Impact of the 26-12-04 tsunami

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• Topography: The 26 natural atolls of the Maldives comprise 1190 islands stretching 823 km north to south. Only 200 islands are inhabited with 88 islands adapted as exclusive resorts.
• Geology: The islands are composed of limestone rocks and are covered with a thin sandy soil. A hard pan layer 0.05 to 0.3 m thick exists within the tidal limits of groundwater levels over much of the island. Although this layer is relatively impervious it is penetrated by some hundreds of wells. Below the hardpan there is coarse to medium coral sand with some larger fragments of corals and shells. The thickness of these uncemented layers is 4-5 m or less near the shore, but reaches 12 m or more near the centre. Below this there is generally coral reef structure. (Grimmelmann, W.F., www.siwin.org)
• Hydrogeology: The size of the fresh groundwater lenses below the islands depends on the rainfall, the width of the island and the permeability of the aquifer. On small islands the depth is only a few meters, on larger islands the depth is probably to about 10 meters. A too high abstraction of groundwater causes upcoming of saline water, rendering well water unsuitable for drinking. The quality of the groundwater also suffers from nitrate and sulphate pollution.
• Vulnerability: Intrusion of saline water during the tsunami flood is to be expected but the volume depends on the duration and the depth of the flooding The salts in soils and aquifer will be removed in an estimated one or two years depending on rainfall quantities
• Water Supply: About 25% of the population depends on groundwater, the rest uses rainwater and desalinated water.
• Damage areas: The flooding did not do much damage to the well structures, but it made the water saline in many wells. Reports were also received of wells which overflowed and of wells in which the sandy well bottom was raised. The pressure wave which came with the tsunami was transmitted through the sub-surface and most probably will have upset the equilibrium between the fresh and saline groundwater below the islands.
• WHO recommendations:
  1) clean groundwater wells with at least one pump unit per island;
  2) measure the water salinity;
  3) carry out hygiene education on cleaning rainwater collection areas and
  4) undertake a feasibility study on groundwater supplies as septic tanks pose a pollution risk.

(Ref: National Disaster Management Center)

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