Geotechniek LandSubsidence Special 2020

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Dr. K. Koster TNO - Geological Survey of the Netherlands, dpt. Geomodelling, Geomodeller

Drs. D. Maljers TNO - Geological Survey of the Netherlands, dpt. Geomodelling, Geomodeller/deputy research manager

Dr. J. Stafleu TNO - Geological Survey of the Netherlands, dpt. Geomodelling, Geomodeller

Dr. M.J. van der Meulen TNO - Geological Survey of the Netherlands, dpt. Geomodelling, Research manager

GEOTOP: A STANDARD IN 3D LAND SUBSIDENCE STUDIES IN THE NETHERLANDS Introduction The Geological Survey of the Netherlands (TNOGDN) has developed a suite of 3D geological subsurface models. In this contribution we address the protocol used for developing one of these models: GeoTOP. We show its applied value in recent subsidence studies, focusing on the coastal plain of the Netherlands, and provide a perspective on its future use by governmental decision makers and other users under the newly implemented Key Register of the Subsurface.

regionally delimited model boundaries. GeoTOP has a grid resolution of 100 x 100 x 0.5 m, and its millions of cells are attributed with lithostratigraphy, lithology, and associated probabilities (Stafleu et al., 2011; 2020). It is based on approximately 455,000 coded borehole descriptions from the national subsurface database DINO, operated by TNO, complemented with 125,000 borehole logs from Utrecht University. At present, GeoTOP consists of eight model areas, covering most the subsiding lowlands.

They result from artificially low phreatic groundwater levels and from overburden pressure generated by man-made overburdens of sand, put in place to ensure that planned buildings or infrastructure will have long-term stability. The low groundwater levels results in aeration of the peat, which facilitates oxidation of organic matter, leading to emission of CO2. In the past, subsidence was locally exacerbated by peat mining for fuel. Altogether, approximately half of the coastal plain has subsided to below mean-sea level.


Drivers of subsidence

Subsidence studies

GeoTOP is a high-resolution voxel model that covers the subsurface down to 50 m below mean sea level (Figure 1). Each voxel is a volumetric cell in a regular, rectangular grid bounded laterally by

Prime subsidence-governing processes in the shallow subsurface are oxidation of peat, as well as shrinkage and compression of Holocene peat and clay. These processes are human-induced.

GeoTOP is the standard tool in land-subsidence studies at TNO-GDN. It covers most of the subsiding coastal plain, has sufficient vertical resolution, and schematizes the distribution of

Figure 1 – The eight model areas of GeoTOP, and zoomed-in view of the peat-rich area of Amsterdam. The voxels are attributed with the most likely lithology. 5 GEOTECHNIEK L AND SUBSIDENCE SPECIAL

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SUMMARY The Geological Survey of the Netherlands maps the Dutch subsurface systematically in 3D. Here we outline the development of GeoTOP, its recent application in subsidence studies, and its use under a newly implemented key register of the subsurface. Recent studies based on GeoTOP confirm that agricultural areas are more prone to subsidence by peat compression and

the peat and clay layers prone to subsidence in a consistent and reproducible way (Van der Meulen et al., 2013). Furthermore, its data format and software infrastructure are well suited for easy combination with datasets on other parameters relevant for subsidence studies, such as density of organic matter, phreatic groundwater level, and CO2 emission. A recently conducted subsidence study addressed forward modelling of peat oxidation and compression under scenarios of progressively lowered groundwater levels in the coastal plain (Koster et al., 2018a). It revealed that urbanized areas are expected to experience less subsidence in the next 15 to 30 years (< 0.4 m) than surrounding

oxidation than major urbanized areas. Subsidence is greatest in the central delta, a rural region that emits a lot of oxidation-related CO2, contributing to the greenhouse effect. Implementation of GeoTOP in the key register enables governmental stakeholders to make the right subsidence-related decisions.

agricultural areas (0.3 to 0.8 m) (Figure 2). This difference is explained by the presence of manmade overburden in urbanized areas, commonly many meters thick, which has pushed the underlying peat into a position well below the water table and has reduced its void ratio. Here, a lowered phreatic groundwater level will not expose the peat to the atmosphere, preventing it from being oxidized. Furthermore, the low void ratio reduces further compressibility of the peat in response to changing hydrostatic pressure. The predicted difference in subsidence behavior does not apply to areas where groundwater is extracted from deep aquifers. In many coastal plains worldwide, cities are subsiding faster than agricultural land.

We designed a workflow to attribute GeoTOP voxels that classify as Holocene peat with values of organic matter density and carbon content (Koster et al., 2018b; Koster et al., 2020). For this purpose, we created a training dataset of hundreds of measurements on the amount of organic matter, sediment, and void ratio of peat layers sampled in different areas of the coastal plain at different depths. Thus, we were able to identify areas most prone to peat oxidation and associated CO2 emission in response to predicted lowering of the phreatic groundwater table. The modeling results show that approximately 10% of about 15 km3 peat situated in the subsurface of the entire coastal plain is organic matter, which would yield some 2 billion tons of CO2 when oxidized. For reference, the current annual CO2 emission in the Netherlands is slightly less, at 1.9 billion tons (CBS, 2019). Clearly, the remaining peat is a major potential source of CO2, easily released when unwise land-management decisions are taken. Most of this vulnerable organic matter is found in the central delta (Figure 3). Other hotspots are major cities in the western coastal plain, including Rotterdam and Amsterdam, where peat is heavily compressed and preserved below man-made overburdens of sand. Following up on deploying GeoTOP for peat compression and oxidation studies, we now aim at parameterizing GeoTOP voxels for predicting clay compaction and shrinkage. In support, we combine GeoTOP with a forward-model workflow that simulates scenarios and stems from compaction studies of deep gas reservoirs (Candela et al., 2020). We also deploy a database containing several thousands of geotechnical measurements collected by TNO-GDN during a nation-wide mapping campaign. Subsidence observations from land levelling, GPS, and satellites (InSAR) are indispen-

Figure 2 – Peat thickness, subsurface lithology, and subsidence through compression and oxidation of peat in the Rotterdam area, for a 0.5 m lowering of the phreatic groundwater level during 30 years (Koster et al., 2018a). For the legend of the subsurface lithology, see figure 1, and for location of the Rotterdam area see figure 3. The hatched areas in the maps denote urbanized zones.



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sable in cross-checking the forward model during data assimilation. Combining GeoTOP and this forward-model workflow is a critical step forward in identifying and quantifying the contribution of each of the processes causing subsidence, and eventually in supporting informed decision making on groundwater management. Knowledge-based decisions balancing conflicting interests of stakeholders, minimizing and mitigating subsidence and associated CO2 emissions, are a key component of sustainable governance.

Key Register of the Subsurface As of 1 January 2020, GeoTOP is part of the Key Register of the Subsurface, and therefore embedded in law. The aim of the key register is to improve the accessibility of subsurface information for a broad spectrum of applications, including spatial planning, construction, and assessment of risks such as land subsidence. Under Dutch law, projects or studies paid for by governments at all levels are required to consult existing and contribute newly collected data and data products to the key register. Now that it is mandatory to consult GeoTOP, its use will intensify and feedback of its accuracy and functionality is expected to increase substantially. GeoTOP’s spatial resolution renders it useful for neighborhood as well as national levels. The model is expected to become the standard for land-subsidence studies financed with money from municipalities, waterboards, provinces and other governmental entities. These stakeholders and their contractors can now deploy GeoTOP to study the effects of changes in phreatic water levels or land use change. They will be able to assess and compare local to national subsidence-related risks associated with different scenarios of watertable lowering and other lowland-management measures.


Figure 3 – Spatial distribution of Holocene peat in the area covered by

We thank Sytze van Heteren (TNO-GDN) for critically reading an earlier version of the manuscript.

GeoTOP (except for the southernmost GeoTOP area), with associated volumes of organic matter and maximum amounts of CO2 emitted in case of oxidation (after Koster et al., 2018b).

References - Candela, T., Koster, K., Stafleu, J., Visser, W., and Fokker, P. (2020), Towards regionally forecasting shallow subsidence in the Netherlands, Proc. IAHS, 97, TISOLS special issues, in press - CBS (2019), Online statistical overview of the Netherlands. - Koster, K., Stafleu, J. and Stouthamer, E. (2018a), Differential subsidence in the urbanized coastaldeltaic plain of the Netherlands. Netherlands Journal of Geosciences, 97, 215-227 - Koster, K., Stafleu, J., Cohen, K.M., Stouthamer, E., Busschers, F.S., and Middelkoop, H. (2018b), Three-dimensional distribution of organic matter in coastal-deltaic peat: Implications for subsidence and carbon dioxide emissions by human-induced

peat oxidation. Anthropocene, 22, 1-9 - Koster, K., Frumau, A., Stafleu., Dijkstra, J., Hensen, A., Velzeboer, I., Esteves Martins, J., and Zaadnoordijk, W.-J. (2020), GreenhousePeat: a model linking CO2 emissions from subsiding peatlands to changing groundwater levels, Proc. IAHS, 97, TISOLS special issues, in press - Stafleu, J., Maljers, D., Gunnink, J.L., Menkovic, A. and Busschers, F.S. (2011), 3D modeling of the shallow subsurface of Zeeland, the Netherlands. Netherlands Journal of Geosciences, 90, 293-310 - Stafleu, J., Maljers, D., Busschers, F.S., Schokker, J., Gunnink, J.L. and Dambrink, R.M. (2020), Chapter 11: Models created as 3-D cellular voxel arrays. 7


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In: Turner, A.K., Kessler, H. and Van der Meulen, M.J. (Eds.) Applied Multidimensional Geological Modeling. Wiley-Blackwell, Hoboken NJ - Van der Meulen, M.J., Doornenbal, J.C., Gunnink, J.L., Stafleu, J., Schokker, J., Vernes, R.W., Van Geer, F.C., Van Gessel, S.F., Van Heteren, S., Van Leeuwen, R.J.W., Bakker, M.A.J., Bogaard, P.J.F., Busschers, F.S., Griffioen, J., Gruijters, S.H.L.L., Kiden, P., Schroot, B.M., Simmelink, H.J., Van Berkel, W.O., Van der Krogt, R.A.A., Westerhoff, W.E., and Van Daalen, T.M. (2013), 3D Geology in a 2D country: perspectives for geological surveying in the Netherlands. Netherlands Journal of Geosciences, 92, 217-241.

dr. Sanneke van Asselen Researcher at Deltares

Gilles Erkens Researcher at Deltares and the Department of Physical Geography, Utrecht University

MONITORING SHALLOW SUBSIDENCE IN CULTIVATED PEATLANDS Introduction This paper relates to, and partly overlaps with, the paper of Van Asselen et al. (2020). A large part of the coastal plain of the Netherlands contains as much as several meters of peat in the subsurface (Erkens et al., 2016). Like many other coastal plains worldwide, the Dutch coastal plain is subject to shallow land subsidence from both anthropogenic and natural causes (Erkens et al., 2016; Van Asselen et al., 2018; and references therein). Subsidence increases flood risk, causes damage to buildings and infrastructure and an overall increase in soil wetness as the surface approaches the phreatic groundwater level. In agricultural areas this translates into a lower soilbearing capacity. In the Dutch coastal plain, subsidence has especially been caused by peat oxidation, peat compaction and peat mining in the Holocene sequence, starting about 1000 years ago (Erkens et al., 2016). Large-scale peat mining continued until the late 19th century, after which subsidence has been mainly caused by peat compaction and oxidation. Peat compaction is the

mechanical process of densification of the soil, caused by loading and/or a decrease of the pore water pressure. Peat oxidation refers to the biogeochemical degradation of organic material by micro-organisms and occurs especially when peat is exposed to oxygen, for example following groundwater level lowering. Peat oxidation also causes emission of greenhouse gasses. The amount and rate of shallow subsidence in organic-rich coastal sequences is determined by (1) geotechnical and biogeochemical properties of organic and mineral facies, (2) structural loading and (3) groundwater level fluctuations. These three aspects generally vary considerably in both time and space. Consequently, the amount and rate of subsidence is spatially and temporally variable: it varies between polders and even within parcels. During recent years, there is a growing incentive to reduce both subsidence and greenhouse gas emissions in cultivated peatlands. To develop effective measures to reduce subsidence, and to be able to monitor results of implemented measures, a subsidence monitoring system is needed that

Figure 1 – Peat thickness map for the study area in northeastern Netherlands (source: Waterboard Drents Overijsselse Delta). Locations of the measuring fields (meadows) at the eight livestock farms, the extensometers and the IGRS station are indicated. 8 GEOTECHNIEK L AND SUBSIDENCE SPECIAL

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captures the temporal and spatial variability of land subsidence, and preferably also discriminates between the contribution of different subsidence processes. Examples of mitigation measures are permanently raising the groundwater level or implementing submerged drainage (Pleijter and van den Akker, 2007). The desired monitoring system should be able to measure subsidence at mm-scale accuracy, since long-term net average land subsidence rates are typically on the order of mm to cm yr-1. Also, the system should not severely impact farming activities. To design and optimize such a system, four different methods are applied to monitor land subsidence of meadows at eight livestock farms in a cultivated peatland area in the north-eastern part of the Netherlands (figure 1). The subsurface of the study area generally consists of a Holocene peat layer (Nieuwkoop Formation; De Mulder et al., 2003) as much as about 3.5 m thick, on top of a thick (tens of meters) Pleistocene sand deposit (mainly Boxtel and Kreftenheye Formations; De Mulder et al., 2003). In the western part of the study area, the peat layer is covered by a few dm-thick clayey top layer (figure 1; Naaldwijk Formation; De Mulder et al., 2003). The methods used include conventional (spirit) levelling, extensometery, LiDAR (Light Detection And Ranging) and InSAR (Interferometric Synthetic Aperture Radar). Levelling is a well-tested and often-used technique for measuring surface elevation and has also been applied in a few cases in peat areas in the Netherlands (Pleijter & van den Akker, 2007). Extensometery is applied worldwide for measuring vertical movement of (sub)surface levels but have rarely been applied in peat areas. Both of these field-based techniques result in accurate (mm-scale) point measurements. The use of LiDAR and InSAR for measuring land subsidence in peat areas is promising but still experimental and the accuracy of the measurements needs testing. These two remote sensing techniques may ultimately result in timeseries of maps with spatial coverage, which is needed to monitor the temporal and spatial variability of land subsidence and the effects of applied mitigation measures. In this paper, the four methods are described, and preliminary levelling, extensometery and LiDAR

SUMMARY To develop a land subsidence monitoring system for cultivated peatlands four measuring techniques are applied in the north-eastern part of the Netherlands, including spirit levelling, extensometery, LiDAR and InSAR. The desired monitoring system should be able to capture long-term spatial and temporal subsidence trends at mm-scale accuracy. Preliminary levelling and extensome-

tery results demonstrate seasonal and shorter-term dynamics with a total vertical movement of up to 35-40 mm in one-year time. A longer (multiple years) monitoring and experimenting period is needed to be able to determine long-term net subsidence (or uplift), and to optimize the subsidence monitoring system for peatlands.

Figure 2 – Schematic representation of the extensometer set-up.

Figure 3 – Integrated Geodetic Reference Station nearby Rouveen (Photo by H. van der Marel).

results and conclusions are presented. InSAR results are not available yet.


The levelling methodology is based on Pleijter and van den Akker (2007). Surface elevations of parts of meadows are measured relative to a reference point, consisting of an iron rod that is founded in the semi-stable Pleistocene sand underlying the Holocene peat layer. Elevations are measured four times a year, using a Leica LS15 levelling instrument and rod, along four section lines of 50 m long and lying 8 meters apart, at a 2 m interval as determined with a measuring tape, resulting in 104 point measurements. The start and end of the section lines are fixed coordinates determined each measuring campaign using a Topcon GRS-1 RTK-GNSS. At local scale, the surface elevation is also measured at ten points closely distributed around the reference point. In each meadow where surface elevations are measured, also the phreatic groundwater level and ditch water level are monitored using standpipe piezometers. The spatial variability of surface elevations in peat meadows is usually higher than the long-term vertical movement of the surface due to peat compaction and oxidation. Surface irregularities are, for example, caused by cow tracks or grass

tussocks. Therefore, a horizontal plate of 10 x 10 cm is fixed to the bottom of the levelling rod to average out the smallest irregularities of the grasscovered surface. For each measuring campaign, the average elevation and the average elevation difference relative to the first measurement campaign in November 2018 (T0) are calculated per measuring field (n =114). EXTENSOMETER

Extensometers are used to measure deformation worldwide (e.g., Poland, 1984; Sneed and Brandt, 2015). However, they have rarely been applied in peat soils. Extensometers can be used to derive point measurements of vertical movement of different (sub)surface levels at mm-scale accuracy, and to determine the contribution of different subsurface layers, and in some cases processes, to total subsidence. In this study, we installed four extensometers that continuously measure the vertical movement of (sub)surface levels (for locations see figure 1). Different types of anchors are used at three or four different (sub)surface levels (figure 2). Anchor level 1 is a lost-cone anchor founded in the Pleistocene sand. This level is stable at the timescales considered (years), and hence, is used as reference level. Anchor level 2 is a Borros anchor positioned just below the average lowest groundwater level. At this level, vertical movement is measured that is mainly caused by processes acting in the saturated peat layer between level 1


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and 2 (B in figure 2), presumably mainly compaction. In the overlying unsaturated zone, peat oxidation is likely to be the dominant process causing longterm subsidence. Anchor level 3 is a small rod pushed into the subsurface just below a clayey top layer, if present (not visualized in figure 2). This level measures the contribution of the entire peat layer (total subsidence A in figure 2 minus contribution of clay layer, if present). Anchor level 4 is a perforated square stainless-steel plate of 0.4 x 0.4 meter positioned at about 5 cm below surface. This level measures vertical movements of the surface relative to level 1 (A in figure 2). All sensors at the different levels are connected to a datalogger installed at the surface (figure 2). The vertical movement of the different levels is continuously and automatically measured at 1hour intervals. LIDAR

LiDAR (Light Detection And Ranging) is a technique that uses laser pulses sent from a laser scanner. The distance from the scanner to the surface or an object on the ground surface is determined by measuring the time gap between emitting the pulse and receiving the reflected pulse. LiDAR surveys result in spatial maps of surface elevation. Timeseries of these maps can be used to analyse vertical movement of the surface. In this study, LiDAR is used to measure surface elevation once every three months at the eight farms. To compare

different measuring platforms, the laser scanner is attached to a small aircraft and a drone. The absolute accuracy of such LiDAR measurements in a rural area is 0-3 cm. To increase this accuracy, which is required to monitor land subsidence, Ground Control Points (GCP) are constructed in

each parcel. A GCP consists of a 50 cm long hollow aluminium rod with a 50 x 50 cm horizontal plate on top of it, that can easily be placed on the founded reference rods used for levelling. All LiDAR measurements in a parcel use the centre of the horizontal plate as reference. Because this centre

has a known height as established by GPS measurements, the accuracy of the LIDAR measurements increases to mm-scale. An AL3-32 Phoenix laser scanner is attached to a DJI M600pro drone. The drone is manually lifted into the air, after which it automatically follows a pre-defined path using barometric and GPS for orientation. All measurements are real-time visualized on a ground station (laptop), using Wi-Fi or 3G, allowing for real-time quality control during the measurements. RTK-GNSS and Motion Sensor data are post-processed to attain the highest possible accuracy of the position of the drone. These data are linked to the LiDAR data and translated into a point cloud dataset, which is used to create a Digital Elevation Model (DEM). Outliers, vegetation and other objects are filtered out of the dataset. Point cloud classification is done using the international standard as utilized by the American Society for Photogrammetry and Remote Sensing (ASPRS). The final processed point cloud dataset is related to the centre of the GCP as explained above.

Figure 4 – Preliminary results of extensometer, levelling and groundwater level measurements

The same method is used for the LiDAR measurements with aircraft. For these measurements, a Riegl VUX1LR laser scanner is fixed to a Tecnam P2010 aircraft, using an Inertial Measurement Unit (IMU) for an accurate determination of the X,Y,Z position. The aircraft flies at 305 m elevation above the ground surface at 130 km h-1. The data point density is on average 10 points per square meter.

(site 05-B; figure 1). Green dots represent average elevation difference relative to T0 (November 2018), with standard deviations indicated by vertical bars. Peat thickness at this site is about 3.3 m.


Figure 5 – Preliminary results of extensometer, levelling and groundwater level measurements (site 09-B; Figure 1). Green dots represent average elevation difference relative to T0 (November 2018), with standard deviations indicated by vertical bars. Peat thickness at this site is about 1.5 m. 10 GEOTECHNIEK L AND SUBSIDENCE SPECIAL

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InSAR (Interferometric Synthetic Aperture Radar) uses satellite radar images of the earth surface. Each year, tens of images are obtained of the same area. Based on phase differences of reflected radar waves, changes in elevation are derived from the radar image time series. The radar signal is reflected by (objects at) the surface. InSAR measurements result in a spatial gridded map of reflections of the surface, which can be translated into a map of change in elevation. Most accurate measurements of elevation (phase) change are derived for permanent objects that consistently reflect the radar signal. Good reflection objects are for example rooftops and roads. Vertical movement of such objects can usually be determined with (sub)mmscale accuracy. In rural areas there are generally a lower number of consistent reflectors, and hence, the use of InSAR is more challenging in terms of attaining a similar accuracy. Increasing the accuracy in rural areas can be done by relating radar data to accurate ground measurements of elevation change by other field techniques and by sophisticated algorithmic data processing by for example better defining temporal correlations. For the use of InSAR for measurements of land subsidence in the study area an Integrated Geodetic Reference Station (IGRS) has been constructed and

installed by Delft University of Technology (figure 3; for location see figure 1). The IGRS basically consists of a GNSS antenna on a rod that is founded in the Pleistocene sand, with radar reflectors and a horizontal plate attached to it. Changes in elevation in the surrounding area, as determined from InSAR data, use the IGRS station as reference level. In addition, the vertical movement of the Pleistocene subsurface, assumed to be stable, can be determined. If the Pleistocene subsurface is not stable, these measurements can be used to correct other land subsidence determinations.

Table 1 - Average vertical movement relative to T0 (November 2018; n=114). Standard deviation in italic. Total vertical movement for a specific field = max rise – max subsidence. All in mm. Field

Feb 2019

Apr 2019

Jul 2019

Oct 2019

Total vertical movement

05-A 05-B 09-A 09-B

8.2 (12.5) 9.7 (12.9) 9.4 (8.0) 7.8 (6.1)

-20.1 (13.4) -13.0 (23.7) 4.4 (10.8) 7.0 (8.8)

-25.9 (11.8) -29.9 (19.2) -10.4 (9.8) -9.4 (7.1)

27.8 (32.7) 2.7 (29.0) 10.1 (33.4) 9.2 (29.9)

34.1 39.6 19.8 17.2

Preliminary results LEVELLING

The results of four measuring fields (two at site 05 and two at site 09; figure 1) are presented in Table 1. Relative to the first measurement in November 2018 (T0), all fields have on average risen about 8 to 10 mm in February 2019. Thereafter, compared to February 2019, all fields show a subsiding trend during spring and summer. The greatest total vertical movement measured in the period November 2018 to October 2019 is about 40 mm (+10 to -30 mm), measured at 05-B (Table 1). EXTENSOMETER

Extensometer results for locations 05-B and 09-B are presented in figures 4 and 5 respectively. These figures also include the averaged levelling values (of elevation differences relative to T0) with standard deviations for these fields. Results of both methods demonstrate a seasonal dynamic with a general rise in autumn/winter and a subsiding trend in spring/summer. Extensometer results demonstrate that shorter-term fluctuations strongly relate to groundwater level fluctuations. At location 05-B, the total vertical movement relative to October 2018 (T0) has been 35 mm (+18 to -17 mm). At location 09-B, the total vertical movement has been 17 mm (+12 to -5 mm). We observe that processes acting in the saturated peat layer significantly contribute to total surface movements (orange line in figures 4 and 5). LIDAR

The amount of vertical surface movement, derived from the elevation difference between the LiDAR maps of April and July 2019, shows a general subsiding trend in this time period (figure 6). The dominating greenish colour indicates a general surface lowering of 0 to 50 cm. However, the eastern (right) part of the northern field generally shows a higher amount of subsidence, while the middle part of the northern field shows a surface rise. This is attributed to mowing activities: the eastern part was mowed shortly before the LiDAR measurements while the middle part was not yet mowed. These observations demonstrate the influence of grass height on LiDAR measurements. Also, striping effects are seen in the resulting elevation maps (figure 6).

Figure 6 – Land subsidence at site 05 (see Figure 1) derived from the difference between LiDAR elevation measurements of April and July 2019.

Experiences and preliminary conclusions A unique land subsidence monitoring site in a cultivated peat area in the Netherlands is presented. Our first experiences with applying the different methods are: - Levelling at farm scale is time-consuming, and therefore, will not be effective at regional scales. - Extensometer results very convincingly show seasonal and short-term (groundwater-related) vertical movements of different sublayers, i.e. processes. - LiDAR results in spatial map but results demonstrate the influence of grass height. This method needs optimization aiming to reduce effects of grass height and increase the accuracy of land subsidence determinations. Moreover, the method is very much weather-dependent (it should not be too windy) and thus more difficult to plan. Also, the use of a drone and/or aircraft may disturb cattle due to noise pollution. - InSAR results are not available yet. Preliminary levelling and extensometry results demonstrate seasonal and shorter-term dynamics with a total vertical movement of up to 35-40 mm in one-year time. This proves that a long (multiple years) monitoring period is needed to be able to determine long-term net land subsidence (or 11


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uplift). Also, a longer record and experiments are needed to develop the most optimal subsidence monitoring system in peatlands.

References - De Mulder, E.F.J., Geluk, M.C., Ritsema, I., et al. (in Dutch): De Ondergrond van Nederland. Geologie van Nederland, deel 7. Nederlands Instituut voor Toegepaste Geowetenschappen TNO, Utrecht. 379 p, 2003. - Erkens, G., van der Meulen, M. J., Middelkoop, H.: Double trouble: subsidence and CO2 respiration due to 1,000 years of Dutch coastal peatlands cultivation, Hydrogeol. J., 24 (3), 551-568, 2016. - Pleijter, M., van den Akker, J. J. H. (in Dutch): Onderwaterdrains in het veenweidegebied: toelichting op de methode en meetinrichting. Wageningen Environmental Research report nr 1586, 2007. - Poland, J.F.: Guidebook to Studies of Land Subsidence Due to Ground-water Withdrawal. UNESCO report, ISBN 92-3-102213-X, 1984. - Sneed, M., Brandt, J. T.: Land subsidence in the San Joaquin Valley, California, USA, 2007–2014, Proc. IAHS, 372, 23-27, 2015. - Van Asselen, S., Erkens, G., Stouthamer, E., et al.: The relative contribution of peat compaction and oxidation to subsidence in built-up areas in the Rhine-Meuse delta, the Netherlands. Sc. Total Environ. 636: 177–191, 2018.

S. Kok Researcher/advisor Research economics, Deltares

A.L. Costa PhD, Researcher/ Advisor Risk and Resilience, Deltares

M. Korff Associate Professor TU Delft Sr. Advisor Geo-Engineering Deltares

METHODOLOGY FOR SYSTEMATIC ASSESSMENT OF DAMAGE TO BUILDINGS DUE TO GROUNDWATER LOWERING-INDUCED SUBSIDENCE IN THE NETHERLANDS Introduction Subsidence of peat and clay soils due to (artificial) lowering of the groundwater table and loading of soft soils is commonplace in the Netherlands, causing extensive damage to exposed and vulnerable assets. Earlier efforts to assess subsidence-related building damage in the Netherlands at the national level predict significant direct damage – ranging from at €17-45 bn cumulative by 2050 (Born et al., 2016; Hoogvliet et al., 2012) - but fail to address probability and impact of climate change, and do not provide a systematic and scalable approach to do so. Improved risk assessment of subsidencerelated damage to buildings would inform public and private actors from national to local scale on current and future risks and stimulate them to

address this issue. Several factors complicate a large scale, realistic risk assessment. On the hazard side, multiple drivers of subsidence and low groundwater tables lead to a high spatial variability of hazard conditions: e.g. local influences on the groundwater table (trees, leaking pipes) and variability in susceptibility of soft soils to settle (e.g. presence of anthropogenic layer). On the side of the assets at risk, databases with key input such as foundation types and historic design practices are incomplete or non-existent, and there is a knowledge gap regarding vulnerability functions (linking building damage to subsidence). In our study we have developed a methodology for systematic regional or countrywide assessment of

two subsidence-related damage mechanisms to buildings: timber pile degradation due to low groundwater levels and differential settlement of buildings on shallow foundations. In our TISOLS paper Costa et al. (2020) we elaborate in more detail on the specifics of the methodology. Furthermore, based on nationwide available data and expert estimates, we executed a quick-scan application of the methodology for the Netherlands in cooperation with KCAF and Stichting CAS.

Approach The risk assessment carried out for the estimation of damage to buildings due to groundwater lowering/ subsidence is construed in a modular, systematic way. For each damage mechanism, we build 3 modules based on the conceptual framework of Hazard – Exposure – Vulnerability according to the definitions by the (UNISDR, 2016), with: I Hazard - characterizing the events causing damage: in this case subsidence and/or low groundwater levels; II Exposure - inventory of the building assets at risk: i.e. probability of building on shallow foundations being exposed to differential settlement, or on timber piles exposed to fungal pile degradation; and III Vulnerability - Defining the degree of physical damage (architectural, functional and/or structural) of the buildings at risk.

Damage Assessment methodology – Application to the Netherlands 1 and 2 illustrate the methodology for the calculation of damages due to timber pile degradation caused by fungi as induced by low groundwater levels and differential settlement of buildings on shallow foundations. 1 gives an overview of key factors for the hazard (H) and exposure (E) modules. For more background on which datasets and assumptions were used to estimate these factors, see Costa et al. (2020). Indicators and functions presented here are relevant to geophysical processes and construction characteristics specific for the Netherlands and cannot be directly applied elsewhere.

Figure 1 – Schematic for the calculation of damages due to timber pile degradation induced by low groundwater levels. 12 GEOTECHNIEK L AND SUBSIDENCE SPECIAL

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SUMMARY subsidence-related damage mechanisms to buildings: timber pile rot due to low groundwater levels and differential settlement of buildings on shallow foundations. Application of the approach is expected to support private and public decision-making on coping strategies e.g. in awareness raising and evaluating interventions.

In the Netherlands, subsidence of peat and clay soils due to (artificial) lowering of the groundwater table and loading of soft soils causes extensive damage. The topic is a major concern to homeowners and public authorities but an integrated risk assessment is currently lacking. In this paper, we propose a modular methodology for the systematic countrywide assessment of two

Timber Pile Degradation HAZARD The decline of the ground water table over time

can be caused by artificial drainage to prevent flooding in areas of continuous subsidence in the Netherlands, or temporary lowering due to droughts, works, or local effects such as trees or leaking pipes. We use an estimate for the low groundwater level based on a nation-wide model.

Exposure Exposure characterizes the buildings that are at risk. Key indicators in this assessment are foundation types, - characteristics and the location of buildings. In the Netherlands, foundations can be broadly classified into three types: shallow foundations, timber piles, and other (mainly concrete) piles. Shallow foundations and timber piles located in ‘soft soils’ are at risk. Timber piles are subject to fungal degradation when they are exposed to oxygen: when the groundwater table is below the top of the timber pile. We establish ‘exposure classes’ for timber piles based on H1 and E6 (0.2), with boundary values derived from guidelines from the Dutch organization for independent foundation research (F3O, 2014a). Table - Exposure classes for timber piles Exposure Boundary Number dry class values of days per year Low GLG – top of wood: > 20 cm Low Medium GLG – top of wood: 5-20 cm Medium High GLG – top of wood: < 5 cm High

Vulnerability The degree of damage to the buildings is related to the pile degradation. Key factors for fungal pile degradation include: the type of the wood and the diameter of the pile, the type of soil (relation to behavior of moisture content) and the duration of drought. The style of timber pile foundation Amsterdam style (two timber piles under a foundation beam) or Rotterdam style (single timber pile foundation) is also expected to relate to different failure modes, but this has not yet been incorporated in the analysis. In the quick-scan application of the damage assessment, we consider the duration of the drought the key factor that defines foundation and in turn

Figure 2 – Schematic for the calculation of damages due to differential settlement of buildings on shallow foundations.

building damage. We assume a linear trend of the damage level with cumulative drought time, ranging from damage level D1 to D5 (Burland and Wroth, 1974) after 10-20 years cumulative drought, i.e. the cumulative number of dry days over a longer time span (KCAF, 2012). We assume the number of cumulative drought years before reaching D5 is dependent on soil type at 1-2 m depth: see E8 in 1.

Differential settlement of buildings on shallow foundation HAZARD & EXPOSURE In general, differential settlement of a building can be directly linked to heterogeneous conditions of the soil, heterogeneous conditions of the building itself or a combination of both. In the assessment methodology we assume that the (shallow) subsidence rate of the soil (H9), when corrected for factors of local hazard (H14) and for building characteristics (E5b), correlates to the damage of 13


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the building. Relevant building characteristics include the quality of masonry of the shallow foundation – based on the age and construction practices at the time – and the presence of a full or partial basement: we assume these factors affect the likeliness of a building to settle differentially rather than in a uniform way under soil subsidence. VULNERABILITY

Vulnerability curves that estimate the development of damage of buildings based on differential settlement of the building over time exist (Peduto et al., 2019). These curves have been developed based on empirical data with available satellite or monitoring measurements of differential settlement at the building level. However, at present we cannot predict whether and to what degree a particular building in a particular location will settle differentially. Therefore, we assign damage classes to each building based on the overall rate of settlement in the area, corrected for the local hazard (H14) and building characteristics (E5b)

Indication of expected damage untill 2050 without climate change

Timber Pile Hazard H1 Map of low ground water level.

Indication of additional expected damage untill 2050 due to climate change (WH scenario)

Shallow Foundation H9 Shallow subsidence rate/ year (excluding subsidence triggered by gas extraction) based on satellite data. E7

Estimate of thickness of the anthropogenic layer


Type of soil at 1-2 meters: represents potential for swell/shrinkage behavior of clay.


Local variability in the susceptibility to settlement as an indicator of the heterogeneity in subsurface.


Correction factor (CF) for the settlement rate: proxy for likeliness uniform settlement will lead to differential settlement, based on information from E7, E8 and E10.

Exposure E0a Map combining the location of all the buildings in the Netherlands. E3

Map identifying the areas of clay and peat where timber pile foundations and shallow foundations are expected.


Defines regional areas of typically similar foundations based on historical practice.


Classifies the land-use type into rural or urban areas.


Estimation of the top of the timber piles per area of E4 (in m below surface level)

E15a Classifies the expected structural quality of the shallow foundation into low, medium and high based on the year of construction.


Defines the type of soil at the top of the timber piles: piles in more permeable soils are expected to be more severely affected by droughts as moisture content drops faster.

E15 b Defines the presence or absence of a uniform or partial basement under the building.


Defines the estimated type of timber of the piles, with a binary classification into spruce and pine based on a threshold of 12m length (<12 M pine, > 12 m spruce)



Estimation of foundation style: Rotterdam or Amsterdam.

Defines the settlement rate correction factor for the building characteristics by aggregating partial correction factors for each of E15a and E15b.

Figure 4 – Spatial distribution of expected damage due to timber pile degradation and differential settlement of shallow funded buildings until 2050 without climate change (left) and the additional expected damage until 2050 due to climate change (note that the scales are relative and not the same for the left and right figure).

factors (1; 2). 2 shows which assumptions we make, taking into consideration the settlement rate classifications from F3O (2014). For both damage mechanisms we use the same damage classes ranging from 1-5, with D1 representing negligible damage, i.e. hairline cracks of < 0,1 mm, and D5 severe structural damage involving partial or complete rebuilding of the structure (Burland and Wroth, 1974) Table - Vulnerability and damage classes for differential settlement of buildings Corrected settlement Damage class rate [mm/year] <2 D2 2-3 D3 3-4 D4 >4 D5

Expected damage costs All information and indicators are collected at the building level, and then scaled up to define the cumulative expected damage level in € in 2050. Based on desk research and interviews with experts active in building restoration sector in the Netherlands, Salerno (2017) estimates restoration costs for Dutch buildings (0.4). Table - Damage classes & restoration costs Damage class Restoration costs/ m3 of building D1 € 3,25 D2 € 15,00 D3 € 53,00 D4 € 184,00 D5 € 670,00

Results The methodology we have discussed above has been applied for the Netherlands, in the context of

Table – Description of key hazard and exposure factors in damage assessment methodology, related to 1 and 2.


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climate change: we calculated expected damage without and with climate change (scenario WH). For timber pile degradation we used projections of change in low groundwater tables: for differential settlement on shallow foundations we made assumptions on how local circumstances (such as shrink/swell behavior of clay soils) will aggravate differential settlement under increasing drought conditions. The results of the analysis are published on This website combines climate risk information at the municipal level to support municipalities in their strive to become climate adaptive by 2050. For each municipality a range of expected damage with and without climate change can be retrieved. 1 shows how expected damage – of both damage mechanisms – is spatially distributed across the Netherlands.

Discussion This study shows that it is possible to incorporate knowledge from different backgrounds that are interrelated in causing damages in one systematic damage assessment approach. However, the uncertainty associated with different contributing factors is still high: there are many knowledge gaps

on characterizing the subsidence and low groundwater level hazard, as well as related to exposure and vulnerability of the buildings. These knowledge gaps can be addressed in future developments. In this light, the Damage Assessment Methodology is particularly suited for large scale applications at this stage: with better-quality information for each of the modules it will become possible to increase the level of spatial detail.

References - Born, G.J. van den, Kragt, F., Henkens, D., Rijken, B., Bemmel, B. van, Sluis, S. van der, 2016. Dalende bodems , stijgende kosten. Den Haag. - Burland, J.B., Wroth, C.P., 1974. Settlement of buildings and associated damage. Settl. Struct. Proc. Conf. Br. Geotech. Soc. 611–764. - Costa, A.., Kok, S., Korff, M., 2020. Systematic assessment of damage to buildings due to groundwater lowering-induced subsidence: methodology for large scale application in the Netherlands, in: Tenth International Symposium on Land Subsidence. - F3O, 2014a. Guideline for investigation and assessment of wooden pile foundations under buildings. - F3O, 2014b. Guideline for investigation and as-


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sessment of shallow foundations under buildings. Hoogvliet, M., Buma, J., Oostrom, N. Van, Brolsma, R., Filatova, T., Verheijen, J., 2012. Schades door watertekorten en -overschotten in stedelijk gebied. - KCAF, 2012. Wat doe ik als bestuurder met (mogelijke) funderingsproblemen? site/images/uploads_pdf/2012.03.19_Hoe_kan_ ik_als_bestuurder.pdf (accessed 1.30.19). - Peduto, D., Korff, M., Nicodemo, G., Marchese, A., Ferlisi, S., 2019. Empirical fragility curves for settlement-affected buildings: Analysis of different intensity parameters for seven hundred masonry buildings in The Netherlands. Soils Found. 59, 380–397. 10.1016/j.sandf.2018.12.009 - Salerno, E., 2017. Evaluation of economic loss related to settlement-induced damage to buildings resting on piled-foundations: case studies in The Nethelands. Università Degli Studi Di Salerno. - UNISDR, 2016. Report of the open-ended intergovernmental expert working group on indicators and terminology relating to disaster risk reduction.

ir. ing. Michel de Koning Senior geotechnical advisor CRUX Engineering BV

ing. Rob R. van Buuren Geotechnical advisor CRUX Engineering BV

ir. Jacco K. Haasnoot Senior specialist, director CRUX Engineering BV

USE OF MONITORING DATA FOR DIKE STRENGTHENING PROJECT KIJK Introduction The goal of the first study was to optimize the predicted land subsidence. The second study was to assess the condition of all buildings along 10 km of dike. The Krimpenerwaard is an area consisting of soft Holocene soil layers with ongoing subsidence mainly caused by changing groundwater levels and the associated drying out and oxidation of peat. The subsidence rate for the crest of the dike is different from that of the hinterland. In both studies INSAR and LiDAR monitoring data were used as primary input.

Monitoring The INSAR measurements consist of settlementdata collected between 2013 and 2016 for different points located on the crest of the dike or buildings located near the dike. The used satellite data is generated and processed by SkyGeo. Also, the Current Dutch Elevation (Actueel Hoogtebestand Nederland, AHN) is used for determining the subsidence rate. The AHN3 map is a digital height map obtained by LiDAR measurement from airplanes or helicopters.

Determination of subsidence of the dike crest The height of the dike is one of the key criteria in the primary dike safety assessment. Land

subsidence is obviously an important factor when the future level of the dike crest is predicted.

of 27% on the amount of soil needed to counteract for the subsidence.

Based on information of the Water authority “Hoogheemraadschap van Schieland en de Krimpenerwaard” (HHSK), the average rate of subsidence for the crest of the dike is about 11 mm/year. For the project this means that for the period 2025-2075, besides effects of sea level rise an additional 0,55 m of subsidence must be considered. Large parts of the dike also do not comply for macro stability which means that every additional load on top of the dike would increase the stability problem. By making a better assessment of the subsidence, the amount of soil needed to balance the subsidence is optimized.

Determination of settlement of the buildings along the dike

A study was undertaken to determine the rate of subsidence based on monitoring data. The first step was to re-analyse the archive data from HHSK which is compared with average subsidence rate determined from the monitoring data, see figure 1. In general was concluded that the INSAR data and the HHSK data give almost the same average subsidence rate for the entire dike of about 7 mm/year. The average subsidence rate based on the LiDAR data is 8,9 mm/year. For further use in the project an average rate of subsidence for the crest of 8 mm/year is used. This means a reduction

The buildings along the dike are sensitive to subsidence and ground deformation, both from natural origin (creep) as well as due to the effects of dike strengthening. The building administration (BAG) and the municipal archives provided valuable information like building year, type of construction and type, material and depth of the foundations. For 56% of all buildings information was available. The annual settlement rate of each building was derived from INSAR data. Combining the INSAR and archive data led to multiple relationships, for example a relation was found between building settlement and foundation type. Using these relationships the foundation and building properties can be assessed for all buildings where the archives are incomplete. In the preliminary design phase of the project four different types of dike strengthening are designed and for all designs the risk of damage due to ground displacements is calculated using FEM and the limiting tensile strength method Netzel [3]. In these calculations the type and material of the foundation are the primary input parameters for this risk assessment.

Figure 1 – Comparison of subsidence rate determined with different measurement methods and HHSK archive data. 16 GEOTECHNIEK L AND SUBSIDENCE SPECIAL

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SUMMARY of soil needed to balance the subsidence and to quantify the building settlement due to natural subsidence.

For the dike strengthening project Krachtige IJsseldijken Krimpenerwaard (KIJK) in The Netherlands two studies have been undertaken for which monitoring data played an important role. The data is used to determine the amount

Conclusion Using INSAR and LiDAR monitoring data it is possible to optimize the subsidence rate for the dike crest from an average of 11 mm/year to an average of 8 mm/year. This means a reduction of 27% on the amount of soil needed to balance for the subsidence. Also, using INSAR data it is possible to determine the settlement rate for almost all buildings of the KIJK project. Combining the building settlement with other data sources it is possible to estimate the foundation type of a building. Although this is not 100% accurate, in combination with safe assumptions it gives a good indication in the preliminary design phase of the project.

Acknowledgements The work presented in this paper is part of the KIJK dike strengthening project in the Netherlands. The authors would like to thank their partners from IBKIJK, BWZ Ingenieurs, Greenrivers and Infram.

Figure 2 – Distribution per foundation type versus settlement rate in mm/year.

Reference [1] Determination of amount of land subsidence based on INSAR and LiDAR monitoring for a dike strengthening project, Michel de Koning, Jacco K. Haasnoot, Rob R. van Buuren, Marco Weijland, Cor Bisschop, Proceedings of TISOLS2020. [2] Using InSAR settlement data in a levee

strengthening project for building settlement risk assessment, R.R. van Buuren, J.K. Haasnoot, M. de Koning, M. Weijland, J.W. van Zanten, Proceedings of TISOLS2020. [3] Limiting tensile strength method, H. Netzel, 2009.

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Wouter Kooijman Senior Consultant Hydrology, Fugro


SOLUTION FOR PREVENTING THE IMMENSE COST OF SUBSIDENCE DAMAGE TO DWELLINGS AND INFRASTRUCTURE Stress test As part of the DPRA, municipal authorities are assessing the current extent of climate resilience and adaptation in their areas. This is focused on the following topics: heat stress, waterlogging, drought and flooding. Fugro’s research mostly concerns the latter three topics. The vulnerability of an area can be assessed using a stress test. It is important to take the future climate into account in public realm planning. This often requires only limited changes to be made to projects currently in the planning pipeline.

Climate-proof urban areas At Fugro, we believe that climate-proof planning necessitates consideration of the ground and water management in all project designs. Climateproof planning of urban areas means balancing spatial planning with the local topography, i.e. where can we build, and where not? And also: how can we deal with a shortage or excess of water? When studying a climate-proof planning proposal, Fugro assesses whether the proposed solution is compatible with the soil and water management practices in the area. If not, then we investigate whether there are options to adapt the area. If this is not possible either, then we suggest that spatial planning choices may need to be made. Fugro can provide support at all stages of this process,

enabling clients to make well-founded decisions about climate-proof spatial planning. Active groundwater level management is a good example of this. Fugro is currently undertaking research on this topic, in association with Deltares (an independent institute for applied research in the field of water and subsurface) and Wareco (a specialist in soil, water and foundations). Large-scale active groundwater level management Urban areas on weak ground can be affected by groundwater levels which are too high or too low. For example, an excessively low groundwater level in a city can lead to immense damage resulting from subsidence and rotting of wooden piles – with the potential to cost tens of billions of euros between now and 2050, according to some estimates. Climate change will further increase these costs unless effective measures are taken. But an excessively high groundwater table can also cause problems, such as damage to roads and houses and other flooding-related issues. Active groundwater level management can solve multiple problems at once.

Drainage and infiltration Active groundwater level management is based on a drainage and infiltration pipe in connection with surface water. If there is too much groundwater, it is discharged to the surface water. During

The Delta Plan on Spatial Adaptation (Deltabeslissing Ruimtelijke Adaptatie or ‘DRA’) is a collective plan, drawn up by municipalities, district water boards, provinces and the central government, to render the Netherlands climate-proof and water-resilient. The Delta Plan expedites and intensifies the efforts to tackle waterlogging, heat stress, drought and the impact of urban flooding. More information:

Figure 1 – Construction site.


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dry periods, the same pipe is used to recharge the groundwater with surface water. This system has already been implemented on a small scale. Eight case studies in several cities have now demonstrated that active groundwater level management can be used on a larger scale. The design is based on factors such as the soil structure, density of the urban area, hydrology, how water reaches the area, and the intended effects.

Short payback period In many cases, the costs of installing and maintaining the system have been found to be less than the cost of the avoided damage. In areas potentially affected by ground settlement due to a low water table, the municipal authorities can often recoup the costs of the system thanks to less damage to municipal assets. For example, active groundwater level management to reduce ground settlement extends the lifespan of infrastructure in the area. The payback period will be even shorter if the installation of the drainage system is combined with sewer installation or replacement. Furthermore, if the avoided damage to foundations of private property is factored in, the costs of active groundwater level management are always easily offset by the savings.

SUMMARY The Delta Plan on Spatial Adaptation (DRA) aims to make the Netherlands climate-proof and water-resilient. Spatial adaptation means changing the public realm so that it can cope with the effects of climate change. As part of the DRA Programme (DPRA), Fugro was commissioned to contribute to a study on active groundwater level management.

In the 21st century, the Netherlands will be faced with four climate-related trends: it will get hotter, it will get drier (in summer), it will get wetter (in winter), and the sea level will rise. The effects and impact of this will be different for each sector. So far, over a hundred climate-related effects have been identified, from the rotting of wooden piles through to roads flooding.

Figure 3 – In dry periods, the surface water flows through a network of drainage

result, the groundwater level does not rise excessively. Source: Rapport: Grootschalig actief grondwaterpeilbeheer in bebouwd gebied, mei 2017, Auteurs: Wareco, Deltares, Fugro.

and infiltration pipes which allows the water to enter the soil. This maintains the groundwater at the required level. The system works the other way round during wet periods: excess rainwater flows through the pipes to the surface water. As a


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