Dutch coordinate system
Onlapping Holocene deposits Ice-pushed ridges
Pleistocene deposits Beach barriers Coastal dunes
Water
Present-day river State boundary
Eastern boundary of marine deposits
Study area
Subsidence due to peat compaction and oxidation in built-up coastal areas
S. van Asselen
1, G. Erkens
2,1, E. Stouthamer
1, H. Woolderink
1, R. Geeraert
3, and M.M. Hefting
3Results (selected)
Study area Approach
We studied subsidence due to peat compaction and oxidation in three built-up areas in the Rhine-Meuse delta (NL). Here, we present peat compaction results*.
1. We made cross sections based on borehole data to reveal the litho- logical composition of the Holcoene sequence below built-up areas.
2. At selected sites, representing different loading histories, we extrac- ted cores and determined variations with depth of the (1) amount of peat compaction, calculated based on organic-matter content and
dry bulk density measurements of compacted and uncompacted peat (Van Asselen (2011); this study*), and (2) effective stress.
Problem
An increasing number of people live on soft-soil coastal
sequences that often contain substantial amounts of peat.
Loading and draining these soils for cultivation causes land subsidence due to peat compaction and oxidation.
This leads to increased flood risk and damage to buildings, infrastructure and agriculture. Especially built-up areas, having densely-spaced assets, are heavily impacted by subsidence, in terms of damage-related costs and impact on livelihood. However, these areas have not yet received the full attention of land subsidence research.
Contact
UU | Faculty of Geosciences | Department of Physical Geography Heidelberglaan 2 | 3584 CS Utrecht | The Netherlands
t.+31 (0)30 253 2754 | s.vanasselen@uu.nl
1
UU-Faculty of Geosciences Department of Physical Geography
2
Deltares Research Institute
3
UU-Faculty of Science Institute of Environmental Biology
Future Deltas
www.uu.nl/futuredeltas
References
Van Asselen, S. (2011). The contribution of peat compaction to total basin subsidence: implications for the provision of accommodation space in organic-rich deltas. Basin Research 23, 239-255.
Conclusions
Quantifying subsidence due to peat compaction
Due to the heterogeinity of Holocene coastal sequences the amount and rate of subsidence due to peat compaction varies in time and space in coastal zones. The amount of compaction is positively related to the (1) overburden weight, (2) time since loading, and (3) organic-matter content of peat. Groundwater table lowering also induces compaction. Peat below urban ground was compacted up to
~50%. We measured subsidence due to peat compaction in built-up area of up to ~2 meters, and subsidence rates, averaged over an 11-year time span, of up to ~14 cm yr-1.
Subsidence potential
In the built-up areas we studied, peat compaction grades varied from ~20-30% in moderately raised urban areas (e.g. parks,
sidewalks) to ~30-50% in heavily raised urban areas (e.g. for buildings). The potential for future subsidence due to peat compaction at these sites is considerable. This is evident from measured compaction of up to ~75% at a nearby site where a similar peat layer as occurs in the study area has been heavily loaded since ca 700 years by a dike embankment (6-7 m thick).
Subsurface-based spatial planning
To sustain projected population growth and urbanization in coastal zones we call for (1) geology-based spatial planning, (2) collection of targeted subsurface information before new developments start (e.g. current compaction grade and organic-matter content), and (3) subsidence-resilient building (e.g. use of lighter construction materials and adapting groundwater tables).
*Full details of this study, including subsidence due to oxidation estimates, are submitted to Earth's Future.
0 50 m
105 104 107 101 106 102 103 108
m O.D.
-1
-2
-3
-4
-5
-6
-7
-8
Peat - wood Peat - sedge
Peat - sedge and reed Peat - reed
Detritus / gyttja Clayey peat / peaty clay Clay (silty)
Clay loam (sandy to silty) Loam (sandy)
Sand
Sandy Clayey Organic Anthropogenic Pleistocene deposits Holocene deposits
Sand peat / peaty sand
W O
Boring location and number / end of boring
Road
KS1_560 KS1_510
KS2_110
KS2_170
KS2_360
KS2_420
Groundwater level (Feb-Mar 2016) Ploughed soil
Core numbers precede:
2015.08
(year.group number)
Core location and number / end of boring
Groundwater level (summer) Respirometer measurement
code
Cross section of the Holocene sequence of the Kanis site. Broadly, an anthropogenic layer of varying thickness overlies an up to 6-m-thick peat layer that is partly dissected by natural clayey overbank deposits.
Variantions in the amount of peat compaction, LOI and effective stress with depth for 2 cores (Kanis site). The amount of compaction is highest below the thick clayey layer at site Kanis 104. Relatively high compaction grades also occur at the top of the sequences, caused by oxidation and compaction in the zone above the groundwater table.
For compacted peat, the amount of peat compaction is positively
related to LOI (Loss On Ignition; indicator for organic-matter content).
The amount of peat compaction is positively related to the effective stress (= total stress - pore water pressure).
0 10 20 30 40 50 60 70 80 90 100
0 10 20 30 40 50 60 70
Average compaction (%)
Effective stress (kPa)
No loading
Agricultural loading (e.g. by livestock, tillage) Natural heavy loading (e.g. natural levee deposits) Moderately raised urban ground (e.g. parks, sidewalks) Heavily raised urban ground (e.g. buildings)
Dike embankment loading
Kamerik 203 Kanis 104 Kockengen 310
Kanis 104 Effective stress (kPa)
Depth (cm below surface) Depth (cm below surface)
Compaction/LOI (%) Compaction/LOI (%)
Effective stress (kPa)Kanis 104
