Effects of peat compaction on delta evolution: A review on processes,responses, measuring and modeling

Peat is most compressible of all natural soils. Compaction of peat layers potentially leads to substantial amounts of land subsidence. Peat is common in many distal parts of Holocene deltas, which are often densely populated. It is known that land subsidence due to peat compaction may have serious societal implications in such areas, as it may cause damage to construction works and lead to land inundation. Effects of peat compaction on the natural evolution of deltas are however poorly understood, whereas this might be an important control on delta evolution at both local and regional scales.The main objective of this paper is to review current knowledge concerning the peat compaction process and its effect on delta evolution in Holocene settings, and to identify gaps in this knowledge. An overview is given regarding: 1) the compaction process, 2) presumed and potential effects of peat compaction on delta evolution, 3) field methods to quantify peat compaction and 4) numerical models to calculate the amount and rate of peat compaction.Peat compaction and formation influence channel belt elevation, channel belt geometry and channel belt configuration. Last-mentioned aspect mostly concerns the influence of peat compaction on avulsion, which is one of the most important processes controlling delta evolution. Interactions between peat compaction, peat formation and avulsion have seldom been studied and remain unclear, partly because factors such as peat type, organic matter content, sediment sequence composition and groundwater table fluctuation are so far not taken into account. Peat compaction and formation potentially influence avulsion as 1) a decrease in accommodation space created by peat compaction underneath a channel causes superelevation and/or an increase in lateral migration, 2) the high cohesiveness of peat banks inhibits lateral migration, which increases bed aggradation, decreases sediment transport capacity and hence increases crevassing frequencies, which possibly evolve into an avulsion, although the low regional gradient in peatlands will hinder this, and 3) peat compaction and oxidation in flood basins following groundwater table lowering leads to relief amplification of channel belts. At delta scale, variations in compaction rates might stimulate the occurrence of nodal avulsions.To quantify effects of peat compaction on delta evolution, and to determine the relative importance of different factors involved, field research should be combined with numerical models describing peat compaction and formation. The model should be validated and calibrated with field data.