3 Post-tsunami assessment and restoration of soil and water

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Other soil issues

In the short term, soil salinity will be the main limiting factor for agriculture after a tsunami or seawater inundation event. However, salinity and sediments can also affect soil chemistry and nutrient availability. Particular attention needs to be paid to coastal peat soils, which can potentially become acid-sulfate soils if exposed by drainage.

Acid-sulfate soils

Tropical floodplains such as those in Aceh may overlie acid-sulfate soils. These soils are harmless while covered in water. However, when they are dry, they produce sulfuric acid, which acidifies soils and surrounding waterways (Sammut and Lines-Kelly 2004). The acidity produced by these soils is often so high that plants cannot grow. Indicators of acid-sulfate soils include red staining from iron particles on the soil surface and along stream banks.

On the west coast, farmers displaced by the tsunami moved inland onto peat soils. Farming in these areas had varying degrees of success, depending on the acidity levels that occurred when the soils were drained and dried out. Some soils became so acid that very little would grow. A basic pH kit can help farmers and advisers to assess acidity. Lime, mulches and organic matter can help reduce acidity levels, although lime can be expensive.

Sodicity

After the 2004 tsunami, soil scientists in Aceh expected sodium in the salt water to attach to clay particles, making the soil sodic and susceptible to erosion. However, soil sodicity proved infrequent, possibly because most soils were sandy, and the puddled and compacted clay rice soils prevented sea water infiltrating to any depth. However, sodicity may be a problem in clay soils that are not compacted. Saline sodic soils were detected on the Andaman and Nicobar islands after the 2004 tsunami (Nayaka et al. 2010).

Tsunami sediments

In Aceh, the tsunami deposited sediment over floodplains, filling in irrigation channels, agricultural drains and rice paddy fields. In many areas, channels and paddy fields had to be restored before any crops could be established. Some agricultural land took months or even years to return to production because farmers could not channel irrigation water in the dry season or drain floodwaters in the wet season. Removing sediment from channels and drains is a high priority after a tsunami. This activity must be coordinated with land surveys, in case changes in land elevations have altered drainage patterns.

At workshops 2 years after the tsunami, farmers said that they would have liked information immediately after the tsunami on sediment removal techniques and how to prioritise which fields to work on first. However, managing sediment is a complex issue that needs site-specific assessment. Removing sediment is a labour-intensive and expensive operation, and may not be necessary in all instances. Some sediment deposits can be left in place or incorporated into the topsoil. In some areas in Aceh, sediments gradually merged into the soil without any intervention, mainly due to self-seeding vegetation that grew through the sediment into the soil below. Where sediment—particularly deep sediment—has to be removed, assistance from government reconstruction and aid groups may be needed. The decision to remove sediment from fields depends on the factors outlined below.

Sediment type

Aceh tsunami sediments ranged from sand and clay to peaty organic matter, coral fragments and seabed mud (Figure 9). Some sediments were highly variable in particle size, comprising mixed gravel, sand and clay. Peaty sediments originating from coastal wetlands proved to be fertile and productive, and so could be left in situ, although monitoring was necessary to ensure that they continued to be fertile.

Sand deposited by the tsunami. Seafloor mud deposited by the tsunami. Organic peat deposited by the tsunami.

Photos: New South Wales Department of Primary Industries

Figure 9 Sediments deposited by the 2004 tsunami in Aceh: sand (top), seafloor mud (bottom left) and organic peat (bottom right)

Clay sediments provided some nutrition to farmer fields once they were incorporated into the soil. In some cases, sediments over acid peat soils buffered the soil acidity, and the peat provided minerals to assist crop growth, so these sediments were initially very fertile.

Tsunami deposits on the Nicobar and Andaman islands consisted of organic matter, fine sand, coral boulders, granules and broken shell fragments (Sarkar et al. 2012). In India, clay and sand sediments were most common (Chaudhary et al. 2006), and were shallow enough to be incorporated into the soil by ploughing.

Sediment depth

Farmers interviewed 2 years after the tsunami said that thin layers of sediment were not a problem for their farming because they could be incorporated easily into the soil below. Farmers did not attempt to grow rice in deeper sediments because their cultivation implements could not penetrate deeper than 20 cm. The Indonesian Bureau of Rehabilitation and Reconstruction found that sandy sediments deeper than 25 cm were too deep to grow rice, but that shallower sandy sediments away from the coast did not affect peanut crops. A sandy sediment deeper than 10 cm could be difficult to incorporate, especially where the underlying soil texture was coarse.

Another reason for shallow sediments being of less concern than deep sediments is that salt levels were observed to be low in shallower sediments, and plant roots could grow through the shallow layer to the soil below. Deep sand or clay sediments posed more of a problem because they were sometimes very saline, as well as being difficult for plant roots to move through. Rice paddies located a short distance inland from the west coast of Aceh were not subjected to the same rate of coarse sediment deposition as coastal paddies on the east coast.

Salinity levels in sediments

Following the 2004 tsunami, sediments varied greatly in their salinity levels, so site-specific assessments were required before any crops could be planted.

Underlying soil type

The soil under the sediment determines the rate of salt leaching from the sediment. Highly permeable sandy soils can leach sediment salts quickly. Clay soils, particularly compacted rice paddy soils, do not leach easily; there is a higher risk of salinity in these sediments, and they may need to be removed.

Farming options

When assessing whether to remove sediments, authorities in Aceh first checked whether villagers had other areas where they could grow crops. Sediment was removed only if the villagers had no other available land. The decision to remove sand or clay sediments was determined by the sediment depth and the underlying soil type.

Sediment removal programs

Two years after the tsunami, farmers identified physical activity as very important in regaining a sense of control. Small-scale sediment removal or incorporation programs provided a useful therapeutic activity, as well as a practical step in preparing fields for planting. Nakaya et al. (2010) proposed a salt removal plan for individual farmers in Thailand that incorporated sediment removal and other practical steps, where required. Mapping of sediment depths was also suggested as a possible activity for farmers. However, this requires resources and coordination that might be too time-consuming to achieve practical outcomes for farming groups.


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