By Simon Dixon, University of Birmingham, UK
Sinkholes regularly appear in city streets around the world, but despite often widespread media interest, there is almost no academic research into sinkholes in urban environments. This is symptomatic of a wider lack of urban-based earth surface research. The world is becoming increasingly urbanised, with the majority of people already living in cities and the proportion expected to rise to 66% by 2050. We are undeniably living in the age of humankind, the “Anthropocene”, but we are still coming to terms with what this means for the planet and for ourselves. Researchers and policy makers have begun to consider the social and environmental impacts of our increased urbanisation. There are also efforts to understand the impact human activity is having on the surface of the earth more broadly – for example, through the creation of anthropogenic landforms like open-cast mines, and by changing erosion processes in rivers through human activity. However, so far there has been little attention paid to the way earth surface processes are slowly altering and morphing the fabric of our cities to create new, startling and potentially dangerous features.
In our new Area paper, we argue, somewhat tongue-in-cheek, that in neglecting to consider how earth surface processes are changing the urban fabric we risk repeating the fate of Ozymandias, the great king in the Shelley poem of the same name. In the poem the famous lines: “look on my works ye mighty and despair” are inscribed on the plinth of a ruined and eroded statue. One way of interpreting this is the king did not consider whether time and earth surface processes would degrade the monuments he constructed. We argue that without considering the forces acting within our cities we cannot understand the way they will change, decay and potentially fall into ruin in the future.
There are several interesting ways we have identified earth surface processes working on our cities, including the development of limestone cavern-type formations in old tunnels running under cities, formed from dissolved concrete. A key component in how earth surface processes develop in a city is the degree of maintenance. Social pressures and conditions which mean parts of a city are neglected or abandoned could allow these physical processes to proceed unchecked. Indeed places such as Detroit, Chernobyl and Hashima Island provide examples of hybrid urban landforms created by decay and weathering.
One important example of hybrid urban landforms is that of urban sinkholes, which although a natural phenomenon, occur in different ways in the urban environment. The formation processes for sinkholes in areas of limestone bed rock are well-documented and understood, and they can be classified partly according to the layer of rock and soil above the limestone. However, in urban environments we have created a very unusual situation where there is a hard, impermeable “rock” (tarmac/asphalt/concrete) sitting on top of a soft layer (soil or “made earth”). Flowing or percolating water can remove the soil, creating a void under the tarmac, which eventually develops into a sinkhole. It is possible this process played a role in the collapse of the Oroville Dam spillway in February 2017, with flowing water removing material under the spillway. The combination of a hard impermeable layer over a soft, easily-erodible layer only really occurs in nature during some volcanic eruptions where pumice is overlain by lava. We therefore have no natural comparisons for how soil piping sinkholes form in cities. Without specifically researching these it is hard to design ways to prevent them from occurring, or devise early warning systems.
Once we begin to think of the whole urban fabric as a human created “landform”, and the buildings, and infrastructure within it as like Anthropocene rock formations or outcrops, multiple research avenues open up. Understanding the processes happening within our cities would obviously help civil engineers and municipal authorities, but potentially also help archaeologists studying ancient ruined cities to interpret the features they find. We argue this exciting new frontier in earth science is fundamentally interdisciplinary, as it is not possible to disentangle the social drivers from the physical processes. It is our hope that researchers will start to view the urban environment in a slightly different way and work to together to explore some of the unknown earth surface processes acting in our cities.
About the author: Simon Dixon is a postdoctoral researcher at the School of Geography, Earth and Environmental Sciences, University of Birmingham. He is currently working a NERC “MegaScours” project looking at river confluences in the world’s largest rivers.
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Dixon, S. J., Viles, H. A. and Garrett, B. L. (2017), Ozymandias in the Anthropocene: the city as an emerging landform. Area. doi:10.1111/area.12358
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Geography Directions 