Peat compaction and formation; key processes controlling alluvia Peat compaction and formation; key processes controlling alluvia Peat compaction and formation; key processes controlling alluvia
Peat compaction and formation; key processes controlling alluvial architecture l architecture l architecture l architecture
S. van Asselen* and E. Stouthamer
****C C C Co o o orrrrrrrreeeessssp p po p o o on n n nd d d diiiin n n ngggg aaaau u utttth u h ho h o o orrrr:::: s.vanasselen@geo.uu.nl. Department of Physical Geography,
Faculty of Geosciences, Utrecht University, P.O. Box 80.115, 3508 TC Utrecht, The Netherlands.
Tel.:+31(0)30 2532779. Website: www.geo.uu.nl/fg/palaeogeography.
In alluvial environments such as floodplains and deltas thick peat layers are often formed in the floodbasins. So far, the role of peat compaction and formation on the evolution of such areas has seldom been investigated and seems to be underestimated. On this poster, possible effects of peat
compaction and formation on alluvial architecture are presented, focussing on its effect on avulsion, which is one of the most important processes controlling alluvial architecture.
CHANNEL BELT GEOMETRY CHANNEL BELT GEOMETRY CHANNEL BELT GEOMETRY CHANNEL BELT GEOMETRY
CONCLUSIONS CONCLUSIONS CONCLUSIONS CONCLUSIONS BACKGROUND
BACKGROUND BACKGROUND BACKGROUND
CHANNEL BELT CONFIGURATION CHANNEL BELT CONFIGURATION CHANNEL BELT CONFIGURATION CHANNEL BELT CONFIGURATION
Cross sections based on logged boreholes from the Cumberland Marshes, Canada (Smith and Perez-Arlucea, 2004).
If the substrate consists of predominantly of peat (and clay), channels with low width/depth ratios low width/depth ratios low width/depth ratios low width/depth ratios develop.
B A
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Possible effects on avulsion avulsion avulsion avulsion:
GW
Flood basin: peat Flood basin: clay Channel belt Crevasse splay Lateral migration
Groundwater table
Degree of compaction (left: low, right: high) Bed aggradation Vertical aggradation
Oxidation
Peat is resistant to fluvial erosion resistant to fluvial erosion resistant to fluvial erosion, which affects channel dimensions: resistant to fluvial erosion
1 If an incising channel encounters an intercalated peat layer it will first erode vertically to the depth of the peat layer, which then, due to its high resistance, prevents further vertical incision resulting in channels with a high width/depth ratio high width/depth ratio high width/depth ratio high width/depth ratio (until the peat layer is eroded).
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Avulsion Crevassing
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GW
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Peat compaction under a channel, induced by the load of the
channel deposits, creates accommodation space. Through time, a decrease in the rate of accommodation space
decrease in the rate of accommodation space decrease in the rate of accommodation space
decrease in the rate of accommodation space created by peat compaction under a channel, results in an increase in lateral
migration and a higher sinuosity of the channel, which increases the risk of crevassing and finally avulsion.
If the rate at which accommodation space is created by peat
compaction under the channel is relatively low, a high sediment load, in combination with the high resistance of peat to fluvial
erosion inhibiting channel enlargement, stimulates bed bed bed bed aggradation
aggradation aggradation
aggradation. This reduces transport capacity, which increases the risk of crevassing and potentially avulsion.
Groundwater table lowering leads to oxidation of peat situated above the groundwater table. This leads to super super super super----elevation elevation elevation of elevation sandy channel belts, which leads to an increase in cross-valley gradients which potentially stimulates the occurrence of
crevassing and avulsion.
CURRENT RESEARCH CURRENT RESEARCH CURRENT RESEARCH CURRENT RESEARCH
4 At a larger scale, invasion of a river system onto a new part of a floodplain is stimulated by I) variations in compaction rate
across a floodplain and II) a sudden drop in gradient a river experiences when it enters a peatland.
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Field research Field research Field research
Field research in the Cumberland Marshes (Canada) and the Rhine Meuse delta (The Netherlands). The amount and rate of peat
compaction is determined in different alluvial settings (floodbasin, natural levee, crevasse splay,…), based on dry bulk density
measurements and reconstructions of initial levels of peat formation (=former groundwater levels). Individual factors influencing peat compaction are studied.
Develop a peat compaction model peat compaction model peat compaction model peat compaction model, which can be implemented in a 3D alluvial architecture model. Field data and modeling results are used to determine effects of peat compaction and formation on
alluvial architecture.
670
1550
0
-1
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depth (m below top levee)
129 130 128
127 125 126
124 123
Mossy River cross section
Mossy River
10 m
N S
14 15
16 17 18
19
20 21
22 23
14) 186 ± 39 (UtC-15429) 140
15) 710 ± 60 (UtC-15430) 670
16) 236 ± 44 (UtC-15431) 160
17) 575 ± 41 (UtC-15432) 550
18) 593 ± 41 (UtC-15433) 625
19) 1133 ± 46 (UtC-15434) 1020
20) 1730 ± 110 (UtC-15435) 1550
21) 1432 ± 43 (UtC-15436) 1325
22) 1838 ± 36 (UtC-15437) 1780
23) 2180 ± 60 (UtC-15438) 2210
Wetland (peat)
Floodbasin deposits (clay, silty clay)
Crevasse sheet deposits (clay loam, loam, silty clay loam, silt loam) Crevasse sheet deposits (sandy loam, loamy sand, sand)
Natural levee deposits (clay, silty clay, clay loam, loam, silty clay loam, silt loam) Natural levee deposits (sandy loam, loamy sand)
Channel deposits (clastic infill) Channel deposits (organic infill) Floating rootmat
Water
Legend
End of coring 105 Coring number
Substratum (fluvial deposits, mainly stiff clay)
Isochrone
Initial level of peat formation
