# Riverbank Stabilization with Bioengineering: Failure Processes and Design Suggestions from a Scaled Physical Model

Solange Leblois, Guillaume Piton, Alexis Buffet, Alain Recking, André Evette

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Solange Leblois, Guillaume Piton, Alexis Buffet, Alain Recking, André Evette. Riverbank Stabilization with Bioengineering: Failure Processes and Design Suggestions from a Scaled Physical Model. Journal of Hydraulic Engineering, 2025, 152 (1), pp.04025047. 10.1061/jhend8.hyeng-14369. hal-05316839

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# Riverbank Stabilization with Bioengineering: Failure Processes and Design Suggestions from a Scaled Physical Model

Solange Leblois<fnref n="1" />; Guillaume Piton, Ph.D.<fnref n="2" />; Alexis Buffet<fnref n="3" />; Alain Recking, Ph.D.<fnref n="4" />; and André Evette, Ph.D.<fnref n="5" />

**Abstract:** Soil and water bioengineering structures (SWBEs) implemented for riverbank stabilization are often designed based on field feedback and expert knowledge. These structures are weaker in the years just after implementation, before vegetation recovery, which limits their reliability and success rate. This study uses a 1:25-scale physical model to compare the failure processes of noncohesive riverbanks protected at their toe with riprap, early-stage fascines, or double fascines. The results highlight the key weak points that should be targeted to further improve SWBE design. In complement to qualitative observations, the topography is measured by photogrammetry before and after flooding with bankfull discharge. Pore pressure is measured in the channel bed and banks during the experiment. The results showed for banks with SWBE toe protection in meanders that (1) scour hole formation drives bank destabilization; (2) due to undermining (i.e., scouring), common SWBEs such as single-bundle fascines are insufficient to stop bank material falling into scour holes; (3) the bank toe can be stabilized by a combination of living structures (e.g., one bundle of fascines) and buried structures that play the role of filter, blocking bank material, and anchorage (here, a second bundle was used); (4) bank slope reduction reduces scour hole depth; and (5) in this scaled physical model without soil cohesion, bank erosion is driven by fluvial entrainment rather than by mass failure or water overpressure in the bank.

DOI: 10.1061/JHEND8.HYENG-14369. This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/.

**Practical Applications:** This article evaluates SWBEs for riverbank stabilization in meanders based on experiments in a scaled physical model. In terms of processes, with low-cohesion banks, such as in gravel-bed rivers, scouring at bank toe drives the failure of banks protected with SWBEs. Scour depth should therefore be estimated during SWBE design to determine the need for protection against undermining. Double fascines limit the extent to which bank material falls into scour holes. Bank slope reduction also decreases scour hole depth. Surface erosion must also be controlled through herbaceous cover or tightly anchored geotextile to ensure vegetation growth. Finally, erosion at the heads of the structures remains high, and further experiments must be conducted to add to design recommendations.

## Introduction

Riverbank erosion is a naturally occurring process that can have negative consequences, including loss of land and associated resources, pollution, and damage to property and infrastructure (Hossain 1993; Piégay et al. 1997; Fox et al. 2016; Vörösmarty et al. 2010). However, streambank erosion is also the main source of sediment in many streams (Belmont et al. 2011; Gellis and Gorman Sanisaca 2018;

Walling and Collins 2005), and is thus required for rivers to function as dynamic systems and ecosystems, creating pioneer habitats and providing bed load (Florsheim et al. 2008; Malavoi and Bravard 2011). Consequently, riverbank protection must be a trade-off accounting for these different aspects and should only be implemented if needed to protect human interests that cannot be moved (Bonin et al. 2013; Pinto et al. 2019). In this context, soil and water bioengineering structures (SWBEs) are an alternative to permanent concrete structures. They are nature-based solutions (NBS), which offer solutions to societal challenges while working with nature as part of an integrated approach (Preti et al. 2022). In this article, we investigate the design of a particular SWBE, called fascines, using a flume experiment.

The stability of riverbanks depends on their geometry and on the mechanical properties of the bank material (Thorne 1982; Duan 2005). Mechanical riverbank erosion is driven by three main mechanisms operating at different levels of frequency and magnitude: fluvial entrainment, mass-failure events, and subaerial weakening and weathering processes (Lawler et al. 1997; Henshaw et al. 2013). These concepts and terms deserve definition. Entrainment corresponds to the abrasion of the riverbank by the river's flow (Chassiot et al. 2020). Mass-failure events refer to rapid motion of sediments when gravitational forces overcome soil cohesion and other restraining forces (Thorne and Tovey 1981). Subaerial weakening and weathering processes are associated with slow-motion weather-related preparatory processes that weaken the bank prior

<footnote n="1">¹Ph.D. Candidate, INRAE, UR LESSEM, Univ. Grenoble Alpes, 2 rue de la papeterie BP 76, Saint-Martin-d'Hères 38402, France (corresponding author). ORCID: https://orcid.org/0009-0008-1662-3065. Email: solange.leblois@inrae.fr</footnote>

<footnote n="2">²Researcher, INRAE, CNRS, IRD, Univ. Grenoble Alpes, Grenoble INP, IGE, Grenoble 38400, France. ORCID: https://orcid.org/0000-0002-0124-0909. Email: guillaume.piton@inrae.fr</footnote>

<footnote n="3">³Technician, INRAE, CNRS, IRD, Univ. Grenoble Alpes, Grenoble INP, IGE, Grenoble 38400, France. Email: alexis.buffet@inrae.fr</footnote>

<footnote n="4">⁴Researcher, INRAE, CNRS, IRD, Univ. Grenoble Alpes, Grenoble INP, IGE, Grenoble 38400, France. Email: alain.recking@inrae.fr</footnote>

<footnote n="5">⁵Researcher, INRAE, UR LESSEM, Univ. Grenoble Alpes, 2 rue de la papeterie BP 76, Saint-Martin-d'Hères 38402, France. Email: andre.evette@inrae.fr</footnote>

<footnote>Note. This manuscript was submitted on October 14, 2024; approved on July 18, 2025; published online on October 15, 2025. Discussion period open until March 15, 2026; separate discussions must be submitted for individual papers. This paper is part of the *Journal of Hydraulic Engineering*, © ASCE, ISSN 0733-9429.</footnote>
to fluvial entrainment or mass failure. Weathering is generated by surface runoff, freeze-thaw, desiccation cycles, and seepage (Chassiot et al. 2020).

Bank erosion is particularly strong on meanders, where secondary currents erode the external bank laterally and at depth, creating corresponding deposits on the internal bank (Lefort and Laffont 2008; Foerst and Rüther 2018; Klingeman et al. 1984). The sharper the bend, the larger the inner deposit, and the deeper the scour hole (Degoutte 2006; USDA 2007b). This process is explained by higher sidewall shear stress-induced turbulence and secondary currents, as shown by Papanicolaou et al. (2007).

Local hydrology also plays an important role. Thus, streambank failures typically occur during the falling limb of a flood hydrograph and are attributed to rapid drawdown of the water table following saturation of the bank during the bankfull stage (Rinaldi et al. 2004; Keown et al. 1981). Failure at drawdown is facilitated by the cumulative effects reported by Rinaldi et al. (2008) and Fox and Wilson (2010). These effects include reduced soil shear strength induced by the water saturation of the bank (Fredlund and Rahardjo 1993; Rinaldi and Casagli 1999), inversion of seepage flow from the bank toward the river if the groundwater level exceeds the river level (Wilson et al. 2007; Fox et al. 2007; Ghiassian and Ghareh 2008; Chu-Agor et al. 2008), and loss of the confining pressure produced by the high level of the stream. With noncohesive bank materials, inversion of seepage flow is a major contributor to failure (Ali et al. 2022).

SWBEs have been used for decades to stabilize bank erosion or superficial landslides (Schiechtl and Stern 1997; Preti et al. 2022). They draw their strength from well-developed vegetation (Gray and Sotir 1996). But in addition to protecting human assets from erosion, and unlike civil engineering alternatives, SWBEs are also hosts to biodiversity and ensure associated ecosystem services (Rey et al. 2019; Rauch et al. 2022; Janssen et al. 2019). Fascines are commonly used in temperate climates, where they offer the advantages of directly retaining bank toe material and of being quick and easy to implement (Zeh 2010; Sotir and Fischenich 2001). A study undertaken in France, where willow fascines are the most commonly used SWBE for bank toe protection (Jaymond et al. 2023), revealed that fascines failed to reach the objective of bank stabilization over time in only 20% of implementations. Among these, 9% of failures were due to bank toe erosion (Leblois et al. 2022, 2024b). Although feedback showed that these structures are effective, they are mainly built on a trial-and-error basis. Up to now, little attention has been paid to good engineering practice. This has motivated several studies in the last few years.

In a pioneering work, Recking et al. (2019) studied fascines in a scaled physical model. Their generic theoretical case produced results that were more qualitative than quantitative. They showed that fascine destruction increases with sharper bends, confirming previous results from Watson et al. (1997), who determined a limit of implementation for willow cuttings on meanders, with a ratio of hydraulic radius to width of almost two. Recking et al. (2019) observed changes associated with morphological patterns but could not describe the step-by-step destabilization dynamics around various bank toe protections. For instance, they observed that flow circulation in the bank combined with pore pressure might contribute to the erosion dynamics, but they could only speculate about the respective roles of channel flow versus (theoretical) pore pressure. In this new work, we continue this investigation by more meticulously addressing similarities in geometry, hydraulics, and sediment transport in order to provide quantitative analysis. In particular, we evaluate pore pressure distribution. Leblois et al. (2024b) presented qualitative results based on field observations and the study of one type of SWBE, one-bundle fascines.

These previous results leave three main questions unanswered. First, how do four types of bank toe protection freshly implemented on meanders, with or without geotextile, react to high in-bank flows that should represent the channel-forming flow? The working hypothesis is that the first step in destabilization is erosion of the toe, exacerbating bank destabilization. Attempts to describe the dynamics of failure raises the second question: what are the respective contributions of fluvial entrainment, mass failure, or seepage in the destabilization process? From the literature, in the context of the noncohesive materials explored in this study, although fascines retain the bank, the working hypothesis is that fluvial entrainment is the major cause of destabilization. Our final question is what design recommendations can be extracted from this study to build more robust SWBE?

The first part of the article presents the flume setup. The second part presents the results of our measurements. Finally, the results are discussed. Our original flume setup gives us access to quantitative results for morphological adjustments around SWBEs implemented in meanders for bank stabilization. From the failure processes measured, we propose a process-based design to facilitate future implementation and improve the success of these structures.

## Material and Methods

### Scaled Physical Model Design

The design of our scaled physical model begins with the definition of a theoretical generic prototype (scale 1:1) (details are given in Supplemental Materials S1). From this prototype, we geometrically scaled ($\lambda = 1:25$) the sediment mixture, the river's form, and the associated riverbank protections. Hydraulic scaling based on the Froude similitude, thus respecting flow resistance similitude (Julien 2018), was used to define the flood intensity and duration and to choose the riverbed slope. The Shields similitude was used to verify the occurrence of sediment transport. Others similitudes were computed for verification to ensure that the scaled physical model was representative (Supplemental Materials S2).

In all the figures in this article, values referring to the 1:25-scale model are written in italics and those referring to the 1:1 generic prototype are written in bold.

### Reference Sites

Our theoretical generic prototype was defined based on 25 sites located on meandering prealpine rivers extracted from the GeniVeg database (Jaymond et al. 2023) (Fig. 1). The selected meanders showed destabilized SWBE on their outer banks. The methods used to define the variables are described in Supplemental Materials S1.

Except for the hydraulic radius of meanders ($R$) and thus the radius of curvature to active riverbed width ratio ($R/W$), the prototype was defined by the median values of the hydromorphological characteristics of the 25 reference sites (Fig. 1). Our generic prototype is thus a rather narrow river ($W = 6.5$ m), with a mild gradient ($s = 0.7$%), a gravel bed channel ($D_{50} = 31$ mm and $D_{84} = 61$ mm) undergoing a typical bankfull discharge equivalent to discharge of a 2-year return flood ($Q_2$), with unit discharge of $3.8 \text{ m}^3/\text{m/s}$. The associated depth, estimated at about 1.2 m, gives this channel a width-to-depth ratio $\approx 10$ when the $3H/2V$ bank slope is accounted for. Consequently, the channel aspect ratio is consistent with the range observed for natural rivers (Dust and Wohl 2012).

To complete the prototype, a rather sharp bend was chosen, $R/W \approx 4$, corresponding to the 25% quantile of the sites sampled. This sharp bend allowed us to explore the limits of the system
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Hydromorphological description of the 25 reference sites on river meanders: unit discharge ($Q_2$/W), radius of curvature to width ratio ($R/W$), active riverbed width ($W$), sediment characterization $D_{50}$ and $D_{84}$, flow rate of a 2-year return flood ($Q_2$) from the SHYREG database (Arnaud et al. 2014), radius of curvature ($R$), and slope of the riverbed ($s$). Each dot corresponds to one of the 25 reference sites. Median values are given in the boxplot.</figcaption></figure>

increasing the constraints applied to the outer bank of the meander (Thorne and Abt 1993).

## Channel Geometry and Grain Size

The 10.5 × 1.5-m flume was sufficient to build a series of three meanders geometrically downscaled from the generic prototype (Fig. 2). Here, $\lambda = 1:25$ is the highest possible scaling factor applicable given the space in the flume and was thus chosen to minimize scale effects (Heller 2011).

A poorly sorted sediment mixture was defined using the geometrically downscaled prototype grain sizes (Fig. 3). To avoid cohesion effects, which are very complicated to rigorously downscale (Thomas et al. 2014; Peakall et al. 1996) and that emerge with grain sizes <0.2 mm, Heller (2011) suggested using a mixture with $D_{50} > 0.5 - 1$ mm. We used $D_{50} = 1.3$ mm.

## Flood Model Features

The experiment was designed with a flood peak at $Q_2$:25 m³/s at prototype scale, or 8 L/s at 1:25. $Q_2$ represents a trade-off, maximizing $\lambda$ with regard to the pump capacities, to reduce scale effects (Heller 2011). Around $Q_2$, representing the bankfull conditions in our study, erosion processes are usually high in unprotected meanders with hydraulic conditions close to or exceeding critical shear stress (Parker et al. 2007).

Runs lasted 2.5 h in the scaled physical model (12.5 h at prototype scale according to Froude scaling). This duration represents a trade-off between excessively short events, such as flash-floods, and long-lasting multiday floods.

Two synthetic hydrographs were tested: either with a fast rising limb and a slow falling limb or vice versa (Fig. 3). With both hydrographs, the meanders were subjected to equivalent hydraulic forcing (same duration of discharge), but the banks were either rapidly saturated and slowly drained or vice versa. Unless specified, the results presented are from the event with a fast rising limb. For all runs, the theoretical threshold of sediment motion, when the shear stress equals the critical shear stress, occurred with a flow of 2 L/s.

## Similitude and Downscaling

For the study focusing on free surface flow and coarse sediment transport, the flow features were downscaled using the Froude and Shields similitude (Yalin 1971; Peakall et al. 1996; Heller 2011). The main dimensionless numbers in the generic prototype and in the scaled physical model were computed at $Q_2$ (Supplemental Materials S2) demonstrating that the following:

* The Froude number $Fr$ stayed slightly subcritical ($Fr \approx 0.9$), which is a reasonable value at peak flow for a channel slope of 0.7% (Piton and Recking 2019; Grant 1997).

* The Reynolds and grain-Reynolds numbers were high enough to maintain a turbulent rough flow, permitting relaxation of their similitude (Peakall et al. 1996).

* The transport stage ($\tau^*/\tau_{cr}^*$) was >2 at flood peak; consequently, intense sediment transport associated with bank erosion could be expected.

* The Weber number was 180, i.e., it exceeded the threshold (120) above which surface tension effects are considered insignificant (Peakall and Warburton 1996).

## Riverbank Designs

The SWBEs evaluated in this study were variations around fascines composed in the field from living branches capable of sprouting new branches and roots, bundled together and placed at the bank toe between two rows of stakes (AMEC 2012; Peeters et al. 2020; Didier et al. 2023). Because the effect of vegetation is difficult to

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Top view of the scaled physical model, showing the three meanders implemented with bank toe protections. The main geometric characteristics at 1:25 scale are in italics. Corresponding values of the prototype are indicated in bold.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (a) Grain-size distribution in the prototype (solid line) and for the poorly sorted sediment mixture purchased for use in the scaled physical model (dashed line); and (b) hydrographs implemented in the scaled physical model with a fast rising limb (solid line) and with a slow-rising limb (dotted line). Values at 1:25 scale are indicated in italics; prototype values are indicated in bold.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Toe protections tested in the flume: (a) without geotextile; and (b) with geotextile.</figcaption></figure>

scale, no living vegetation was implemented in the flume. The study therefore focused on the destabilization processes at the time of implementation, before vegetation growth, when mechanical resistance was at its lowest (Evette et al. 2018; Pinto et al. 2019).

The bank slope was at $3H/2V$, which is the maximum slope recommended for SWBEs (Gray and Sotir 1996). Four bank toe configurations were tested: no protection, one-bundle fascine, two-bundle fascine, and riprap (Fig. 4). Each bank configuration was tested twice: with and without a geotextile. Staples were used to fix the geotextile behind the bank toe protection and to confine it at the top of the bank, so that it covered the entire bank. For each test, the same bank configuration (e.g., fascine with geotextile), was implemented on all three meanders of the flume, to simultaneously verify the consistency of observations in three similar tests (Fig. 2).

Each element composing the bank toe protections tested was geometrically scaled (Table 1 and Fig. 5). The theoretical prototype dimensions were defined based on SWBE guidelines (Gray and Sotir 1996; Schiechtl and Stern 1997; Bonin et al. 2013; Peeters et al. 2020). The riprap consisted of two layers of stones—in width and in depth—making it as deep as the double fascine to allow comparison.

The toe protection was implemented on the bank at the level of the green line. This feature of river banks is defined by Baird et al. (2015) as “horizontal elevation marks that divide between the frequently wet toe zone and the zone where riparian vegetation can

<table><caption>**Table 1.** Bank toe protection design based on SWBE guidelines for the generic prototype and the 1:25 scaled physical model</caption><thead><tr><td>Material</td><td>Prototype (cm)</td><td>Scale physical model (cm)</td></tr></thead><tbody><tr><td>Stake diameter</td><td>8–15</td><td>0.3–0.6</td></tr><tr><td>Stake length</td><td>200</td><td>8</td></tr><tr><td>Branch diameter</td><td>2–6</td><td>0.08–0.24</td></tr><tr><td>Branch length</td><td>200–300</td><td>8–12</td></tr><tr><td>Bundle diameter</td><td>30–40</td><td>1.2–1.6</td></tr><tr><td>Bundle length</td><td>200–500</td><td>8–20</td></tr><tr><td>Steel wire diameter</td><td>0.2–0.3</td><td>0.01</td></tr><tr><td>Lateral spacing between stakes</td><td>23–40</td><td>1.2–1.6</td></tr><tr><td>Longitudinal spacing between stakes</td><td>80–100</td><td>3.2–4</td></tr><tr><td>Riprap mean diameter</td><td>30–50</td><td>1.5</td></tr><tr><td>Geotextile hole size</td><td>2–4</td><td>0.1</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Transverse profiles in the 1:25 scale model (in italics) of the four bank toe configurations tested: (a) no protection; (b) riprap; (c) single fascine; and (d) double fascine. Values in bold are the corresponding lengths at prototype scale (1:1).</figcaption></figure>

establish on the banks. [...] , it can often be easily identified in
the field by the distinct change from vegetation to substrate.” For
the reference sites, the green line corresponds to the mean annual
flow level where the first shrubby vegetation is generally observed
(Schiechtl and Stern 1997) (Fig. 5). The middle of the upper bundle
of the fascine was fixed at this level. The fascine was therefore com-
pletely submerged at Q₂ (Fig. 5).

Measurements

Five types of measurements were taken during the experiments:

* Three fixed cameras took pictures of each of the three meanders every 5 s to qualitatively monitor erosion and failure.

* Digital elevation models (DEMs) were produced by structure-from-motion (SfM) photogrammetry (Westoby et al. 2012) using additional pictures taken before and after each experiment (discussed further subsequently).

* Pore pressure in the bed and banks was measured at each flow discharge (discussed further subsequently).

* The water depth was measured manually to cross check the prior friction law-based computation (Supplemental Materials).

* The water discharge was controlled by the pump control system.

Digital Elevation Models

DEMs of each meander before and after each experiment were ob-
tained using SfM software (Agisoft Metashape version 1.8.4). Input
target coordinates were measured by theodolite with mm accuracy
and data were post-processed using Cloud Compare (version 2.11.3).
DEM of difference (DOD) were computed to reveal erosion and
deposition patterns with the associated volumes for predefined
areas of riverbank and riverbed [Fig. 6(a)].

The absolute change in volume was estimated (m³/m²) to allow
comparisons between experiments and between meanders [Fig. 6(b)].

The ±1 · 10⁻⁴ m³/m² uncertainty of this volumetric analysis was
below the typical changes measured, which ranged from 1 · 10⁻³
to 5 · 10⁻² m³/m².

The complicated three-dimensional (3D) patterns of these proc-
esses were simplified in a proxy of the elevation change, taken as
the 95th percentile of positive or negative height change. The scour
hole depth (negative change) estimated by this method refers to the
vertical distance from the original riverbed level to the deepest
point of erosion caused by water flow. Its corollary is that the inner
bed deposit height is the positive change from the original riverbed
level to the highest point of sediment deposition. The average un-
certainty of the DEM was estimated to be <1 mm based on crossed
controls with reference points measured with a theodolite, which is
much less than the centimeter-scale erosion and deposition reported
[Fig. 6(b)].

Pore Pressure Measurement

Pore pressure sensors were placed at the bottom of the flume in
each meander at several transversal and longitudinal locations
[Fig. 7(a)]. These sections of plastic pipe (diameter = 20 mm)
were capped with a thin mesh to prevent sediment intrusion and
connected to smaller water-filled pipes. The pore pressure at vari-
ous locations was measured by reading the free surface level on a
single fixed grid for each meander [Fig. 7(b)].

In each meander (meander index: *m* in 1–3), 12 sensors were
distributed over three transects perpendicular to the flow (transect
index: *t* in 1-3) and at four transversal positions (position index: *p*
in 1-4): river center, bed side, bank toe, and bank side [Fig. 7(a)].
Measures were taken at every flow discharge with increments of
0.5 L/s (index: *f* in 1-16), during the rising and falling limbs of
the hydrograph [Fig. 3(b)].

The results present a total of 9,216 measures: *m* = 3
meanders × *t* = 3 transects × *p* = 4 positions × *f* = 32 discharges
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Contour layer definition for (a) DEM analysis; and (b) example of DoD highlighting erosion (in blue) and deposition (in red) patterns, for a run with single fascine toe protection and geotextile.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** (a) Pressure sensors distribution inside a meander at four positions over three transects, namely, river center ($p_1$), bed side ($p_2$), bank toe ($p_3$), and bank side ($p_4$); and (b) system of pressure reading. The interstitial flows were read on each of the 12 pipes with millimeter accuracy by comparing menisci levels with background graph paper.</figcaption></figure>

(0.5 to 8 L/s in 0.5-L/s steps, i.e., 16 values, repeated twice due to the rising and falling limbs of the hydrograph) × 4 bank toe configurations × 2 geotextile possibilities.

All pressure values ($P$) were normalized to the dimensionless variable $P^*$ [Eq. (1)]. In $P^*$, pressure changes due to discharge variations ($f$) and differences in elevation due to variations in the downstream position in the flume ($m, t$) were collapsed by subtracting the transverse minimum value at each discharge step. The pressure difference was made dimensionless by dividing it by the bank full water depth $d_{observed} = 7$ cm. Thus, $P^*$ reveals the pressure gradient between positions in transects [p in Fig. 7(a)] for the various bank configurations.

For each flow discharge ($f$), $P^* = 0$ corresponds to the lowest relative pressure measured inside a transect ($t$) on a specific meander ($m$), whatever the bank configuration. A high $P^*$ means that the relative pressure inside the transect was higher at the given position ($p$) than at other positions in the same transect, whatever the bank configuration, reflecting a high pressure gradient in the transect [t in Fig. 7(a)]

$$P_{m,t,f,p}^* = \frac{P_{m,t,f,p} - \min(P_{m,t,f})}{d_{observed}} \quad (1)$$

## Results

### Qualitative Results: General Observations of Responses to Flood Conditions

Qualitative results were obtained from observations of pictures of the various toe protections after each run (Table 2). Leblois et al. (2024a)'s videos and Figs. S1 and S2 present all the bank configurations studied before a run, at flood peak and after the experiment.

In the absence of geotextile covering, the bank systematically suffered deep and large erosion [Fig. 8(a)]. This erosion was already considerable at flood peak and continued during water recession (Fig. S1). Material above and behind the toe protection was eroded. In all configurations with a geotextile, erosion was extensive at the upstream and downstream ends of the geotextile [Fig. 8(d)]. Bank erosion also occurred behind the geotextile, although sediments were not small enough to pass through the geotextile [Fig. 8(b)].

Nevertheless, the presence of the geotextile considerably changed the results observed, with banks appearing more stabilized overall (Fig. S2). The configuration without bank toe protection and only geotextile presented a wide diversity of behavior [Figs. 8(f–h)]. On the first meander, the geotextile was ripped off and subsequently
<table><caption>**Table 2.** Summary of the morphological adjustments in meanders in all bank configurations in terms of bed and bank erosion</caption><thead><tr><th>Configuration</th><th>Bank erosion</th><th>Bed erosion</th></tr></thead><tbody><tr><td>Without geotextile</td><td>Erosion is greatest [Fig. 8(a)].<br>Erosion is only slightly reduced in the presence of a double fascine; the configurations with no protection, riprap, or fascine show the same extent of bank erosion (Fig. 9).</td><td>The scour depth is approximately two-fold lower than with geotextile (Fig. 10).<br>Erosion is close to 0 m³/m² without toe protection or with riprap and increases gradually when a single or double fascine are installed (Fig. 9).</td></tr><tr><td>With geotextile</td><td>Without toe protection, the results are widely dispersed [Figs. 8(f-h)].<br>Erosion with riprap or double fascine as toe protections approaches 0 m³/m². In the configuration with a single fascine, a slightly higher level of erosion is observed (Figs. 9 and 10).</td><td>The riprap configuration presents dispersed volumes of erosion resulting from the variability in the installation of the geotextile anchorage behind the riprap [Figs. 8(e) and 9].<br>The configurations with riprap and fascine are associated with the same extent of bed erosion. The double fascine, which presents the smallest bank erosion, has the highest bed erosion (with or without a geotextile) (Fig. 9).</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Reactions of bank protections to bankfull discharge: (a) bank erosion; (b) toe and bank erosion; (c) bundle falling; (d) erosion at heads and toe; (e) riprap destructuring and toe erosion; and various reactions of geotextile to flooding, including (f) ripped off and buried; (g) erosion at heads; and (h) remained in place.</figcaption></figure>

buried under sediments [Fig. 8(f)]. On the second meander, the bank was strongly eroded at the head and toe of the geotextile [Fig. 8(g)]. On the third meander, the geotextile remained well fixed [Fig. 8(h)]. Only mild erosion of the riverbank occurred at its head. From direct observations, this variability could be linked to how well the geotextile was anchored at bank toe.

The bed erosion pattern was also changed by the geotextile: deeper scour holes were observed at bank toe in configurations with compared with those without (Figs. S1 and S2). The scour hole at the toe below the geotextile protection was mainly located between the apex of the meander and its downstream end. This observation confirmed predictions based on the guidelines. When geotextile was combined with other protection, the toe erosion dug below the fascine bundle protection [Fig. 8(c)] or around the bottom layer of riprap [Fig. 8(e)]. Without a geotextile on the bank, the erosion pattern was different [Figs. 8(b and e)].

## Quantitative Effects of Bank Protections on Geomorphic Adjustments

### Volume Distribution

Table 2 summarizes the observations relating to sediment erosion with the four toe protections with or without geotextile. In essence (Fig. 9), without geotextile, the banks were massively eroded whatever the toe protection, and the bed was subject to marginal to minor erosion. With geotextile, bank erosion was decreased, although to a variable extent (between configurations and also for a given configuration, demonstrating some random variations). The bed was consistently more scoured when the bank was not eroded. The general pattern of bed erosion, whatever the bank configuration, was the inverse of the pattern of bank erosion. Thus, when bank erosion was low, bed erosion was high (and vice versa).
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Volume of sediments eroded during each run ($Q_2$), from the bank and bed sides as defined in Fig. 6. The four bank toe protections were tested on three meanders (three bars) with geotextile in gray and without geotextile as solid. Volumes (m³/m²) were estimated at prototype (1:1) scale. There are different vertical axes because bank erosion was much more extensive than bed erosion.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Topographical transverse profiles of each experimental configuration before (continuous line) and after (dashed line) the flood event. The values represent the 95th percentile of positive or negative change in bed elevation between before and after the bankfull stage. They are extrapolated to 1:1 scale and expressed in meters. The intervals give the minimum and maximum results between the three meanders.</figcaption></figure>

Erosion with a fast rising limb tended to be higher [average factor 1.4 ± 0.06 (mean ± standard deviation)] than with a slow rising limb. These two hydrographs [Fig. 3(b)] were tested in two configurations: no protections and a single fascine with a geotextile.

## Topographic Evolution

The transverse profiles shown in Fig. 10 schematically represent the elevation change proxy compared with the initial conditions (95% quantile) in all bank configurations as measured after the experiment. This representation allows us to illustrate the overall results and focus on scour depth, which is a key design criterion.

From these images, the bank was best stabilized by the configuration with a geotextile and a double fascine as toe protection.

Without a geotextile, complete bank erosion down to the top level of the toe protection was observed whatever the toe configurations. The inner-bank deposition, forming a gravel bar, was also higher without the geotextile. The river channel actually shifted laterally in the direction of the outer bank, mimicking meander migration in the field. Bed erosion at the toe protection remained high only for the double-fascine configuration.

With the geotextile, the riprap and single-fascine configurations presented similar profiles at the end of the experiments. The configuration with geotextile and without toe protection had higher
inner bed deposition, lower scour hole depth, and higher bank erosion than the riprap or single-fascine configurations. In contrast, the final transverse profile, with double fascine, showed a lower inner bed deposition, a deeper scour hole, and reduced bank erosion compared with other configurations.

From all the configurations tested (Fig. 10), we can conclude that the better the bank stabilization, the deeper the scour hole and the smaller the inner bed deposit.

## Hydraulic Pressure Distribution inside Meanders

Pore pressure measurements revealed no difference in the pressure behavior between the rising and falling limbs of the hydrographs. The pressures were also measured for two hydrograph shapes

[Fig. 3(b)]. Here again, no difference was observed between the fast rising limb and the slow falling limb.

Fig. 11(b) presents $P^*$ values to allow comparison of the pore pressure between configurations and transversal positions. For example, hydraulic pressure at a specific position (e.g., river center), for a specific configuration (e.g., double fascine with geotextile) was, on average, lower than $P^*$ values for another position (e.g., bank side) with the same bank configuration (with geotextile and double fascine), or $P^*$ values for the same position (river center), but with another bank configuration (e.g., no toe protection without geotextile). The main observations that can be drawn from the thousands of measurements are as follows:

* The pressure varied depending on the technique implemented to protect the riverbank. At each position, pore pressure increased

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** (a) Schematic representation of a riverbank with fascine before (continuous line) and after (dashed line) a flood event; (b) pore pressure gradient between bank configurations (i.e., toe protection type and presence/absence of geotextile) depending on the position in the transversal transect (i.e., river center, bed side, bank toe, and bank side); and (c) schematic conceptual analyses of the pressure gradient presented in plot (b).</figcaption></figure>
from double fascine to fascine to riprap to no protection. There was a large difference between a double fascine and no protection.

* With similar bank toe protection, the configurations with the geotextile presented lower pressure values than those without a geotextile.

* With the geotextile, the fascine and riprap presented similar ranges of values.

* Pore pressure tended to increase transversely from the river's center to its bank side. There was an overpressure on the river-bank compared with the riverbed.

A closer look revealed a gradient only from bed side to river center for configurations combining both toe protection and a geotextile, a gradient from bank toe to river center for configurations with toe protection and without a geotextile, and a gradient from bank side to river center for configurations without toe protection.

# Discussion

## Process-Based Design of Bank Protections

To transcribe the effects of various bank toe protections on bank destabilization dynamics, various destabilization processes are summarized in Fig. 12, and the key lessons drawn from our experiments are discussed.

All types of erosion observed in this study with the various bank configurations have been previously observed in a scaled physical model (Recking et al. 2019) and in the field (Anstead and Boar 2010; Peeters et al. 2018; Leblois et al. 2022).

Without a geotextile, strong bank erosion was driven by entrainment following overflow over the toe protection. Migration of the river channel toward the outer bank due to secondary currents and internal deposition (Odgaard 1989; Keown et al. 1981) caused outer bank erosion and thus reduced the bank slope in combination

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Destabilization dynamics of outer banks in meanders equipped with bank protections: (a) postbankfull stage top view without geotextile; and (b) front view with geotextile.</figcaption></figure>
with shallow erosion at the bank toe (Fig. 10). In the field, cohesion of bank material or the presence of vegetation cover would limit fluvial entrainment following overflow. Our scaled physical model thus represents an upper bound of this process, mimicking a site where the bank is composed of only bare, noncohesive alluvial material, such as would be the case with a gravel bar.

With a geotextile, whatever the toe protection used, the erosion process leading to overall bank destabilization started with the formation of a deep scour hole at bank toe. When local erosion is deep, bank toe protections may fall into the scour hole and plants may be uprooted, as described by Edmaier et al. (2011). Because local erosion drives failure of the bank toe protection, the general shear stress values at reach scale proposed in design guidelines are not sufficient to comprehensively define stable SWBEs (Schiechtl and Stern 1997; Leblois et al. 2024b). Field feedback has consistently highlighted that for SWBEs implemented to stabilize riverbanks, in particular for fascine and willow spiling, the most prominent issue is undermining, i.e., erosion below the structure (Anstead and Boar 2010; Leblois et al. 2022), which restricts future vegetation growth and the durability of the protection.

With a geotextile in combination with deeply anchored toe structures, such as riprap or double fascines, bank erosion was reduced compared with the configuration with a single-bundle fascine (Fig. 9) because sediments could not pass through or below the bank toe structure. This created the conditions for the second process contributing to bank destabilization dynamics (single fascine in Fig. 12). However, successful bank stabilization, such as achieved with a double fascine, led to a deeper vertical scour at the bank toe (Fig. 10). The relation between bank slope reduction and vertical scour development at the bank toe was also visible on configurations without a geotextile. In such cases, only local erosion occurred around fascines, and no deep vertical erosion developed. Thus, the riverbank slope reduction is a major design factor to consider when seeking to limit bank toe erosion.

In the model, the maximal water depth in meanders measured at $Q_2$ represented two-fold the mean water depth in a linear upper section at $Q_2$. This result agrees with results for the Red River from Thorne (1997), who reported ratios between two and four for $2 < R/W < 4$. According to Blanchet and Morin (1990), the recommended design for the base level of a riprap protection with a bank slope of $3H/2V$ goes as deep as 9 to 11 cm at our laboratory scaling, which corresponds to 2.25 to 2.75 m at field scale. In other words, with a fixed bank, the vertical scour could theoretically descend to these depths without the (time and space) constraints of the experiment. This represents at least two times more than what was observed in the flume. The fascine piles were 8 cm long in the scaled physical model and were laid deep enough to stabilize the fascine (Fig. S2).

Riprap laid as deep as the bundles of the double fascine only sometimes fell into the scour hole, resulting in bank erosion [Figs. 8(e), 10, and S2)]. The better stabilization achieved with the double fascine compared with riprap was probably linked to the anchorage of the bundles and the rigidity of the structure. The implication in terms of design for SWBE bank toe protections is that they must block bank sediments from falling into the scour hole. They must therefore be dense between the green line and the estimated scour depth for the flood which they are designed to withstand. However, it is recommended that the scour depth be estimated from both field measurements and multiple theoretical scour estimations (USDA 2007b; Maynord 1996), because, as predicted by Blanchet and Morin (1990), scour depths in the flume may lead to overestimations.

To compare morphological adjustments with other scaled physical models and further field observations, the scour hole depth was

<table><caption>**Table 3.** Ratio of the scour hole depth, represented by the 95th percentile of negative change in topography after the bankfull stage in the scaled physical model with sediment dimensions $D_{90}$</caption><thead><tr><td>Toe protection</td><td>No geotextile</td><td>Geotextile</td></tr></thead><tbody><tr><td>None</td><td>1</td><td>6</td></tr><tr><td>Riprap</td><td>2</td><td>9</td></tr><tr><td>Single fascine</td><td>4</td><td>9</td></tr><tr><td>Double fascine</td><td>7</td><td>12</td></tr></tbody></table>

normalized with the $D_{90}$ (3.2 mm in our runs) of the sediment mixture [Table 3 and Figs. 3(a) and 10]. When the riverbank was stabilized, this dimensionless scour depth was approximated as 10 for a bankfull discharge in the scaled physical model. Further research will be required to estimate whether a simple criterion such as 10 to 15 times the $D_{90}$ could be used in gravel-bed rivers to rapidly estimate the required anchorage to ensure bank stability. In these studies, the equations to estimate scour depth from Degoutte (2006), USDA (2007b), and Blodgett (1986) account for grain sizes.

The extent of bank erosion and scour hole development was slightly higher in the specific case of experiments with rapidly increasing hydrographs [Fig. 3(b)]. Similarly to immediately after river restoration projects in the field, the experiment was started without hydromorphological preadjustment. Consequently, strong erosion was observed with a fast rising limb. In the case of a slow rising flood limb, the riverbed may have had the time to morphologically adjust and organize before reaching high shear stress. As a result, the bed and bank erosion would be comparatively reduced. In terms of design, it is important to remember that after reshaping, and until morphological adjustment has occurred, a river is more unstable and sensitive to floods and more prone to bank destabilization (Malavoi and Bravard 2011).

As observed in Figs. 8(f–h) and based on the variability of the volume eroded in the presence of a geotextile (Fig. 9), the inclusion of the geotextile in installations had a strong impact on riverbank stability in the scaled physical model. This observation highlights the care that must be taken when using geotextiles. The geotextile must be bonded to the toe protection and buried deeply in the bank behind (Leblois et al. 2022; Zeh 2010). Other SWBEs, such as wil-low sprout mattresses, could be tested in the flume to compare their effects with the positive effects and limitations of geotextiles.

In summary, SWBEs in meanders or where there is a risk of incision may be useful but must be designed to resist the consequences of the scour hole that develops below the green line. At least two main methods can be used to reduce bank scouring (Bigham 2020): (1) structures deviating water flow resulting in the reduction of water velocity at riverbanks and of the centrifugal force of the secondary currents induced (Matsuura 2004; Odgaard and Mosconi 1987; Biswas and Barbhuiya 2018); and (2) structures directly protecting the bank, which should also ideally increase the roughness and therefore reduce the water velocity and formation of eddies due to turbulence.

The first type of protection corresponds to stream bards, vanes, bendway weirs, and spur dikes (Baird et al. 2015; USDA 2007a). These structures are often made of riprap, but they can also be NBS made of large pieces of wood, such as log jams, which enhance ecological restoration responses (Bureau of Reclamation and USACE 2015; Shields et al. 2004; Wheaton et al. 2019). The second type of structures refers to buried riprap key (Bonin et al. 2013; USDA 1996), and could be extended to NBS in the form of buried trunks, root wads, or fascines to improve restoration cobenefits (Yochum and Reynolds 2020; ÖWAV 2021; Cramer 2012; Wood and Jarrett 2004; Rafferty 2017). In western Europe, interventions
for riverbank stabilization combining vegetation establishment on riverbanks with large wood techniques to stabilize the bank below the green line are not common.

## Processes Dominating Bank Erosion

Fluvial entrainment, mass failure, and seepage are the main processes leading to bank erosion in scaled physical models (Lawler 1992; Chassiot et al. 2020). To dissociate the effects of these processes in this study, we measured hydraulic pressure.

The capillary effects in bank material contributing to the pressure measurements are detailed in Supplemental Materials S4. The test showed that these effects or reading errors account for <5% of the maximal pressure values presented in Fig. 11(b).

Applying the equation for the hydraulic gradient in meander, Eq. (2) [from USACE (1994)], and setting coefficient $C = 1$, the water level in outer meanders ($\Delta h$) could be superelevated by up to 16 mm compared with the river's center in the scaled physical model, due to centrifugal forces and secondary currents. The centrifugal effect could thus explain 76% of the maximal pressure gradient, given that 22 mm was the maximal value measured in a transect [Fig. 11(b)]. The superelevation in a meander's extrado could explain more of the measured hydraulic gradient in most cases since they presented a lower pressure gradient

$$ \Delta h = C \frac{V^2 W}{gR} \qquad (2) $$

The hydraulic pressure gradient extended from the lateral position at which the bank was eroded [Fig. 11(c)] down to the river's center. The amplitude of the water level superelevation is proportional to the square of the flow velocity in the direction tangential to the meander's shape [Eq. (2)]; thus, it will be higher in free flows than in riverbank material. No pressure gradient was observed in riverbanks stabilized by configurations combining toe protection and a geotextile [Fig. 11(c)].

The pressure difference between configurations (lowest for the double fascine and highest for no protection) was probably linked to morphological changes in the distinct setups. In fact, in the setup with a double fascine, due to the scour hole, the riverbed was lower than in the no-protection configuration. With greater bed erosion, the water level driving the hydraulic pressure was consequently lower.

In the absence of clay in the bank material, which would decrease its permeability, interstitial flows were strong enough to adjust pore pressure in real time based on the varying flow level and bed and bank morphology. As a consequence, there was no transient excess pore pressure in the bank during the falling limb after the flood peak, whatever the recession speed. A pore pressure excess in the bank side could increase bank erosion due to seepage during flood withdrawal when the running water no longer compresses the bank (Rinaldi et al. 2004). The behavior would be expected to be different if low-permeability material or established vegetation were present.

To summarize the exploitation of pressure measurements in meanders, in our model:

* capillary effects are negligible;

* pressure behavior between the rising and the falling limb of the hydrograph is the same, meaning that there are marginal retention and seepage effects;

* pressure behavior is the same whatever the speed of the falling limb of the hydrograph applied, meaning that retention and seepage played no part in the erosion measured;

* centrifugal forces and secondary current effects explain the hydraulic pressure gradients due to water level superelevation in free surface flow; and

* hydrostatic pressure explains the flat pressure behavior in bank material.

Subaerial weakening processes (Chassiot et al. 2020) were not taken into account in this study. Seepage, as a weakening process affecting earth deposits or as a mass-failure trigger, has a low impact in the bank configuration studied. In the absence of cohesion, the bank is not suitable for the study of cantilever or slip-type failures (Thorne and Tovey 1981; Thorne 1982). Therefore, no mass-failure phenomena were observed.

The first part of the discussion emphasized the large amount of direct bank erosion due to overflow in the absence of geotextile, and at the bank toe in the presence of the geotextile. Therefore, as confirmed by the pressure behavior in meanders and as expected for noncohesive sediments (Couper 2003; Julian and Torres 2006; Hooke 1979), the main bank erosion process in this scaled physical model is fluvial entrainment. The various bank protections tested did not increase the effect of seepage due to water retention in the bank nor the effect of mass failure.

## Conclusion

This study used a scaled physical model to determine the dynamics of destabilization of riverbanks implemented with SWBEs.

In terms of the dynamics of bank destabilization in meanders, even with bank toe protection, scour hole formation is the starting point of bank failure and needs to be controlled; once the scour hole has formed, bank sediments are free to fall in and to be transported by the flows. This removes the soil support for the SWBE and vegetation, which cannot grow in the absence of a substrate. The scour hole depth must be estimated and measured when designing the SWBE. A double fascine can help to limit bank erosion following formation of a scour hole. Other techniques implemented below the green line—where stable vegetation cannot grow—such as buried trunks, root wads, or riprap, could also be implemented to fix the bank toe and riverbed.

In the absence of direct bank protection—such as geotextile or herbaceous cover—surface erosion leads to bank destabilization too. Furthermore, the reduction of bank slope proved to be a very efficient means to reduce scour hole depth in our model.

Finally, fluvial entrainment led to most erosion in banks implemented with a SWBE in our flume. Effects would probably be comparable in gravel bed channels with sufficiently low clay content in their bank material to prevent excess pore pressure-related bank failures during flood recession. In such cases, riverbank drainage is not necessary prior to SWBE implementation. The absence of plant growth or cohesion in the well-sorted sediment mixture used in our scaled physical model creates a possible gap between the results presented and field experiments. Further field measurements or numerical modeling could help to fill this research gap.

## Data Availability Statement

Some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

## Acknowledgments

We would like to thank all the people who helped with flume installation and ensured the smooth running of the channel: Christian
Eymond-Gris, Hervé Bellot, Firmin Fontaine, Marie Didier, and Emma Schmitt. This work has been funded by the Office Français de la Biodiversité, INRAE AQUA's Department, and Suez Consulting.

## Author Contributions

Solange Leblois: Conceptualization; Data curation; Formal analysis; Funding acquisition; Methodology; Visualization; Writing – original draft; Writing – review and editing. Guillaume Piton: Conceptualization; Methodology; Project administration; Supervision; Writing – original draft; Writing – review and editing. Alexis Buffet: Data curation; Methodology; Resources. Alain Recking: Conceptualization; Project administration; Supervision; Writing – original draft; Writing – review and editing. André Evette: Conceptualization; Funding acquisition; Project administration; Supervision; Writing – original draft; Writing – review and editing.

## Notation

The following symbols are used in this paper:

$C$ = superelevation formula coefficient (–);

$D_{XX}$ = particle size that XX% of the samples are equal to or smaller than in a given sediment size characterization using the Wolman pebble count procedure (m);

$d$ = water depth (m);

$Fr$ = Froude number (–);

$g$ = acceleration of gravity: 9.81 (m/s²);

$P$ = pressure values measured (m/m);

$P^*$ = dimensionless pressure gradient (–);

$Q$ = water discharge (m³/s);

$Q/W$ = water unit discharge (m³/m/s);

$R$ = radius of curvature (m);

$Re$ = Reynolds number (–);

$Re^*$ = grain Reynolds number (–);

$S$ = riverbed slope (m/m);

$V$ = mean water velocity (m/s);

$W$ = active riverbed width (m);

$\Delta h$ = water level superelevation in meander's extrado due to centrifugal force and secondary currents (m);

$\lambda$ = scaling factor = 1:25;

$\tau$ = shield by shear (N/m²);

$\tau^*$ = dimensionless Shields stress (–);

$\tau^*/\tau_{cr}^*$ = transport stage ratio (–); and

$\tau_{cr}^*$ = critical Shields stress (–).

## Supplemental Materials

There are supplemental materials associated with this paper online in the ASCE Library (www.ascelibrary.org).

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