Rapid urbanization and the demise of wetlands around Dhaka, the mushrooming capital of Bangladesh, has made the city more vulnerable to flooding and other natural disasters.
"Our wetlands are our only surface-water drainage system. As the wetlands are receding, Dhaka is becoming more vulnerable to water-logging and flooding,” Mohammed Ataur Rahman, director for the Centre for Global Environmental Culture (CGEC) told IRIN.
The city is located in the world’s largest delta system, the Ganges-Brahmaputra or Sunderbans Delta, a network of rivers and streams more than 350km wide.
“Nowadays, even an inch of rain will cause water-logging in Dhaka. Heavy rainfall can easily flood the whole city,” Rahman said.
The fastest growing city
Dhaka, a megacity with an estimated population of more than 12 million, has experienced a near four-fold increase in its number of residents over the last 25 years, and is expected to reach close to 18 million in 2015, according to the World Bank. In 1980 the population stood at just 3.26 million.
The rate of expansion far outstrips the pace of infrastructure development, while efforts to accommodate the needs of the city’s ballooning population take a rising toll on the environment.
A new study covering the development of the city from 1960 to 2008, to be published by the Bangladesh Centre for Advanced Studies (BCAS), notes that about 52 percent of the lowlands and 33 percent of the water bodies - rivers, lakes, etc. - around Dhaka have been lost to urbanization.
The BCAS research revealed that in just six years, from 2005 to 2011, the wetlands adjacent to Dhaka shrank from 5.85 sq km to 3.95 sq km when local water bodies and lowlands were converted to commercial, industrial and residential zones.
Weak enforcement of government regulations has made illegal encroachment onto wetlands a “very common” practice, activists say. Research shows that typically, the process starts with bamboo posts being driven into the bed of the water channel, after which huts and shops are erected on them, and landfill is then allowed to accumulate.
“We are losing our wetlands to the unplanned growth of the city and to unscrupulous land grabbers,” said Halima Begum, associate professor of the Department of Urban and Regional planning at Jahangirnagar University, “These wetlands are crucial to the environment. They help in recharging our groundwater storage, and also provide the channel for rain- or floodwater to run off."
About 87 percent of Dhaka’s residents use groundwater, mostly from deep tube-wells - in which a pipe of 100mm-200mm is bored into the ground until it reaches the water table - while the rest of the people use treated surface water, according to the Dhaka Water Supply and Sewage Authority.
Photo: David Swanson/IRIN
|Flooding is a regular occurrence in Dhaka|
The depletion of Dhaka’s wetlands is also having an adverse effect on the local climate. The BCAS study found that the average temperature in the metropolitan area is 2 degrees Celsius hotter than in the peri-urban zones surrounding it, turning the city into a "heat island".
The study indicates that maximum and minimum temperatures in Dhaka city have steadily increased, bringing hotter summers and cooler winters. In the last 100 years the average temperature in Dhaka has increased by 0.50 degrees Celsius, and in the next 50 years is expected to increase by another 1.5 to 2 degrees Celsius.
“Loss of open spaces also means the densification of built-up zones. These are essentially collections of bricks, concrete and metal - materials that retain heat. As a result, Dhaka is getting hotter every year,” Halima Begum said.
Dhaka ranks third in the list of cities most vulnerable to coastal flooding due to climate change. Traditionally, the rainy season starts in mid-June and continues for two months, but this is no longer the case. The change makes Dhaka’s drainage system vital to its existence, and the wetlands are an essential part of its drainage system.
Drainage system failure
“The rainy seasons [in Dhaka] are getting shorter but more intense. The wetlands serve as flood basins for the city - they help the rainwater to run off. With the depletion of the wetlands, the city’s drainage systems are no longer able to function properly, resulting in frequent water-loggings,” said Mohammad Abu Syed, a research fellow of BCAS.
"The wetlands are our water catchments. They recharge our groundwater aquifers. As the wetlands are disappearing, the aquifers are no longer resupplied with water, resulting in vacuums underneath the city,” CGEC’s Rahman explained.
“These vacuums greatly increase the potential damage risk from an earthquake. We have already drained the first layer of groundwater aquifers and are now digging into the second layer. If there is an earthquake, the top layer of soil can sink down, taking the city with it.”
To protect and preserve the wetlands and agricultural areas of the city, the BCAS study recommends the immediate enforcement of environmental regulations and urban management strategies.
Halima Begum warned: “We need speedy resolution of these issues, or else the very existence of Dhaka might be threatened.”