Nasa report: Over 66,000 hectares lost to Padma since 1967

The Padma river has eroded over 66,000 hectares of land since 1967, Nasa Earth Observatory said. 

“Over 66,000 hectares of land have been lost -- roughly the area of Chicago, one of the biggest cities in the US -- to erosion caused by the river, said the report published by the Nasa Earth Observatory in August.

The report titled "The Shape of Erosion" compares natural-colour satellite images showing the changes to the shape and width of the river since 1988.

According to a report published by the Nasa in August, the extreme erosion patterns of the river are known to have two main causes. 

Firstly, it is a natural, free-flowing river with little bank protection, other than some occasional sandbags to protect buildings. Secondly, the bank sits on a large sand bed that can be eroded quickly.

Scientists measure erosion on the Padma river by noting differences in its width, depth, shape, and overall appearance on satellite images. 

Each "twist and zigzag" is said to tell a different geologic story about the river. 

Over the years, researchers have observed an increase in the river's "sinuosity" and "braiding." Sinuosity is the tendency of the river to snake back and forth in an S-shape across its plain. Rivers with high sinuosity are labelled "meandering."

Such rivers are said to evolve as the flow wears away the outer edges, widening the channel. 

The flow on the inner edge has less energy, allowing more sediment to be deposited there. 

Sometimes meandering rivers leave scars where the water once flowed, as can be seen in the 2014 image of the Harirampur region.

The sediments can come from a variety of sources. One theory suggests that some of the sediments are remnants from a landslide triggered by an earthquake in 1950. Researchers think the coarser material (like sand) took half a century to pass through the river.

Over the past three decades, the river has changed from a relatively narrow, straight line to meandering to braided and, most recently, back to straight. 

In the image sequence, the most noticeable change occurs upstream near the Harirampur upazila region, which experienced the most erosion. 

A large flood rose over these banks in 1998, exacerbated by the opening of the Farakka barrage in India, which released more water into Bangladesh, the Nasa report said.

Further downstream, meandering bends eroded the land near Char Janajat. The river's curves became most extreme from 1995-1996. 

The curve started to develop in 1992, began to decline in 2002, and has since disappeared.

The report also touches on the construction of the Padma Bridge, and how it may be affected by river erosion. 

The report said: "There are some concerns that erosion could threaten the construction of the bridge, although some researchers believe it could actually stabilize the land and reduce erosion once it is finished."

In recent years, Padma's erosion rates, sinuosity, and braiding have actually decreased. 

Erosion is said to have slowed as the meandering bends disappeared due to sedimentation and something called "chute cutoff"-when the water flows across the land instead of following the curve of the river. "But that does not mean the area is free from erosion," the report concludes.