Subsidence Overlay
What is the subsidence overlay
The subsidence overlay is a grid overlay, and part of the subsidence collection of overlays. It shows which places in the 3D world are subject to subsidence due to oxidation and/or compaction of peat.
How the subsidence overlay calculates
Subsidence is currently composed of 2 forms of reduction of peat: oxidation and compaction. These forms of subsidence are both relevant for the complete picture of subsidence, but are, in principle, calculated via separate formulas. The results of these formulas are added together to get the total amount of subsidence.
Oxidation
Peat, when exposed to oxygen, can oxidize. In this process the peat combines with the air to form CO2, reducing the total mass and volume of the peat. The amount of oxidation depends on the clay thickness, because clay may insulate the peat, preventing it from oxidizing. It also depends on the (lowest) ground water level in relation to the surface of the land.
For information on the exact calculation of subsidence due to oxidation, see the article on Subsidence calculation.
Compaction
Peat is a porous and relatively soft terrain type, meaning it can be compressed. Based on the amount of peat in the ground, the density of the top layer, and the net height increase.
For information on the exact calculation of subsidence due to compaction, see the article on Subsidence calculation.
Multi-year calculations
Subsidence is calculated in 1-year steps. For each year, the amount of subsidence is calculated. That amount is then used to recalculate the input parameters for the overlay. The next 1-year step is then calculated.
For more information on the way the multi-year calculation is performed, see Subsidence calculation.
How to configure the subsidence overlay
When the subsidence overlay is added to a project, there is some minimum information required for a calculation. The compaction calculation already functions based on data present in the engine, but requires a running session and specific actions to be taken. The oxidation calculation can show results without taking actions, provided that the required data is loaded in.
Oxidation
The following data must be added to a project for the oxidation calculation:
| Attribute | Description | Name to import as |
|---|---|---|
| Water level | The areas with the relevant data must have a water level attribute to mark them as water level areas. | WATER_LEVEL |
| Ground water level | A ground water level, in meters from the surface of the land. The larger the value, the dryer the soil is. Should be greater than 0. (Can also be 0, but there will be no subsidence then.) | GLG |
| Clay thickness | The thickness of the layer of clay on the top of the soil, in meters. The thicker the layer, the less subsidence occurs. Should be 0, or greater than 0. | CLAY_THICKNESS |
There are 2 ways of getting this data into your project: by importing it (i.e. your own data) into the project as areas with attributes, or by manually drawing the appropriate areas and adding attributes to those.
It is possible to load in your own GeoJSON file with information which is relevant to the oxidation calculations. When this data is loaded in, results of the calculations will become visible in the 3D world when the overlay is selected.
To import you own file, you must prepare a GeoJSON file with the specified data in the correct format. The file should contain the data listed above. When the file is prepared, it can be imported via the option in the ribbon, or by dragging the file into the Engine. The file must be imported as areas. When importing, the attributes must be renamed to the attribute names listed above.
- Drag your GeoJSON file into the editor
- Select "Import as areas
- Rename the ground water level attribute to "GLG"
- Rename the clay thickness attribute to "CLAY_THICKNESS"
- Rename the water level attribute to "WATER_LEVEL"
- Select "Send"
- Select the subsidence overlay
- Select "refresh grid"
When creating the data manually in the editor, you must add at least one area to the project, and to the 3D World. The appropriate attributes (listed above) must be added to the area.
- Add a new area to the project
- Draw the area into the 3D World
- Add the attribute "WATER_LEVEL" to the area, with value "-10"
- Add the attribute "GLG" to the area, with value "-0,9"
- Add the attribute "CLAY_THICKNESS" to the area, with value "0,1"
- Select the subsidence overlay
- Select "refresh grid"