Groundwater rise

Groundwater rise in the western part of the Rhine-Meuse delta is closely related to sealevel rise, and the technique to reconstruct a curve of sealevel rise or a curve of local groundwater rise is identical. Because paleo-groundwater levels cannot be directly measured, an indicator has to be found, that depends on the level of the groundwater table. The reconstruction technique involves the measurement of two parameters:

In fact, all curves should be regarded as curves of local groundwater rise. If a curve is made close to the sea, it can be regarded as an approximation of (relative) sealevel rise.

Figure 1 Roots of basal wood peat, overlying Pleistocene sand of the Kreftenheye Formation in an excavation near Bergharen.

The first reliable curve of Holocene relative sealevel rise in the Netherlands was made by Jelgersma (1961). She used samples of Holocene basal peat (that is peat directly overlying permeable Pleistocene sand, Fig. 1) to reconstruct the rise of sealevel, assuming that peat formation was related to sealevel rise. The peat samples were radiocarbon dated. The principle of the method is illustrated in Fig. 2. Presumptions are:

If one of these presumptions is not valid, then data points may deviate from the general trend. This could happen for example in a closed depression, where peat formation starts earlier. In that case a datapoint obtained from that location will lie above the trendcurve.

Figure 2 Principle of making a curve of Holocene sealevel rise, based on basal peat samples. d = depth of basal peat samples relative to NAP (Ordnance Datium), t = age of the samples, obtained by radiocarbon dating. A curve drawn through the data points in the graph is the local rise of groundwater. To approximate sealevel rise, a curve has to be drawn below the data points, because peat is formed above sealevel.
Figure 3 Curve of relative sealevel rise in the Netherlands (Jelgersma 1961). After 6000 yr BP, relative rise of sealevel is largely caused by land subsidence. Data points in Zeeland are generally higher in the diagram than datapoints from the northeren Netherlands. This is due to a difference in land subsidence.
Figure 4 Late Glacial eolian dune (Dutch: donk), that is still visible in the present-day landscape. The flanks of the dune are coverd with peat and (fluvial) clay.

There are two other important considerations:

The sealevel curve obtained by Jelgersma (1961) (with some later slight modifications) is shown in Fig. 3. It shows, that sealevel rose sharply during the Holocene, until approximately 6000 yr BP. Then the ice caps on North America and Scandinavia had melted, and sealevel rise slowed. In the Netherlands, relative sealevel rise after 6000 yr BP is largely caused by land subsidence. An interesting aspect of her curve is, that samples taken in Zeeland are generally at a higher level in the diagram. This is due to subsidence, being smaller than elsewhere. Note that all samples are of Holocene age; the oldest peat samples were collected from the bottom of the North Sea.

A problem related to the method illustrated in Fig. 2 is, that the groundwater table slopes down seaward. In other words, samples from the same age occur at slightly higher levels further inland. Therefore the technique illustrated in Fig. 2 will only give reliable results for sealvel reconstruction if the Pleistocene surface slopes steeply over a short distance. Such is the case in Late Glacial eolian dunes (Dutch: donken, Fig. 4) that occur all over the delta. These dunes are often up to 15 m high, and their flanks are covered with peat (Fig. 5). These are ideal locations for the construction of local groundwater curves.

Figure 5 Late Glacial eolian dune in the Alblasserwaard. Its flanks are covered with peat. Because the peat is not influenced by compaction of the substratum, it can be used to construct a curve of local groundwater rise.

Van Dijk et al. (1991) used this technique to make local groundwater rise curves for several locations in an E-W cross section of the Rhine-Meuse delta. They showed, that lcoal groundwater curves were indeed at a higher level in the diagram if samples were taken further inland (Fig. 6). Drawing isochons in an E-W cross section, enabled them to make a graph of groundwater rise for the Rhine-Meuse delta (Fig. 7).

Figure 6 Local curves of groundwater rise at several locations in the Rhine-Meuse delta, in an E-W cross section. Based on basal peat samples from Late Glacial eolian dunes (Berendsen 2004).
Figure 7 E-W cross section of the Rhine-Meuse delta with isochrons of the rise of the groundwater table. Originally after Van Dijk et al. 1991, adapted by Törnqvist (1993).
Figure 8 Average rise of sealevel, tidal amplitude and compaction in the western Netherlands. The use of windmills to improve drainage induced a sharp increase of compaction (originally from various sources, from Berendsen 2005).

Fig. 7 has been shown to be a very powerful tool for many other studies. For example, it can be used for archeological research, because it indicates when archeological sites started to drown. It can also be used to date channel belts. Because there exists a relationship between the groundwater gradient, and the gradient of the top of the sand in a channel belt (GTS line), the elevation of natural levees relative to the groundwater level can be calculated. This has demonstrated that natural levees decrease in height in a downstream direction. Fig. 8 shows the original elevation of peats (samples are taken from compaction-free surfaces). If samples from the same age of compacted peats in floodbasins are compared with samples from uncompacted peats, the total amount of compaction can be calculated. This technique is applied in the Ph.D. compaction study (start: 2006). From earlier studies it is known, that compaction in the western part of the delta can be as high as 4 m.

Figure 9 Reconstruction of local groundwater curves, after Cohen (2003).

Model of groundwater rise

Cohen (2003, 2005) used the same technique as Van Dijk et al. (1991) to reconstruct the rise of the groundwater table over the entire western part of the delta (Fig. 9) and demonstrated the effects of neotectonic movements on groundwater curves. He reconstructed groundwater rise at many locations in an E-W cross section (Fig. 10), but also at the margins of the delta, and made a three-dimensional model of Holocene groundwater rise.

Figure 10 Location of groundwater rise data points, and resulting curves of groundwater rise (Cohen 2003).

Cohen's (2003, 2005) model of groundwater rise is based on a 3D geostatistical interpolation that used >300 basal peat radiocarbon dates, from compaction-free positions at sites all over the delta. His data show, that groundwater slopes not only in an E-W direction, but also from the margins to the central part of the delta (Fig. 11). A graphic representation of the 3-dimensional model can be downloaded (zipped AVI; 3,23 MB).

Figure 11 Area drowning by groundwater level rise, approximately 6600 cal yr BP (after Cohen 2003). Download the 3-dimensional model (zipped AVI; 3,23 MB).

The model allows to construct accurate curves of local groundwater rise for any location in the western part of the Rhine-Meuse delta, within the coordinates X=75 km in the west (coastline at The Hague) to X=175 in the east (Wageningen) and from Y=410 km in the south (Biesbosch, Den Bosch) to Y=460 km in the North (Leiden, Utrecht). There is no output for locations outside the limits of the delta plain. Based on depth, the model can be used to estimate:

Vertical accuracy of the model is better than 0.5 m, timing is accurate to 200 yr. Because of human activity, there is hardly any Late Holocene basal peat preserved in the delta. Therefore, no interpolation results younger than 2200 cal yr BP are given. For more details, see Cohen (2003). If you want to make your own local groundwater rise curve, look up the coordinates of a desired location on the topographical map of the Netherlands, feed them into the model and generate the groundwater data from the model. You can export the data from the model as a csv file and produce your own local curve in, e.g., Excel.

The data can also be used to estimate:

In case you need additional information or maps, contact Dr. K.M. Cohen.

Literature

  1. Berendsen, H.J.A. (2004), De vorming van het land. Inleiding in de geologie en geomorfologie. Fysische geografie van Nederland. Assen: Koninklijke Van Gorcum. Vierde, geheel herziene druk, met CD-ROM.
  2. Berendsen, H.J.A., & E. Stouthamer (2001), Palaeogeographic development of the Rhine-Meuse delta, The Netherlands. Assen: Van Gorcum. 270 pp.
  3. Cohen, K.M. (2003), Differential subsidence within a coastal prism. Late-Glacial - Holocene tectonics in the Rhine-Meuse delta, the Netherlands. Netherlands Geographical Studies 316, 172 p. KNAG/Faculteit Ruimtelijke Wetenschappen Universiteit Utrecht.
  4. Cohen, K.M. (2005), 3D Geostatistical interpolation and geological interpretation of paleo-groundwater rise in the Holocene coastal prism in the Netherlands. In: Giosan, L. & J.P. Bhattacharya (Eds.), River Deltas - Concepts, models, and examples. SEPM Special Publication 83, p. 341-364.
  5. Cohen, K.M., E. Stouthamer & H.J.A. Berendsen (2002), Fluvial deposits as a record for neotectonic activity in the Rhine-Meuse delta, The Netherlands. Netherlands Journal of Geosciences / Geologie en Mijnbouw 81 (3-4), p. 389-405.
  6. Hoek, W.Z. & S.J.P. Bohncke (2002), Climatic and environmental events over the Last Termination, as recorded in The Netherlands: a review. Geologie en Mijnbouw / Netherlands Journal of Geosciences 81, p. 123-137.
  7. Jelgersma, S. (1961), Holocene Sea Level Changes in the Netherlands. Thesis Leiden, Mededelingen Geologische Stichting, CVI-7.
  8. Törnqvist, T.E. (1993), Fluvial sedimentary geology and chronology of the Holocene Rhine-Meuse delta, The Netherlands. Netherlands Geographical Studies 166, 169 p. KNAG/Faculteit Ruimtelijke Wetenschappen Universiteit Utrecht.
  9. Van Dijk, G.J., H.J.A. Berendsen & W. Roeleveld (1991), Holocene water level development in The Netherlands' river area; implications for sea-level reconstruction. Geologie en Mijnbouw 70, p. 311-326.