There is a story in the Daily Mail cited by the GWPF which talks about subsidence due to groundwater extraction. For example, North Jakarta Indonesia has sunk four meters in the last 35 years, with other parts of the city also affected, and the impact of subsidence combined with heavy rain and high tides can be seen in the photo at right.
The gist of the study is that in some cities, subsidence is now exceeding sea level rise.
It is something to think about and cite the next time there is an alarming story about sea level “inundating” some city with a coastal flood.
Here are some excerpts and an abstract:
Forget global warming and melting polar caps – groundwater extraction is causing cities to SINK beneath sea level
- Ground is dropping up to 10 times faster than the sea level is rising in coastal megacities, a new study says
- Scientists at Deltares Research Institute in Utrecht studied subsidence in five coastal cities, including Jakarta, New Orleans and Bangkok
- North Jakarta has sunk four metres in the last 35 years – a fall of 10 to 20cm per year and experts have called on governments to take action
- Land subsidence is contributing to larger, longer and deeper floods
- Total damage due to subsidence worldwide is estimated at billions of dollars a year and is set to increase
I located the abstract for the research:
Sinking coastal cities
Dr. Gilles Erkens, Deltares | Utrecht University
G. Erkens, Deltares Research Institute, Utrecht, The Netherlands and Faculty of Geoscience, Utrecht University, Utrecht, The Netherlands, T. Bucx, Deltares Research Institute, Utrecht, The Netherlands
R. Dam, Deltares Research Institute, Utrecht, The Netherlands
G. de Lange, Deltares Research Institute, Utrecht, The Netherlands
J. Lambert Deltares Research Institute, Utrecht, The Netherlands
Abstract
In many coastal and delta cities land subsidence now exceeds absolute sea level rise up to a factor of ten. A major cause for severe land subsidence is excessive groundwater extraction related to rapid urbanization and population growth. Without action, parts of Jakarta, Ho Chi Minh City, Bangkok and numerous other coastal cities will sink below sea level. Land subsidence increases flood vulnerability (frequency, inundation depth and duration of floods), with floods causing major economic damage and loss of lives. In addition, differential land movement causes significant economic losses in the form of structural damage and high maintenance costs. The total damage worldwide is estimated at billions of dollars annually. As subsidence is spatially different and can be caused by multiple processes, an assessment of subsidence in delta cities needs to answer questions such as: what are the main causes, how much is the current subsidence rate and what are future scenarios (and interaction with other major environmental issues), where are the vulnerable areas, what are the impacts and risks, how can adverse impacts can be mitigated or compensated for, and who is involved and responsible to act? In this study a quick-assessment of subsidence is performed on the following mega-cities: Jakarta, Ho Chi Minh City, Dhaka, New Orleans and Bangkok. Results of these case studies will be presented and compared, and a (generic) approach how to deal with subsidence in current and future subsidence-prone areas is provided.
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Large as it may be I don’t think I would consider New Orleans a ‘mega city’.
Withdrawing groundwater in countries that receive bountiful rainfall is a non-starter for subsidence. The WEIGHT of the city is enough of an explanatory factor. Indeed, this alone would express the groundwater out into the sea all by itself. New Orleans is built on saturated clays. The Mississippi itself would supply more water for recharge than needed if this was an issue there (I don’t believe it is).
Now a bit of soil mechanics: If it is simply compaction, the subsidence rate slows over time and eventually the compacting layers would hold up the city at a certain point. And now water balance: 20cm per year subsidence would be ~120million m^3 of water withdrawn using just the city area. Rainfall on the city area is 2000mm (2 metres – 10 times the amount alledgedly used) and recharge availability for aquifers would actually be many times this since recharge is drawn from the entire watershed. It seems to me that they would have adequate rainfall for most of their uses. Now lets see about this.
http://en.wikipedia.org/wiki/Water_supply_and_sanitation_in_Indonesia
“For example, the main water source for Jakarta is the Jatiluhur Dam on the Citarum River 70 km southeast of the city. For those who are self-supplied (GP: largely outside of Jakarta) or receive water from community-based organizations, SHALLOW groundwater and springs are by far the main sources of water on most islands. On Sumatra and Irian, however, rainwater harvesting is also an important water source.[8]”
In my experience, people who get adequate rain know how to quench their thirst.
Thinking a little outside the square here, the Dutch are the ultimate in dyke building and flood defences. It was they that originally set up Jakarta, they were called back to their original work by Indonesia only a couple of years ago. The original drainage channels were full of rubbish, dumped cars etc.
This article no doubt true to a large degree, could be a clever way for the Dutch to gain some serious infrastructure work in many places. Just saying.
There are also other things to think about regarding sea level rise and inundation.
Groundwater abstraction contributing to 1/4 of sea level rise and atoll island degradation.
and not one word about sedimentation…
How ridiculous to claim the rain in Australia…
…and think this doesn’t add to sea levels
http://media.nola.com/environment/photo/gulf-dead-zone-july2011jpg-c281807bdf052bf8.jpg
Jakarta isn’t just sinking, the local bureaucracies are AWOL. Every year it floods, every year people clamour about doing something about it, and every year nobody does anything. Streams flow from the hills which are cleared of vegetation with no dams or sluices or proper water management, and the areas downstream clog up with rubbish, silt, and over-development. its stormwater runoff from over-development and clogged waterways without supervision. The water that comes down gets trapped and floods as much by water mismanagement as it does by sinking ground.
This has been known for some years and, if memory serves, it’s mostly due to ground water removal and large scale building construction in areas where ground water has been removed. But you never get to hear that. It’s all due to CO2.
Blaming ground water removal for land sink is spurious. First ground water is found in the interparticle spaces and cracks in the rocks. Removing the water will not cause the voids to close.
Djakarta is on a very volcanically active plate boundary so my first choice for the cause would be volcanic, some underground magma movement.
It’s not *the* cause, just a factor, as with many things.
John Marshall:
That is what I thought, but I have learned different. The susidence in the vicinity of Galveston Bay is due to groundwater removal from aquifers that have shale and sand interfingering and apparently water is expressed from shales into the sand as water is extracted by wells. This leads to compression of the shales, according to the volume of water expressed.
Posted about this before on another thread.
The subsiding Big Easy has been going on for 60 years or more. Grew up there and saw it.
The delta does not flood much anymore due to levees and the diversion of the Big Muddy using the Old River project up the river, which diverts a significant amount of water to the west.
The hydrostatic pressure under NOLA has been going down for all that period. My uncle’s home built in the early 50’s had the concrete slab exposed by 1980 or so. The ground was sinking. There are also pictures of street signs and such from south of NOLA that show the subsiding soil.
As far back as the late 1950’s the river becme “tidal” (SP?). The level at Jackson Square would rise and lower. Didn’t quite back up, but it was clear that it was not like 100 years before we built all the levees the Corps built and the canals used by the oil companies.
Pozzuoli, a few miles west of Naples (Italy), sits inside an old but active caldera, Campi Flegri. The town of Pozzuoli rose some 6m a few years ago. The authorities expected an active vent to open. Luckily the land subsided some months later. This was due to magma movement within the magma chamber below the town. Roman ruins in the town are evidence of past upheavels.
Looking at this from a layman’s point of view, the draw down of water coupled with the weight added and the motion of traffic above is causing settling of the earth, As we draw down the water each year from the aquifers during summer and it rebuilds in winter it is the weight and motion on the surface which causes liquefaction of the underlying soils. This liquified soil then migrates into cracks and crevasses slowly but surely compacting.
Add to this natural earth quake activity along with stress of the earths surface caused by Continental twisting, pressures created by earths rotation and the earths mantle of liquid rock you simply have the natural cycle of earths crust forming, deforming, and regeneration.
Much like the glaciers which caused whole continents to subside mans pointed increase of weight in cities is a natural compactor. The answer then is to spread out that weight and water consumption over larger areas to slow this process down.
This will not go over well with the enviro-wackos and Agenda 21 folks who want to compact people into cities and limit their movements while leaving huge swaths of land untouched and unused.
The drawing down, as Bill H posts above, is called compaction, an interaction of water and gravity on the particles. With compaction comes dewatering, when the weight of compacted rock particles closes voids and extrudes water. These processes lead to sinking like the coastal region of Bangladesh, part of the worlds largest alluvial fan. Liquifaction is a different process due to agitation of water saturated loose particles like sand and is often caused by earthquakes. Even walking over such ground can cause liquifaction and the walker trapped.