Practical applications of river and delta studies

The study of rivers is not a purely scientific hobby, but also has great practical significance. In this section a few practical applications of river studies are illustrated.

Maps

Maps are important for planning, construction, environmental studies, nature conservation, water control, scientific research, recreation, agriculture, forestry, archeology, and many other aspects of society. In archeological prospection, the map of Berendsen & Stouthamer (2001) is frequently used. In addition it was used to make the IKAW. The Netherlands has been mapped in great detail, but recent high-accuracy digital elevation maps based on laser-altimetry show that existing maps can still be improved significantly. An effort is under way to update the Berendsen & Stouthamer (2001) map, and to increase accuracy for use on a 1:10,000 scale.

Figure 1 Digital elevation map of part of the IJssel Valley in the Netherlands, showing a meander cutoff. Based on the AHN (Rijkswaterstaat-AGI 2004).
Figure 2 Digital elevation map of the area southwest of Montfoort (blue = low, yellow = intermediate, brown = high), compared to the map of Berendsen & Stouthamer (2001). Various channel belts and crevasse splays can be seen in the image (black lines); most have been mapped very accurately. However, one channel belt, meandering from E to W in the middle of the image was missed entirely. Based on the AHN (Rijkswaterstaat-AGI 2004).

Seepage

Figure 3 Ignoring subsurface geology may lead to unwanted wells.

In a fluvial environment, subfossil sandy channel belts are the 'pipelines' for underground transport of water. Ignoring the subsurface geology may lead to serious problems, when the impermeable overburden is removed (Figure 3).

Where channel belts cross the dikes, seepage may undermine the dike, and dike bursts may be the result (Figure 4). In the Netherlands, approximately 80 % of all the dike bursts can be related to this phenomenon. To know exactly where the subfossil channel belts are, detailed geological-geomorphological maps are needed. This is one of the reasons why the Province of Gelderland sponsored the construction of sand depth maps of the Rhine delta. Channel belts that occur near the surface can still be seen in the present landscape, and can now be mapped easily by using the AHN (Actueel Hoogtebestand van Nederland), a digital elevation map. Channel belts at a depth of > 4 m below the surface (that are usually invisible on the AHN) can only be mapped with great difficulty, and a dense network of drillings is necessary to produce reliable maps. At present, the best available map of the Rhine-Meuse delta is the 1:100,000 geological-geomorphological map by Berendsen & Stouthamer (2001). The digital version of this map is still extended and updated continuously.

Figure 4 Dike burst of the Diefdijk in the Netherlands, as a result of seepage, undermining the dike (Berendsen 2005, after Verbraeck 1970).

Construction

The depth of the sand is also important for construction purposes. Because much of the western Netherlands consists of clay and peat, virtually all buildings in that area need to be founded on the Pleistocene substratum. The depth of the Pleistocene increases from approximately 2 m in the east to over 15 m in the west. This implies, that foundations on poles need to be made to support tall buildings (Figure 5), especially in the western part of the country. Construction costs of foundations increase with the depth of sand layers that are able to support buildings. In the cities of Amsterdam and Rotterdam, almost all the houses need to be built on pole foundations. Foundation costs decrease significantly if buildings can be founded on shallow sand bodies like channel belt sands.

Figure 5 Length of poles for foundations, and percentage of houses that have to be founded on poles in the Netherlands (Berendsen 2005, after Van Weele 1979).

Avulsions

Avulsion (shift of a river course to another location on the floodplain) is an important process in fluvial and deltaic environments. Avulsion leads to a redistribution of discharge, and can have deadly consequences if it occurs rapidly. For example, in 1887, over 2 million Chinese died as a result of an avulsion of the Yellow River. In 1931 another avulsion of the Yellow River killed 3.7 million people. To prevent such disasters, it is important to understand the nature and causes of avulsion.

The Rhine-Meuse delta offers a great opportunity to study avulsions on a time scale of millennia, because the delta has been mapped in great detail, and time control is excellent. This means, that the avulsions can be dated, and processes can be quantified over time scales of millennia. Over the past decade, we have learned a lot about factors influencing avulsions, but there is still a lack of fundamental understanding of the causes of avulsion. In the Mississippi River, that has a much larger drainage basin than the Rhine (Figure 6), avulsions are equally important. The redistribution of sediment and discharge to other parts of the floodplain may also drastically alter the evolution of the coast (Figure 7). In the Mississippi delta, delta lobes of various ages can be distinguished. Another well-known example of avulsions is found on the Kosi fan, south of the Himalaya mountains (Figure 8), but avulsions are common in virtually all fluvial and deltaic environments.

Figure 6 Drainage basin of the Mississippi River.
Figure 7 Avulsions of the Mississippi River.
Figure 8 Avulsions on the Kosi fan (Bridge 2003).

Alluvial architecture

The three-dimensional geometry, proportion and spatial distribution of the various types of alluvial deposits, is generally referred to as alluvial architecture. Alluvial architecture is important in the oil and gas industry, because sandy fluvial deposits often contain natural resources (Figure 9). To simulate alluvial architecure, models are used to predict where sandy deposits occur, and how thick they are. These models are still very inadequate. Attempts are being made to improve presently existing models. This is a potentially important line of research, that may enhance future exploration of natural resources.

Figure 9 Example of simulated alluvial architecture (Mackey & Bridge 1995). The yellow blocks represent sandy channel belts.

Flooding

The last inundation in the Netherlands took place in 1953, when the dikes in the southwestern part of the country were breached at 600 locations, as a result of a storm surge. Over 1800 people drowned, and an area of 2000 square kilometers was inundated. After this disaster the tidal inlets were closed off by large dams (Figure 10).

Figure 10 Dams close off the former tidal inlets in the southwestern part of the Netherlands.

The last inundation as a result of a dike breach in the river area occurred in 1926 in the Land van Maas en Waal. The 1995 flood led to extreme seepage in some locations (Figure 11) and almost resulted in disaster. A total of 200,000 people were evacuated. Dikes are now designed to withstand a 1 in 1250 years flood, but with the predicted climatic change, this may be insufficient, and dikes may have to be strengthened again. Additional measures are also taken, e.g., lowering the level of the embanked floodplains, removal of obstacles, and even widening of the embanked floodplains. During the flood of 1995, water levels stood just 50 cm below the top of the dikes (Figure 12). Unfortunately, political ignorance has allowed building in the embanked floodplains.

Figure 11 Flooding of the embanked floodplains near Zaltbommel.
Figure 12 The embanked floodplain of the River Waal was completely inundated during the 1995 flood of the river Rhine. This almost resulted in disaster, and 200,000 people were evacuated. Seepage from the embanked floodplain to the polder may undermine the dike, and eventually cause a dike breach. The most vulnarable spots are those, where subfossil sandy channel belts cross the dike. In this case, seepage is controlled by sand bags.

Literature

  1. Berendsen, H.J.A. (2005), Landschap in delen - Overzicht van de geofactoren. 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. Mackey, S.D., & J.S. Bridge (1995), Three-dimensional model of alluvial stratigraphy: theory and application: Journal of Sedimentary Research v. B65 (1), p. 7-31.