There are several types of damage:

• Direct crop destruction by uprooting, salt poisoning, flood, etc...

• Erosion and scouring that modifies the topography, land levelling and the elimination of bunds (for paddy fields)

• Soil fertility losses when upper layer is washed away

• Deposition of salted sediment

• Salt infiltration

• Trash and debris accumulation.

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NO, the intensity of damage provoked by a tsunami depends on three main characteristics at that particular location: the energy of the flood, the type of soil coverage and vegetation, and the soil hydraulic properties including drainage capacity.

The energy of the stream coming in and stream receding at a particular location influences the erosion and physical destruction. The energy is mainly related to the velocity of the stream but also to the stream load in sediment and debris.

As a result of the high energy along the first inland stretches it is likely that erosion is very high, however, when the wave penetrates further inland the energy decreases (velocity diminishing) and that results in a deposit of sediments. This is for instance what has been preliminary stated in Northern Sumatra (Indonesia).

The type of soil coverage in the affected fields influences the velocity of the water. A bare soil is likely to experienced high velocity and therefore higher erosion, whilst a dense coverage and small trees will absorb energy and reduce the velocity of the water, dense cereals such as rice are likely to be overturned ensuring some protection to the soil (at least for a while).

The duration of the sea water flood obviously depends on the capacity of drainage. Well drained areas are likely to be flooded for only a limited time (several hours), while water logged areas can be under sea water for weeks. Thereby resulting in contamination through the infiltration of saltwater.

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The immediate and critical priority in the case of sea water floods is to ensure quick drainage in order to get rid of the salt water on the fields and surrounding water bodies.

The second priority is to restore the physical integrity of the fields: the removal of debris and trash, the restoration of appropriate topographical conditions, for paddy fields it means ensuring land levelling and reconstruction of bunds where they have been washed way.

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YES salt sediment deposit is almost always associated with a tsunami. In fact the study of this deposit allows palaeontologist to trace similar historical events. For instance in Japan, a recent study on 13 tsunamis which occurred between 1026 and 1933, show that the thickness of deposit on the flooded soil may vary from 30 cm up to 1 meter depending on the run-up of the tsunami.

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There are two options to be taken into consideration: immediate removal of the deposit or integration into the existing soil profile with salt removal. Practically only the latter is acceptable.

Removal of the deposit requires technical and financial means which makes it out of reach for most. Removing a layer of 10 cm of deposit would be the equivalent to removing about 1000 m3 of deposit per hectare, i.e. about 1500 tons. This in turn would have to be removed to a nearby site. Even if it was possible all of this would have to be done with the help of costly scrapers, trucks, etc.

Instead, a more viable option is:

1) Integration in the soil profile of finer elements as the the only way to deal with the deposit, after ensuring removal of coarser elements (rocks, debris, trash,...)

2) Accompanioed with leaching of the salted profile, in this case through regular watering by rainfall and/or irrigation will slowly eliminate salt traces.

Of course this deposit is likely to modify the texture and structure of existing soils, and that might create some difficulties in the next cropping seasons. For instance when a sandy deposit from the sea shore is mixed with the heavy clay soil of paddy fields.

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