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Subsidence due to peat decomposition in
the Netherlandskinematic observations from radar interferometry
Miguel Caro Cuenca, Ramon Hanssen, Freek van Leijen.
Fringe 2007, Frascati, Italy
Netherlands, Area Overview
The Hague
Amsterdam
Rotterdam
Utrecht
‘Randstad’: a near-circular series of towns
Rotterdam
The Hague
Leiden
Haarlem Amsterdam
Utrecht
The ‘GREEN HEART’
• Typical Dutch landscape: peat grassland polders (west NL)
• fen-meadows, drained peat soils, natural and artificial lakes,
ditches, reed swamps and quaking fens.
• To keep land suitable for agricultural use, peat area has to be
drained
• Drainage resulted in subsidence !polders with fen-meadows are
now 1-2 m below sea level.
• In between the fen-meadows, deep polders with a clay soil are
found. These deep polders used to be large lakes, which have
been reclaimed in the 17th century for agricultural use. Presently,
these polders are 2-6 m below sea level (Best and Bakker, 1993).
• The most characteristic cultural landscape of the Netherlands: peatgrassland polders (west NL)
• fen-meadows consisting of wet pasture lands with drained peat soilsalternated by natural and artificial lakes, ditches, reed swamps andquaking fens.
• current fen meadows have originated from the drainage of a large peatsystem dating back from 1800 B.C. To keep the land suitable foragricultural use, the peat area has been drained deeper in recentdecades. This drainage has resulted in a subsidence of the soil and as aresult the polders with fen-meadows are now 1-2 m below sea level. Inbetween the fen-meadows, deep polders with a clay soil are found.These deep polders used to be large lakes, which have been reclaimedin the 17th century for agricultural use. Presently, these polders are 2-6m below sea level (Best and Bakker, 1993).
As in other parts of the country, water tables in Noord-
Hollands Midden are controlled to facilitate agriculture,
building of housing, infrastructure and other land-uses
and to avoid damage and inconveniences caused by
water. However, problems with water surpluses as
well as water deficiencies have had large economical
consequences in the area recently. Based on
predictions from climate change scenarios, the
problems in the area are expected to increase in the
future
• Modern fen-meadow: this is the current situation with ‘‘counter-natural’’ watermanagement. Waterlevels are higher in summer (40 cm below ground level) thanin winter (70 cm below ground level). Thearea can be used for (extensive)agricultural practices and is suitable for meadow birds. However, because of therelatively low water levels year round, the peat will oxidise and the soil willsubside.
• Historical fen-meadow: a more historical situation with management aimed at amore natural water level fluctuation: the groundwater level varies between 40 cmbelow soil surface in summer and 20 cm below soil surface in winter. Agricultureis still possible, however, less intensive than in the modern peat pasture scenario.The area is still suitable for meadow birds. Soil subsidence will still occur, but lessrapidly than in the modern fen-meadow scenario.
• Dynamic mire: water levels will fluctuate between 40 cm above soil surface inwinter and more or less at the soil surface in summer. The area is not suitable foragriculture any more and as a result the meadow birds will largely disappear. Thearea will consist of reed beds, carrs, quaking fens and open water. Nature valuesbelonging to these habitats will develop. The area will be suitable for storage ofwater in periods of heavy precipitation.
Where’s the peat?
• 300.000 core drillings of Holocene layer over
Netherlands (Van der Linden et al. 2000)
• Large part of these drillings in the fen-meadow areas
• 4 samples
Legend
Sand
Peat
Clay
Different core drills
Surface
Water table
-40 cm
Water table
-80 cm
No subsidence No subsidence
if water>-40 cm
Subsidence
Peat compaction
(=subsidence) for a water
table of -40 cm b.s.
Estimation of expected subsidence based on interpolated
drillings
No subsidence
250 m
grid cell
Reclaimed
lakes: peat
removed:
No subsidence
[MM/Y]
National land subsidence prognosis
>8 cm/y
3-7 cm/y
0-1 cm/y
0.2-0.5 cm/y
1-2 cm/y
3-4 cm/y
6-8 cm/y
>8 cm/y 0-0.2 cm/y
0 cm/y
Uplift 0.2 cm/y
Uplift >0.2 cm/y
Land subsidence scenario until 2100
• Peat thickness increases towards center of Green Heart:
max. 7 meters.
• The terrain subsides due to peat oxidation, ~1cm/yr.
• The ground water level is high to control the subsidence
due to peat oxidation.
PeatPeat
Ground water table
Ditch
Peat oxidation process
- Peat is composed of organic material.
- Peat oxides when it is in contact with the air, i.e.
reduces in volume producing the consequent
subsidence
- Then ground water level get closer to the surface.
- The water is pumped out to have a dry ground.
The ‘Green Heart’
• Most of the soil contains peat.
• Peat thickness increases towards the center of the area: max. 7 meters.
• Terrain subsides due to peat oxidation, expected ~1cm/yr.
• Only vadose zone (above groundwater table)
• ! ground water table kept high to control subsidence
(in the year 2700: all peat is gone !! ! Land is 7 m lower !
Main question: can we observe this
proces from space?
• Hypothesis 1: focus on linear compaction rate
(complication: are PS representative for shallow
subsurface)?
Compaction
Compaction
Instable
foundation
Stable layer
Specular
Dihedral
A B C D
Main question: can we observe this
proces from space?
• Hypothesis 1: focus on linear compaction rate
(complication: are PS representative for shallow
subsurface)?
• Hypothesis 2: if proces is peat-related, then there
should be strong seasonal signal due to ground water
changes
Radar (PSI) analysis
Linear deformation , Results
Center of Green Hart subsides at
a rate of 1.7mm mm/yr with
respect to The Hague
Amsterdam
Utrecht
The Hague
Rotterdam
PS results: Linear Deformation contribution,
[mm/yr]
After adding the seasonal effects to the
model the linear deformation is less
visible.
The center of the Groene Hart subsides
0.9 mm /yr respect to The Hague
Amsterdam
Utrecht
The Hague
Rotterdam
Amsterdam
Utrecht
The Hague
Rotterdam
PS results: Linear Deformation contribution,
[mm/yr]Amsterdam
Utrecht
The Hague
Rotterdam
Center of the Groene Hart subsides ~2
mm /yr relative to The Hague
0 mm/y
-3 mm/y
Confirmation through study of the seasonal
effects.
summer
winter
I. Hoving, EUROPEAT 2006.
Ground water level variation in Green Heart
(…or, is the green heart beating? …or dead?)
Deformation model, linear + seasonal
• A is the amplitude,
• T the period 1 year fixed,
• t0 time offset relative to master image,
(August)
Values of the amplitudes and time offsets
dependent of reference point!
t0<0
… winter spring summer autumn winter…
t0>0
August
PS results: Seasonal contribution, results
Amplitude, A Time offset, t0
The histogram of the time offset reveals 3
different deformation regimes
Sqeezing t0 histogram may lead to optimal
reference point!
Seasonal contribution, Amplitude [mm].
There is correlation of the
amplitudes of the scatterers
located in the peat areas.
1.5
0.0
3.0
mm
Seasonal contribution, time series
An area of radius 500 m was chosen
to study the time-series.
The noise was reduced by averaging
the results of the PS located
inside the area.
The reference, also an area and not
a single point, was at The
Hague.
Ref. at
The Hague
(Deliberate offset)
Time [yr] respect master(23/08/1995)
Conclusions
• Something seems to be happening…
• Probably effects related to shallow subsurface; peat?
• Linear effect: peat compaction?
• Seasonal effect: groundwater level changes?