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?