Land subsidence is the lowering of the land surface relative to a fixed reference point, such as the Amsterdam Ordnance Datum (NAP). The ground is subsiding in large parts of the world, including the Netherlands. This is caused by deformation of the subsurface at different depths. 

Natural processes, such as tectonic movements and isostasy, play a role in the deep subsurface. In addition, human activities, such as the extraction of gas, salt or groundwater, can contribute to land subsidence. 

In areas where the subsurface consists of peat, clay or other compressible sediments, land subsidence occurs mainly due to the compaction of these soft layers. As a result, the volume of the soil decreases, causing the land surface to lower.

Processes that cause land subsidence

Compaction of the upper soil can result from various natural and human processes. Compaction is the compression of soil layers under the influence of a load or a change in pore water pressure in the soil. Compaction can occur due to

• loading of the subsurface by buildings, infrastructure or raised ground levels;
• lowering of the groundwater level;
• groundwater extraction.

When the groundwater level drops, the pore pressure in the soil decreases and the effective stress increases. As a result, soil particles are pressed closer together, leading to settlement and land subsidence.

In peat soils, a lower groundwater level allows oxygen to penetrate deeper into the soil. This accelerates the decomposition of organic material by microorganisms. This process, also known as peat oxidation, results in a loss of soil volume and consequently in a lowering of the land surface.

Shrinkage occurs when clay or peat soils dry out. Evaporation and drainage reduce the moisture content, causing the soil to contract. This process can be temporary or permanent and contributes to land subsidence.

Ripening is the process by which young, water-rich sediments gradually change after drainage from a soft, wet soil into a soil with a firmer structure. During this process, drainage, shrinkage and structural development occur, accompanied by a lowering of the land surface. Ripening can be accelerated by the presence of suitable vegetation, such as reeds.

In addition to processes in the shallow subsurface, processes in deeper soil layers can also contribute to land subsidence. Examples include tectonic movements, glacio-isostatic adjustments and mineral extraction.

Natural rates of land subsidence are generally a few millimetres per year. In densely populated coastal and delta regions, such as the Netherlands, these processes are often accelerated by human activities, including groundwater extraction, water level lowering, mineral extraction and loading of the subsurface. As a result, local subsidence rates of several centimetres per year can occur (Stouthamer & Van Asselen, 2015).

Consequences of land subsidence

The consequences of land subsidence vary by area and depend on land use, soil composition and water management. Important consequences of land subsidence include:

• an increased risk of waterlogging and flooding;
• higher costs for pumping and water management;
• damage to buildings, foundations, roads, railway lines and pipelines;
• an increasing risk of salinisation due to upward seepage of saline groundwater;
• a decrease in agricultural productivity;
• loss of soil carbon and greenhouse gas emissions, particularly in peatland areas;
• increasing infrastructure management and maintenance costs.