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Jakarta has sunk by up to 4 meters, forcing Indonesia to build a new capital

Yesterday, Indonesian President Joko Widodo announced a plan to move the country’s capital from Jakarta to a new location in Borneo. The reason? Jakarta is bursting at the seams—and sinking.

Different sections of the city—home to 10 million people within an urban area of 30 million—are subsiding at different rates, but most fall in the range of 3 to 10 centimeters every year.

Over the years, that has added up to as much as four meters of surface elevation change. This has wreaked havoc on building foundations and other infrastructure. And as Jakarta sits on the coast, where a number of small rivers meet the sea, the flooding hazard is also real. (The fact that sea level is rising doesn’t help.) That includes high-tide seawater flooding but also stormwater flooding as rain captured by the sprawling city’s pavement struggles to drain seaward.

Why the instability? Jakarta is a case of humans doing the wrong things in just the right place. River sediments deposited at the coast in places like this are naturally somewhat compressible. (It’s possible the bedrock beneath is moving a little bit and contributing, as well.) The actual weight of all the buildings and other construction at the surface is acting to compact the sediment a little, not unlike tamping down loose sand or soil in your yard. The biggest factor, though, is excessive groundwater pumping.

Within the sediment beneath Jakarta are several stacked aquifer layers that water can be pumped out of. Between the aquifer layers are impermeable capping layers. The use of well water in and around the city has caused the groundwater levels in the aquifers to drop tens of meters.

Because groundwater lives in the little spaces between grains of sediment, it actually helps support the grains and keep those spaces open. As water level drops, the drained spaces lose that support and can collapse in, compacting the sediment. In addition, the water pressure inside the impermeable capping layers can also drop during all this. This allows them to compress in a more reversible way—more like an air mattress deflating slightly.

Enlarge / Average subsidence rates in Jakarta from 1974-1991 (left) and 1991-2010 (right).

People face this same issue in many places. Cities like New Orleans and Venice, to name a couple problematic examples, are sinking partly due to groundwater extraction. And Shanghai has experienced more than two meters of subsidence due to groundwater pumping and construction on compressible sediments—just like Jakarta. California’s Central Valley is famous for the incredible amount of subsidence that has occurred as the region’s agriculture has tapped the aquifers for irrigation.

Indonesia’s plan is to start a new capital city in an undeveloped portion of the East Kalimantan province of Borneo. Reuters reports that President Widodo’s goal is to begin relocating the 1.5 million civil servants working in Jakarta in 2024—an endeavor that would cost around US$33 billion overall.

Sumber: Arstechnica



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