【2009.09.22 地球观察】Sinking River Deltas — Irrawaddy River: Image of the Day

rhapsody

http://earthobservatory.nasa.gov/IOTD/view.php?id=40204

Rivers flow across the world’s continents, picking up soil as they grow and move towards the ocean. When rivers pour into the sea, the dirt and rocks sink as fresh water disperses into salt. Over time, rich flat land, a delta, builds around the river mouth, usually forming a triangular shape. Deltas are rich agricultural land and provide easy access to water and water-based transportation. As a result, many of the world’s largest deltas are heavily populated. Nearly half a billion people live on or near deltas, and that could be an increasingly serious problem, says James Syvitski, a researcher at the University of Colorado, and his collaborators.

In a paper published in Nature Geoscience on September 20, 2009, Syvitski and his colleagues warn that deltas throughout the world are sinking, largely because of human activity. One of the deltas in danger is the Irrawaddy River delta in Burma (Myanmar), shown in this image. The image is an elevation map, made from data collected by the Shuttle Radar Topography Mission. Higher elevations are white, while lower elevations are depicted in green. The Irrawaddy Delta is clearly flat, much of it below five meters in elevation. This low elevation makes the delta particularly prone to flooding from storm surges. In April 2008, Cyclone Nargis flooded much of the delta, killing thousands. As deltas sink, storm surges can move farther inland, affecting more people.

Syvitski and his colleagues used shuttle radar topography data compared to historic maps to assess the condition of 33 of the world’s largest deltas. They found that many of the deltas were sinking, and as a result, the delta area vulnerable to flooding could increase by 50 percent in the twenty-first century. Disastrous storm surge flooding like that seen in the wake of Cyclone Nargis or Hurricane Katrina could become more common.

The deltas are sinking for a number of reasons. Deltas grow when they receive new sediment, often during seasonal floods. Rivers split into many channels that wander across the delta, distributing sediment. In this image, small dark green channels reveal where the Irrawaddy once meandered across its delta before finding its current path, which is marked in blue. Higher-elevation white squiggles are probably the canopy of trees growing along small water channels. But wandering rivers are very inconvenient for people, who build canals to divert the water and prevent flooding in cities built on the delta. People also build dams along major rivers to control flooding and generate power. The sediment in the river water gets trapped behind the dams. In the end, less sediment reaches the deltas, so they are not regenerated. Even during severe floods, the authors found that little sediment was being added to many of the deltas.

Even as the deltas are not being replenished with fresh soil, sediment in the delta is compacted or erodes away, naturally causing the delta to sink. The delta may sink farther if oil, gas, or water are pumped out of the delta. If the delta sinks faster than it gets new soil, it becomes increasingly prone to floods, both from the river itself and from storm surges. Even if the delta is building, it must build faster than sea levels are rising, or it will, in effect, sink.

References
Syvitski, J.P.M., Kettner, A.J., Overeem, I., Hutton, E.W.H., Hannon, M.T., Brakenridge, G.R., Day, J., Vorosmarty, C., Saito, Y., Goisan, L., and Nicholls, R.J. (2009, September 20). Sinking deltas due to human activities. Nature Geoscience. doi:10.1038/ngeo629.

rhapsody

Many of the world's largest deltas are densely populated and heavily farmed. Yet many of their inhabitants are becoming increasingly vulnerable to flooding and conversions of their land to open ocean. The vulnerability is a result of sediment compaction from the removal of oil, gas and water from the delta's underlying sediments, the trapping of sediment in reservoirs upstream and floodplain engineering in combination with rising global sea level. Here we present an assessment of 33 deltas chosen to represent the world's deltas. We find that in the past decade, 85% of the deltas experienced severe flooding, resulting in the temporary submergence of 260,000 km2. We conservatively estimate that the delta surface area vulnerable to flooding could increase by 50% under the current projected values for sea-level rise in the twenty-first century. This figure could increase if the capture of sediment upstream persists and continues to prevent the growth and buffering of the deltas.

rhapsody

反正也不长，干脆自己领了；先看球去，回头再翻^^~

rhapsody

已发：http://bbs.m4.cn/thread-197917-1-1.html