only those related to yours.

Hazard data

For the hazard data, you are provided with the result of the probabilistic hazard modelling.

For tephra:
- Probability maps (%) to exceed specific tephra accumulation (Probability maps layers)
- Probabilistic isomass maps (kg/m\(^2\)) for a given probability of occurrence of the hazard (Probabilistic isomass maps layers)
- In the folder GroupX (i.e., not in QGIS), there is also an Excel file called tephra_hazard_curves.xlsx containing the hazard curves for specific locations in your area. The location of the points for the hazard curves is shown in the layer Points group X
For lava:
- Probability of lava flow inundation (Lava inundation probability layer)
- A lava flow outline corresponding to an inundation probability ≥ 0.1% (Flow outline layer)

In addition, we have prepared a multi-hazard hazard map (Multi-hazard hazard map layer) that defines four zones based on the assumptions below. Hazard zones I-V are stored in the HazardZone field.

Hazard zone	Hazard threshold
Zone I	Tephra accumulations ≥ 1 kg/m² (considering a 25% probability of occurrence of the hazard)
Zone II	Tephra accumulations ≥ 10 kg/m² (considering a 25% probability of occurrence of the hazard)
Zone III	Tephra accumulations ≥ 100 kg/m² (considering a 25% probability of occurrence of the hazard)
Zone IV	Tephra accumulations ≥ 300 kg/m² (considering a 25% probability of occurrence of the hazard)
Zone V	Probability of lava flow inundation ≥ 0.1% (i.e., one chance in a thousand)

Exposure data

The QGIS project also contains exposure data for all groups, which you can view as the result of a preliminary, rapid assessment of the exposed elements on the island. All exposure layers contain hazard information extracted for the specific location of each asset contained as two fields of the attribute table:

lava_probability: Probability of lava flow inundation (%)
tephra_load_25pct: Tephra load (kg/m\(^2\)) extracted for a 25% probability of occurrence

All groups contain a layer with the buildings footprints (Buildings and population layer) with the number of residents in each household (POP_resid field). In addition, you are provided with specific layers for each group related to the system you must consider.

Exercise

The objective of the exercise is to produce a poster that presents the results of your hazard assessment and put them in the perspective of the nature and number of assets exposed to the volcanic hazards. Your presentation should cover the following topics:

Explain the underlying eruption scenario and the main assumptions used to conduct the probabilistic hazard modelling
Present the result of the hazard assessment in the perspective of the resulting exposed asset. Find a narrative and be creative - communicating results is an integrant part of the challenge!
Answer the hazard-related question specific to your group
Provide any recommendation regarding what attributes are missing/required to characterise vulnerability, and narrow down a few that you will investigate during the next field campaign (i.e., tomorrow)

Work as a team

Keep in mind that each group represents a multidisciplinary team. It is important to work as a team and exploit the strength of each member. Before rushing head first into the exercise, please take a moment for all of you to i) read the exercise until the end and ii) open the GIS project, look at the layers and review their attributes. When you have a global understanding of the exercise and the available data, it is probably a good idea to discuss splitting tasks between group members.

Task I: Hazard assessment

The objective of this task is to present the assumptions behind the probabilistic hazard modelling you performed for both tephra fallout and lava flows. Refer to the online classes on tephra and lava for references.

Required output

Present the assumptions behind the eruption scenarios - you can use some of the figures produced in class if needed
Reproduce the multi-hazard hazard map on two transparent papers, both for the island scale and for the local area defined for your own group
Draw the hazard curves for the locations associated with your group. It is probably a good idea to clearly show these locations on a map too!
Explain the spatial distribution of hazards (e.g., in which direction is the hazard going? what is the shape of the hazard footprint? what controls direction and shape? how far from the source is the hazard footprint extending from the vent?)
Put the hazard assessment in the context of exposure (see Task 2)

Know your audience!

Remember that you are communicating assumptions behind and results of the hazard assessment to PEVOLCA, who are not experts in the topic. Know your audience: carefully choose what assumption you want to communicate. Also remember that the symbology used in maps conveys some important information. Don't get lost in over-technical details!

Task II: Exposure analysis

The objective of this task is to identify the population and asset exposed to tephra fallout and lava flows. For this, you will need to quantify how many people and assets are contained within each zone of the multi-hazard hazard map. All groups should quantify i) the number of buildings and number of residents located in each hazard zone. Then, each group should quantify the exposure of their attributed system.

Take some time to investigate the provided exposure layers. Namely:

What is the geometry of each layer (i.e., point, polyline, polygon)?
What is the spatial distribution of the assets (e.g., density, clusters)?
What is contained in the attribute tables? For instance, what are the values in each field? Are they numerical or categorical?

Methodology

In QGIS, start by displaying the Processing Toolbox:

For Windows/Linux: Ctrl + Alt +T
For macOS: Cmd + Option + T

Conceptually, two steps are required to analyse exposure:

Extract the hazard zone at the location of each asset
Quantify how many assets are located in each hazard zone

1. Extract the hazard zone for each asset

This step retrieves the hazard zone in which each asset is located and adds it as a new feature to the exposure layer. The hazard zone is stored in the HazardZone field of the Multi-hazard hazard map layer. For polyline geometries, note that the method retrieves the maximum hazard zone along an edge.

In the Processing Toolbox, search for the Join attribute by location tool:

Join to feature in: The exposure layer
Choose Intersect
By comparing to: The multi-hazard hazard map
Fields to add: Select HazardZone

This creates a new file of the same geometry as the input exposure layer that contains a new field called HazardZone. You can copy the symbology from the multi-hazard hazard map and paste it on the newly created layer for comparison. For this:

Right-click on Multi-hazard hazard map and choose Style > Copy Style > All Style Categories
Right-click on the exposure layer and choose Style > Paste Style > All Style Categories

Temporary layers

If you don't specify a path and a name to the output layer generated by tools in QGIS, it will automatically save them as temporary layers. To save them, right-click on the layer in the Layer panel and choose Make permanent.

height — Join attribute by location window.

2. Quantify asset exposure

This step quantifies the exposure of populations asset in each hazard zone. Think carefully which operator is required for each type of exposure layer. Typically, point geometries must be aggregated using the count function. Some polygon geometries also need a count (i.e., number of buildings), but perhaps other aggregation function need to be used in different contexts (e.g., population per building). For polyline geometries, we are typically interested in the exposed length.

In the Processing Toolbox, search for the Aggregate tool:

Input layer: The layer created at the previous step
Group by expression: The categorical field used for aggregation (i.e., HazardZone)
In the Aggregate window, delete all fields if any are present and add a first field structured as such:
- Source expression: HazardZone
- Aggregate function: first_value
- Name: Hazard zone
Then add the the field you would like to aggregate.
- To count assets, you can select any field in source expression
- For other operators, you need to select the relevant field

This creates a new layer that has no geometry information. Inspect its attribute table to display the result of the aggregation.

Required output

Only one task: find a creative way to display and communicate the results of your exposure analysis. Plots are a good idea, or maybe tables? In any case, make sure that the fonts are large enough for the audience to read them

What type of plot?

Several types of plots exist - and each type of data calls for a different one. For instance:

Histograms are useful to show the distribution of continuous values
Barplots can help ranking values and work well with categorical data
Spider plots can serve a similar purpose
Donut or pie charts are good to illustrate a proportion of a whole

Also - make the plots as easy to read as possible: adding the raw value to some of the plots is often a good idea.

Task III: Specific hazard questions

We saw that gas can be an important hazard both during and after an eruption. Modelling the gas hazard goes beyond the scope of this exercise, but please take a moment to reflect on the following questions:

Group	Question
Group 1	Which hazardous gases are more probable to find out associated with an eruptive event?
Group 2	Which hazardous gases you expect to find in the (a) crater fumaroles (b) and in the flanks of the volcano?
Group 3	What parameters can control the dispersion of gas in the craters? And in the diffuse degassing areas?
Group 4	Which are the volcanic gases that may act as toxic and irritant?
Group 5	Which are the volcanic gases that may act as asphyxiant?

Required output

You do not need to add anything on the poster, but answer the question attributed to your group orally during your presentation

Task IV: From exposure to vulnerability

Ok, we should have done it before we actually conducted the hazard assessment in class and it should have taken more than two days, but hey, let's pretend ↩