Rhine-Meuse Delta studies

Faculty of Geosciences - Department of Physical Geography, Universiteit Utrecht

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Avulsions

Figure 1 Development of an avulsion from a crevasse splay (schematic, after Stouthamer 2001).

Avulsion is the abandonment of a part or the whole of a channel belt by a stream in favour of a new course. In meandering rivers, an avulsion is defined as a channel belt shift over a length of more than two meander loops (Figure 1). In the process of avulsion, the old channel is abandoned either instantaneously or gradually. Avulsion occurs in all kinds of fluvial environments, in braided, and meandering, as well as anastomosing rivers (Figure 2).

Figure 2 Anastomosing river pattern in an avulsion belt of the Saskatchewan River (picture by N. Smith).

Although avulsions have been studied extensively over the past decade, the avulsion process itself is still poorly understood. It is generally believed to be caused by differential sedimentation leading to superelevation of the channel belt above the floodplain.

Avulsion types

Different types of river avulsions are recognised in the literature (Stouthamer 2001):

Figure 3a Full avulsion (Stouthamer 2001).

Figure 3b Partial avulsion (Stouthamer 2001).

• full avulsion: the new channel takes over the entire discharge of the old channel (Figure 3a).
• partial avulsion: a new channel is formed, but the old channel continues to exist (Figure 3b). Full and partial avulsion seem be distributed randomly over time (Figure 4). During the time the two channels coexist, the avulsion location is a bifurcation, which is a topic presently studied by M. Kleinhans. The formation of the river IJssel is an example of a partial avulsion. Some bifurcations seem to remain stable for a long time. If they become unstable, they will eventually result in an avulsion.

  

  Figure 4 Full and partial avulsion in Rhine-Meuse delta (Stouthamer 2001).

• instantaneous avulsion: the river shifts abruptly (within 200 years) and the old channel is abandoned (Figure 3a). The new river takes over the entire discharge;
• gradual avulsion: the formation of a new channel takes more than 200 years. Instantaneous and gradual avulsions occurred in approximately equal numbers in the Rhine-Meuse delta during the Holocene (Figure 5).

  

  Figure 6 Nodal and random avulsion (Stouthamer 2001).

  

  Figure 5 Instantaneous and gradual avulsion in the Rhine-Meuse delta (Stouthamer 2001).

  

  Figure 7 Local and regional avulsion (Stouthamer 2001).

• nodal avulsion: more than one avulsion occurs at approximately the same location over time (Figure 6). Most avulsions in the Rhine-Meuse delta are random avulsions, however, the most important ones are nodal avulsions. These were concentrated near the faults bordering the Peel Horst.
• random avulsion: the avulsion occurs randomly across the floodplain (Figure 6),
• local avulsion:the new channel reoccupies the previous channel further downstream (sometimes called avulsion by channel reoccupation, Figure 7),
• regional avulsion: an entirely new channel is formed (sometimes called avulsion by diversion into the flood basin, Figure 7). In the Rhine-Meuse delta, local avulsions are more common than regional avulsions. This is due to the limited number of tidal inlets during the younger part of the Holocene.
• failed avulsion: a new channel forms, but does not survive, because the old channel continues to exist, and eventually takes over the entire discharge again (Figure 8). Failed avulsions are common in the Holocene fossil record.

  

  Figure 8 Failed avulsion: the new channel does not succeed in taking over the discharge, but gets abandoned before the life span of the old channel ends (Stouthamer 2001).

In the Rhine-Meuse delta, avulsions were studied by Berendsen (1982), Törnqvist (1993), and most extensively, by Stouthamer (2001). Stouthamer (2001) reconstructed the avulsion history of the Holocene Rhine-Meuse delta (The Netherlands), on a timescale of millenia.

Methods used

• detailed maping and reconstruction of paleogeographic maps (see palaeogeography). Avulsion sites were inferred from the paleogeographic reconstruction. For example, comparison of the palaeogeographic maps of 6000 yr BP and 5000 yr BP (Figure 9), shows, that a major avulsion occurred near the town of Wijk bij Duurstede.
• gradient lines of all channel-belts. These allowed determination of gradients, paleo-flow direction, relative age of channel-belts, and time correlation of undated channel-belt fragments. The younger the channel belt, the higher the gradient line plots in a distance versus elevation diagram (Figure 10).

At least 91 avulsions occurred over the past 10,000 years, of which 82 could be dated with an accuracy of ± 200 14C years.

Figure 9 Major avulsion of the Rhine, approximately 5500 years BP, near Wijk bij Duurstede (Berendsen & Stouthamer 2001).

Figure 10 Gradient lines of the top of sand in three channel belts. The gradient line of the youngest channel belt is highest in the diagram (Berendsen 1982).

Factors influencing avulsion location

The location and shifting of Holocene avulsion sites in the Rhine-Meuse delta in space and time (Figure 11) are related to:

Figure 11 Space-time diagram of avulsions in the Rhine-Meuse delta (Stouthamer 2001). Avulsion locations are mainly determined by: sealevel rise (7500 yr BP until 3000 yr BP), neotectonics (3000 yr BP until 1700 yr BP), increased discharge and sedimentation (2800 yr BP until 1500 yr BP), and human influence (1000 yr BP until the present).

• Relative sealevel rise. In the Early Holocene, avulsions could not take place, because rivers were still incised. Around 7500 yr BP, avulsions occurred in the western part of the present delta as a result of backfilling of the Late Weichselian valley. Between 7500 and 3700 yr BP, the zone where avulsions occurred shifted inland as a result of relative sealevel rise (Figure 11).
• Neotectonics. Between 4900 and 1700 yr BP, the location of avulsion sites seems to have been influenced by neotectonic movements of the upthrown Peel Horst. Four out of six avulsion nodes in the Rhine-Meuse delta were located in the Peel Horst fault zones, and a large number of avulsions occurred on faults.
• Increased discharge and/or within-channel sedimentation. From 2800 until about 1500 yr BP, avulsion sites were located all over the delta (Figure 11). During this time, the number of channels was high, and avulsion frequency reached a maximum, at a time when aggradation rate decreased with a reduction in the rate of sealevel rise. After 2000 yr BP meander wavelength of alluvial channels increased considerably. The increased meander wavelength and the high avulsion frequency are attributed to increased bankfull discharge, or within-channel sedimentation (leading to channel widening), or both.
• Human influence. Between 1000 and 650 yr BP, all the rivers were embanked (Figure 11), and avulsions could no longer take place. The few that occurred were induced by humans. The relative importance of factors influencing avulsions is summarized in Figure 12.

Figure 12 Relative importance of factors influencing the location of avulsions (Stouthamer 2001).

Avulsion parameters

Avulsion parameters are: avulsion duration, avulsion frequency, period of existence, and interavulsion period. These avulsion parameters (Figure 13) greatly influence alluvial architecture, because they determine channel density and interconnectedness. In the Rhine-Meuse delta the number of coeval channels (Figure 14) is related to the avulsion frequency (Figure 15). Instantaneous and gradual avulsions were almost equally important in the Rhine-Meuse delta (Figure 5).

Figure 15 Avulsion frequency determines the number of coeval channels (Stouthamer 2001).

Figure 13 Temporal variation of the most important avulsion parameters in the Rhine-Meuse delta: avulsion frequency, number of coeval channels and period of existence.

Figure 14 Number of coeval channels over time (Stouthamer 2001). The largest number of channels occurred in the western part of the delta, between 6500 yr BP and 4500 yr BP, and between 2500 yr BP and 1000 yr BP. Between 4000 yr BP and 3000 yr BP, the number of channels was very low. This was a period of extensive peat formation.

Figure 16 Interavulsion period of channel belts in various deltas (Stouthamer & Berendsen, submitted). In the Rhine-Meuse delta the interavulsion period fluctuates strongly, but remains on average constant. For the Yellow River and Po River the interavulsion period decreased over time. For the Kosi River it first decreased, then increased again. Note that the amount of data from the Rhine-Meuse delta is significantly greater than for the other river deltas.

Figure 17 Avulsion frequency in the Rhine-Meuse delta during the Holocene (Stouthamer 2001).

Figure 18 Period of activity of channel belts in the Rhine-Meuse delta (Stouthamer 2001).

Avulsion duration

The avulsion duration (Figure 5) fluctuates between less than 200 and 1250 cal years and averages 335 cal years. The avulsion duration shows no significant trend over time and remained constant until at least 1900 cal yr BP. Avulsion duration seems not to be influenced by aggradation rate.

Avulsion frequency

A high avulsion frequency (Figure 17) occurred from 8000 to 7300 cal yr BP (a total of 17 avulsions, i.e., 2.43 avulsions/100 years). During this period the avulsion frequency was related to the high rate of sea-level rise, which induced fluvial sedimentation in the present near-coastal area. After 7300 cal yr BP avulsion frequency decreased as a result of a continuously decreasing rate of sea-level rise. From 7300 to 3200 cal yr BP avulsion frequency was low; 35 avulsions took place within 4100 years (0.85 avulsions/100 years). Approximately 5000 cal yr BP the coastal barriers became closed and large-scale peat formation occurred. This resulted in fixation of the river channels and low cross-valley gradients, reducing the chances for avulsion. Between 3200 and 1400 cal yr BP avulsion frequency was high again (a total of 34 avulsions occurred, i.e., 1.89 avulsions/100 years) as a result of increased discharge and/or within-channel sedimentation, which enhanced chances for avulsion.

Period of activity and interavulsion period

The period of activity of channel belts (Figure 18) shows no significant trend on the time scale of the Holocene. It is highly variable and averages 1280 ± 820 cal yr. Average interavulsion period is shorter than the period of activity of channel belts, and is approximately 945 cal years. In the Rhine-Meuse delta, on the time scale of the Holocene, interavulsion period and avulsion duration are on average constant. Therefore, the number of coeval channels is directly related to avulsion frequency. Available evidence from other rivers and deltas around the world, however, suggests that the relationship between these avulsion parameters is not everywhere the same. Almost 25 % of all avulsions result in reoccupation (Figure 19).

Figure 19 Avulsions and reoccupations of previous channel belts (Stouthamer 2001).

Avulsion deposits

In the Rhine-Meuse delta, the majority of avulsion sites is accompanied by crevasse-splay deposits (formed by breaches of natural levees, Figure 20) with a relatively large areal extent. These crevasse-splay complexes occur near the base of the overbank deposits of the newly formed channel belt (Figure 21) and are similar to 'avulsion belt deposits' described by Smith et al. (1989; 1998). The lithology of the crevasse-splay deposits ranges from coarse sand to silty clay. Sand is concentrated in crevasse splay channels, and silty and sandy clay occurs as distal deposits (Figure 22) and as levee deposits between the channels. The total volume of sand is small, relative to clay and silt.

Figure 20 Formation of a crevasse splay in the Columbia River, British Columbia, Canada (picture by H.J.A. Berendsen).

Figure 21 Cross section of the proximal part of a crevasse splay (Stouthamer 2001). Characteristic is the wide and thin sandbody.

Figure 22 Cross section of the distal part of a crevasse splay (Stouthamer 2001). Characteristic are the narrow and thick sandbodies.

Dating evidence of the abandoned and newly formed paleo-channel is always required to verify if avulsion really occurred, although the presence of large-scale sandy crevasse-splay deposits in combination with two main meandering channels often indicates avulsion (Figure 23). Based on a 2-D outcrop or just one cross section, it is impossible to determine whether a splay complex was associated with an avulsion or not, because the architecture and lithology in both cases can be identical.

Figure 24 Two different types of reoccupations. Type 1: the new channel reoccupies a previously existing channel on the floodplain. Type 2: the new channel reoccupies its own channel further downstream (Stouthamer 2001).

Figure 23 Proximal deposits associated with full avulsion often show crevasse deposits (Stouthamer 2001).

Not all avulsion locations are characterized by crevasse-splay deposits. There are two possible explanations for this: (1) splay deposits were eroded by the newly formed channel, or (2) splay deposits were never formed. This may be a result of reoccupation of an old channel. In this case, avulsive flow is immediately appropriated by the pre-existing channel and no large-scale splay formation occurs (Figure 24). Alternatively, splay deposits were not formed because avulsion resulted from headward erosion.

Alluvial architecture

Figure 25 Alluvial architecture resulting from avulsion (Heller & Paola 1996).

Avulsion is a key process in the formation of deltas and alluvial fans, and strongly influences alluvial architecture (Figure 25): the three-dimensional geometry, proportion and spatial distribution of the various types of alluvial deposits, e.g. channel-belt and overbank deposits, in sedimentary basins. Definition of alluvial architecture requires extensive exposures, and/or high-reolution seismic data, and/or many closely spaced cores or borehole logs, and accurate dating. Because the Rhine-Meuse delta is by far the most detailed studied delta in the world, alluvial architecture data from this sedimentary environment can be used as an analog for other (recent and ancient) deltas in the world.

Alluvial architecture is important in the oil and gas industry, because sandy fluvial deposits often contain natural resources. As detailed data in oil fields are often lacking, it may be necessary to 'fill in' three dimensional space in order to produce a complete (and often hypothetical) representation of alluvial architecture (Bridge 2003). To do this, models are used to interpret and predict alluvial architecture. Most models of alluvial architecture are either process-based (process-imitating, e.g., Mackey & Bridge 1995) or stochastic (structure-imitating). A discussion of the limitations of these models is given by Bridge (2003).

An attempt to improve the models is presently carried out by the fluvial group (Karssenberg, Stouthamer, Bridge, Cohen & Berendsen), while other members of the group study the geometry of sandbodies (Gouw) and the influence of compaction on avulsion (Stouthamer).

Literature

1. Berendsen, H.J.A. & E. Stouthamer (2000), Late Weichselian and Holocene palaeogeography of the Rhine-Meuse delta (The Netherlands). Palaeogeography, Palaeoclimatology, Palaeoecology 161 (3/4), p. 311-335.
2. Berendsen, H.J.A. & E. Stouthamer (2001), Palaeogeographic development of the Rhine-Meuse delta. Assen: Van Gorcum, 270 pp. Enclosures: 3 coloured maps and a CD-ROM. Contents and Summary
3. Berendsen, H.J.A. & E. Stouthamer (2001), Palaeogeographic evolution and avulsion history of the Holocene Rhine-Meuse delta, The Netherlands. Netherlands Journal of Geosciences / Geologie en Mijnbouw 81 (1), pp. 97-112.
4. Bridge, J.S. (2003), Rivers and floodplains. Forms, processes and sedimentary record. Oxford: Blackwell Publishing.
5. Heller, P.L., & C. Paola (1996), Downstream changes in alluvial architecture: An exploration of controls on channel-stacking patterns: Journal of Sedimentary Research, v. 66, no. 2, p. 297-306.
6. 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.
7. Miall, A.D. (1996), The geology of fluvial deposits. Berlin: Springer Verlag, 582 p.
8. Stouthamer (2001), Holocene avulsions in the Rhine-Meuse delta, The Netherlands. Ph. D. Thesis, Utrecht University. Netherlands Geographical Studies 283, 224 p.
9. Stouthamer, E. & H.J.A. Berendsen (2000), Factors controlling the Holocene avulsion history of the Rhine-Meuse delta (The Netherlands). Journal of Sedimentary Research 70 (5), p. 1051-1064.
10. Stouthamer, E. & H.J.A. Berendsen (2001), Avulsion frequency, avulsion duration, and interavulsion period of Holocene channel belts in the Rhine-Meuse delta, The Netherlands. Journal of Sedimentary Research, 71 (4), p. 589-598.
11. Stouthamer, E. (2001), Sedimentary products of avulsions in the Holocene Rhine-Meuse delta, The Netherlands. Sedimentary Geology 145, p. 73-92.

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Design: René Berendsen --- Programming and layout: Arnoud Berendsen --- Scientific content: Henk Berendsen --- Sandwiches and milkshakes: Helen Berendsen --- Last updated: April 14, 2007