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Abstract

The western part of the Netherlands has a typical Dutch landscape with fen-meadows that consist of wet<br>pasture lands with drained peat soils alternated by natural and artificial lakes, ditches, reed swamps and quaking<br>fens (see fig. 1). This area has been and is still being continuously drained, so as to keep the land dry and suitable for agriculture, pasture and residence. Water levels are artificially controlled in the region by local water management authorities.<br><br>This drainage has resulted in subsidence of a couple of meters over the last centuries. As a result, the polders with fen-meadows now lie 1–2 m below sea level. In addition to that, we also find deep polders which used to be large lakes and were reclaimed in the 17th century for agricultural use. Presently, these polders are 2-6 m below sea level. <br>The continuing subsidence of the surface in the polders and the rise in sea level caused that about 25% of the Netherlands is now being situated below mean sea level (up to 6.7 m). Without dikes and dunes 65% of the land would be flooded daily. This situation makes the Netherlands vulnerable to storm surges and river floods.<br>The ’Green Heart’ (Groene Hart) is the rural center of the Dutch Randstad, surrounded by the biggest cities in the Netherlands: Amsterdam, The Hague, Rotterdam and Utrecht. The soil of the Green Heart contains mainly sand, peat and clay. The ground water level is controlled in order to avoid fast subsidence due to peat oxidation and at the same time to maintain a dry surface. Peat is composed of organic material which oxides when it is in contact with air, reducing in volume and consequently resulting in subsidence, and therefore, bringing the surface gets closer to the ground water. Thus, in order to have a dry ground suitable for agriculture, construction and recreation the land is periodically drained. <br><br>Observing precise subsidence rates of peat and marsh lands using geodetic techniques is notoriously difficult, due to the difficulty of installing fixed benchmarks in this type of soil. Moreover, because of the soft soils, modern buildings have pile foundations, with pilings up to 25 m long, reaching to stable pleistocene sand layers. Consequently, while subsidence due to shallow surface compaction continues, most new buildings remain relatively stable. Figure 2 shows the subsidence rates estimated by 2 for a ground water level of -40 cm below surface. The results were obtained using boreholes. The area with the maximum deformation rates corresponds to the Krimpenerwaard, where a deformation of -5 to -11 mm/yr is expected. <br><br>In this contribution we investigate the use persistent scatterer interferometry, (PSI) 1 to study shallow deformation in wetlands in The Netherlands. PSI utilizes a time series of space borne radar scenes to select scattering objects whose reflecting properties remain fairly constant over time and are therefore minimally affected by noise. The information about deformation is extracted from the interferograms, which contain the phase differences between two radar images.<br><br>The PSI technique as developed at the TU Delft 3 is based on creating a first-order network of measurements, using the most coherent objects to estimate and remove atmospheric artifacts. Then a denser second order network is built from which the full deformation velocity field is derived.<br>One of the major limitations of PSI techniques is that we cannot be certain about the object we observed. In any case, the position of the object is known with an error of about 10 m. However, PSI overcomes the limitations of traditional geodetic methods. It provides a very dense distribution of measurements (~ 100/km2 in urban areas, sensor dependent) and high observation frequency (once a month or higher, depending on satellite requirements).

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/content/papers/10.3997/2214-4609-pdb.150.A01
2010-01-20
2021-01-25
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http://instance.metastore.ingenta.com/content/papers/10.3997/2214-4609-pdb.150.A01
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