Sinking cities

Sinking cities are urban environments that are in danger of disappearing due to their rapidly changing landscapes. The largest contributors to these cities becoming unlivable are the combined effects of climate change (manifested through sea level rise, intensifying storms, and storm surge), land subsidence, and accelerated urbanization. Many of the world's largest and most rapidly growing cities are located along rivers and coasts, exposing them to natural disasters. Metropolitan areas were built on flat flood plains due to their suitability for agriculture, urban development, and international trade; however, these flood plains are often geologically young (only several thousand years old), and human activities such as groundwater extraction and urban development can trigger rapid subsidence, creating compound risks alongside sea level rise. As countries continue to invest people, assets, and infrastructure into these cities, the loss potential in these areas also increases. Sinking cities must overcome substantial barriers to properly prepare for today's dynamic environmental climate.

Background and history

Development The vast majority of sinking cities are located in coastal lowlands. These areas are particularly vulnerable to climate related hazards, but since ancient times, have also been preferred areas for human settlement. Soil fertility, availability of fresh water from rivers, and accessibility due to flat topographical relief have long made coastal plains valuable agricultural resources. In addition, their proximity to waterways allowed for the development of port cities, which became hot spots of trade and economic activity. Port cities played a central role in the expansion of regional and global economies, particularly from the early modern period through the industrial era. Cities, such as New York City, grew rapidly due to their strategic locations surrounded by water. New York City, and its easy access the Hudson River and along Atlantic trade routes, quickly became a gateway for international commerce, migration, and development. As a major port city, New York City experienced exponential population growth and industrial development throughout the 19th and 20th centuries. However, large portions of the city were built on reclaimed land and filled wetlands, which makes it more susceptible to instability and subsidence over time. Over time, we are already seeing the effects of this from gradual land subsidence.

Causes The growing physical risks to many coastal cities stem from a combination of factors relating to rapid urbanization, climate change, and land subsidence. Many of these natural hazards are largely anthropogenic in origin. In many cases, the fundamental aspects that lead to sinking cities become tightly interwoven, and over time, are increasingly difficult to resolve.

Urbanization

For the first time in human history the majority of people live in urban areas. The United Nations estimates that approximately 68% of the world's population will be living in urban areas by 2050. Urbanization has vast implications including the urban planning, geography, sociology, architecture, economics, and public health of a region. The rate at which urbanization occurs is also important. Slower rates of urbanization allow city planners time to make thoughtful planning decisions. Once cities reach maturity, it can take decades for local governments to develop, fund, and execute major infrastructure projects to alleviate the issues brought on by rapid urbanization.

Land subsidence

Subsidence is the sudden sinking or gradual downward settling of the ground's surface with little or no horizontal motion. Land subsidence can have both direct and indirect repercussions for cities. Direct impacts are often in the form of structural damage to major infrastructure systems, including water management networks, buildings, and highways. Land subsidence also further adds to the growing risk of coastal flooding, and oftentimes, the net rate of subsidence exceeds that of sea level rise. In Bangkok, the Gulf of Thailand is rising 0.25 cm per year, but the city is sinking at a far faster rate, up to 4 cm per year. This downward settlement significantly increases flood vulnerability which can ultimately lead to major economic damages and loss of lives.

Causes

Water is a vital resource that people need to survive. However, with the increase of the human population, more water is being extracted from underground aquifers which creates large subsidence problems. Due to the dense populations along river deltas, industrial development, and relaxed or no environmental protections, river waters often became polluted. This has become an ever more common phenomena in coastal mega-cities, particularly in Asia. Many cities are unable to afford costly water treatment systems and are forced to rely heavily on groundwater. In the aquifers, there are rock and sediments of different sizes, creating different pores of varying sizes which capture water underground. As the ground is loaded, most often through increased development, the soil compresses and land begins to subside. Depending on the geology of the region, subsidence may occur rapidly, as in many coastal plains, or more slowly depending on bedrock depth. Furthermore, oil, natural gas, and other hydrocarbons, can be found in rock pores underground. These natural resources, however, can also cause land subsidence when extracted. High buildings can create land subsidence by pressing the soil beneath with their weight. The problem is already felt in New York City, San Francisco Bay Area, Lagos.

Examples Venice is often referenced as an example of a city suffering from subsidence, however, it is a relatively minor case with mostly historical origins. More serious are the Asian metropolises with concentrations of millions of people living at or even below mean sea level. Some cities, such as Tokyo, have developed sophisticated techniques for measuring, monitoring, and combating land subsidence. But many other large cities (Hanoi, Haiphong, Yangon, Manila, etc.), particularly in developing nations, have no record of their subsidence, which is far from under control. Many cities do not possess the resources necessary to conduct complex, and often expensive, geological, geotechnical, and hydrogeological studies required to accurately measure and model future land subsidence.

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