Last month, when news came that half of China’s major cities, including coastal and inland are sinking [i], to many it sounded like some minor obscure event in some far off corner of the world. But the event is not only major and worrying, also relevant to many parts of the world, including many parts of India. Our situation may even be worse in some areas. But first let us understand what was this news about. (Feature Photo above of recent Land Subsidence in Kashmir. Photo from The Times of India).

China The report in Nature and Science was about new research highlighting that nearly half of China’s major cities were experiencing land subsidence or sinking, including coastal and inland cities. This assessment is based on study of 82 cities across China, each with a population over 2 million. 10% residents of China’s coastal cities, meaning between 55 and 128 million people could be living below sea level by 2120, facing flooding and irreparable damage. 16% area of China’s major cities are sinking at the rapid rate of 10 mm per year. About 45% of the area was sinking at moderate annual rate of over 3 mm. The affected cities included the coastal and inland cities, including capital Beijing.

The researchers used radar pulses from satellites to measure the changes in the distance between the satellite and the ground to examine how its elevations changed between 2015 and 2022. On the list are cities in the largely land-locked southwest, such as Kunming, Nanning and Guiyang. These cities are not the most densely populated or industrialized, yet they still experience significant subsidence.

Other parts of the world: Subsidence is not limited to China, however. Jakarta [ii] is sinking so rapidly that Indonesia is proposing a new capital city. Parts of Jakarta sank by over a meter in a decade. Another study published in February 2024 found that around 6.3 million square kilometers of land worldwide was at risk of continued sinking [iii]. One of the most affected nations is Indonesia. Of the 44 major coastal cities currently suffering from the problem, 30 were in Asia.

Manila, Ho Chi Minh City, New Orleans and Bangkok are similarly at risk. Parts of Iran’s capital city Tehran [iv], home to 13 million people, are subsiding by 25 centimetres each year. Netherlands is famous for the fact that about 25% of the land has subsided below sea level. By 2040, about 20% of the world population is projected to be living on sinking land.

Dhaka is an example of a city that started to discover that it subsided after the flood frequency increased. In this rapidly expanding city, data on subsidence and its impacts is currently largely lacking. Large-scale extractions cause groundwater levels to fall by 2–3 m/ yr. At present, 87 % of the supplied water is from groundwater, and it has been acknowledged that a shift to surface water instead is necessary. But it is difficult to achieve as most of the surface water here is polluted.

One of the important driving forces behind sinking coastal cities is sea level rise. The Gulf of Mexicohas experienced twice the global average rate of sea level rise since 2010, an analysis of satellite data shows. Few other places on the planet have seen similar rates of increase, such as the North Sea near the United Kingdom.

Both the century-long tide gauge data and the more recent altimetry data reveal a rapid decadal acceleration of Sea Level Rise during 2010–22 along the U.S. East Coast and the Gulf of Mexico coast. Thus, since 2010, Sea Level Rise or relative subsidence is very abnormal and unprecedented [v]. While the swift rate of sea level rise could eventually taper, the higher water that has already arrived in recent years is here to stay.

Galveston (Texas), has experienced an extraordinary rate of sea level rise — 8 inches in 14 years. Experts say it has been exacerbated by fast-sinking land. High-tide floods have struck at least 141 times since 2015, and scientists project their frequency will grow rapidly.

In the United States, over 44,000 sq km of land across 45 states has been directly affected by subsidence, with more than 80% of the cases relating to groundwater extraction. Sinking ground and rising sea levels will put more than half a million people at risk of repeated flooding across 32 U.S. coastal cities [vi] — including New York, Boston, San Francisco and Miami — new research shows.

Monitoring Subsidence To assess land subsidence, the first step is to establish if a certain area is actually subsiding. This may not be evident from the field, particularly if subsidence is non- differential and no structural damage (cracks, tilting) is observed in buildings or infrastructure. Typically, the loss of elevation, which may have been observed, is interpreted as the result of climate-driven sea level rise instead of the result of subsidence. Land surface elevation measurements need to be combined with local in-situ measurements in order to be able to unravel the total subsidence signal.

To determine land subsidence rates, accurate measuring techniques are required. Continuous subsidence monitoring provides the necessary insight into changes in the topography of the urban area. Some of the methods used for this include: optical levelling; Global Positioning System (GPS) surveys; Laser Imaging Detection and Ranging (LIDAR); Interferometric Synthetic Aperture Radar (InSAR) satellite imagery. All these techniques measure land surface elevation change, but give no information on the source of the subsidence.

To assess what are the contributing factors and their extent to subsidence in any city, a detailed research will be required. The next step will be predicting future land subsidence and possible damage due to them, using modelling and forecasting tools for different scenarios for different contributing factors for future.

The reasons Subsidence can be for natural and anthropogenic reasons. In urban areas, human induced land subsidence dominates the total subsidence. Subsidence happens when land sinks relative to sea level, usually owing to extraction of subsurface water, rock or other resources including oil, gas, coal. Underground tectonics can also contribute to subsidence. Largest contribution is possibly from over extraction of groundwater. In coastal areas, however, the rising sea levels due to melting of glaciers and snow-mass and expansion due to higher temperatures are also major contributors to relative subsidence.

The new research links a range of natural and human factors to sinking, such as the depth of a city’s bedrock, groundwater depletion [vii], the weight of buildings, the use of transport systems and underground mining. Other studies have found that excessive groundwater extraction is a key cause of severe land subsidence [viii] in cities across the world. In cities such as Macao and Hong Kong where no groundwater is used, subsidence mainly comes from consolidation — downward movement as a result of soil being compressed — after land reclamation.

Currently, global mean absolute sea- level rise is around 3 mm/ yr, and projections until 2100 based on Intergovernmental Panel on Climate Change scenarios expect a global mean absolute sea-level rise in the range of 3– 10 mm/ yr. However, currently observed subsidence rates in coastal megacities are in the range of 6–100 mm/ yr. This just shows that sea level rise is one of the many contributors to coastal subsidence.

The study concludes, “In many coastal and delta cities land subsidence now exceeds absolute sea level rise up to a factor of ten.”

Role of Big Dams in Subsidence of Delta Cities: There is also the Role of Big dams in subsidence of delta cities [ix]. It has been established that one of the main reasons behind delta subsidence is drastically reducing sediments reaching the delta. Studies estimate that during the past century, there has been a 94% reduction in Krishna’s sediment reaching the delta, 95% reduction from historic load in Narmada, 80% reduction in Indus, 80% reduction in Cauvery, 96% reduction in Sabarmati, 74% reduction in Mahanadi, 74% reduction in Godavari, 50% reduction Brahmani, etc.[x]

For example, the Ganges–Brahmaputra delta is one of the largest in the world. As wind and rain erode the Himalayan mountain range in the catchment, massive rivers carry more than a billion tonnes of sediment into the Bay of Bengal each year. In some places, the layer deposited since the most recent ice age is more than one kilometre thick. As in all deltas, this loose material compacts easily, causing the land to sink slowly and the relative sea level to rise. The high tides and storms also erode the delta. In the past, sediment carried downstream each year would have refreshed the delta. But large dams built upstream have diverted water and choked the flow of sediments, so the coastal land is no longer being rebuilt. A 2009 study found that 85% of the world’s largest deltas suffered severe flooding in the first decade of the twenty-first century. The embankments that protect the land from the river and sea can also rob the delta of fresh supplies of sediment.

Earthquakes The quake in 1762, estimated at magnitude 8.8, caused land around the South-Eastern city of Chittagong in Bangladesh to sink by several metres; in the Sundarbans it seems to have caused at least a 20-centimetre drop. Seismologists think that another major quake is overdue in this tectonically unstable region, and that when it comes it will devastate poorly built high-density cities such as Dhaka and Chittagong. It could also cause patches of the delta to drop more in one fell swoop than they have over decades of slow sea-level rise and sediment compaction.

Climate Change brings Double whammy As cities sink, global sea levels are also rising, owing to the effects of climate change [xi] as noted above. This double whammy will cause 22–26% of China’s coastal lands to drop below sea level by 2120.

Climate change might exacerbate sinking in other ways, such as by affecting where and when it rains — or doesn’t rain. For example, droughts can lead to accelerating of groundwater use, thus bringing higher and faster subsidence.

Consequences Non-differential land subsidence increases flood vulnerability (frequency, inundation depth and duration of floods), with floods causing major human, social and economic damages. Examples of cities that have increased flood risk as a result of subsidence include Jakarta, Ho-Chi-Minh and Bangkok. Subsidence can also lead to sea water intrusion, contaminating groundwater.

Moreover, differential land movement causes significant economic losses in the form of structural damage and high maintenance costs for infrastructure. It affects roads and transportation networks, hydraulic infrastructure, river embankments, sluice gates, flood barriers, pumping stations, sewage systems, buildings, and foundations. In general there is disruption in water management. Examples of cities in which structures are damaged include New Orleans (USA), Venice (Italy) and Amsterdam (the Netherlands). In Netherlands, over exploitation of gas in the north also led to increased seismic activities.

The total damage worldwide is estimated at billions of dollars annually. Indications are that both subsidence and damage thereof are on increasing trend. Subsidence also means that there will be increased impact of storm surges, hurricanes and typhoons and changes in precipitation.

Making an estimation of costs associated with subsidence is complex. Some broad estimates are available. For instance, in China, the average total economic loss due to subsidence is estimated at around USD 1.5 billion per year, of which 80–90 % is from indirect losses. In Shanghai, over the period 2001–2010, the total loss cumulates to approximately USD 2 billion. In the Netherlands, it is calculated that damage to foundations (as a result of subsidence) has been more than EUR 5 billion thus far, and might reach EUR 40 billion in 2050.

Solutions There are a number of examples of cities where subsidence has reduced or stopped after successful mitigation measures were implemented. Tokyo slowed its sinking from a rapid 240 mm a year in the 1960s to about 10 mm a year in the early 2000s, after passing laws that limited groundwater pumping. In Bangkok-Thailand, regulation of and restrictions on groundwater extraction have successfully reduced severe land subsidence.

Shanghai, China, which sank by a staggering 2.6 metres between 1921 and 1965, reduced its annual rate of sinking to about 5 mm after implementing a series of environmental regulations. Here groundwater levels were restored with active recharge techniques. The Shanghai case shows that, with active and substantial recharge, sustainable groundwater use is achievable, without severe subsidence, provided that average yearly pumping rates are in balance with the average yearly recharge. This shows a direction for other Chinese cities facing subsidence as brought out by new research.

Delayed response may introduce additional subsidence even after implementing mitigation measures. For measures taken to reduce subsidence or its impacts, the effectiveness of these efforts should be monitored.

There are two possible policy strategies for subsiding cities: mitigation and adaptation. Any policy will need to include both. Mitigation works for human-induced subsidence. Typical mitigation measures include restrictions of groundwater extraction, artificial recharging aquifers. Similarly restricting exploitation of gas or other resources when that is the reason for subsidence. Building with lighter materials decreases the load on soft soils, thereby decreasing consolidation and subsidence. Sediment release or decommissioning of upstream dams may help for delta cities starved of sediment brought by rivers.

An adaptation strategy could also be considered where mitigation does not work or for residual subsidence or for the period when mitigation measures are still in works.

Land subsidence in India In India, we do not have systematic monitoring or information gathering to provide a comprehensive picture of land subsidence and their causes. However, considering India is the world’s largest user of groundwater and that use continues to go up, and considering that groundwater has been India’s water lifeline for over 4 decades by now, the possible dimensions of sinking in India should worry us. India is also third largest dam builder in the world and possibly biggest dam builder currently. We need to urgently start monitoring and measuring sinking across the country. We should also be estimating the impact of dam building on subsidence in deltas.

Here we can look at some snapshots of the situation subsidence situation in India.

As mentioned above, considering the amount of sediment trapped in upstream dams and thus the sediment not reaching the delta areas, the situation of delta areas including delta cities should worry us.

The most (in) famous of the sinking event in recent times in India is in Joshimath town in Chamoli district of Uttarakhand [xii]. Here the role of under construction hydropower project is suspected to have contributed to sinking, besides other factors, but this issue remains unresolved.

There have been series of land subsidence events, the latest of which happened in May 2024 in districts like Ramban and Doda and now in Udhampur in Jammu & Kashmir in the fragile Himalayan region. These are being attributed to a multitude of factors including blasting and cutting of hills for roads and railway tunnels, proliferation of hydroelectric projects.[xiii]

In April 2024, two major incidents of land subsidence took place in Rajasthan [xiv], alarming both geologists and the general public. Both incidents took place in desert districts, raising suspicions of a possible connection between them. On April 16, 2024, one and a half bigha of land collapsed in Sahajrasar village of Lunkaransar tehsil of Bikaner district. At the time, a train full of passengers was passing by, narrowly escaping the sinking ground. The subsidence formed a pit about 70 feet deep, which, according to villagers, has since deepened to about 80-90 feet.

The second incident occurred on May 6, 2024, in Nagana village of Barmer district, where two parallel cracks appeared in the ground over an area of about one and a half kilometres. A geological team has submitted its preliminary report to the administration on these incidents occurring in the two districts of the Thar Desert blaming groundwater extraction among other reasons. A detailed report is forthcoming, based on satellite imagery, water data, and other technical information.

There is thus a lot that we need to study and do in India to avoid sudden sinking feeling.

Himanshu Thakkar (https://sandrp.in/, ht.sandrp@gmail.com)

Note: Hindi translated version of this was published by Srote Features in two parts on June 19 2024:

– https://www.srotefeatures.in/srote/2024/06/%e0%a4%a7%e0%a4%82%e0%a4%b8%e0%a4%a4%e0%a5%80-%e0%a4%ad%e0%a5%82%e0%a4%ae%e0%a4%bf-%e0%a4%95%e0%a4%be%e0%a4%b0%e0%a4%a3-%e0%a4%94%e0%a4%b0-%e0%a4%aa%e0%a4%b0%e0%a4%bf%e0%a4%a3%e0%a4%be%e0%a4%ae-1/
– https://www.srotefeatures.in/srote/2024/06/%e0%a4%a7%e0%a4%82%e0%a4%b8%e0%a4%a4%e0%a5%80-%e0%a4%ad%e0%a5%82%e0%a4%ae%e0%a4%bf-%e0%a4%95%e0%a4%be%e0%a4%b0%e0%a4%a3-%e0%a4%94%e0%a4%b0-%e0%a4%aa%e0%a4%b0%e0%a4%bf%e0%a4%a3%e0%a4%be%e0%a4%ae-2/

[i] https://www.nature.com/articles/d41586-024-01149-7

[ii] https://www.nature.com/articles/437312a

[iii] https://learningenglish.voanews.com/a/study-nearly-half-of-large-chinese-cities-are-sinking/7581574.html

[iv] https://www.nature.com/articles/d41586-018-07580-x

[v] https://journals.ametsoc.org/view/journals/clim/36/13/JCLI-D-22-0670.1.xml

[vi] https://www.livescience.com/planet-earth/climate-change/32-us-cities-including-new-york-and-san-francisco-are-sinking-into-the-ocean-and-face-major-flood-risks-by-2050-new-study-reveals

[vii] https://www.nature.com/articles/466308a

[viii] https://piahs.copernicus.org/articles/372/189/2015/