Subsidence due to peat decomposition in the Netherlands kinematic observations from radar interferometry

Transcription

1 Subsidence due to peat decomposition in the Netherlands kinematic observations from radar interferometry Miguel Caro Cuenca, Ramon Hanssen, Freek van Leijen. Fringe 2007, Frascati, Italy

2 Netherlands, Area Overview Amsterdam The Hague Utrecht Rotterdam

3 Randstad : a near-circular series of towns Haarlem Amsterdam Leiden The Hague Utrecht Rotterdam The GREEN HEART

4 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).

5 The most characteristic cultural landscape of the Netherlands: peat grassland polders (west NL) fen-meadows consisting of wet pasture lands with drained peat soils alternated by natural and artificial lakes, ditches, reed swamps and quaking fens. current fen meadows have originated from the drainage of a large peat system dating back from 1800 B.C. To keep the land suitable for agricultural use, the peat area has been drained deeper in recent decades. This drainage has resulted in a subsidence of the soil and as a result the 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).

6 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

7 Modern fen-meadow: this is the current situation with counter-natural water management. Waterlevels are higher in summer (40 cm below ground level) than in winter (70 cm below ground level). Thearea can be used for (extensive) agricultural practices and is suitable for meadow birds. However, because of the relatively low water levels year round, the peat will oxidise and the soil will subside. Historical fen-meadow: a more historical situation with management aimed at a more natural water level fluctuation: the groundwater level varies between 40 cm below soil surface in summer and 20 cm below soil surface in winter. Agriculture is 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 less rapidly than in the modern fen-meadow scenario. Dynamic mire: water levels will fluctuate between 40 cm above soil surface in winter and more or less at the soil surface in summer. The area is not suitable for agriculture any more and as a result the meadow birds will largely disappear. The area will consist of reed beds, carrs, quaking fens and open water. Nature values belonging to these habitats will develop. The area will be suitable for storage of water in periods of heavy precipitation.

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14 Where s the peat? 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

15 Different core drills Surface Water table -40 cm Water table -80 cm Legend Sand Peat No subsidence No subsidence if water>-40 cm Subsidence Clay

16 Estimation of expected subsidence based on interpolated drillings Peat compaction (=subsidence) for a water table of -40 cm b.s. [MM/Y] 250 m grid cell No subsidence Reclaimed lakes: peat removed: No subsidence

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18 Peat thickness increases towards center of Green Heart: National land subsidence prognosis >8 cm/y 3-7 cm/y 0-1 cm/y Land subsidence scenario until 2100 >8 cm/y cm/y 6-8 cm/y 0 cm/y 3-4 cm/y Uplift 0.2 cm/y Uplift >0.2 cm/y 1-2 cm/y cm/y

19 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. Peat Ditch Peat Ground water table

20 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!

21 Main question: can we observe this proces from space? Hypothesis 1: focus on linear compaction rate (complication: are PS representative for shallow subsurface)? Specular Dihedral Stable layer Instable foundation Compaction Compaction A B C D

22 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

23 Radar (PSI) analysis

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25 Linear deformation, Results Center of Green Hart subsides at a rate of 1.7mm mm/yr with respect to The Hague Amsterdam The Hague Utrecht Rotterdam

26 [mm/yr] The center of the Groene Hart subsides 0.9 mm /yr respect to The Hague Amsterdam Amsterdam The Hague The Hague Utrecht Utrecht Rotterdam Rotterdam

27 PS results: Linear Deformation contribution, [mm/yr] Amsterdam Center of the Groene Hart subsides ~2 mm /yr relative to The Hague -3 mm/y The Hague Utrecht 0 mm/y Rotterdam

28 Confirmation through study of the seasonal effects. Ground water level variation in Green Heart ( or, is the green heart beating? or dead?) winter summer I. Hoving, EUROPEAT 2006.

29 Deformation model, linear + seasonal A is the amplitude, T the period 1 year fixed, t 0 time offset relative to master image, (August) Values of the amplitudes and time offsets dependent of reference point! t 0 >0 t 0 <0 winter spring summer autumn winter August

30 PS results: Seasonal contribution, results Amplitude, A Time offset, t 0 The histogram of the time offset reveals 3 different deformation regimes Sqeezing t 0 histogram may lead to optimal reference point!

31 Seasonal contribution, Amplitude [mm]. There is correlation of the amplitudes of the scatterers located in the peat areas mm 3.0

32 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. t gue Time [yr] respect master(23/08/1995) (Deliberate offset)

33 Conclusions Something seems to be happening Probably effects related to shallow subsurface; peat? Linear effect: peat compaction? Seasonal effect: groundwater level changes?