Flood events in coastal cities are on the increase. The trend is usually blamed on rising sea levels and extreme weather events, caused by anthropogenic climate change. While water levels are rising, some cities are also sinking. And human activity may partly be to blame.
Urban subsidence can be caused by soil drainage, construction projects, or the extraction of groundwater, oil, gas and coal. Subsidence can result from natural processes, but most of the problematic land subsidence is induced by human activity.
Subsidence not only increases flood risk, it also damages buildings and infrastructure and disrupts water management.
Many cities around the world are affected, including New Orleans, Manila, Venice and Dhaka, and each city faces a unique set of causes and challenges. Although these cities can learn from one another, solutions need to be found locally.
Only thirty years ago, Shenzen was a small fishing village outside of Hong Kong. It now hosts a population almost as large as London and boasts an economy to match Shanghai. This rapid growth resulted in a land grab from the ocean. Rocks from nearby hills were piled into the ocean and covered in apartments and offices. Many of these buildings are now unusable due to the ensuing land subsidence. The top floors of some tower block are now leaning over so dramatically that they are almost touching.
Dr. Peng Liu has been working at Shenzen Key Laboratories to map the subsidence across the city. Using a radar-based method, InSAR time-series, he is able to detect elevation changes of less than a centimetre. His results show that, in general, the greatest subsidence is occurring in coastal areas and areas of reclaimed land. Two distinct lines of subsidence also stand out on his map, tracking the construction of underground rail lines to facilitate Shenzen’s growing transport demands. Dr. Peng Liu thinks that if the reclaimed land had been allowed time to “settle down”, then the current problems could have been avoided.
The view of Shenzen City from Hong Kong. Source wikicommons
We can begin to tackle subsidence by curbing the human activities that cause it: reducing the rate at which we pump out resources from beneath cities, injecting fluids back into aquifers, even using lighter building materials. For natural land subsidence the best solution is to adapt, by reintroducing sediment to deltas, building up levees, and improving spatial planning.
Geologists in particular can contribute to the fight against land subsidence. Gilles Erkens, from Ultrecht University, notes that although surface observations are plentiful, there is a dearth of measurements being made in the ground. In-situ measurements are essential to better understand the local causes of subsidence. Surface models and maps made in Earth Science departments can help improve prediction. Geologists can also lend advice to policy-makers to facilitate geology-based spatial planning.
Sea level rise has received considerable attention, but subsidence is often overlooked. Gilles Erkens thinks that this may partly be an administrative problem. Nobody is responsible for subsidence at a governmental level, and there are often no dedicated research institutions on the subject. But with rising sea levels and continued urban growth, land subsidence isn’t a problem that can be ignored for long.
Land subsidence triggered by anthropogenic processes such as groundwater abstraction is only one example of a human process triggering a natural hazard. Joel Gill (GfGD Director) will be giving a presentation at EGU on Friday about the range of interactions that can occur between 17 anthropic-processes and 21 natural hazards. The talk will take place on Friday, 14-14.15 in Room G8.
