Subsidence Overlay

How the subsidence overlay calculates

Subsidence is currently composed of 2 forms of reduction of peat: oxidation and subsidence. 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.

The amount of subsidence due to oxidation is calculated by the following empirical formula:

Subsidence = GLG * a - clay thickness * b - c

Settlement

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.

The amount of subsidence due to settlement is calculated by the following formula:

Subsidence = (Peat fraction * PEAT_A + Top layer * TOP_LAYER_A) * log10(days)
+ Peat fraction * PEAT_B
+ Top Layer * TOP_LAYER_B
+ Height Increase * HEIGHT

This formula is based on provided expert data in the form of a reference table, indicating the amount of subsidence based on the parameters used in the formula above. The formula's results conform to the reference table to within a fifth of the margin of error of the original table.

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.