Project topics and materials
Get Complete Project Material File(s) Now! »
Bedload dynamics and deposition in abandoned channels using an experimental bifurcation setup
This chapter introduces flume experiments conducted in a bifurcation setup to test controls on channel abandonment (bifurcation angle, initial bed slope, varying free surface slope and the presence of an erodible sand bed) with a specific focus on their effect on sand plug length, volume and deposition processes. The first section (2.1), which is the transcript of an article published in Earth Surface Dynamics (https://esurf.copernicus.org/articles/8/275/2020/), focuses mostly on the bifurcation angle:
SZEWCZYK, L., GRIMAUD, J.-L., COJAN, I., 2020. Experimental evidence for bifurcation angles control on abandoned channel fill geometry. Earth Surface Dynamics, 8, 275-288. https://doi.org/10.5194/esurf-8-275-2020.
The second section (2.2) describes similar experiments that were conducted in the aim of determining the effects of different forcings, i.e. the presence of an erodible sand bed and the modification of the free surface slope in one distributary channel, on the sand plug geometry.
Experimental evidences for bifurcation angles control on abandoned channel fill geometry
Abstract
The nature of abandoned channels’ sedimentary fills has a significant influence on the development and evolution of floodplains and ultimately on fluvial reservoir geometry. A control of bifurcation geometry (i.e., bifurcation angle) on channel abandonment dynamics and resulting channel fills, such as sand plugs, has been intuited many times but never quantified. In this study, we present a series of experiments focusing on bedload transport designed to test the conditions for channel abandonment by modifying the bifurcation angle between channels, the flow incidence angles and the differential channel bottom slopes. We find that disconnection is possible in the case of asymmetrical bifurcations with high diversion angle (≥ 30) and quantify for the first time an inverse relationship between diversion angle and sand plug length and volume. The resulting sand plug formation is initiated in the flow separation zone at the external bank of the mouth of the diverted channel. Sedimentation in this zone induces a feedback loop leading to sand plug growth, discharge decrease and eventually to channel disconnection. Finally, the formation processes and final complex architecture of sand plugs are described, allowing for a better understanding of their geometry. Although our setup lacks some of the complexity of natural rivers, our results seem to apply at larger scales. Taken into account, these new data will improve fluvial (reservoir) models by incorporating more realistic topography and grain size description in abandoned channels.
Introduction
Abandoned channels are ubiquitous features of the alluvial plain, which have a huge impact on the fluvial system evolution and properties. First, abandoned channels form local topographic lows that trap sediments (Aalto et al., 2008; Lauer & Parker, 2008; Dieras et al., 2013) and host wetlands (Novitzky et al., 1996; Ward et al., 1999). Second, the fine-grain fraction of their filling may influence active channels migration, as clays are more resistant to erosion than sandy sediments (Howard, 1992; Smith et al., 1998; Berendsen & Stouthamer, 2000; Schwendel et al., 2015). Last, abandoned channels are filled with sediments of varied permeability, which may impact flow path in active alluvial plains (Flipo et al., 2014) and ultimately in the resulting geological reservoir (Miall, 1996; Willis & Tang, 2010; Colombera et al., 2017; Cabello et al., 2018). Indeed, recent studies have shown that sedimentary fills are complex bodies and may contain coarser sediments than initially assumed (Hooke, 1995; Toonen et al., 2012; Dieras et al., 2013). When integrated to reservoir flow simulations, these coarse deposits may drastically change the connectivity of otherwise isolated sand bodies (e.g., point bars; Donselaar & Overeem, 2008).
Currently, abandoned channels are studied on the field (Hooke, 1995; Constantine et al., 2010a; Dieras et al., 2013) but less so in numerical models and experiments. Different styles of abandonment are observed in fluvial systems (i.e., cutoffs, avulsions), implying the formation of sedimentary fills of various grain sizes and geometries (Allen, 1965; Toonen et al., 2012; Fig. 2.1). A common thread to existing models is that abandonment is the consequence of the formation of a wedge-shaped sand plug in one of two channels shortly after a bifurcation (Fig. 2.1). Disconnected channels are then mostly filled by fine-grained overbank flood sediment (Bridge et al., 1986; Plint, 1995; Bridge, 2003). The coarse deposits are introduced beforehand as bedload, i.e., as long as there is a connection with the active channel. The dynamics at the bifurcation during the disconnection phase have therefore a key control on the sediment architecture of later abandoned channels (Bertoldi, 2012; Bolla Pittaluga et al., 2015; Constantine et al., 2010a; Kleinhans et al., 2013).
Figure 2.1: 3-D sketch showing the occurrences of deposits associated with abandoned channels in an alluvial plain.
Based on field studies, the geometry of the bifurcation, particularly the upstream bifurcation angle, is thought to control the duration of (dis)connection and therefore sand plug accretion and geometry (Fisk, 1947; Shields et al., 1984; Shields & Abt, 1989), but most authors agree that bifurcations remain overlooked in alluvial plains (Constantine et al., 2010a; Kleinhans et al., 2013).
Existing numerical and experimental studies focus on the parameters controlling discharge and sediment partitioning at bifurcation (Bulle, 1926; de Heer & Mosselman, 2004; Kleinhans et al., 2008, 2013; Salter et al., 2018, 2019) and bifurcation (in)stability (Bertoldi & Tubino, 2007; Bolla Pittaluga et al., 2003, 2015; Iwantoro et al., 2019). To our knowledge, no study currently exists that focuses specifically on quantifying the condition(s) for abandoning channels at a bifurcation and on the resulting sediment architecture.
In this work, we study experimentally and quantify for the first time the influence of bifurcation geometry, specifically the diversion angle, on fluvial channel abandonment. We focus on (1) abandonment potential and the associated processes and (2) the extent and geometry of the sedimentary bodies formed by bedload deposition in abandoned channels, i.e., sand plugs and sandbars.
Methods
Experimental design
Experiments were carried out in the Geomorphic Lab of the Centre de Géosciences of MINES ParisTech, Fontainebleau. A modular flume with fixed walls was built. It was composed of three branches: one inlet and two distributary channels connected through a bifurcation area (Fig. 2.2). Each channel had a width W of 4 cm and a length of about 75 cm. The global slope of the experiment was 1.48% while the slopes in distributaries 1 and 2, respectively, S1 and S2, varied with the configuration (Table 2.1). The bifurcation area was modular to allow different angles between the inlet and the distributary channels. Three angles were considered (Table 2.1, Fig. 2.2). The bifurcation angle α was the angle between the two distributaries. The incidence angles β1 and β2 were the angles between the inlet channel and the stream-left and -right distributaries, respectively (Fig. 2.2). When β1 = 0, β2 = α and corresponded to a diversion angle, as usually defined.
The experiments started in an empty flume and typically lasted 90 to 100 min. Input water and sediment discharges were constantly fed at rates of 300 and 0.6 Lh-1, respectively. Both fluxes were calibrated beforehand to allow formation of sedimentary structures without filling the flume too quickly, allowing observations to be made. The water was delivered through a head tank to reduce turbulence in the incoming flow. The water was dyed in blue using food colorant in order to enhance contrast in pictures. The sediment was a well-sorted, rounded to sub-angular, fine (d50= 209 μm) Fontainebleau sand. Sediment traps allowed quantifying the volume of sediment that bypassed the distributaries.
Figure 2.2: Overhead view of the experimental setup together with the different angles considered and the levee breach setup.
Table 2.1: List of experiments and associated parameters.
Figure 2.3: Evolution of the experiments. (a-c) Overhead pictures of the setup for the symmetrical (a) and asymmetrical configurations without (b) and with levee breach (c). (d-f) Evolution of water discharge measurement at the output of the distributary channels for symmetrical (d-e) and asymmetrical (f-g) configurations. (g) Asymmetrical 90° configurations with varying slope ratios S2/S1.
A total of 16 experiments were designed to explore the influence of bifurcation angle α values ranging from 30 to 90° and β1 and associated β2 values ranging from 0 to 90° (Exps. 1 to 16) (Table 2.1). A first set of four symmetrical (β1 = β2 = 1/2α, Exps. 3, 7, 11 and 14) (Fig. 2.3a) and five asymmetrical (β1 ≠ β2, Exps. 1, 5, 9, 12 and 15) (Fig. 2.3b) configurations was built. A second set of seven experiments (Exps. 2, 4, 6, 8, 10, 13 and 16) replicated the configurations of the first set (except for Exps. 11 and 14) with the addition of a removable wall placed at the entrance of distributary 1 parallel to the orientation of distributary 2 (Figs. 2.2 & 2.3c) (Appendix A). The wall was removed after the system had reached equilibrium (identical input and output sediment discharges) to simulate a levee breach (Fig. 2.3c). In all asymmetrical configurations (except for Exps. 1 and 2), distributary 1 was straight (β1 = 0) so that β2 was a diversion angle (Table 2.1).
A final set of three experiments (Exps. 17, 18 and 19) was designed to determine if the observed effects of a given diversion angle could be counterbalanced by a slope variation in the deviated distributary. The experiments had the same planar geometry as Exp. 15 (α = β2 = 90°, slope ratio S2/S1 = 0), but the slope at the bottom of distributary 2 was modified so that three additional different slope ratios S2/S1 could be tested (i.e., respectively, 0.68, 0.87 and 1.73 in Exps. 17, 18 and 19).
Data acquisition
Free surface elevation was periodically measured in all channels. Water discharge was measured out of the two distributaries using a system similar to that of Salter et al. (2019). Water was flowing out of the channel into a cylinder with a hole at the bottom small enough to allow variation of the level in the cylinder. The weight evolution was measured within the cylinder using a digital scale and converted into discharge using a calibration curve. Figure 2.3d, e, f and g show the resulting water discharge partitioning for experiments without removable wall. Pictures of the flume were taken every minute by an overhead camera to observe sand bodies’ formation and measure their length. The sand body’s total length, i.e., including both subaerial and submarine parts, was measured from pictures. Sand plug construction was reported by increments of 15 min for asymmetrical configurations without levee breach (Fig. 2.4). Sand plug length was measured at the last location where it extended over the whole channel width and at its downstream limit. In the following, the mean of these two measurements is used to speak of sand plug length (Fig. 2.4). The final digital elevation models (DEMs) of the deposits were computed from 3-D photogrammetric surveys taken by two cameras mounted on a mobile rail using the Agisoft PhotoScan Professional v1.4 software (Fig. 2.5). The DEMs (precision of 0.4 to 0.5 mm) were used to produce mean longitudinal elevation profiles and to measure the longitudinal slope of sediment deposited in disconnected channel 2 (Fig. 2.6). Finally, sand plug volume calculated from DEMs and sand plug length L– divided by channel width W – were compared to bifurcation angles and slope ratios (Fig. 2.7).
Table of contents :
Abstract
Résumé
Acknowledgments
Contents
List of Figures
List of Tables
Introduction
Chapter 1: Channel abandonment and infilling – Literature review
1.1. Introduction
1.2. Fluvial systems
1.3. Floodplain dynamics and resulting sedimentary facies
1.3.1. Channel migration deposits and associated facies
1.3.1. Overbank deposits and associated facies
1.3.3. Relation to abandoned channels
1.4. Channel abandonment processes
1.4.1. Avulsions
1.4.1.1. Avulsion types
1.4.1.2. Avulsion initiation
1.4.1.3. Avulsion styles
1.4.2. Chute cutoffs
1.5. Current models for abandoned channels fill
1.5.1. Channel fill nature and geometry
1.5.2. Current knowledge on processes controlling channel infilling
1.5.2.1. Regional controls on channel abandonment and fill
1.5.2.2. Bifurcation dynamics and its control on disconnection
1.5.2.3. Closed-stage channel filling processes
1.6. Conclusion
Chapter 2: Bedload dynamics and deposition in abandoned channels using an experimental bifurcation setup
2.1. Experimental evidences for bifurcation angles control on abandoned channel fill geometry
2.1.1. Abstract
2.1.2. Introduction
2.1.3. Methods
2.1.3.1. Experimental design
2.1.3.2. Data acquisition
2.1.4.2. Sand plug formation dynamics and architecture
2.1.4.3. Controls on sand plug length and volume
2.1.5. Discussion
2.1.5.1. Bifurcation angle control on abandonment
2.1.5.2. Bifurcation angle control on sand plug extent
2.1.5.3. Mechanism for channel abandonment
2.4.5.4. Comparison with field cases and upscaling
2.1.5.5. Sand plug architecture integration to reservoir modelling
2.1.6 Conclusion
2.2. Influence of additional forcing parameters on plug geometry
2.2.1. Erodible sand bed
2.2.2. Differential base level and associated backwater dynamics
2.2.2.1. Experimental design
2.2.2.2. Results
2.2.3. Interpretation
2.3. Conclusion
Chapter 3: Experimental abandonments in curved channels geometries: insights on the architecture of cutoff bedload fills
3.1. Introduction
3.2. Methodology
3.2.1. Experimental design
3.2.2. Data acquisition
3.3. Results
3.3.1. Bedload deposits in the case of local avulsions
3.3.2. Effects of cutoff channel width on bifurcation (un)stability
3.3.3. Bedload deposits in the case of cutoff incision
3.3.4. Controls on channel fill architecture
3.4. Discussion
3.4.1. Geometrical controls on channel equilibrium
3.4.2. Deposition process and architectural elements of abandoned channel fills
3.4.3. Controls on bedload fill deposits geometry and volume
3.1.4. Experimental predictive model for channel plug length and volume
3.5. Conclusion
Chapter 4: Gravel fill dynamics and depositional patterns in chute cutoffs channels of a bedload dominated river: the Ain River, France
4.1. Abstract
4.2. Introduction
4.3. Study area
4.3.1. Geomorphic setting
4.3.2. Studied channels
4.4. Methods
4.4.1. Channel fill sedimentary bodies mapping
4.4.1.1. DEM
4.4.1.2. Aerial pictures analysis
4.4.1.3. Observations and data collection on the field
4.4.2. Estimation of the bedload deposits in the abandoned channel
4.5. Results
4.5.1. Channel facies description and interpretation
4.5.2. Channel fill architecture in the studied channels
4.5.2.1. Open-stage channel: the CHA site
4.5.2.1. Closed-stage channels
4.5.3. Geometrical controls on planar geometry and volume of channel fills
4.6. Discussion
4.6.1. Depositional processes of the bedload fill
4.6.2. Grain-size changes associated with plug formation
4.6.3. Geometry, extent and volumes of channel plugs
4.6.4. Cutoffs initiation and bifurcations stability
4.7. Conclusion
Chapter 5: Long-term preservation of coarse-grained fill deposits: tests on field observations
5.1. Introduction
5.2. Reconstruction of a pluri-centenal abandoned channel fill architecture: the Vimpelles avulsion channel (Seine, France).
5.2.1. Geographical and geological contexts
5.2.2. Methods
5.2.3. Channel fill present architecture
5.2.4. Coarse-grained fill deposits long-term preservation
5.2.4.1. Evidences of several phases of activity
5.2.4.2. Recent moderate-energy reworking events
5.3. Coarse-grained fill deposits preservation in several systems
5.3.1. Studied abandoned channels
5.3.1.1. The Boire Torse channel
5.3.1.2. The Rijnstrangen channel
5.3.2. Results and discussion
5.4. Cutoff channel plug length prediction using aerial pictures
5.4.1. Study area
5.4.2. Channel selection and measurement method
5.4.2.1. Data source
5.4.2.2. Channel selection
5.4.2.3. Measurement method and uncertainties
5.4.3. Results and discussion
5.5. Conclusion
Chapter 6: Conclusion and perspectives
6.1. Main contributions of the PhD
6.1.1. Experimental results
6.1.2. Comparison with field examples
6.2. Future work and perspectives
6.2.1. Experimental setup improvement
6.2.2. Complexification of the flume experiments
6.2.3. Transitional phase and heterolithic deposits study
6.2.4. Channel fill under tidal influence
References
