Wednesday, December 3, 2014
The Dangerous Geomorphology of Deltas
Deltas were always going to be a problem. Deltas form where rivers dump sediment into the ocean, and form vast low-lying plains that lie only a few feet above sea level. Because deltas are flat and well-irrigated, delta areas like the Ganges River Delta in Bangladesh (above) are some of the most densely populated areas on Earth. If sea levels rises by about a meter over the next 80 years, as climatologists with the UN IPCC project predict, hundreds of millions of people will be displaced and millions of acres of farmland will be lost.
If that wasn't bad enough, deltas have another problem. Because deltas are built by annual floods, the sediments will consolidate over time so that the elevation of deltas tend to decrease over time unless new floods deposit fresh sediment across the delta every few years. But human-constructed levees and other flood control infrastructure along rivers flowing across deltas stop floods, and keep the sediment within the river channel. At places like the Mississippi Delta (below), the levee systems help direct the sediment down the channel all the way to the ocean. The flood control projects have helped preserve a long skinny "birds foot" delta, but have starved the surrounding parts of the delta of sediment. The result is the rate of local sea level rise at deltas with levee systems like those on the Mississippi River may be 2-3 times greater then the background rate of global sea level rise, because the land itself is locally sinking. By trying to protect delta areas from river floods, we have unintentionally made them more susceptible to being drowned by rising sea levels.
The Mississippi River Delta, USA