Q-LavHA

With the knowledge gained on topographic controls and modeling from the previous exercise, we will now apply the Q-LavHA¹ model to estimate the hazard associated with lava flow inundation. This part of the exercise assumes that:

• You have understood the philosophy behind Q-LavHA according to this guide.
• You have already installed QGIS and Q-LavHA.
• You have loaded the provided QGIS project

Alternatively, the Q-LavHA user manual contains some important information about how to configure the model and the data, so we encourage you to have a look at it at some point.

Objectives

• Perform probabilistic lava flow modeling for several vents in La Palma using Q-LavHA.
• Account for the uncertainty in vent locations in hazard assessments.

Exercise

Running Q-LavHA for single vents

In QGIS, start by activating the Vents for Exercise layer group. These vents correspond to those indicated in the Table below. Q-LavHA requires projected UTM coordinates, which are shown in the Easting and Northing columns (x and y columns, respectively). The UTM zone for La Palma is 28N, or EPSG:3268.

Vent	Latitude	Longitude	Easting (m)	Northing (m)	EPSG
Vent 1	-17.83989	28.62507	222340	3169746	32628
Vent 2	-17.86626	28.61262	219728	3168426	32628
Vent 3	-17.84316	28.58782	221922	3165623	32628
Vent 4	-17.81585	28.60777	224646	3167772	32628
Vent 5	-17.84649	28.49699	221357	3155562	32628

Each of you has been assigned a vent. Please run the exercise only for the vent assigned to you!

Coordinate systems

Here, you are provided with input data that all have the same coordinate system. Before modeling lava flows for your own projects, make sure that you first reproject all your data to a unique CRS.

Model setup

Let's run Q-LavHA for the selected vent. For now, we will assume that we know the exact vent location. Open the Q-LavHA window, and let's look at the parameters:

Vent location tabLava flow parameter tabOutput files tab

1. In the DEM Selection box, select the Data/Lava/DEM_Qlavah.tif. In the DEM selection window, make sure you change the type of files from .asc to .tif.
2. As a Vent Type, choose Point for now.
3. Add the relevant Coordinates as specified in the table above.

1. Lava Flow Propagation: This section sets up the behaviour of the lava flow, as described in the theory section.

   • Leave the default values of Hc and Ht.
   • Use the H16 option: This means that the algorithm will first search for a topographic low within the first 8 adjacent pixels, and will extend the search to the next 16 ones if not found.
   • Use Probability to the square

2. Lava Flow Length Constrain: This section defines when a lava flow stops. You can switch between the various options for an illustration of the different behaviours. As discussed above, use a Manhattan Distance with a length of 10000 m.

3. Simulations: Use a Number of iterations of 1500. This represents the number of simulated lava flows, or \(N_r\) in the equations above.

1. Set an Output path and a name to the output layers.
2. Save the Parameters.

Q-LavHA

You are now ready to run the simulation!

Vent geometry

Q-LavHA offers different source geometries, such as a fissure, or a vent opening susceptibility map. The choice of these options depends on the case study, and their use is described in the Q-LavHA user manual.

Styling outputs

Upon completion of the model run, the output file is directly added to the layer panel. Let's change its visualisation:

• Open the Layer Properties panel by double-clicking on the output layer in the Layers panel.
• From the Symbology tab on the left:
  • Under Color Rendering, uncheck the Colorize option.
  • Under Render type, choose Singleband pseudocolor.
  • Set the min and max values to 0 and 0.5, respectively.
  • Change the Color ramp to Magma
• Click ok.

The output file now shows the spatial distribution of inundation probability varying between \(\gt 0\) and \(0.5\) (i.e., 50%).

Copying layer symbology

In QGIS, you can simply copy the display properties of one layer to other layers:

• In the Layers panel, right-click on the layer you want to copy the style from and select Styles > Copy style.
• Now right-click on the layer you want to apply to style on and select Styles > Paste style.

Question 1: Single-vent simulations

1. In one sentence, describe what the color at any given pixel expresses.
2. How do modelled flows compare to the closest historical lava flows in terms of length and width?
3. Analyse and discuss the spatial distribution of inundation probability. How do they compare to the path of steepest descent?

Using several vents

In the case of fissure eruptions, one eruptive episode can consist of multiple satellite vents. This was certainly the case for the 2021 eruption of La Palma. In fact, as you can see from the 2021 Vents layer in QGIS, CEMS identified that ~10 vents opened over a 500 x 500 m area. Here, we won't attempt predicting the number or the location of each vent. Instead, we will model lava flow inundation from a grid of vents within a surface area and explore how vent location affects the lava flow simulation hazard.

Probability of vent opening

Note that with this approach, we consider an equal probability of vent opening within this area. We will consider this as an acceptable assumption for this exercise, but this is likely an over-simplification for real-case scenarios.

Define the area

We will define a potential vent opening surface as a 500 x 500 m square centered on the reference coordinates of your attributed vent. Note that UTM coordinates are in metre: you can therefore simply identify the x and y coordinates these four points by addition and subtraction:

• Lower left corner
• Lower right corner
• Upper right corner
• Upper left corner

Model setup

• From QGIS open Q-LavHA. Most of the parameters should already be filled. Otherwise, you can use the Load parameters option and load a previous run.
• Set a Distance between vents of 100 m. Since we defined a 500x500 m area, Q-LavHA will therefore model lava flow inundation from 25 vents.
• Enter the 4 coordinates defining your area.
• In the Lava Flow Parameter tab, change the Number of Simulations to 100. This will help save some computation time.
• Make sure you change the output name.

Q-LavHA

You are now ready to run the simulation!

Apply the same symbology to the surface area runs as you did for the point run.

Question 2: Vent location uncertainty

1. Compare the run with its single-vent counterpart. How do they differ and why?

Prepare your hazard maps

The raw geotiff files produced by Q-LavHA are your main hazard files. If you choose to work in QGIS you can further export them to maps using the Print Layout tool (→ doc & tutorial).

Food for thoughts

Forecasting the flows from the 2021 eruption

Compare the Q-LavHA run with the flow outlines from the 2021 eruption below. Note that the Q-LavHA run was performed using the same 500 x 500 m source area as used above.

Q-LavHA runsFlow outlines

Question 3: Interpreting hazard forecasts

1. From what you know about both the 2021 eruption and the dynamics of lava flows, compare and discuss the hazard forecast and the actual deposit. What are Q-LavHA's strengths and limitations?

Limitations

The method presented here is one amongst many attempts to estimate the hazard from lava flow inundation. This model relies on no to some degrees of physics, but inherently attempts to quantify uncertainties. It is important to recognise that although this model is useful and has been validated, it still bypasses some critical aspects of lava flows including time, effusion rates or morphology. Again, models are useful as long as used in the range of their limitations and for an appropriate purpose.

Summary

In this exercise, you have:

• Assessed the hazard of lava flow inundation using probabilistic modeling.
• Included the uncertainty on vent location in hazard estimates.
• Understood the limitations of model predictions when compared to past events.

References

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1. Mossoux, S., Saey, M., Bartolini, S., Poppe, S., Canters, F., Kervyn, M., 2016. Q-LAVHA: A flexible GIS plugin to simulate lava flows. Computers & Geosciences 97, 98–109. https://doi.org/10.1016/j.cageo.2016.09.003 ↩

2. Harris, A.J., Rowland, S. FLOWGO: a kinematic thermo-rheological model for lava flowing in a channel. Bull Volcanol 63, 20–44 (2001). https://doi.org/10.1007/s004450000120 ↩