Unique site for monitoring land subsidence in the Netherlands
Published: 15 July 2019
The site is the first step on the road to a national monitoring network that will produce systematic and frequent measurements of land subsidence. The comprehensive monitoring of land subsidence helps to determine the effectiveness of mitigation measures such as submerged drainage. It also provides a picture of the rate of land subsidence, and how it varies within an area and over the course of the year. The monitoring network will ultimately resolve the vexed issue of whether the Dutch peat area really is subsiding by 8 mm a year.
Land subsidence and the polders are inextricably linked
Once the Netherlands started reclaiming peatland and draining land, subsidence inevitably became part of the Dutch landscape. The water table fell, the peat oxidised, and the ground level sank. Even so, the rate and extent of land subsidence has never been systematically documented. Today, land subsidence as a result of peat oxidation represents a challenge to agriculture. It causes rewetting and carbon emissions, making the achievement of the climate goals more difficult and undermining the goal of establishing sustainable farming practices without putting the quality of life of farmers, the general and the environment at risk. As a result, we urgently need to be able to determine the extent and rate of land subsidence in space and time.
Systematic measurement required for effective technical and policy measures
The monitoring site in Rouveen will now measure land subsidence systematically and frequently for the first time. The measurements can be used to unravel the main processes that cause subsidence. An understanding of those processes is indispensable as a basis for effective technical and policy measures. Gilles Erkens: ‘Rouveen is the first site in a national monitoring network. If we measure land subsidence as seriously and systematically as sea level rise, then we can learn from the successes and mistakes of the past and identify the right approach for each region. So we will need monitoring sites in both rural and urban areas.’
Four different ways to determine the effectiveness of submerged drainage
At Land subsidence will be measured at the Rouveen site for at least five years using four different techniques: land surveys, extensometers, LiDAR and InSAR. Applying four different methods – both conventional and innovative – allows for the comparison and optimisation of the methods. The aim is to establish an accurate (mm scale) measurement method that covers the area being monitored. The monitoring location is spread over eleven farms and each farm has a reference plot and a submerged drainage plot established with the help of, among others, the Drents Overijsselse Delta water management authority. The water authority and the farmers want to know how effective the submerged drainage system is that was installed in the spring and summer of 2018 to reduce land subsidence. The drains allow ditch water to infiltrate the peat during dry periods, reducing the fall in the groundwater level. During wet periods, the drains provide extra drainage to ensure that the water levels in the land do not rise too high. In this way, the phreatic water table is levelled out, staying closer to the ditch level. The result is the reduction of land subsidence and carbon emissions.