Yahoo News 20 Sep 09;
PARIS (AFP) – Two-thirds of the world's major deltas, home to nearly half a billion people, are caught in the scissors of sinking land and rising seas, according to a study published Sunday.
The new findings, based on satellite images, show that 85 percent of the 33 largest delta regions experienced severe flooding over the past decade, affecting 260,000 square kilometres (100,000 square miles).
Delta land vulnerable to serious flooding could expand by 50 percent this century if ocean levels increase as expected under moderate climate change scenarios, the study projects.
Worst hit will be Asia, but heavily populated and farmed deltas on every continent except Australia and Antarctica are in peril, it says.
On a five-tier scale, three of the eleven deltas in the highest-risk category are in China: the Yellow River delta in the north, the Yangtze River delta near Shanghai, and the Pearl River Delta next to Guangzhou.
The Nile in Egypt, the Chao Phraya in Thailand and the Rhone River delta in France are also in the top tier of danger.
Just below these in vulnerability are seven other highly-populated deltas, including the Ganges in Bangladesh, the Irrawaddy in Myanmar (Burma), the Mekong in Vietnam and the Mississippi in the United States.
These flood plains and others all face a double-barrelled threat, reports the study, published in the journal Nature Geoscience.
On the one side, a range of human activity -- especially over the last half-century -- has caused many delta regions to subside.
Without human interference, deltas naturally accumulate sediment as rivers swell and spread over vast areas of land.
But upstream damming and river diversions have held back the layers that would normally build up.
Intensive subsurface mining has also contributed mightily to the problem, notes the study, led by James Syvitski of the Institute of Arctic and Alpine Research at the University of Colorado.
The Chao Phraya delta, for example, has sunk 50 to 150 millimetres (two to six inches) per year as a result of groundwater withdrawal, while a 3.7-metre (12-foot) subsidence of the Po Delta in Italy during the 20th century was due to methane mining.
Indeed, oil and gas mining contribute to so-called "accelerated compaction" in many of the most vulnerable deltas, according to the study, the first to analyse a decade's worth of global daily satellite images.
The other major threat is rising sea levels driven by global warming.
In a landmark report in 2007, the UN's Intergovernmental Panel on Climate Change (IPCC) predicted oceans would rise by 18-59 centimetres (7.2 and 23.6 inches) by 2100.
More recent studies that take into account the impact of melting icesheets in Greenland and Antarctica have revised that estimate upwards to at least a metre (39 inches) by century's end.
The already devastating impact of such increases will be amplified by more intense storms and hurricanes, along with the loss of natural barriers such as mangroves.
In the Irrawaddy delta the coastal surge caused by Cyclone Nargis last year flooded an area up to six metres (20 feet) above sea level, leaving 138,000 people dead or missing.
"All trends point to ever-increasing areas of deltas sinking below mean sea level," the researchers concluded.
"It remains alarming how often deltas flood, whether from land or from sea, and the trends seems to be worsening."
World's River Deltas Sinking Due To Human Activity, Says New Study
ScienceDaily 20 Sep 09;
A new study led by the University of Colorado at Boulder indicates most of the world's low-lying river deltas are sinking from human activity, making them increasingly vulnerable to flooding from rivers and ocean storms and putting tens of millions of people at risk.
While the 2007 Intergovernmental Panel on Climate Change report concluded many river deltas are at risk from sea level rise, the new study indicates other human factors are causing deltas to sink significantly. The researchers concluded the sinking of deltas from Asia and India to the Americas is exacerbated by the upstream trapping of sediments by reservoirs and dams, man-made channels and levees that whisk sediment into the oceans beyond coastal floodplains, and the accelerated compacting of floodplain sediment caused by the extraction of groundwater and natural gas.
The study concluded that 24 out of the world's 33 major deltas are sinking and that 85 percent experienced severe flooding in recent years, resulting in the temporary submergence of roughly 100,000 square miles of land. About 500 million people in the world live on river deltas.
Published in the Sept. 20 issue of Nature Geoscience, the study was led by CU-Boulder Professor James Syvitski, who is directing a $4.2 million effort funded by the National Science Foundation to model large-scale global processes on Earth like erosion and flooding. Known as the Community Surface Dynamic Modeling System, or CSDMS, the effort involves hundreds of scientists from dozens of federal labs and universities around the nation.
The Nature Geoscience authors predict that global delta flooding could increase by 50 percent under current projections of about 18 inches in sea level rise by the end of the century as forecast by the 2007 Intergovernmental Panel on Climate Change report. The flooding will increase even more if the capture of sediments upstream from deltas by reservoirs and other water diversion projects persists and prevents the growth and buffering of the deltas, according to the study.
"We argue that the world's low-lying deltas are increasingly vulnerable to flooding, either from their feeding rivers or from ocean storms," said CU-Boulder Research Associate Albert Kettner, a co-author on the study at CU-Boulder's Institute of Arctic and Alpine Research and member of the CSDMS team. "This study shows there are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone."
Other study co-authors include CU-Boulder's Irina Overeem, Eric Hutton and Mark Hannon, G. Robert Brakenridge of Dartmouth College, John Day of Louisiana State University, Charles Vorosmarty of City College of New York, Yoshiki Saito of the Geological Survey of Japan, Liviu Giosan of the Woods Hole Oceanographic Institute and Robert Nichols of the University of Southampton in England.
The team used satellite data from NASA's Shuttle Radar Topography Mission, which carried a bevy of radar instruments that swept more than 80 percent of Earth's surface during a 12-day mission of the space shuttle Endeavour in 2000. The researchers compared the SRTM data with historical maps published between 1760 and 1922.
"Every year, about 10 million people are being affected by storm surges," said CU-Boulder's Overeem, also an INSTAAR researcher and CSDMS scientist. "Hurricane Katrina may be the best example that stands out in the United States, but flooding in the Asian deltas of Irrawaddy in Myanmar and the Ganges-Brahmaputra in India and Bangladesh have recently claimed thousands of lives as well."
The researchers predict that similar disasters could potentially occur in the Pearl River delta in China and the Mekong River delta in Vietnam, where thousands of square miles are below sea level and the regions are hit by periodic typhoons.
"Although humans have largely mastered the everyday behaviour of lowland rivers, they seem less able to deal with the fury of storm surges that can temporarily raise sea level by three to 10 meters (10 to 33 feet)," wrote the study authors. "It remains alarming how often deltas flood, whether from land or from sea, and the trend seems to be worsening."
"We are interested in how landscapes and seascapes change over time, and how materials like water, sediments and nutrients are transported from one place to another," said Syvitski a geological sciences professor at CU-Boulder. "The CSDMS effort will give us a better understanding of Earth and allow us to make better predictions about areas at risk to phenomena like deforestation, forest fires, land-use changes and the impacts of climate change."
Adapted from materials provided by University of Colorado at Boulder, via EurekAlert!, a service of AAAS.
Sinking River Deltas Threaten Millions
livescience.com Yahoo News 21 Sep 09;
Most of the world's low-lying river deltas are sinking due to human activity, making them increasingly vulnerable to flooding from rivers and ocean storms and putting tens of millions of people at risk, a new study finds.
Researchers have long warned that the mass human migration to coastal areas in recent decades puts more and more people at risk of death from major storms. About 500 million people in the world live on river deltas.
A 2007 Intergovernmental Panel on Climate Change report, which concluded that many river deltas are at risk from sea level rise. The new study indicates other human factors are causing deltas to sink significantly.
Why? The sinking of deltas from Asia and India to the Americas is made worse by the upstream trapping of sediments by reservoirs and dams, man-made channels and levees that whisk sediment into the oceans beyond coastal floodplains, and the accelerated compacting of floodplain sediment caused by the extraction of groundwater and natural gas, the scientists say.
Clear example
New Orleans is one example of a city that sits in a sinking delta, a problem known long before the city was ravaged by hurricane Katrina.
The new study, led by professor James Syvitski at the University of Colorado at Boulder, concluded that 24 out of the world's 33 major deltas are sinking and that 85 percent experienced severe flooding in recent years, resulting in the temporary submergence of roughly 100,000 square miles of land.
The results were detailed in the Sept. 20 issue of the journal Nature Geoscience. The study is part of a $4.2 million effort funded by the National Science Foundation to model large-scale global processes on Earth like erosion and flooding. Hundreds of scientists from dozens of federal labs and universities around the nation are involved.
Syvitski and colleagues predict that global delta flooding could increase by 50 percent under current projections of about 18 inches in sea level rise by the end of the century, as forecast by the 2007 Intergovernmental Panel on Climate Change report. The flooding will increase even more if the capture of sediments upstream from deltas by reservoirs and other water diversion projects persists and prevents the growth and buffering of the deltas, according to the study.
"We argue that the world's low-lying deltas are increasingly vulnerable to flooding, either from their feeding rivers or from ocean storms," said CU-Boulder Research Associate Albert Kettner, a co-author on the study. "This study shows there are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone."
Storm surges
The team used satellite data from NASA's Shuttle Radar Topography Mission, which carried a bevy of radar instruments that swept more than 80 percent of Earth's surface during a 12-day mission of the space shuttle Endeavour in 2000. The researchers compared the SRTM data with historical maps published between 1760 and 1922.
"Every year, about 10 million people are being affected by storm surges," said CU-Boulder's Overeem, also an INSTAAR researcher and CSDMS scientist. "Hurricane Katrina may be the best example that stands out in the United States, but flooding in the Asian deltas of Irrawaddy in Myanmar and the Ganges-Brahmaputra in India and Bangladesh have recently claimed thousands of lives as well."
The researchers predict that similar disasters could potentially occur in the Pearl River delta in China and the Mekong River delta in Vietnam, where thousands of square miles are below sea level and the regions are hit by periodic typhoons.
"Although humans have largely mastered the everyday behavior of lowland rivers, they seem less able to deal with the fury of storm surges that can temporarily raise sea level by three to 10 meters (10 to 33 feet)," wrote the study authors. "It remains alarming how often deltas flood, whether from land or from sea, and the trend seems to be worsening."
'Millions at risk' as deltas sink
Richard Black, BBC News 21 Sep 09;
Most of the world's major river deltas are sinking, increasing the flood risk faced by hundreds of millions of people, scientists report.
Damming and diverting rivers means that much less sediment now reaches many delta areas, while extraction of gas and groundwater also lowers the land.
Rivers affected include the Colorado, Nile, Pearl, Rhone and Yangtze.
About half a billion people live in these regions, the researchers note in the journal Nature Geoscience.
They calculate that 85% of major deltas have seen severe flooding in recent years, and that the area of land vulnerable to flooding will increase by about 50% in the next 40 years as land sinks and climate change causes sea levels to rise.
"We argue that the world's low-lying deltas are increasingly vulnerable to flooding, either from their feeding rivers or from ocean storms," said Albert Kettner from the University of Colorado in Boulder, US.
"This study shows there are a host of human-induced factors that already cause deltas to sink much more rapidly than could be explained by sea level alone."
Most of the at-risk river basins are in the developing countries of Asia, but there are several in developed nations as well, including the Rhone in France and the Po in Italy.
The Po delta sank by 3.7m during the 20th Century, mainly from methane extraction, the researchers say.
Sinking feeling
The researchers drew on data from various space missions including the Shuttle Radar Topography Mission, an 11-day project run from the shuttle Endeavour in 2000, and the Moderate Resolution Imaging Spectroradiometers (Modis) carried on two Nasa satellites.
Combined with historical records and measurements of sea level rise, this allowed the team to view how fast land was sinking in some deltas, and to look at the various factors that might be responsible.
Of the 33 major deltas studied, 24 were found to be sinking.
Possibly the worst affected is the Chao Phraya, the river that flows through Bangkok. In some years, parts of the delta have sunk relative to sea level by 15cm (six inches).
This is significantly more than the global rate of sea level rise as a consequence of climate change (1.8-3.0mm per year).
The flow of sediment down to the Chao Phraya delta has been almost entirely blocked, the researchers report - by taking water out for irrigation, damming the river, and directing the main flow through just a few channels.
Normally, this sediment would add to the height of the land, a process known as aggradation.
Taking water from aquifers for drinking, industry and agriculture is also compacting the ground, making it sink.
As the ground falls and sea level rises, people become more vulnerable to inundation during storms.
"Every year, about 10 million people are being affected by storm surges," said Irina Overeem, another of the study team from the University of Colorado.
"Hurricane Katrina may be the best example in the US, but flooding in the Asian deltas of the Irrawaddy in Burma and the Ganges-Brahmaputra in India and Bangladesh have recently claimed thousands of lives as well."
The team identifies the Mekong and the Pearl River delta near Hong Kong as places where similar disasters are likely in future.
THE HIGH-RISK LIST
# Deltas with "virtually no aggradation (supply of sediment) and/or very high accelerated compaction"
# Chao Phraya, Thailand
# Colorado, Mexico
# Krishna, India
# Nile, Egypt
# Pearl, China
# Po, Italy
# Rhone, France
# Sao Francisco, Brazil
# Tone, Japan
# Yangtze, China
# Yellow, China
