A natural levee is a typical wedge-shaped deposit adjacent to a river channel. Given its location and distinctive features, the levee can serve as a key to revealing depositional processes of the coupled channel to floodplain system preserved in the rock record. Levee–floodplain topographic evolution is also closely linked to river avulsion processes which can cause catastrophic floods. Nonetheless, the levee geometry and its aggradation pattern on the floodplain have not been fully incorporated in the study of avulsion. Here, we present a levee-building model using advection settling of suspended sediment to reproduce the evolution of a fluvial levee over floods and to examine the effects of boundary conditions on levee geometry and the grain-size trend of the levee deposit. We further investigate river avulsion frequencies and potential channel reoccupation associated with the grain-size distribution of overbank sediment flux and the overflow velocity into the floodplain, both of which can control the levee geometry, especially the aggradation rate at the levee crest. In the modeling results, the levee develops (1) a concave-up profile, (2) an exponential decrease in grain size of the deposit away from the main channel, and (3) a relatively steeper shape for coarser sediment supply and vice versa. The subsequent scaling analysis supports that the input grain size to the floodplain and levee profile slope are positively correlated with the avulsion frequency, whereas the overflow velocity is inversely proportional to the avulsion frequency. In connection with the avulsion styles and levee geometry, we suggest that relatively steeper levee slopes tend to promote more reoccupations of preexisting floodplain channels as protecting abandoned channels from topographic healing, but relatively gentler levees are likely to create a new avulsion channel as their remnant channels are more vulnerable to the removal of topographic memory. The insights drawn from the current modeling work may thus have potential implications for reconstructing paleoenvironments in regard to river sediment transport and flood dynamics via levee deposits. Based on the roles of natural levees on the avulsion frequency and channel reoccupation, the flood hazards triggered by river avulsions as well as the alluvial architecture in sedimentary records can be better assessed.
During floods, rivers overflow into floodplains, facilitating the deposition of suspended sediment due to the loss of flow competence and transport capacity. The reductions in competence and capacity are responsible for the decreases in depositional rate and grain size of sediment away from the rivers and form distinctive wedge-shaped natural levees along the channel margins. Because of their unique location at the boundary, levees may represent an important linkage between the mainstream and overbank processes (Allen, 1965; Brierley et al., 1997; Wolman and Leopold, 1957). According to Brierley et al. (1997), this linkage between the channel and floodplain facies could further play a critical role in assessing river types that cover the geometry, size, and distribution of the channel deposits in stratigraphic records.
Despite the geomorphologic and stratigraphic importance of levee deposits, there have been a limited number of publications on fluvial levee depositional processes. Indeed, a few early numerical models of overbank suspended sediment transport and depositional processes have been carried out. These models include James (1985) and Pizzuto (1987), which have been a great aid in comprehending the mechanisms of overbank configuration (e.g., floodplain topography) by quantifying the depositional patterns of suspended sediment and grain-size distribution across the channel margin to the floodplain. Similarly, some studies have probed the detailed evolution of levee geometry based on the field investigations of natural levees (Adams et al., 2004; Cazanacli and Smith, 1998; Ferguson and Brierley, 1999; Filgueira-Rivera et al., 2007; Gugliotta et al., 2018; Johnston et al., 2019; Pierik et al., 2017; Skolasiñska, 2014; Smith and Pérez-Arlucea, 2008). Cazanacli and Smith (1998) described the geometry and lateral grain-size distributions of fluvial levees, where the differences in levee shape and slope are attributed to non-uniform deposition of coarse overbank sediment. Ferguson and Brierley (1999) stated that the stream power determined by valley width is essential for levee accretion and floodplain stripping, and thus the preservation potential of levee deposits. Recent work by Pierik et al. (2017) found that the dimensions of levees and their changes in time are associated with both environmental forcing (e.g., suspended sediment influx and flood intensity) and initial geomorphic conditions (e.g., flood basin configurations). Even with the earlier findings, there is still a need to ascertain the primary driver of levee geomorphology to accurately delineate and interpret field data of modern river systems and ancient fluvial records.
Moreover, there have been no attempts to establish a fluvial levee-building model accounting for the river avulsion processes by measuring depositional patterns of levee deposits, which would be also an important step forward to understanding the influence of natural levees in connecting in-channel and floodplain evolution. River avulsion, an abrupt relocation of a river from an established channel to a fresh or formerly abandoned channel, is one of the important processes for river dynamics and fluvial stratigraphy. It has long been known that the channel perching, in which the flow potential energy would increase as a result of the in-channel bed and levee crest aggradations, leads to lateral instability and consequently channel avulsion (Bryant et al., 1995; Imran et al., 1998; Mohrig et al., 2000). After multiple floods, levee deposits can simultaneously be accumulated as much as the in-channel deposits and serve as a local superelevation, the relief between levee crest and minimum low point of the nearby floodplain (Bryant et al., 1995; Heller and Paola, 1996; Mohrig et al., 2000). The study by Mohrig et al. (2000) suggested that comparing channel depth with levee crest height (i.e., normalized superelevation) can be regarded as an avulsion criterion. It is traditionally thought that the river would avulse when the levee crest height reaches approximately one channel depth. The aggradation rates of both local superelevation and in-channel bed can be governed by the water and sediment supplies in the main channel and overbank flows, changes in flood regime, and other factors including cohesion and vegetation type, which in turn impact the temporal and spatial patterns of avulsions (Chadwick et al., 2020; Ganti et al., 2016; Mohrig et al., 2000; Stouthamer and Berendsen, 2001; Tooth et al., 2007). Some researchers have recently proved that even climate change and anthropogenic effects such as land use and deforestation, can enforce dramatic changes in channel avulsion behaviors and increase the possibilities of catastrophic flooding disasters in densely populated communities near river systems (Chadwick et al., 2020; Mishra and Sinha, 2020; Pearce, 2021; Slingerland and Smith, 2004). Yet, the understanding of river avulsion associated with floodplain architecture, especially with levee morphology is still insufficient. With these concerns in mind, it would be a great task to unravel the relationship between river switching and levee-building processes linked to the flood conditions for predicting modern avulsion processes and diminishing the threats posed by avulsion flooding events (Hajek and Wolinsky, 2012; Valenza et al., 2020).
In this study, we develop an advection-settling, suspended sediment transport model to quantitatively determine what are the main controls on the geometry, depositional rate, and grain-size sorting in fluvial levee evolution during flooding. In particular, previous studies of levee formations (e.g., Cazanacli and Smith, 1998; Filgueira-Rivera et al., 2007; Hudson and Heitmuller, 2003) highlighted that levee morphology is determined by flood-basin dynamics, such as overflow hydraulics, the grain-size distribution of suspended sediment, and the maximum channel water stage. Therefore, to investigate the flood-basin dynamics under simplified conditions, a total of five tests are designed in which the overflow velocity, grain size, entrainment, and the flood level condition are varied. We then explore river avulsion frequencies and channel reoccupation associated with the levees constructed differently under the various input sediment grain sizes and flood-flow discharges and demonstrate the contributions of levee deposits to the mechanisms governing the river avulsion behaviors. Lastly, we address the implications of our findings to modern river systems and fluvial rock records.
Schematic of a levee with the Rouse profile used in the model. The extent of suspended sediment supplied to the floodplain (
Consider an initially flat floodplain adjacent to a flooded channel, the
overflow carries suspended sediment from the channel to the floodplain, and
the suspended sediment settles to build a levee (Han and Kim, 2022a, b) (Fig. 1). The mass conservation of suspended sediment takes the following form:
The total sediment flux at the channel–floodplain boundary (at the boundary
between the channel margin and the floodplain),
We make an erodible substrate at the beginning, supposing suspended sediment
in the initial flood flow to be uniformly mixed and settled over the entire
floodplain instantaneously (i.e., the time for levee deposition is much
longer than that for the flood inundation over the floodplain). The initial
concentration of suspended sediment in the floodplain can be defined as:
The depositional rate for the
For the entrainment rate of the
Combining the governing equation, Eq. (1) with the Exner equation of
conservation of bed sediment yields the following form for the time evolution of the bed elevation
In order to test our numerical model, we apply approximate field-scale
parameters based on previous studies in the Vistula River at the Smolice
station, southern Poland (Pruszak et al., 2005; Wyżga, 1999). The
dimensions and flow properties of the channel and floodplain have been chosen based on the field observations of flow hydraulics and natural levee deposits in the Vistula River (Wyżga, 1999). A constant overflow depth in the floodplain (
Initial boundary conditions for the 5 Test runs.
Derived from Eqs. (2) and (3), we integrated suspended sediment concentrations in the channel for each grain size above the floodplain
elevation, which are used for defining an overbank sediment flux at the
channel–floodplain boundary,
The main purpose of our test model is to gain a first-order understanding of
the fluvial levee evolution under various but simple boundary conditions. We
focused on changes in the levee profile associated with the overbank flow
velocity and the median grain size of suspended sediments. We also dealt with the effects of entrainment and flood water level on the levee evolution and stratigraphic development in our model. All these parameters are summarized in Table 1. A total of 5 tests were performed until the levee crest in each run (at the channel-floodplain boundary) reaches a height of 2 m. The total simulation times (
The cross-sectional views of predicted levee topography are presented in Fig. 3. The dashed black and solid red lines are the results of the prototype model (Test 1) and other test runs, respectively. The elevation of Test 2 at the proximal distance near the levee crest was slightly lower than Test 1, whereas, at the distal locations, Test 2 has slightly higher elevations (Fig. 3a). However, the differences (caused by the entrainment) are quite subtle. In Fig. 3b, the results do not indicate any noticeable differences between Tests 1 and 3 with the different flood level conditions. The total simulation times for both two models are also the same (Table 1). The levee profile of Test 4 using a higher flood velocity, shown in Fig. 3c, is gentler in slope and takes a longer time to build up the levee crest of 2 m in height. Test 5 with an increase in grain size produces a steeper slope of levee despite the longer total run time compared with Test 1 (Fig. 3d).
Levee surface profiles taken at the end of each model. The dashed black line represents the results of Test 1 and the red lines are other test results.
Figure 4 indicates changes in the levee surface elevation over time at the
proximal location (grid node
Time series of levee elevations at the proximal (
The plots of variations in grain sizes
The grain-size (
The model calculates
Time series of grain sizes
The model produces concave-up surface profiles and shows proximal to distal fining trends, both of which are the typical features of natural levees (Brierley et al., 1997). As described in the field case studies of Cazanacli and Smith (1998) and Filgueira-Rivera et al. (2007), a faster overflow velocity would cause suspended sediment to transport farther across the floodplain building a relative gentler levee slope, while coarser grain sizes would be deposited closer to the channel margin and produce a steeper levee. Throughout the test runs, we found that levee evolution is not much different from the prototype when the model used entrainment (Test 2) or flood level conditions (Test 3). However, when the overflow velocity or the grain size of incoming suspended sediment to the floodplain increases (Tests 4 and 5), the levee shape significantly becomes gentler or steeper than the prototype model, which is mainly consistent with the observations reported in previous documents (Cazanacli and Smith, 1998; Filgueira-Rivera et al., 2007). Herein, we attribute the levee geometry and its grain-size trend to variable external forcings, such as flood hydraulics and suspended sediment supply.
Test 2, including the entrainment processes of sediment from the bed, is
shown in (a) of each figure from Figs. 3a to 6a. Compared to Test 1, the
aggradation rates for Test 2 are lower, especially at the proximal location
and thus the final levee profile is also slightly lower at the proximal
location while the final elevations are higher at the distal location (Figs. 3a and 4a). The overall grain sizes,
The prototype model has a constant water depth across the evolving levee on the floodplain, thereby the water surface is in phase with the sediment–surface topography. In contrast, Test 3 is a case of constant flood level which means the water elevation in the channel always defines the flood level across the floodplain (Fig. 1). Both test models use the advection settling of suspended sediment, but the setups in terms of the lateral water-surface slopes are similar to the “wide and dry” floodplain versus “narrow and wet” floodplains reported by Adams et al. (2004) as two flooding styles on the floodplain. The former represents a fast overflow along with a gradient in the water surface over the floodplain and the latter is associated with a filling of flood in a relatively narrow river valley, leading to no significant water surface gradient. In Test 3, as the levee gradually grows, the flood depth increases further away from the levee crest decreasing the flood velocity over time and thereby causing decreases in aggradation rates at the distal locations compared to Test 1. Meanwhile, the overflow depth at the levee crest is constant because the in-channel deposition is assumed to be equal to the levee crest aggradation (Fig. 4b). However, in general, there are no meaningful changes between the prototype and Test 3 in terms of the profile shape, aggradation rate, and grain-size distribution during the total run time until the levee crest reaches 2 m high (Fig. 3b through Fig. 6b). We inferred that in our model the water level does not affect the topographic and grain-size characteristics of the levee significantly in a way that increases in the water depth to the distal direction are compensated by decreases in the suspended concentration (cf. Figs. S2 and S3). Furthermore, if the flood level is equal everywhere in the floodplain, i.e., the hydraulic gradient is minimal so the flood flow should not be significant, and thus diffusion would be possibly dominant (Adams et al., 2004).
To evaluate the effect of hydraulic characteristics of flood in overbank
deposits (James, 1985; Pierik et al., 2017; Wyżga, 1999), we set Test 4
such that it has a 1.5 times higher flood discharge which brings a 1.5 times
higher flow velocity at the channel–floodplain boundary since the water depth is kept constant. This faster flow is more efficient to transport coarser sediment further into the floodplain on account of an increase in its competence. It also results in higher aggradation rates at the distal location than that at the proximal location in comparison with those of the
prototype model, which is reflected in the gentler levee-profile slope
(Figs. 3c and 4c). The gentler slope in Test 4 produces a larger volume
under the profile compared to that in Test 1, which implies it needs more
time to be filled until the levee crest height reaches 2 m in height. In the
same context, the faster flow with regard to the grain-size distribution
over the levee has more influence on the coarser grain size,
Increasing grain size from
Based on the findings from the current model, we estimate a possible linkage between avulsion frequency and levee geometry under given flood and grain-size conditions and explore subsequent associations of levee geometry with potential channel reoccupation. As river avulsion is known to be highly sensitive to the depositional patterns and adjacent floodplain morphology (Hajek and Edmonds, 2014; Jerolmack and Mohrig, 2007; Mohrig et al., 2000; Slingerland and Smith, 2004), this section can provide a first-order role of levee morphodynamics in the avulsion processes and, in turn, a source of insight regarding alleviation of damages from natural hazards related to river avulsion.
It is generally accepted that natural levee growth acts as an avulsion threshold (Jones and Schumm, 1999) whereby avulsion can initiate when an adjacent levee crest elevates about one channel depth in rivers, which is defined as a critical superelevation (Bryant et al., 1995; Jobe et al.,
2020; Mohrig et al., 2000). In Fig. 7a, we apply this avulsion threshold to
further quantify the avulsion frequency in our numerical modeling by measuring the total run time until the levee crest reaches one channel depth (
Modeling results for
To elucidate the correspondence between the levee geometry and avulsion
frequency, we use a geometric scaling analysis of avulsion frequency. Avulsion time scale,
Through the results of our numerical modeling and scaling analysis, it turns out that in the levee deposits, not only the critical superelevation is associated with the avulsion threshold, but the characteristic levee slope should be also taken into account in the avulsion processes. Some researchers suggested a floodplain slope ratio as an alternative approach for an avulsion criterion (Guccione et al., 1999; Jones and Schumm, 1999; Mackey and Bridge, 1995; Slingerland and Smith, 1998, 2004; Tornqvist and Bridge, 2002). The floodplain slope ratio is measured as a cross-valley slope relative to the down-channel or down-valley slope. In a large sense, the characteristic levee slope can be a proxy for the cross-valley component of the floodplain slope ratio. These avulsion controls, the floodplain slope ratios and superelevation from the modern rivers, are compared in the study by Mohrig et al. (2000). The authors observed that the distribution of normalized superelevation heights is less scattered than that of the floodplain slope ratios. This scatteredness compared to that in the floodplain slope ratios is even up to two orders of magnitude less (Mohrig et al., 2000), but still exists, and can be further explained by our relationship between the avulsion frequency and levee geometry. As in previously published studies (Bryant et al., 1995; Mackey and Bridge, 1995), a high sedimentation rate in the main channel leads to a high avulsion frequency. In this instance, the levee deposits also rapidly aggrade toward the local superelevation building steep levee slopes, consistent with our modeling results (see Fig. 7 and Eq. 13). The channel in turn can jump into a new flow path before preferentially constructing the distal part of the levee (i.e., backloading) and relatively steep levees would be less disturbed and remain in the abandoned channel producing wide variations in the floodplain slope ratio (Filgueira-Rivera et al., 2007). Hence, instead of adopting a single criterion, it is reasonable that both the superelevation and characteristic levee slope are taken into account to evaluate the channel avulsion processes (Tornqvist and Bridge, 2002).
When a river avulsion occurs, flow typically migrates into a preexisting channel or excavates a new flow path in the vicinity of a parent channel searching for low spots with the highest gradient advantage across a basin (Sahoo et al., 2020; Slingerland and Smith, 2004). In the sense that the former channels can serve as “attractors” to avulsing channels (Heller and Paola, 1996; Jerolmack and Paola, 2007; Mohrig et al., 2000; Reitz et al., 2010; Slingerland and Smith, 2004), it is common to recognize that most avulsion paths are prone to reoccupying the previously abandoned channels within the history of ancient avulsions, and in the modern rivers such as the Mississippi and Red River avulsions (Aslan et al., 2005; Edmonds et al., 2016; Hajek and Edmonds, 2014; Jerolmack and Paola, 2007). Hajek and Wolinsky (2012) suggested that the extent of proximal levee deposits located along the channel margin may also have an influence on avulsion behavior. We therefore propose that the levee geometry, i.e., the levee slopes along the abandoned channels have a critical role in the distribution of sediment on the floodplain that contains relicts of abandoned channels. It means, depending on the shape of the remnant levees, they can act to barricade the previously occupied channels promoting more channel reoccupation (Jerolmack and Paola, 2007; Mohrig et al., 2000).
Schematic of abandoned channel infillings under
In the case of relatively steep levee slopes, the steep levees would extend to only limited distances to the floodplain, and consequently modify less of the initial local relief between the levee crests and adjacent floodplain (Fig. 8a). Hence, the abandoned channel topography surrounded by the high gradient levees can be protected from the influx of flood deposits. Furthermore, the steeper levees are associated with higher avulsion frequency, as described in Eq. (13), which indicates the avulsion would happen faster and leave the abandoned channel with less time to be filled. The abandoned channels maintain their topographic lows because they are filled with overbank deposits more slowly, and thus they would readily capture the flow and coalesce it into a new pathway (Hajek and Wolinsky, 2012; Mohrig et al., 2000). If so, it may increase the possibility for an active channel to find the topographic lows of preexisting channels and reoccupy them. The model of Jerolmack and Paola (2007) demonstrated that channel reoccupation repeatedly occurs within a limited number of active channels called the “active channel set”. This active channel set may occur in concert with steeper levee slopes along the floodplain channels taking advantage of remaining local conduits and producing multistory sand bodies (or vertical stacked patterns) in the ancient avulsion deposits (Jerolmack and Paola, 2007; Sahoo et al., 2020; Slingerland and Smith, 2004).
However, with the gentler levees, preexisting abandoned channels can be more vulnerable to being modified and smoothed, which is called “channel healing” or “annealing” as opposed to preserving the former topography (Guccione et al., 1999; Reitz et al., 2010; Slingerland and Smith, 2004). It is expected that relatively gentler levee deposits compared to the steeper ones would reach farther across the floodplain and increase the adjacent floodplain elevations, resulting in frequent overbank deposition into the abandoned channels (Fig. 8b). The low avulsion frequency predicted by Eq. (13) also intensifies topographic healing on any relict channels by providing more time for infilling with the overbank sediment. Once the abandoned channel topography is covered (i.e., smoothed by deposition), it tends to be left out for a long time deprived of any chance to encounter the active channels (Jerolmack and Paola, 2007). Due to the removal of topographic memories, a new channel is thereby more likely to incise the floodplain surface or to stay in the parent channel position even over the critical superelevation resulting in multilateral sand bodies in the avulsion stratigraphy (Jerolmack and Paola, 2007; Jones and Hajek, 2007).
The linkage between the levee geometry and channel reoccupation is applied to the following two field observations: one is a modern avulsive system and the other is from ancient fluvial strata. The former one, from published data by Valenza et al. (2020), investigates how the channel avulsion style evolves from the upstream to downstream reaches in modern rivers. In the study, they classify the avulsion styles into annexational and progradational avulsions (cf. Fig. 8); annexational avulsion occurs when the current flow returns to the remnant floodplain channels, and progradational avulsion occurs when a new avulsion channel is made with floodplain deposition (Edmonds et al., 2016; Hajek and Edmonds, 2014; Jones and Hajek, 2007; Slingerland and Smith, 2004). They quantify 63 avulsions across three sedimentary basins of the Andes, Himalayan, and New Guinean basins and suggest that most avulsions close to the mountain fronts generate annexational styles on braided rivers, while the avulsions of relatively further downstream basins mainly make progradational avulsions on meandering rivers. Valenza et al. (2020) provided several plausible reasons causing the shift in avulsion style, such as downstream changes in slope and downstream fining caused by selective deposition. Given these two changes, we speculate that the levee geometry may affect their reoccupation likelihoods of preexisting channels that are susceptible to the floodplain topography and surface roughness, e.g., depressions on the floodplain (Edmonds et al., 2016; Hajek and Edmonds, 2014; Hajek and Wolinsky, 2012; Jerolmack and Paola, 2007; Mohrig et al., 2000; Slingerland and Smith, 2004). Near the mountain fronts, any amount of coarser sediment overflows into the adjacent floodplain on account of selective deposition along the main stream. Moreover, a flooding type of upstream is traditionally characterized by intense rainfalls in a short period of time. The localized upstream flooding rapidly raises the overflow depth and pours out into the floodplain so that considerable coarser suspended sediment aggrade on the levee deposits. As a result, the upstream flash flooding and coarser grains would form non-cohesive steep slopes of levees which can defend formerly abandoned channels as topographic lows and create a favorable condition for annexational avulsions (Cazanacli and Smith, 1998; Hajek and Edmonds, 2014). The finer sediment at the downstream basins, on the contrary, is able to transport across the floodplain for a longer duration as the downstream flooding generally has a prolonged inundation period with a gradually increased flood level on a large scale. The suspended sediment would spread over a great distance, building cohesive levee deposits with a gentler shape (Cazanacli and Smith, 1998; Hudson and Heitmuller, 2003) and accelerating topographic healing of any scours or relic channels on the floodplain. By means of the erased topographic memories caused by the gentler-sloped levees, new streams have difficulty reoccupying the abandoned channels. Additionally, the cohesion of levee deposits due to the finer sediment in the downstream basins impedes the destruction of the robust levees on the current channel, which in turn will promote floodplain deposition and more progradational avulsions (Valenza et al., 2020).
The latter case of field observations is the Upper Cretaceous alluvial to coastal plain deposits of the Blackhawk Formation in Wasatch Plateau, Central Utah, USA. Previous studies on this ancient fluvial strata have identified that the fluvial sand bodies in the upper Blackhawk Formation contain vertically stacked and laterally offset channelized patterns in response to large-scale avulsion processes (Flood and Hampson, 2014, 2015; Hampson et al., 2012, 2013; Sahoo et al., 2020). Sahoo et al. (2020) highlighted that in terms of channelized sand bodies, their internal architectures, paleochannel mobility, and their distribution and stacking patterns in strata are correlated with each other. They interpreted that vertically stacking single-story sand bodies indicate channel reoccupations with low channel mobility, and isolated or lateral offset patterns of multilateral sand bodies represent the regional avulsions (randomly choose their new flowpath) with high mobility of channels (Heller and Paola, 1996). In our model, this can be explained as a result of the geomorphic difference in alluvial ridges. Since the channel mobility,
Our 1D levee-building model is used to examine the dynamic evolution of fluvial levees describing their resultant topography and grain-size trends over time. Even though our model offers the fundamental mechanisms of levee geomorphology and their relationship to river avulsion processes, challenges have remained to fully reflect the complexity of depositional processes in the levee–floodplain complex. Like other 1D hydrodynamic models, the current model cannot thoroughly reproduce the dynamics of flood conditions since the model can overestimate the inundation extents by ignoring the friction parameters and infiltration processes (Tayefi et al., 2007). Additionally, we do not account for other factors associated with vegetation, cohesion, and preexisting floodplain topography, which can affect the suspended sediment load across the floodplain and drainage development in the flood basin, and eventually impact the levee formations and the infilling of abandoned channels (Boechat Albernaz et al., 2020; Branß et al., 2022; Kleinhans et al., 2018; Mohrig et al., 2000).
The current model is based on the following simplifying assumptions: (1) a constant suspended flux with normally distributed grain sizes that flow into the floodplain and (2) a cross-sectional levee profile that evolves symmetrically in both sides of the channel. Within the natural river systems, bedload sediment transport can be important for some levee building processes, especially in meandering rivers. The alluvial ridge which contains levee deposits can be developed by both suspended and bedload sediment deposition and would be varied by the size of the bank (inner and outer bank) and channel lateral migration rate (Ielpi et al., 2020; Toonen et al., 2012; van Dijk et al., 2013). The fluvial levees are thus generally unpaired along sinuous channels due to the differences in lateral erosional and depositional processes (Skolasiñska, 2014; Wyżga, 1999). However, the current model has not yet incorporated other components of the alluvial ridge that can be amalgamated with point bar deposits or crevasse splays, etc., which may possibly alter the levee geometry rather than a simplified levee deposit. Still, we believe that isolating levee formation from the channel bank dynamics can allow us to infer the effects of levee geometry on the avulsion behaviors and abandoned channel fills on behalf of the alluvial ridge topography. We also note that the aggradation rates set equal for the in-channel and levee crest can be another limitation in the model, both of which can be varied in nature and influence the overbank grain-size distribution and the avulsion conditions (Ganti et al., 2016; Nicholas et al., 2018). This complexity in channel–floodplain hydrodynamics can cause discrepancies between our simplified modeling approach and natural levee evolutions. Nevertheless, our 1D levee-building model does provide significant implications for further understanding of the levee formation and its possible linkage with avulsion processes and may give an important basis for enhancing future models along with more accurate field parameters.
The fluvial levee evolution under various boundary conditions was investigated by using the numerical levee-building model with the advection
settling of suspended sediment. The current levee-building model allows us
to establish what determines the levee geometry and delineate the relationship between the levee geometry and avulsion behaviors. Briefly,
our main conclusions can be summarized as follows:
Overflow discharge and incoming sediment grain size into floodplain exert first-order controls on the levee geometry. The results show that a relatively gentler shape of the levee is associated with the faster flooded flow and a steeper slope is associated with coarser suspended sediment. The levee geometry that can reflect the flood hydraulics and/or grain-size distribution in the channel may work as a good indicator of the paleo-environment in the stratigraphic records. There is a significant correlation between the avulsion frequency and levee geometry in respect of overflow properties. The avulsion frequency is proportional to the characteristic levee slope and median grain size of overbank suspended sediment but negatively correlated with overflow discharge. With a high avulsion frequency, a steeper levee is more likely to lead to the reoccupation of the previously abandoned paths. In contrast, a gentler levee with low frequency causes smoothing of the abandoned channel topography, and then a new path would be made on the floodplain. We propose a new approach concerning levee morphology to understand the transition of river avulsion styles in the modern avulsive system reported by Valenza et al. (2020). From upstream to downstream, the levee geometry which can be modified due to downstream fining and decreasing stream power, is potentially involved in the shift of the avulsion style from annexational to progradational avulsions. We further suggest that the geomorphic difference in alluvial ridges may be related to the channel mobilities and their stacking patterns of sand bodies and explain the case of the upper Cretaceous Blackhawk Formation in Wasatch Plateau, Central Utah, USA described in Sahoo et al. (2020). Even though more field data will be needed to fully test our hypotheses, the implications can nourish our knowledge of avulsion processes linking with the levee geometry. Therefore, this study may encourage additional studies of natural levees for better prediction of avulsion behaviors and their flood risks.
The following list includes variables with symbols
The MATLAB codes used in the one-dimensional levee-building model and associated tests are openly available at
The modeling output data are archived and openly available at
The supplement related to this article is available online at:
JH designed, conducted the computer code with the help of WK; analyzed the model results; and wrote the manuscript. WK motivated, conceptualized the study; developed methodology; acquired funding and supervised the research; and revised the paper. Both authors contributed to the discussion and editing of the manuscript writing.
The contact author has declared that neither of the authors has any competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We thank the National Research Foundation of Korea (NRF) and the Yonsei University Research Fund for research support. We also thank the editor Paola Passalacqua and two reviewers, Douglas Edmonds and one anonymous referee for thoughtful discussions and constructive comments which tremendously helped to improve and clarify this paper.
This research has been supported by the Basic Science Program through the National Research Foundation of Korea (grant nos. NRF-2020R1A2C1006083 and NRF-2017R1A6A1A07015374) and in part by the Yonsei University Research Fund of 2021 (grant no. 2020-22-0507).
This paper was edited by Paola Passalacqua and reviewed by Douglas Edmonds and one anonymous referee.