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
(
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 and Parker, 2008; Dieras et al., 2013) and host wetlands (Novitsky 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 and 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 and 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 and Overeem, 2008).
3-D sketch showing the occurrences of deposits associated with abandoned channels in an alluvial plain.
Currently, abandoned channels are studied on the field (Hooke, 1995; Constantine et al., 2010; 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. 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. 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., 2010; Kleinhans et al., 2013).
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 and Abt, 1989), but most authors agree that bifurcations remain overlooked in alluvial plains (Constantine et al., 2010; Kleinhans et al., 2013).
Existing numerical and experimental studies focus on the parameters controlling discharge and sediment partitioning at bifurcation (Bulle, 1926; de Heer and Mosselman, 2004; Kleinhans et al., 2008, 2013; Salter et al., 2018, 2019) and bifurcation (in)stability (Bertoldi and 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.
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). Each
channel had a width
Overhead view of the experimental setup together with the different angles considered and the levee breach setup.
List of experiments and associated parameters.
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 L h
Evolution of the experiments.
A total of 16 experiments were designed to explore the influence of bifurcation
angle
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 (
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 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. 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. 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. 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. 6).
Finally, sand plug volume calculated from DEMs and sand plug length
Planar growth of sand plug from overhead pictures.
Final topography of Experiment 5 showing the active (1) and
disconnected (2) channels.
Mean longitudinal elevation profiles of the sand plugs for
increasing
Relationships between sand plug length and volume, and the
incidence angle
A sediment transport law was calibrated to compare the experimental results
with theory using the methods of Seizilles et al. (2013) and Delorme et al. (2017).
A series of runs were carried out with constant water and sediment feed
rates in a 3 cm wide flume. The experiment was repeated with different
sediment discharges
In experiments where a disconnection was observed,
Each experiment began with a short (15–20 min) phase of progradation of the sediment down to the bifurcation point. The sediment started forming a sandbar downstream of the sediment feeder and then split into alternate bars that migrated through the inlet channel. Once the sediment reached the bifurcation, it was partitioned into the distributaries, except when a removable wall was present. After an adjustment period, water discharge was considered at equilibrium when it remained constant – or slightly varied around a constant value – in each channel (Fig. 3). The associated water partitioning could be equal (i.e., identical in both branches) or unequal (i.e., different discharges in both branches). Sediment bypassing each distributary channel at equilibrium was roughly proportional to discharge partitioning. A distributary channel was considered disconnected when no bedload movement was observed. Usually, such disconnection occurred before water discharge equilibrium was attained (Fig. 3f and g).
In the case of symmetrical bifurcations (Fig. 3a, d and e), no
disconnection occurred. For low bifurcation angle (
For high bifurcation angles (
All asymmetrical configurations reached equilibrium with unequal water
discharge partitioning (Fig. 3f and g). In Exps. 1 and 2 (
Levee breach experiments reached final equilibrium 5 to 10 min after distributary 1 was opened. Their final equilibrium state was very similar to that of the experiments made without levee breach in the same configuration (Fig. 3b and c). In asymmetrical experiments with levee breach (Exps. 6, 10, 13 and 16), sediment remained in transit through distributary 2 before the opening of distributary 1. This allowed the deposition of alternate bars (Fig. 3c) in distributary 2. In the asymmetrical experiments without levee breach (i.e., Exps. 5, 9, 12, 15, 17, 18 and 19), all sediment that entered distributary 2 was deposited to form a sand plug (Figs. 3b and 5b).
Figure 3g shows discharge partitioning under varying slope ratios
During sand plug formation, discharge gradually decreased in distributary channel 2, until a constant value was attained (Fig. 3f) and no more sediment motion was observed. Sand plug growth processes and final architecture were very similar in all these experiments, regardless of the incidence angle (Fig. 4).
The formation of the sand plug initiated on the external side of the diverted channel, immediately after the bifurcation (Fig. 4). Visual inspection showed that in this area flow velocity was the lowest, allowing the deposition of sand that initiated the first fixed sandbar. This first bar anchored to the external bank of the channel and quickly grew downstream and towards channel centerline (Fig. 4a). It then widened and lengthened until its growth stopped. Other bars then formed from the sides of previously formed sandbar(s) and stretched downstream, resulting in a composite sand plug (Figs. 4a and 8). When sand was deposited over the entire width of the channel, a thalweg formed across the sand plug. It allowed sediment transfer downstream until it was buried by a sandbar that disconnected the whole channel. Overall, a slight decrease of sand plug growth speed with diversion angle is observed.
In the case of levee breach experiments (Exps. 6, 10, 13 and 16), alternate bars formed in distributary 2 before the opening of distributary 1 (Fig. 3c). After the levee breach, the discharge abruptly decreased in distributary 2, initiating the rapid (2–5 min) formation of the sand plug. In the meantime, a transient knickpoint formed along the inlet channel to allow for bed slope adjustment. Parts of the existing alternate bars at the bifurcation were thus reworked (Fig. 8). A small plug, mostly consisting of the reworked deposits, formed rapidly at the entrance of distributary channel 2.
Sand plug long slope increased with the diversion angle (from 2.5 % to
7.8 %; Table 1). This was the most visible in the no-forcing scenarios, as
there was no interference from previously deposited alternate bars (Fig. 6a).
Bars after the plug itself showed the shallower slopes (Fig. 6b). In
the case of a levee breach, the sand plug slope was slightly steeper when the
diversion angle was lower than or equal to 45
Relationships between incidence angle
Without levee breach, the sand plug was the only bedload deposit in the
disconnected distributary (Exps. 5, 9, 12 and 15) (Figs. 3b and 5b). Its
volume steadily decreased with
In this study, channel disconnection was possible for highly asymmetrical
bifurcation (i.e.,
In symmetrical experiments, water discharge shifted from a soft avulsion
regime with roughly equal partitioning (e.g., Salter et al., 2018) to an
unequal partitioning as bifurcation angle
Numerous field studies intuited a relationship between the incidence angle and the length of the sand plug, stating that a low incidence angle produced a longer sand plug (Fisk, 1947; Allen, 1965; Gagliano and Howard, 1984; Shields et al., 1984; Shields and Abt, 1989; Dieras et al., 2013). This was explained by the fact that bedload is easily diverted into the channel at low angles and on a longer distance before deposition. With increasing bifurcation angles, a smaller fraction of the bedload enters the channel, resulting in shorter and smaller sand plugs. Our experiments show that bedload partitioning is proportional to discharge partitioning and that final discharge partitioning is controlled by the diversion angle (Fig. 3). As a result, sand plug volumes and lengths linearly decrease with diversion angle (Figs. 7a–b). Sand plug volumes and lengths are also modulated by the slope of the abandoned channel (Fig. 7c). Sudden events such as levee breaching create shorter and less voluminous sand plugs, as less bedload material is mobilized to build the sand plug due to the faster disconnection (Fig. 7a and b).
The fast disconnection and limited sand plug length and volume observed in the case of a levee breach (Fig. 7a and b) can also be explained by the rapid entrenchment of the flow in channel 1. The newly opened channel 1 has a slope advantage as no aggradation occurred during the first phase of the experiment. As the result of incision and knickpoint retreat, a threshold that prevents part of the bedload to enter distributary 2 is created (Slingerland and Smith, 2004) and the flow is preferentially funneled in the distributary channel 1. It is worth noting that the effect of levee breaching on sand plug length and volume decreases slightly when the incidence angle increases (Fig. 7a and b), with a slight convergence towards higher angles. The slight difference in regression line values between the no-forcing and levee breach scenarios also hints that although the sand plug extent and volume are controlled by the diversion angle, different triggers might initiate sand plug formation depending on the scenario.
As bedload partitioning varies with discharge partitioning in our setup, a
smaller fraction of bedload enters the deviated distributary and this
fraction diminishes in time with discharge, until disconnection. In a flume
with comparable geometry, Bulle (1926) measured the bedload partitioning in
distributaries and found that around 90 % of the bedload was steered in
the deviated distributary for angles ranging from 30 to 150
Beyond the influence of the bifurcation angle, other mechanisms have been invoked to explain channel disconnection: the Shields number of the system upstream of the bifurcation and its effects on the aspect ratio (Wang et al., 1995; Bertoldi et al., 2009; Bolla Pittaluga et al., 2015) and bars' presence (Bertoldi and Tubino, 2007; Bertoldi, 2012), as well as the sinuosity upstream of the bifurcation and the slopes in each distributaries (Kleinhans et al., 2013; Van Dijk et al., 2014). Based on the results of this study, one may argue that it is the difference of channel bed slope associated with the bifurcation geometry – and not the bifurcation geometry itself – that leads to abandonment. The experiments presented in Fig. 3g show that this is not the case in the experiments with asymmetrical bifurcations, as a high diversion angle is a sufficient condition for hydraulic disconnection. Having a reasonable slope advantage to the diverted channel does not reverse the final outcome; it only affects the time needed to disconnect the channel and the extent of the deposits.
Another proposed mechanism is that low discharge in one channel induces
sedimentation, which would in turn further reduce the discharge in the
channel by plugging it or by changing the bed slope, creating a feedback
loop leading to disconnection (Zolezzi et al., 2006; Bertoldi, 2012). Based
on this study, this is discounted as abandonment would have been observed
when discharge partitioning was unequal from the beginning in symmetrical
configurations with incidence angles above the 30
Eddies were observed on the external bank of the channel, immediately
downstream of the bifurcation, in which water was slowed and sediment
deposited. Bulle (1926) observed that these eddies were produced by flow
separation at the bifurcation. These eddies were also found in numerical
modeling by de Heer and Mosselman (2004) and van der Mark and
Mosselman (2012), and observed by Constantine et al. (2010) on the Sacramento River
and were named the “flow separation zone”. The latter authors observed that the width of the flow
separation zone increases with the incidence angle value. In our
experiments, the wider flow separation zone in the diverted channel led to
the formation of the first stage of the sand plug against the external bank
of the diverted channel just downstream of the bifurcation (Fig. 4). This in
turn led to a further reduction of the flow diverted in the channel,
favoring deposition and growth of the sand plug, until the channel got
disconnected. At low values of
Although our model represents a very simplified setup, it has been purpose built to study the influence of bifurcation geometry on disconnection and on bedload deposits partitioning and geometry in abandoned channels. As such, it complements previous studies focusing on equilibrium configurations relative to water and sediment flows (Bolla Pittaluga et al., 2003, 2015; Bertoldi and Tubino, 2007; Edmonds and Slingerland, 2008; Bertoldi et al., 2009; Salter et al., 2018, 2019). River channel section and slope are thought to adjust to water discharge (see Métivier et al., 2017 and references therein). For instance, Lacey's law states that river width scales with the square root of water discharge (Lacey, 1930). Both the equilibrium slope of the inlet channel and the slopes of disconnected channels observed in the experiment are consistent with the threshold theory and remain within the uncertainty range (Fig. 8b). However, in the case of a levee breach, equilibrium slopes are gentler than the theoretical equilibrium slope, and in the case of no-forcing scenarios, slopes are steeper. These deviations would confirm that abandonment trigger was dominated by channel entrenchment in the former case and by plug construction in the latter. As our experimental observations are compatible with Lacey's law and the threshold theory, even in such constrained settings (i.e., fixed wall), they are likely to apply in nature.
An intermediate scale between our flume experiment and natural systems is
the irrigation system channels, which also have fixed width. Intake plugging
is an issue that many water management engineers face, and the question of
optimal diversion angle value has been intensively studied. Novak et al. (1990)
and Munir (2011) state, for instance, that a 30 to 45
In natural cases, avulsion and disconnection are usually accompanied by the enlargement of the new dominant channel path (Kleinhans et al., 2008) and eventually change of bifurcation angle with time (Bertoldi, 2012). Both impact sand plug construction. In our experiments, channel walls were fixed and the dominant flow immediately occupied a channel equal in width to the inlet channel. As a result, disconnection rates were likely faster than when the channel could erode its bank to adjust its shape, similarly to natural systems. Such delay would probably favor the building of larger sand plugs in abandoned channels. To our knowledge, no relationship between diversion angle and sand plug length has been quantified on the field yet. Constantine et al. (2010) found no significant correlation between the incidence angle and the emerged length of the sand plug based on aerial photographs. However, they did find a negative correlation between incidence angle and gravel fill depth below water measured at the apex of abandoned meanders. Such measurements are more comparable to our results (i.e., taking into account the submerged part of the sand plug) and seem to be adequate when investigating on the field for a relationship between incidence angle and bedload deposits. The fact that the subaerial plugs length in abandoned channels is not related to bifurcation angles may be due to fine-grain deposition over the sand plug after disconnection. More field investigations would be required to test this hypothesis.
The sand plug formation processes observed experimentally in this study are the same independently of the bifurcation geometry and occurrence or absence of levee breach (Figs. 4 and 9). Sand plugs are not simply sediment wedges deposited at the mouth of the disconnected channels (Fisk, 1947; Allen, 1965) but complex bedload features formed by bars amalgamation (Fig. 9). Sand plugs have a major slope break separating the thicker upstream part that actually plugs the channel and the downstream part and lateral width variation on the downstream part (Fig. 9). Inherited topography affects the sand plug final architecture (Fig. 6b). For instance, alternate sandbars may be found isolated in the channel. During disconnection, the sand plug may rework some of the previously deposited bars to form transitional bars (Figs. 3c and 9). Together, they form a consistent coarse-grain plug that extends inside the paleo-river path.
Conceptual architecture of sand plug derived from experimental
observations: overhead view
In natural cases, the successive episodes of construction would imply the presence of grain-size variations in the internal structure of the sand plug. These heterogeneities would include permeability baffles formed at the interface between sandbars during low energy phases and possible erosional surfaces formed during phases of high energy, increasing permeability. In these cases, the internal structure of a sand plug could be comparable to that of a complex point bar (Deschamps et al., 2012; Cabello et al., 2018) and thus form a good reservoir in itself.
Finally, the presence of bedload deposits in disconnected channels
demonstrates how the abandoned channel fills could be connectivity bridges
rather than permeability barriers, especially in the upstream part of
abandoned channels (Larue and Hovadik, 2006; Donselaar and Overeem, 2008).
This could have a significant impact on fluvial river models. Indeed, common
models for meandering channels migration (e.g., Parker et al., 2011) usually
assume that abandoned channels are 100 % filled by mud plug, with
consequences on the erodibility of alluvial plains and thus channel
migration (Howard, 1996). Using a dependence of sand plug length and volume
on diversion angles will influence the overall connectivity in reservoir
modeling (i) at the scale of channel fills and potentially (ii) at the scale
of channel belts. For instance, sand plug lengths vary between 5 and 12
Based on the series of experiment designed to force channel abandonment
under constant water and sediment discharge, we find the following:
Above a diversion angle threshold of 22.5 Sand plug length and volume linearly decrease with the diversion angle. The incidence angle controls the width of the flow separation zone in
the diverted channel. When the incidence angle is high enough, sand plug
formation begins in this zone and its presence creates a feedback loop
leading to further deposition until disconnection occurs. The sand plug is a complex structure formed by amalgamated and
interconnected sandbars of various lengths, widths, elevation and slope,
which may increase the connectivity of fluvial reservoirs, in particular
between otherwise isolated point bar deposits.
Movies of the experiments and topography data (point clouds) of the final state of the experiments are available in the OSF data repository (
The supplement related to this article is available online at:
LS and JLG built the flume and designed the experiments with input from IC. LS carried out all experiments. All authors contributed to writing the manuscript.
The authors declare that they have no conflict of interest.
We are indebted to Aurélien Baudin, Cyril Leipp, Yasmina Habaoui, David Marquez and Loic Marlot for their help during the building of the flume. We thank Joël Billiotte, Cyril Castanet, François Métivier, Gerard Salter and Damien Huyghe for fruitful discussions on this work, as well as Richard Hale and John Shaw for their positive reviews. We also thank Lisanne van Rijn for her careful review of our English. This work is part of the first author's PhD thesis and the FLUMY program for channelized reservoirs.
This research has been supported by the Institut du Carnot M.I.N.E.S. (grant no. 2017-1700557).
This paper was edited by Paola Passalacqua and reviewed by Richard Hale and John Shaw.