Land subsidence requires creative approach

Published: 11 June 2014

The future of delta cities such as Jakarta, Bangkok and Tokio is threatened by sea-level rise, but even more by land subsidence of up to several metres a century. Raising dikes fails to solve the underlying problem: falling groundwater levels. The only sustainable remedy is to combine urban functions with a high water table.
Een garage in New Orleans, waarbij de gevolgen van bodemdaling duidelijk zichtbaar zijn.

A garage in New Orleans, where the impact of subsidence is clearly visible

Land subsidence central during debate

During the “Delta city sustainability” debate that Deltares organised on 4 June in the Kunsthal in Rotterdam, pictures were shown that raised the question of whether the limits have not already been reached. The potential consequences were made clear in equally dramatic pictures showing New Orleans post-Katrina and New York post-Sandy.

Groundwater extraction is one cause

The causes are known. Groundwater extraction leads to the consolidation of peat layers, and the land subsides, often in an irregular way because peat layers in the subsurface are intermingled with sand and clay. As it emerged during the debate, there were good reasons why the original settlements upon which Dutch towns are based were built on sand banks or river clay. The problems began as they extended onto peat land. As a result, towns like Gouda have to spend tens of millions annually on raising streets, and Rotterdam has to resort to freezing the subsurface to prevent settlement damage.

Real action still required to tackle land subsidence

Although the causes are clear and although this is becoming an increasingly urgent problem throughout the world, the problem of land subsidence is not yet being tackled properly. In part, this is a ‘governance’ question, relating particularly to a lack of clarity about how costs and benefits, responsibilities and liabilities, should be distributed. In New Orleans, for example, it turned out that nobody was responsible for looking after the groundwater.

Search for solutions limited to hydraulic engineering interventions

Another obstacle is the direction taken by the search for solutions. This search is often limited to hydraulic engineering interventions, even though the underlying causes such as drinking water supplies or leaky sewage systems are not addressed, let alone that people consider ways of using cities as sponges and drastically raising groundwater levels to stop land subsidence.

A new polder model suitable for exporting

Those sustainable solutions are available: during the debate, for example, there was a discussion of the ‘blue-green streets’ devised for New Orleans. The development, but above all the implementation, of those solutions requires a new approach in which knowledge and creativity from a range of disciplines will be integrated in a design process that will be conducted in collaboration with authorities and residents. In effect, this is a new polder model that can not only be used in the Netherlands, but also be exported.