Subsidence of Peat Soils – Flooding Risk in South-East Asia

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Courtesy of Wetlands International

There is an increase in awareness of the carbon emissions resulting from peatland drainage and peat fires. There is in general however a lack of recognition link between carbon loss and peatland subsidence, i.e. lowering of the soil surface.

In the tropics the process of subsidence after drainage is very fast due to high temperatures. Flood risks are also more severe because of high precipitation. Continuous peatland subsidence may eventually result in increasing flooding and possibly in production loss in agriculture and silviculture as has been shown in studies both in South-east Asia and beyond (Hooijer et al. 2012).

What is soil subsidence?
Subsidence is the lowering of the soil surface as the result of physical compression of the peat and loss of carbon due to oxidation and erosion. Peat soils comprise of 10% accumulated organic materials and 90% of water. When drained, the peat oxidizes and all peat above the drainage level will eventually be lost (see Figure 1).

Figure 1: Cross-section of a peat dome in natural situation and after drainage (Hooijer et al 2006/2010).

Flooding risks
Subsidence and the related flood risk is a wellknown and inevitable phenomenon in all places in the world where lowland peatlands have been converted to drainage-dependent land-uses. Examples include the UK (Somerset), USA (Sacramento Delta, Everglades), northern Germany, Denmark and the Netherlands where a large part of the highly populated west is situated below sealevel as a result of soil subsidence.

Research results peat soil subsidence SE Asia
In western Indonesia (namely Sumatra and Kalimantan) and Malaysia many of the peat swamp forests have been drained for palm oil or pulp wood plantations. In the first five years after drainage, peatland subsidence is typically 1 to 2 metres. In subsequent years, this stabilizes to a constant 3 to 5 cm per year, resulting in a subsidence of 2-3 metres in 25 years and 4-5 meters within 100 years.

Wetlands International works with Deltares in better quantifying the peatland subsidence problem in SE Asia. These insights in peatland drainage impacts and subsidence rates are largely taken from Hooijer et al. (2010) and Hooijer et al. (2012).

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