The effect of peat compaction on the evolution of alluvial plains

Alluvial plains like deltas often contain thick peat layers. Peat has several distinctive properties compared to inorganic sediments, which affect processes and the geometry of alluvial rivers. One of these properties is the high compressibility of peat, which potentially leads to high amounts of land subsidence. Peat compaction likely plays an important role in the evolution of alluvial plains, for example by providing extra accommodation space, which affects temporal and spatial fluvial sedimentation patterns. It has also been suggested that differential peat compaction affects floodplain gradients, and hence may affect the occurrence of avulsion. In such ways, peat compaction influences the alluvial architecture of both modern and ancient alluvial sequences, which is important knowledge for e.g. the exploration of natural resources such as oil and gas.

Still, there is a lack of field data to test such hypothesis. Therefore, field research, using new methods to quantify peat compaction, was carried out in the Cumberland Marshes (Canada) and the Rhine-Meuse delta (The Netherlands), where processes occurring on respectively decades and millennia timescales were studied. Most important factors controlling the amount of subsidence due to peat compaction are 1) the organic matter content of peat, 2) stress imposed on a peat layer by loading, 3) thickness of the compressible substrate, 4) relative vertical position in a peat layer, and 5) peat composition. Much compaction (=thickness reduction / original thickness) occurs shortly after loading; within decades up to 43% compaction might occur. In the Rhine-Meuse delta, where peat layers are up to ~8 m thick, peat compaction has led to subsidence of up to 3 m.

On short timescales (101-102 years), high amounts of peat compaction locally occur due to loading by crevasse splay or natural levee deposits. As the created additional accommodation space is rapidly filled by increased fluvial sedimentation or by peat accumulation, no gradient advantages are created which could drive avulsion. Peat compaction below channel deposits rather leads to vertical aggradation and fixation of river channels. In addition, lateral migration is inhibited by river banks consisting of mainly peat, which is highly resistant to fluvial erosion. On longer timescales, when the maximum compaction potential of peat layers in an area has largely been reached, delta lobes become prone to switch to another part of the alluvial plain.