Susceptibility of Holocene deltaic sequences for subsidence due to peat compaction

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Sanneke Van Asselen, Esther Stouthamer

Chair(s): Esther Stouthamer

Tuesday 30 june 2015

14:05 - 14:25h at Asia (level 0)

Themes: (T) Special session, (ST) Deltas from multiple pressures to integrated solutions

Parallel session: 6C. Special session: Deltas - from multiple pressures to integrated solutions

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Land subsidence is a major threat to hundreds of millions of people living in deltas worldwide. Natural drivers of subsidence include tectonics, isostasy and sediment compaction. Human-induced subsidence is mainly due to groundwater table lowering for land reclamation, loading by for example buildings, and extraction of oil, gas and groundwater. Subsidence due to compaction is highly variable in time and space, mainly depending on the composition of the subsurface. Holocene delta sequences are usually very heterogeneous, consisting of mixtures of sand, silt, clay and peat. Because peat is most compressible of all natural soils, especially sequences comprising thick peat layers are very susceptible for subsidence due to compaction. Peat compaction is induced by (natural or anthropogenic) loading and/or groundwater table lowering. We quantified natural peat compaction rates based on field research at different key locations in the anthropogenic Rhine-Meuse delta (NL) and a natural inland delta in the Cumberland Marshes (CA), and used these data to calibrate a new peat compaction model. Results demonstrate local subsidence due to peat compaction of up to 3 m at Holocene timescale in the Rhine-Meuse delta. Predicted subsidence rates due to peat compaction are up to 15 mm/yr at decadal timescales, following loading. The main factors influencing the susceptibility for subsidence due to peat compaction are (1) the thickness of peat layers in the subsurface, (2) the organic matter content of these layers, and (3) the effective stress, which is a function of the weight of the overburden and pore water pressure. In addition, the Holocene sedimentation and groundwater table history is extremely important, because this determines if peat layers have experienced past compaction, which influences the present susceptibility to additional subsidence due to compaction. All these variables, and in particular their spatial variability, should be taken into account in developing strategies to prevent or mitigate subsidence due to peat compaction in deltas.