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20 February, 2018 00:00 00 AM
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Dealing with subsidence

The uninterrupted phenomena of subsidence may lead to a catastrophe
Syeda Faria Rasul
Dealing with subsidence

Bangladesh was formed a delta mainly of alluvial deposits and a combination of stronger rocks bordering the south east and north east which were the consequence of tectonic collapse of the Eurasian and Indian plates.  So the soil composition and constituency remains a question respective to strength related to current building code. Dhaka a densely populated city housing approximately 15 million people is quite a handful for the administration. It has a wide array of issues to deal with it.

The perils are obvious. It is overcrowded, pollution of land, water and air, poverty, resource reduction, space constraint, demand for unlimited power and others. Not to forget earthquakes!

Focusing on Dhaka, located in a seismic zone with the presence of soft soils, it amplifies the ground motion and having plenty of low lying lands in the city area filled up for urban construction to meet the population growth, it has not been properly compacted. Soil boring data including Standard Penetration Test (SPT) values from 34 sites around Dhaka have been collected to estimate equivalent dynamic properties of the top 30 m soil. Results obtained suggest that the maximum spectral acceleration ratio of the response spectrum should be around 2.75 for soil type S2 and around 3.75 for soil type S3. The current building code is thus on the unsafe side. The results presented indicate the necessity of revising the existing national building code.

A concern associated with urban sprawl that deserves more attention than ever is water management and associated ground water extraction. In order to quench the water supply requirement ground water pumping has led to large scale drawdown where arsenic-contaminated shallow ground water has potential to migrate downward and affect the deep aquifers.

There are 11 million tube wells in Bangladesh out of which about 5 million are highly arsenic contaminated. About 80 million people of the affected districts are at risk and the total number of patients suffering from Arsenicosis is about 7000 and about 200 dead in the last few years. Arsenic contamination is widespread and polluting the shallow aquifer in the region, hence, eventually those people who drink water from shallow tube wells are most likely to get affected with arsenic contamination and arsenicosis.

Water bodies which are the strength of city water supply are being dumped with sewage and consequently any refilling through the hydrological cycle is being wasted. The increase in surface water pollution has led cities like Dhaka to be more and more reliant on ground water.

In Dhaka about 87 per cent of the city supply depends on groundwater. A research by Bangladesh Agricultural Development Corporation (BADC), said that Dhaka’s groundwater level had dropped to 52 metres below mean sea level in 2011, compared with 46 metres in 2004. The excessive withdrawal caused vacuum underground creating possibility of subsidence, a major issue that is being grossly neglected by authorities and citizens alike.

Over the years, the natural drainage channels of cities have been encroached upon and urban water bodies, which were the main recharge points to soak up the monsoon rainwater, are filled up and built upon. Additionally, drainage systems in many of the towns were built some years ago and have not been expanded to keep up with the population. Worse, existing drains are not maintained and desilted.

The traditional water sources for many towns were mainly ponds, canals and dug-wells (pre-independence: before 1971). Only very few towns had piped water supply. Major piped water supply to many towns started in 1980 only where the Department of Public Health and Engineering (DPHE) was responsible for the supply. Gradually the ‘paurashabhas’ became in-charge of water service delivery. In order to address the water supply of bigger cities WASA was established. Cities like Dhaka and Chittagong used river water in small proportion to supplement the groundwater. In absence of suitable aquifer, towns like Gopalganj, Sunamganj and Pirojpur use river water. Lake water is also a source for towns like Rangamati .

97 per cent of people in Bangladesh have turned to getting their water from underground sources over the past 20-30 years. This started when the Bangladesh government and few organisations tried to stop the problem of diarrhoeal diseases that has been a predicament in Bangladesh throughout history. These organisations concluded that the epicentre of the disease was sewage contamination of the surface water, and so they started digging thousands of wells to get to the groundwater.

Researchers say the groundwater resource available is not sufficient to meet the demand of the cities and towns. Excess extraction causes the groundwater level to decline at a rate of more than 2.0 m inside the city of Dhaka and more than 1.0 metre near Buriganga River. As per model study of 1997 the depth of groundwater level was 25.0 metres below ground level (mbgl), by year 2006  to 35.0 mbgl and by the year 2016 it has been lowered to 45.0 mbgl .

Due to over extraction of groundwater which is sadly still a common practise, land subsidence has been noted in different major places of the city. Groundwater has been abstracted for urban and industrial water supplies (mostly in Dhaka, Narayanganj and Gazipur areas). Most rapid and higher rates of decline occur over Dhaka city and adjacent areas where groundwater abstraction is the highest in the country. These aquifers are replenished (known as recharge) each year during the monsoon season when rain and flood water finds its way into the aquifer slowly percolating down through overlying soils and sediments. The rate of recharge varies depending on the property of soil and geology of the area. Unfortunately, the recharge rate over Dhaka city is much slower than that of the adjacent floodplain areas.

Whether caused by natural or human activities, subsidence often has a number of serious consequences for human societies. Probably the most dramatic, of course, is the disappearance of whole sections of land, as occurred in Alaska's Good Friday earthquake. Today, the sudden appearance of sinkholes in Florida is no longer unusual news. In many cases, these sinkholes appear because the removal of groundwater has left limestone caves that are unable to support the land overlying them.

Even relatively modest subsidence can also damage a variety of human structures.

Buildings are weakened and collapse, railway lines and roads are twisted and broken, and underground sewer, power, and water lines are torn apart. Due to its ability to destroy property on a large scale, subsidence is a very expensive type of mass wasting that also poses some risk to human lives.

Thus, it has to be reiterated that subsidence is being neglected grossly and if ground water over extraction continues at the current pace Dhaka, being on a fault line, is heading for unfathomable disasters. More so because our unanxious, and happy-go-lucky population are very much in oblivion.

The writer is a journalist

 

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Editor : M. Shamsur Rahman

Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

Editor : M. Shamsur Rahman
Published by the Editor on behalf of Independent Publications Limited at Media Printers, 446/H, Tejgaon I/A, Dhaka-1215.
Editorial, News & Commercial Offices : Beximco Media Complex, 149-150 Tejgaon I/A, Dhaka-1208, Bangladesh. GPO Box No. 934, Dhaka-1000.

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