Delta Subsidence: An Imminent Threat to Coastal Populations
Authors: Charles W. Schmidt
Journal: Environmental Health Perspectives (2015)

ea-level rise from a warming climate threatens to inundate coastlines around the world. 1 But some of the world's most vulnerable coasts-those fringing flat delta plains, mainly in Southeast Asia-face the far more immediate threat of sinking land. 2 Induced mainly by human activities on a local rather than global scale, this phenomenon, known as land subsidence, can outpace sea-level rise substantially.Indonesia's biggest city, Jakarta, is sinking at an average rate of 5-10 cm per year, 3 much faster than the global rate of sea-level rise, which clocks in at 3.2 mm per year, according to the recent estimates. 1Should subsidence in Jakarta continue unabated, the city could sink up to 6 m by the end of the century, according to JanJaap Brinkman, a water management specialist with Deltares Research Institute in Delft, the Netherlands.Roughly half a billion people live in delta regions threatened by subsidence, and concerns for their well-being are mounting. 4For instance, in 2007 Jakarta experienced catastrophic flooding-portions of the city were inundated for weeks, 200,000 people were displaced, and roughly 1,400 were hospitalized for waterborne diarrheal diseases and by dengue fever, which is carried by mosquitoes that thrive in standing water. 5Marc Bierkens, a professor of hydrology at Utrecht University, the Netherlands, says Jakarta sits in a subsiding bowl where "floodwaters pool up with nowhere to go."Apart from the increased risk of floods and associated diseases, experts consulted for this article say subsidence threatens health in other ways.It accelerates the contamination of freshwater resources with saltwater, making them unsuitable for drinking and agriculture.Subsidence stresses gas lines, sewage pipes, and other infrastructure, which can crack as the land buckles and heaves, increasing the risk of explosions and contamination of surface and groundwater.Finally, the stress of the threat to drinking water supplies, homes, and livelihoods can adversely affect people's sense of well-being.Subsidence has been slowed in cities such as Tokyo 6 and Bangkok, 7 and the lessons learned there are galvanizing efforts to tackle the problem elsewhere.It's also a multifaceted problem, and researchers are investigating site-specific causes of subsidence in a search for targeted solutions.But in many deltas, time is running out, warns Gilles Erkens, a senior researcher at Deltares Research Institute and Utrecht University."In many cases, we simply don't have ten more years to wait for more data," he says. Deltaic ProcessesThe world's deltas were built up mostly by aggradation, or the deposition of fertile river sediments over thousands of years. 8As such, they comprise important food-producing areas that attract large populations.The Mekong Delta, for instance, which is now subsiding at an average rate of 1.6 cm per year, 9 is one of the world's major rice exporters 10 and home to more than 20 million people. 9Unlike rocky continental coasts, delta plains tend to be soft and easily compressed.They're often propped up by underlying oil, gas, or fresh groundwater that flows through the pores of sediment deposits.As those resources are extracted, the sediments compress, and the land shrinks like a dried sponge.Some sediments, especially those rich in organic matter, such as peat, also oxidize when they dry.Oxygen binds with carbon in the soils, creating carbon dioxide that is released to the atmosphere.Deprived of the carbon lost to this reaction, the soils lose mass and compact. 11