Over 66,000 ha lost to Padma since 1967 Over 66,000 ha lost to Padma since 1967 NASA report says More than 66,000 hectares (256 square miles) of land
have been lost-roughly the area of Chicago, one of
the biggest cities in the US - to erosion caused by
the Padma River since 1967.
That is according to an August 2018 report published
by the NASA Earth Observatory, the arm of the US
space agency that is focused on the planet we live
in.
The extreme erosion patterns of the Padma are known
to have two main causes. First, it is a natural,
free-flowing river with little bank protection, other
than some occasional sandbags to protect buildings.
Second, 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. The NASA
report, "The Shape of Erosion", compares natural-
color satellite images showing the changes to the
shape and width of the Padma since 1988.
Each "twist and zigzag" is said to tell a different
geologic story about the river. The images were
acquired by NASA's Landsat satellites: the Thematic
Mapper on Landsat 5, the Enhanced Thematic Mapper
Plus on Landsat 7, and the Operational Land Imager on
Landsat 8. All images were acquired in January and
February, during the dry season, reports UNB.
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 labeled "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 (dam) in India, which released
more water into Bangladesh, according to the NASA
report.
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. "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," it
says.
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.
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