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Land assessment

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Impacts of subsidence on coastal areas

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Damage and recovery capacity

Tsunami in perspective

IMPACTS OF SUBSIDENCE ON COASTAL AREAS:
DRAINAGE AND SALT RELATED ISSUES

The earthquake and resulting tsunami of December 2004 were occurred along the Sunda-Andaman Arc of the boundary where the Indo-Australian Plate subducts under the Burma Micro-Plate of the Eurasian Plate. The movement along this divide did not only cause a major earthquake, but because of subduction of the former plate under the latter, lands just east to the zone plate boundary sustained both uplift and subsidence. This vertical movement is reported to be several meters in the islands close to the arc. Major uplift and subsidence has been reported in the Andaman and Nicobar islands as well as the islands off the Aceh Coast and Aceh mainland as well. Since the tsunami several groups of scientists have been investigating the nature and degree of vertical movement along with horizontal displacement that also took place. On average the movement is around 2 meters, up to 5 meters or more on some islands. In Aceh, where subsidence is visible, it is less than one meter. Signs of uplift and subsidence were immediately visible, however. Inhabitable or arable lands that were close to the sea are now submerged, either permanently or with high tide. Coconut trees that were close to the coast and were not uprooted by the tsunami are now standing in the sea. Jetties are deeper or submerged. Uplift is showed by the new occurrence of shallow coral or coral being exposed to the air. Mangroves are standing dry in some places.

Left: West Aceh The land here subsided 1 to 2 m during the earthquake; the treetops were snapped off by the tsunami. Source: USGS: http://soundwaves.usgs.gov/2005/03/
Right
:Meulaboh (West Aceh ): Coconut trees withstood the waves, but are now with their feet in the water due to a rise in mean sea level

Impacts on Agriculture

Subsidence and uplift are permanent. Land lost to the sea (Class D of the FAO Damage Classification) cannot be reclaimed. The immediate impact of subsidence is thus that farmers have to be relocated if they lost their lands, and also in these areas that are now very close to sea and will have continuing problems with lateral seawater intrusion.

There are other less visible slow-onset problems that can be expected, however:

1)  Because of the subsidence the drainage system has changed. Field and channel drainage can be submerged, because of higher water level in estuaries and river mouths due to the influence of the higher sea water table. Submersion reduces the drainage capacity and may cause problems of water logging and salinisation.

2)  The relative rise of the sea level will cause a change in water balance between the fresh water layer and the saline water layer. Generally, saline water will push the fresh water lense further to the inland. This can affect a strip of several hundreds of meters. Especially in areas where groundwater is pumped for irrigation this effect has to be carefully monitored to prevent saline water intrusion and degradation of water quality.

Other problems related to lateral and vertical movement of the islands are slow changes in island morphology that may occur over time through processes of scouring and sedimentation. This could have various effects on agricultural land. All these processes are slow and will not be easily determined. Assuming that in-depth research is not possible everywhere it is best to monitor regularly and investigate in detail when problems become apparent.

 

 

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© FAO, 2005