# Soil and water bioengineering on moutain streambanks: Lessons learnt from extensive feedback on projects in the Alps

Juliette Rousset, Guillaume Piton, Marie Lou Hirschy, Delphine Jaymond, André Evette

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## ► To cite this version:

Juliette Rousset, Guillaume Piton, Marie Lou Hirschy, Delphine Jaymond, André Evette. Soil and water bioengineering on moutain streambanks: Lessons learnt from extensive feedback on projects in the Alps. Ecological Engineering, 2026, 226, pp.107910. 10.1016/j.ecoleng.2026.107910. hal-05490431

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# Soil and water bioengineering on mountain streambanks: Lessons learnt from extensive feedback on projects in the Alps☆

Juliette Rousset<sup>a, *,</sup> Guillaume Piton<sup>b</sup>, Marie Lou Hirschy<sup>a</sup>, Delphine Jaymond<sup>a</sup>, André Evette<sup>a</sup>

## ARTICLE INFO

Dataset link: Extensive feedback on soil and water bioengineering projects in the Alps (Original data)

**Keywords:**

Soil and water bioengineering
Alpine streambank
Shear stress
Failure analysis
erosion control
Restoration

## ABSTRACT

Channel mobility is essential to maintaining a river's healthy biogeomorphic and ecological functions. Although, local erosion processes can sometimes require stabilization to protect infrastructure. Soil and Water Bioengineering (SWB) techniques use vegetation as living construction materials to provide effective erosion control while preserving riparian ecological functions. Designing these structures depends significantly on expert knowledge and technical documentation that shares best practices and experiences, and provides specific hydraulic stress resistance thresholds. There is currently only limited documentation regarding the use and mechanical resistance of SWB on mountain streambanks where the harsh climate and steep slopes put a strain on vegetation growth and recovering, which in turn makes it challenging to design effective SWB structures. This study aims to fill this gap by providing an exhaustive record of SWB works throughout the French Alps, with additional sites in Italy and Switzerland. Hydraulic parameters, vegetation dynamics, and failure mechanisms were assessed across 48 SWB works. Drawing on our expertise, we assessed the key factors governing the implementation and success of SWB works in mountain streambanks.

Our survey revealed that the works could be classified along a "naturalness" gradient, with vegetation playing different structural roles. Riprap was commonly used at the bank toe and technical choices depended mostly on the cultural context of the watershed and the local knowledge of SWB rather than on the hydromorphological context or the assets to be protected. Ultimately, this study enhanced our understanding of the hydraulic stresses experienced by these structures and the most prevalent causes of failure.

## 1. Introduction

Bank erosion is a natural process essential for supplying sediment materials to rivers (Fox et al., 2016) and for maintaining the river's healthy biogeomorphic functions (Florsheim et al., 2008; Newson Malcolm and Large, 2006), which must be preserved (European Commission, 2000). Where buildings or infrastructure are threatened by bank destabilization and cannot be relocated, bank protection can be implemented through conventional engineering techniques (e.g., riprap or gabions) or Soil and Water Bioengineering techniques (SWB). The latter preserve the natural biotic and abiotic structure of the streambanks by stabilizing them with living and dead plant material integrated into structures inspired by functional, i.e., self stabilizing, natural streambanks (Bonin et al., 2013; Preti et al., 2022; Schiechtl, 1997). Vegetation stabilizes bank substrates through root development (Simon et al., 2013)

and protects the surface soil with a flexible aerial system that increases rugosity and creates a carpet effect (Gray and Sotir, 1996). In addition to protecting streambanks from erosion, SWB techniques offer numerous co-benefits to the ecosystem (Janssen et al., 2019; Symmank et al., 2020; Tisserant et al., 2021), thus justifying their classification among the so-called Nature-Based Solutions (Cohen-Shacham et al., 2016; Preti et al., 2022). The ability of SWB structures to resist disturbances improves over time (Leblois et al., 2016; Peeters et al., 2018), and they become fully effective two to four years after completion, once the vegetation re-establishes itself (Li and Eddleman, 2002).

Historically, these techniques have mostly been developed empirically, based on technical experience, expert feedback and observations of naturally stable banks. Numerous technical guides provide qualitative recommendations for implementing SWB (AMEC, 2012; Baird et al., 2015; Gray and Sotir, 1996; Peeters et al., 2020; Schiechtl, 1997),

<footnote><sup>a</sup> Univ. Grenoble Alpes, INRAE, LESSEM, 2 rue de la papeterie BP 76, F-38402 Saint-Martin-d'Hères, France</footnote>

<footnote><sup>b</sup> Univ. Grenoble Alpes, INRAE, CNRS, IRD-Grenoble INP, IGE, Grenoble, France</footnote>

<footnote>* This article is part of a Special issue entitled: 'SBEE2025' published in *Ecological Engineering*.</footnote>

<footnote>* Corresponding author.
E-mail address: juliette.rousset.1@inrae.fr (J. Rousset).</footnote>
supplemented by a number of scientific publications on quantitative criteria such as flow velocity, shear stress or stream power computed for an average stream cross-section, and maximal known discharge undergone by the works (Leblois et al., 2022, 2024; Recking et al., 2019). The latter studies provide ranges of stress resistance for different types of SWB techniques at different lifetimes. Depending on shear stress and other site features, they provide indicative values to help managers to select suitable techniques. However, most of the existing literature focuses on lowland rivers.

European mountain rivers have been submitted to extensive anthropogenic pressure in recent decades (Comiti, 2012). As a result of economic development, buildings, roads and infrastructure have spread, often encroaching on rivers. In this context, structures at risk due to bank erosion have been protected through traditional measures, for example, with riprap and concrete used to stabilize streambanks and prevent erosion (Petts, 1989). Managers who seek to apply alternative techniques that promote bank stabilization while preserving certain ecological functions, such as SWB ones, encounter significant technical challenges when implementing them on mountain streambanks. Furthermore, alpine mountain rivers have nivo-glacial hydrologic regimes. Consequently, spring snowmelt raises water levels, hindering vegetation recovery, while lower temperatures (Körner and Paulsen, 2004) and shorter growing seasons (Körner et al., 2023) at higher elevations further slow plant growth. High seasonal flows combined with steep channel gradients, frequently between 1 and 10%, give these rivers significant sediment transport capacity and high shear stress (Wohl, 2010). Mountain rivers exhibit a wide diversity of channel morphology, mainly driven by slope, channel confinement, flow, sediment supply and vegetation (Montgomery and Buffington, 1997). These rivers have coarse sediment beds and loads and their flows experience changing regimes – super-critical and subcritical with many hydraulic jumps – thus differing them from lowland rivers and implying high stress along the banks. On the steeper alpine streams, the natural banks are generally stabilized by boulders intermingled with woody vegetation (Rousset et al., 2025b). Therefore, as a nature-inspired concept, one possible SWB approach would be to use vegetated riprap along the banks or at the bank toe, with riprap of a diameter consistent with the natural forms observed on the surrounding banks.

Due to the empirical design inherent to SWB techniques, it is essential to refer to specific documentation that considers the unique characteristics of mountain streams. However, the sources, in both gray and scientific literature, that report on SWB works in specific mountainous areas are quite limited. In the previous century, the Austrian engineer, H. M. Schiechtl, published several books and guides in English and German, describing techniques and species to use in this context (Schiechtl, 1992; Schiechtl and Horstmann, 1980). In 2013, a European Union InterReg project in France and Switzerland produced a technical guide in French that summarized the hydromorphological functions in mountain streams, introduced species, and presented four pilot projects and feedback on two structures located between 400 and 1300 m a.s.l. (Bonin et al., 2013). Finally, in 2014, the Swiss Bioengineering Association magazine (Ingenieurbiologie) published an issue containing three papers on SWB structures located at around 700 m a.s.l on Swiss Alpine rivers (Bettler and Herzog, 2014). However, this literature is too limited to present an exhaustive view of SWB practices and does not include information on the mechanical stresses that these structures can undergo along mountain streambanks, even though the torrential regime of certain mountain streams can lead to particularly high stresses.

The lack of guidelines and feedback from the field is a serious obstacle to the development and democratization of SWB techniques. In spite of this, some river managers have already implemented SWB projects based on documentation for lowland rivers and their own knowledge of how alpine rivers function. Feedback from these few existing structures on their successes and failures could provide valuable information to improve and refine high-mountain SWB techniques.

The objective of this paper is to assess the success of mountain SWB

projects and to learn how they fulfill their stability functions in response to the specific biotic and abiotic environmental stresses of mountain streams. An interdisciplinary approach was adopted to investigate how biogeomorphological factors influence the design, implementation, and performance — whether successful, degraded or failed — of the different SWB techniques. We aim to evaluate the degree of naturalness of these techniques in relation to the biogeomorphological and hydraulic conditions, local vulnerability and social context. Furthermore, the study seeks to refine and update the threshold values of hydraulic stresses that such structures are capable of withstanding in mountainous environment. To do this, we provide a large-scale technical feedback on SWB structures on riverbanks in the European Alps, specifically in France, with few works in Italy and Switzerland. It aims to identify key lessons to guide managers in designing suitable SWB structures. To achieve this, we analyzed 48 projects for structural integrity, vegetation development and the hydraulic stress the structures were able to withstand.

## 2. Materials and methods

### 2.1. Site selection

Sites were selected throughout the European Alps, though mainly in France, with additional sites in Italy and Switzerland. A minimum elevation threshold was defined based on the lower limit of the montane belt, varying according to exposure from 600 m a.s.l. for north-facing slopes to 900 m a.s.l. for south-facing slopes. Above this threshold, streams have a hydrological regime influenced by snow and glacial melt, with a channel slope typically steeper than 1% (Wohl, 2010) accompanied by characteristic vegetation (Körner, 2003). Below this elevation threshold, SWB techniques are already well-documented, but the riparian vegetation and hydromorphology differ significantly, with generally milder slopes and a less harsh climate.

We contacted the sixteen river basin agencies in the French Alps to ensure a comprehensive inventory of the SWB techniques implemented there. In Switzerland, the selected sites were those included in the Géni'Alp interreg project (Bonin et al., 2013). In Italy, we established contact with the managers in the Bolzano and Piemonte regions, therefore our study reflects SWB practices only within these two areas. Each selected site was surveyed between June and July 2024.

We selected sites with SWB structures where vegetation played a structural role by contributing, either partially or entirely, to stream-bank stabilization (Schiechtl and Stern, 1997). The different techniques were classified into four types along a decreasing gradient of naturalness, i.e., the structural importance of the vegetation decreases progressively and the use of mineral elements takes on a more central role (analogous to Cavaillé et al., 2013) (Fig. 1):

- Type 1: **Pure bioengineering** structures encompass SWB techniques in which stability is fully ensured by vegetation. These techniques involve the use of living plant material, such as cuttings, stakes or plants, organized in fascines, brush layers or wattle fences (Gray and Sotir, 1996; Schiechtl and Stern, 1997). The main techniques were: fascines consist of live cuttings tied together in linear cylindrical bundles and installed in shallow trenches; brush layers are long live cuttings embedded into the streambank between layers of soil; brush mattresses are long live cuttings laid with branches parallel to the slope and covered with a shallow layer of soil to form a mattress (Li and Eddleman, 2002); brush matting is a tangled mass of large branches, twigs and trunks solidly attached together; and finally, wattle fences are live cuttings interwoven around stakes driven vertically into the bank (Lachat, 1994).

- Type 2: **Cribwalls** are structures made of logs and filled with live brush layers, boulders and sediment (Krymer and Robert, 2014).

- Type 3: **Mixed bioengineering** techniques include methods that combine riprap bank toe protection with pure bioengineering techniques on the banks (Cavaillé et al., 2013). This category also
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Distribution of SWB techniques along the naturalness gradient and definitions of each structure type. Underlined words correspond to the technique illustrated in the photo. The same color code will hereafter be used in all figures.</figcaption></figure>

included vegetated riprap, which was riprap with cuttings inserted between the blocks.

- Type 4: **Riprap with vegetation** is a technique where vegetation plays a secondary role in stability. The top of the riprap extends higher than the biennial flood level (Q2). We chose to include this type of structure in our sample to illustrate the diversity of the techniques implemented in the study area and to discuss the use and sizing of the riprap toe protection used in the mixed techniques. This category is at the extreme end of our naturalness gradient, where mineral materials, as opposed to vegetation, are mostly used.

## 2.2. Structural descriptions

### 2.2.1. Structure type, objectives and status

To describe the bioengineering techniques precisely, we examined the structural components of each project. We then classified the structures along the naturalness gradient according to the type of technique used (Fig. 1 and Table 1). We interviewed the practitioners to identify protection assets, which we classified into five categories based on anthropic importance (Building > Road > Forest road > Agricultural land and parks > No asset, see Appendix A), and traced the history of the structures, including their construction phase and notable past events such as droughts or floods. These discussions have evolved into more informal conversations with some of the experts about topics such as decision making and design of SWB. The insights gained from these exchanges are presented in the Discussion section, although they were documented less systematically than the primary Results.

The stability of the structures was assessed based on observed

failures, divided into two categories: 1) mechanical failures such as bank toe scouring, and 2) failures related to poor vegetation establishment according to the criteria defined by Leblois et al. (2022), including, for instance, an unsuitable choice of species. In addition, a mechanical status index was established based on the type and severity of the failures: “good” (absence of failure), “degraded” and “failed” (no longer fulfilling bank protection). The degraded category included structures that still fulfilled their stabilization function but exhibited signs of degradation that could lead to long-term failure (e.g., local scouring, small upstream or downstream erosion). These initial failures might worsen, particularly in the event of a major flood or insufficient vegetation regrowth, e.g., due to drought. It has been demonstrated that SWB structural failures are often initiated by local erosion expanding from inside the structure according to non-linear patterns (Citation).

Every 50 cm, the woody species in contact with the transect were counted and identified. The position of the transect was chosen to be representative of the vegetation observed on the SWB structure. At least one transect was always set up at the bank toe, and additional transects were included at some sites where the vegetation composition at the top of the bank was very different, in order to cover the full range of the existing woody species diversity. For sites less than 25 m long, the transect survey was carried out along the entire length of the structure.

The results from this line-point intercept survey were then used to estimate vegetation cover and to quantify the frequency of the dominant species (Halbritter et al., 2020). Woody species diversity along the transects was assessed through two metrics: species richness (S), defined as the total number of species present at the site; and the Simpson Index (Simpson, 1949), which accounts for both species richness and relative abundance. The Simpson Index ranges from 0 to 1, with values closer to 1 indicating greater homogeneity within the stand. Given our focus on abundant species, this index was used to represent presence/absence relationships, as it assigns greater weight to more dominant species. We used the *diversity* function from the Vegan package in R to estimate the Simpson index values (Oksanen, 2013).

<caption>**Table 1**</caption>

SWB projects distributed according to technique SWB naturalness gradient for all 48 structures.

<table><thead><tr><th>Type</th><th>Techniques</th><th>Total</th><th>Sites N°</th></tr></thead><tbody><tr><td rowspan="4">Pure bioengineering</td><td>Wattle fence</td><td>1</td><td>1</td></tr><tr><td>Fascine</td><td>5</td><td>2-7</td></tr><tr><td>Brush matting</td><td>2</td><td>8, 9</td></tr><tr><td>Brush mattress</td><td>3</td><td>10-12</td></tr><tr><td rowspan="3">Crib wall</td><td></td><td>8</td><td>13-20</td></tr><tr><td>Riprap + Brush layers</td><td>10</td><td>21-30</td></tr><tr><td>Riprap + Brush mattress</td><td>8</td><td>31-38</td></tr><tr><td rowspan="3">Mixed bioengineering</td><td>Riprap + Fascine</td><td>1</td><td>39</td></tr><tr><td>Vegetated riprap</td><td>2</td><td>40, 41</td></tr><tr><td>Riprap + cribwall</td><td>4</td><td>42-45</td></tr><tr><td rowspan="2">Riprap with vegetation</td><td>Riprap + Brush layers</td><td>2</td><td>46, 48</td></tr><tr><td>Riprap + cribwall</td><td>1</td><td>47</td></tr></tbody></table>

## 2.3. Stream hydro-geomorphological characteristics

We also characterized the geomorphic and vegetation features of the stream channels following Rousset et al.'s (2025b) approach tailored to natural mountain-stream banks. The methodology is described below.

### 2.3.1. Geomorphological measurements

Bed slope (i) was determined at the channel axis with a TruPulse® 300× laser telemeter, with centimeter accuracy, according to a linear fit
of six points. The topographical points were measured at key locations along a distance ten times the width of the channel to account for possible slope breaks. Where the stream flow was too great to stand in the stream, the slope was estimated from LiDAR data and GeoPortal information (Institut national de l'information géographique et forestière, 2024) (Sites: 20, 28, 30, 33, 45 and 48). Two to three cross-sections perpendicular to the SWB project were measured with the above-mentioned laser method at each site. To best reflect the channel morphology and the SWB structure, the first point was taken at the top of the bank, followed by points at each significant change in slope and/or structural unit until the top of the opposite bank was reached. Where the flow was too great to cross the stream, the streambed section was approximated assuming a rectangular profile (Sites: 8, 11, 17, 28, 33, 39 and 40). We estimated bank slope with a linear fit on the horizontal and vertical total distances measured on the bank. To characterize bed sediments, we measured the b-axis (the limiting dimension when a particle passes through a measuring mesh) of 100 particles randomly selected from the whole bed at the reach scale, following the Wolman (1954) surface counting method. Grain size distribution was characterized by quantiles, i.e., X% of the particles with a diameter smaller than $D_x$.

### 2.3.2. Discharge data

Three different reference discharges were used to compute hydraulic variables: the 2-year return period $Q_2$ and the 100-year return period $Q_{100}$ peak discharges, which provide the hydrological context of the stream, and the historical maximum discharge experienced by the structure $Q_{max}$. For the French sites, the 2-year and 100-year peak discharges, as well as the catchment area at the site location ($A_{work}$), were extracted from the SHYREG database (Arnaud et al., 2016). The SHYREG method estimates flood discharge quantiles based on a regionalized hydrological model calibrated on the closest downstream gauging station, coupled with a stochastic rainfall generator running over 100,000 simulated years. It provides estimates for the entire French river network, every few kilometers or at each stream confluence. For each French site, we obtained historical discharge data and the catchment area ($A_{station}$) at a nearby gauging station from Hydroportal (https://www.hydro.eaufrance.fr). We then extracted the maximum discharge experienced at the gauging station since the year the SWB structure was built ($Q_{maxstation}$). The maximum discharge experienced by the structure was estimated with the basin transfer method (Eq. 2) (Oudin et al., 2008).

$$Q_{max} = Q_{maxstation} \cdot \left( \frac{A_{work}}{A_{station}} \right)^{0.8} \qquad (2)$$

For the Swiss sites (7, 19, 29), the three discharges were computed following Eq. (2) from data from a hydraulic station located on the Rhone River (https://www.hydrodaten.admin.ch). For the Italian sites, $Q_{100}$ and $Q_{max}$ were computed following Eq. (2) applied to a time series of instantaneous discharges obtained from the hydraulic stations listed on the provincial websites (https://www.arpa.piemonte.it, https://heme.province.bz.it).

### 2.3.3. Hydraulic computations

For each stream cross-section, hydraulic radius ($R$, m) and channel width ($w$, m) were computed for varying flow levels, at 5 cm intervals. Water velocity ($U$, $m.s^{-1}$) was determined with Ferguson's variable power equation (Ferguson, 2007) (Eq. 3) based on parameter values proposed by Rickenmann and Recking (2011). Eq. (3) is especially well-suited for gravel-bed streams with coarse sediments (D'Agostino and Michelini, 2015). As morphological stability is commonly controlled by the coarse particle fraction, we used $D_{84}$ as the representative particle diameter for our computations (MacKenzie et al., 2018). Discharge was defined as the flooded cross-section area multiplied by water velocity. The hydraulic radii associated to $Q_2$, $Q_{100}$ and $Q_{max}$ were used to

compute reference shear stress and dimensionless Shields stress (Eqs. 5 and 6, respectively) (Table 1). The specific stream power was evaluated with the width for the $Q_2$ (Eq. 4). The values were obtained by matching the Ferguson-estimated discharges, linked to a hydraulic radius and stream width, with the reference discharges. For $Q_2$ and the historical discharge $Q_{max}$, flows were confined in the channel in all excepted one site (Appendix C). Neglecting floodplain overflowing is thus reasonable.

Shear stress ($\tau$, $N.m^{-2}$) relates the energy developed by a stream to the resistance offered by the bed (Leighly, 1932), and specific stream power ($\omega$, $W.m^{-2}$) reflects the flow power which drives sediment transport capacity. Both notions are commonly taken into account when designing stable channels, in particular, shear stress is used to size streambank protection and to guide managers in selecting SWB techniques. The Shields stress ($\theta$) is a dimensionless shear stress that captures the ratio of the stabilizing force (submerged weight of the particle) to the destabilising force (shear stress, $\tau$) for a particle of diameter $D$ and density $\rho_s$. The critical Shields number $\theta_{cr}$, which represents the value of the Shields parameter of incipient sediment motion, was computed with a slope-dependent function suggested by Recking (2009) for the coarse grains in poorly sorted sediment mixtures (Eq. 7). The transport stage, i. e. the ratio between $\theta$ and $\theta_{cr}$, was used as a proxy for potential sediment mobility. With a transport stage of $\theta/\theta_{cr} > 1$, the stream channel can be considered mobile (MacKenzie et al., 2018). For each reference discharge, we averaged the values of the three cross-sections.

$$\frac{U}{\sqrt{g \cdot i \cdot R}} = \frac{16 \cdot R / D_{84}}{\sqrt{42 + 6 \cdot (R / D_{84})^{5/3}}} \qquad (3)$$

$$\omega = \frac{\rho \cdot g \cdot i \cdot Q_2}{w(Q_2)} \qquad (4)$$

$$\tau = \rho \cdot g \cdot R \cdot i \qquad (5)$$

$$\theta = \frac{\tau}{(\rho_s - \rho) \cdot g \cdot D_{84}} \qquad (6)$$

$$\theta_{cr} = 0.56 \cdot i + 0.021 \qquad (7)$$

with $D_{84}$, the particle size of the bed such that 84% of the particles are smaller in diameter ($m$); $g$, gravitational acceleration ($9.81m.s^{-2}$); $i$, bed slope ($m/m$), is assumed to be equal to the energy slope following the uniform flow hypothesis; $\rho$, water density ($1000kg.m^{-3}$); $\rho_s$, particle density ($2650kg.m^{-3}$).

## 2.4. Statistical analyses

To assess the relationships between context, and geomorphological and ecological quantitative variables, non-parametric Spearman correlation tests were performed. We used Kruskal-Wallis tests with a degree of freedom of three to assess the significant influence of certain variables on site distribution along the naturalness gradient. If the test indicated a significant difference, we performed a Dunn's post hoc test to identify the specific groups that differed. Statistical analyses were performed with the R software, version 4.1.0 (R core team., 2022).

## 3. Results

The results for each project are presented in Appendix A, and two photos of each structure are provided in Appendix B.

### 3.1. Overview of mountain SWB structures

#### 3.1.1. Structure type, location and age

A total of 48 SWB projects were appraised. Twelve structures were pure bioengineering, 23 were mixed bioengineering, 8 were cribwalls and 3 riprap with vegetation (Table 1, Fig. 2). These structures were
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** SWB project locations based on data from river basin agencies. The number of structures is shown at the center of the pie charts, while the colors indicate the number of structures according to the naturalness gradient. For each administrative entity, the name of the main river is provided in the table.</figcaption></figure>

located between 600 and 1763 m a.s.l., predominantly within the montane belt (i.e., 600–1600 m a.s.l.), with three structures located in the subalpine belt (i.e., 1600–2200 m a.s.l.) (Fig. 3A). Elevation did not appear to be a determining factor in explaining the naturalness gradient (H = 0.5, *p* = 0.92). The length of the structures varied from 10 to 200 m

(Appendix A). These initial results showed which SWB techniques were used and implemented in the French Alps.

The structures were built between 1993 and 2024, of which half were constructed between 2011 and 2020 (Appendix A). In France, among the nine pure bioengineering projects identified, seven were

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Distribution of SWB structures according to naturalness gradient (i.e. from riprap with vegetation to pure bioengineering) in relation to context variables: elevation, where dashed lines indicate the delimitation of the north and south-facing montane and subalpine belts (A); grain size (B); transport stage, where dashed lines correspond to the threshold of motion, (C); specific stream power (D); bed slope (E); and bank slope, where the two statistically significantly different groups are identified by “a” and “b”, (F).</figcaption></figure>
located in the Southern Alps (Fig. 2). The three riprap with vegetation projects were all found in the Northern French Alps. Branch mattress and vegetated riprap techniques were only present in Italy (Appendix A). Some sites in South Tyrol (the Bozen area) had riprap toes with tilted and disordered blocks where fine sediments had been deposited and colonized by herbaceous vegetation. These structures were more than ten years old. Specific construction materials were observed in some locations: riprap reinforced with metal cables in Italy (sites 33, 37 and 38) and plastic geotextiles in some cribwall structures (sites 18, 43 and 48). It seems that the use of certain techniques and the riprap implementation at the bank toe were specific to certain areas.

### 3.1.2. Geomorphological context and associated techniques

Most of the projects were located in alluvial valley bottoms, with a channel width of 13 ± 6.8 m (mean ± standard deviation), while a few were confined between valley slopes (Sites: 1, 7, 17, 30, 43, 45, 46) (Appendix A). Bed slopes ranged from 0.3% to 11%, and particle size varied between 25 and 91 mm for $D_{50}$ and between 59 and 320 mm for $D_{84}$ (Fig. 3B, E). Neither bed slope nor particle size explained the choice of the technique used in the structure ($H = 5.9, p = 0.11$ and $H = 1.5, p = 0.68$), although for slopes >6%, only mixed techniques and cribwalls were found, though only four structures existed on such steep streams. To a certain degree, drainage area determined the type of technique used, with cribwalls found on smaller streams (< 60 km²) and mixed bioengineering used on larger areas (20–300 km²). For a 2-year flood event, 45 out of the 48 sites had a transport stage >1, indicating high particle mobility in most channels (Appendix A). Pure bioengineering techniques were used with transport stages ranging from 0.4 to 3.3. However, mineral-based techniques (mixed bioengineering and riprap with vegetation) were more commonly applied to streams with higher transport stages. Sites with a transport stage above 1.5 were predominantly mineralized (Fig. 3C). The same trend was observed for specific stream power (Fig. 3D). Neither of these trends, however, was statistically significant ($H = 3.3, p = 0.35$ and $H = 5.8, p = 0.12$); they primarily indicate the effects of both slope and drainage area. In essence, mineralized techniques were implemented in very different contexts on active mountain rivers, the distribution of technique types along the naturalness gradient was independent of grain size and bed slope, and only slightly dependent of specific stream power or transport stage. Generally speaking, vegetative and mixed techniques were observed on gentler bank slopes, whereas cribwalls and riprap with vegetation were found on steeper slopes ($H = 17.1, p < 0.001$) (Fig. 3F). In essence, the stream's geomorphological activity did not to justify the use of a more or less mineral technique. In contexts with steeper slopes, greater specific power or larger sediment size, mixed techniques or cribwalls were used preferentially, but in more moderate contexts, there was no preferential use of certain techniques along the naturalness gradient (Fig. 3).

### 3.1.3. Elements at risk protected by the SWB structures

The SWB structures we studied were intended to stabilize slopes to protect elements at varying degrees of risk (Appendix A, Fig. 4). Pure bioengineering structures, cribwalls and mixed techniques were all used regardless of the assets needing protection (Fig. 4). The type of asset did not clearly determine the type of technique, although mixed techniques tended to be more frequent with higher-value assets and were more commonly used for road protection, which represented the most frequent asset type (15 out of 48 sites). All three vegetated riprap structures were employed to protect high-value assets, either roads or buildings.

### 3.1.4. Vegetation

Across all the surveyed sites, we identified 46 woody species, with species richness per site ranging from 1 to 16. Vegetation recovery was observed on 47 of the 48 appraised sites, with bank toe vegetation cover ranging from 12% to 100%. Only one site (site 44) showed no vegetation recovery: the cuttings dried out during the first year. Willows (Salix spp.)

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Frequency of occurrence of elements at risk along the SWB naturalness gradient for all 48 structures.</figcaption></figure>

were present at all of the sites, with total willow cover ranging from 12% to 100%. The most frequently recorded species was *Salix purpurea L.* (43 sites), followed by *S. eleagnos Scop.* (35 sites), *S. daphnoides Vill.* (22 sites), *S. myrsinifolia Salisb.* (19 sites), *S. triandra L.* (18 sites) and *S. cinerea L.* (12 sites). The most common accompanying species were *Alnus incana L.* (12 sites) and *Cornus* sp. (10 sites).

Statistical analysis did not reveal any significant relationship between species richness or the Simpson diversity index and the naturalness gradient ($H = 4.3, p = 0.23$ and $H = 2.7, p = 0.44$). A high variability in species number was observed within the cribwalls: from one to 17, with seven species present on average. Pure bioengineering and mixed bioengineering structures included between one and ten species, with an average of five and seven, respectively. Riprap with vegetation had between two and four species.

Invasive alien species were recorded. *Reynoutria* spp. was present at four sites. *Artemisia verlotiorum Lamotte*, *Buddleja davidii Franch*, *Impatiens glandulifera Royle*, *Robinia pseudoacacia* L. and *Solidago gigantea Aiton* were all found at least once at varying types of sites.

### 3.2. Structural stability and observed failures

Fig. 5 shows the different failure modes observed in the field for each type of technique ranked according to the naturalness gradient (from less to greater naturalness from top to bottom). On the right, the bars indicate the proportion of the structures in each type according to mechanical status and vegetation contribution to stability. For example, for the six fascines (number of structures indicated on the far right), six failure modes were observed (shades of gray), including upstream end erosion on four of the six fascines. One fascine was destroyed (failed mechanical status, in hatched gray on the right), but the other five had a good mechanical status, with a key contribution of vegetation to stability (in green).

At first sight, it can be seen that the techniques with higher naturalness (bottom rows) tend to experience more mechanical failures (more gray cells) but fewer problems with vegetation recovery. Obviously, vegetation contribution to the structure's ability to stabilize is stronger for techniques with higher naturalness.

Out of the 12 pure bioengineering sites, 50% demonstrated good mechanical status, while 25% were degraded and another 25% had failed. Notably, the three failed projects were pure bioengineering structures. For the eight cribwall sites, 87.5% exhibited good mechanical status, and mixed bioengineering showed an 88% success rate across the 25 sites. Additionally, riprap with vegetation achieved 100% success at the three sites where the technique was used.

The three failed structures were wattle fence, fascine and brush matting and all exhibited bank toe erosion associated with torn-out stakes, as well as erosion upstream, downstream and on the upper
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** The frequency of occurrence of each failure process across the 48 sites (number of techniques with a failure/total number of techniques, 48) according to the SWB techniques, is shown in the middle section in shades from gray to black. Failures occurred with all the techniques except two, which are unique to fascines (asterisks). The right-hand column shows the mechanical status of the structures (hatched segments) based on the vegetation contribution index (colors represent Key, Partial or Marginal importance). The number of sites corresponding to each technique is indicated on the far right.</figcaption></figure>

streambank. The fascine failed mechanically due to inadequate vegetation management, as the willows were damaged in the spring by road-side maintenance equipment (Site 5) (Fig. 6).

The seven structures with a degraded mechanical status showed various types of failure depending on the techniques used: brush matting and brush mattresses lacked toe protection, thus leading to scouring (Sites 8, 10 and 12) (Fig. 6B3, B4); early damage due to significant sediment transport during floods caused damage to cuttings, with deposits of coarse sediment observed (Sites 21 and 22); there was significant erosion at the fascine toe within the first years after installation (Site 39); willow trees destabilized a cribwall structure due to their excessive lean (Site 16).

We also observed some minor failures on the 38 sites with a good mechanical status (i.e., on sites where the bank was still protected). The most often observed minor failures included: desiccation of vegetation at the top of the bank (Sites 13, 18, 23, 25, 39 and 44) (Fig. 6B1) and the absence of an appropriate technique for the upper bank, leading to erosion above the structure (Sites 3 and 6) (Fig. 6B2). Additionally, the vegetation substrate linking the rock toe protection and the bioengineering structure used for the upper bank can disappear because of internal erosion or the riprap moving due to scouring. This sometimes caused the vegetation to dry out or the geotextiles to settle unevenly, which in turn hindered vegetation development (Sites 21, 23, 24, 25 and 39) (Fig. 6B4). Then, the lack of surface protection exacerbated the erosion at the connection and at the upper part of the riprap. Finally, poor anchoring of the ends of the structure into the bank promoted upstream and downstream erosion, which could outflank the structure (Sites 1, 5, 6, 7, 8, 9, 11, 13, 17, 24 and 26).

All of the successful sites had a willow cover ranging from 92% to 100%. Sites presenting failures but with a good mechanical status (i.e., ensuring bank stability) had a willow cover >55%. Whereas sites with failures and a degraded mechanical status had a willow cover ranging from 30% to 100%. Failed structures had a cover <30%. In essence, poor vegetation establishment was often associated with ongoing

degradation (sometimes driving it, sometimes following it).

Failed or degraded structures were identified on streams with a transport stage between 1 and 2.5 (Fig. 3C). In contrast, structures with a good status were observed on streams with a transport stage ranging from 0.8 to 3.4. Although all failed sites experienced flow events with strong scouring capacity, some sites with well-established vegetation endured even stronger events and remained intact.

### 3.3. Mechanical resistance of existing structures

Fig. 7 shows the shear stress experienced by the structures at all 48 sites and compares them with the shear stress associated with a 2-year (Q2) and 100-year (Q100) return period flood. The time between construction and the maximum flood (i.e., maximum shear stress event) is indicated after the site ID. A wide range of situations can be seen. Four sites with a good mechanical status had already experienced stresses similar to those of Q100 (Sites 14, 23, 26 and 42). The degraded or failed structures were necessarily not the ones subjected to the highest levels of τ (i.e., maximal shear stress withstood by the structure), providing that degradation and failures can thus appear even on sites that did not experience a strong event.

The 11 pure bioengineering structures experienced τ ranging from 16 to 224 N/m² (Fig. 7, Appendix A). In the case of the wattle fence and two fascines, the τ experienced exceeded those induced by Q2. Among the 11 pure bioengineering sites, all five with a good mechanical status were more than four years old at the time of the flood, whereas, out of the seven sites that were degraded or failed, five were less than two years old. Cribwalls were subjected to a wider range of τ, from 25 to 504 N/m², with five out of eight structures enduring floods > Q2. Among the ten riprap toe with branch layers structures, τ ranged from 65 to 354 N/m², and only one remained unexposed to a τ value exceeding Q2. The computed τ for brush mattresses were lower, ranging from 31 to 200 N/m², and only one site experienced a τ value above Q2. Vegetated riprap and riprap combined with fascines experienced τ between 83 and 148 N/
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Failed and degraded sites showing the main failure modes. The rectangles in the photographs give the site number, age at maximum discharge and year the structure was implemented for each site.</figcaption></figure>

$m^2$, remaining below the Q2 value. Riprap combined with cribwalls were not subjected to $\tau$ higher than that endured by cribwalls alone - only two structures experienced $\tau$ exceeding Q2 values. Riprap with vegetation were subjected to $\tau$ exceeding those of Q2 values, ranging from 140 to 409 N/m$^2$.

The range of $\tau$ for a Q2 theoretically experienced by the structures did not differ among the SWB types arranged along the naturalness gradient (H = 7.04, p = 0.07). For Q100, there was a slight though significant difference between pure bioengineering and riprap with vegetation structures (H = 2.53, p = 0.034). Riprap with vegetation were implemented at sites experiencing higher $\tau$.

## 4. Discussion

The technical feedback in our study provides a clear overview of SWB practices implemented along mountain streambanks in the French Alps, complemented by case studies from Switzerland and Italy. The 48 study sites cover a wide range of stream hydraulics and particle dynamics and provide examples of various techniques, which we classified along a naturalness gradient. Our hydraulic analyses provide new mechanical resistance values for a wide range of techniques, and our observations highlight different failure modes for structures in these specific environments.
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Maximal shear stress probably experienced at each site (dots) and reference shear stress for Q2 and Q100 (squares). Each site is identified by an ID number and the age of the structure when it experienced the maximal shear stress (far left). The sites are arranged by technique, ranked from least natural (top) to most natural (bottom). Within each technique, the sites are ranked according to bed slope.</figcaption></figure>

## 4.1. How are SWB used in the European Alps? Practices and biogeomorphological contexts from France, Italy and Switzerland

A lack of guidelines has limited the use of SWB on Alpine streams. However, we found a wide range of techniques that stabilize the banks of streams with steep gradients >2% and high sediment mobility (Fig. 3). Many of these techniques have been successful (Fig. 8A), with some undergoing significant hydraulic stress (Figs. 7, 8B). Overall, the naturalness gradient did not correspond to the biogeomorphological variables; an increase in sediment transport or stream power did not lead to the use of increasingly mineralized techniques. Riprap at the bank toe is common, but this choice of technique was not explained by the biogeomorphological context, as reflected by particle size or bed slope, of the streambank requiring protection. Mixed techniques were generally preferred to protect high-value infrastructure, but in some cases, steep

banks and high-risk areas were protected with pure bioengineering. Conversely, on gentle slopes with minimal risks, mixed techniques were sometimes also employed. Prior to our fieldwork, we assumed that the presence and significance of riprap at the bank toe would increase with increasing site vulnerability; however, this tendency was not observed across our sample (Appendix A, Fig. 4).

Seven of the eight pure bioengineering sites in France were located in the southern Alps, within two administrative areas (Fig. 2). This distribution reflects practices associated to specific managers: the choice of a more or less natural technique seemed to depend on the technical culture and SWB skills of the managers or contractors executing the work. This trend was also confirmed for Italy, where vegetated riprap was only found in the province of Torino (Fig. 8C), while all the sites in the province of Bolzano were brush mattresses (Fig. 8D). The managers justified their choices based on local expertise, and in some areas, the
<figure><img src="image_11.png" /><figcaption>(B) Riprap toe and brush layers (construction – major flood event – after flood event) – Site 28 – 3.5 %</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>(C) Vegetated riprap – Site 40 – 1.2 % – downstream view, front view, zoom – pictures taken during the field visit, July 2025</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(D) Destructured riprap toe evolution with brush mattress (construction – 3 years – 19 years) – Site 36 – 2005 – 2 %</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>(E) Cribwall with reserve (construction – 2013 years – 13 years) – Site 45 – 8.5 %</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 8.** Illustrations of some technical success stories.</figcaption></figure>
influence of research (the works by Prof. Florineth in South Tyrol (Florineth, 1982)). Furthermore, the Brunico area is unique within the Bolzano province, as it has the only river basin agency with a dedicated SWB team (personal communication from C. Ghiraldo, July 2024). Implementing SWB techniques in mountain streams seems to depend, in part, on cultural history (Evette et al., 2009) and locally invested personalities with international influence (such as Prof. Florineth and H. Schiechtl in Austria). These observations were confirmed during informal discussions we had around the structures with some managers. Two of them mentioned a personal lack of knowledge as one of the main obstacles to implementing pure bioengineering techniques (Fig. 2, areas A, B, L). Some justified their use of mixed techniques as a response to hydraulic and morphological stresses they deemed too severe for pure bioengineering (Fig. 2, areas A, P, O). Others justified mixed techniques as a way to take fewer risks and achieve better social and political acceptance (Fig. 2, areas A, L). Those who tend to use pure bioengineering techniques mainly emphasized economic and landscape considerations as the basis for their choices (Fig. 2, areas C, E, M). These discussions highlight that the choice of more natural techniques have been strongly influenced by cultural, political and local knowledge and expertise. However, we did not systematically collect informal comments throughout the study area and these conclusions were not based on collected data, so it is difficult to generalize. These informal discussions align with the sociological results found for lowland rivers, and reinforce the evidence of sociocultural obstacles to developing bioengineering techniques, demonstrating that mainstreaming these techniques requires that practitioners adopt a new posture and develop new soft skills (Moreau et al., 2024).

Differences in practices were also evident in the design of riprap toes. In some areas, the riprap is unstructured, allowing for the deposition of fine particles and vegetation to colonize the structure, which leads to better integration with the bank over time (Fig. 8D). To mimic stream-banks naturally stabilized by boulders, the void-filled riprap technique fractures the riprap and fills the voids between the riprap with river-washed natural material (cobbles, gravels, sands and silts) to create stable structures at the bank toe that are easy to revegetate (Wulliman and Johns, 2011). In contrast, other geographical areas in our study favor smooth riprap with no roughness. Some managers justified this choice by claiming that construction sub-contractors are better trained to produce "smooth, clean" structures (Fig. 8C) than ones that look natural, which are considered more "chaotic" (Fig. 8D).

Riprap with vegetation, a technique close to civil engineering, was found on nearly vertical banks, where it was impossible to soften the slope (Fig. 1). In this case, stability was mostly ensured by the riprap structure, and vegetation was mainly seen as a vector for biodiversity. However, in this type of structural configuration, vegetation does not restore transversal ecological connectivity due to the steep banks and mineral materials (Boncourt et al., 2024). Cribwalls were also chosen as an alternative on steeper banks, allowing plant communities to develop, but they still remained too vertical to totally restore riparian habitat and connectivity (Fig. 8E).

## 4.2. Lessons learned about SWB structures

Thirty-eight of the 48 sites had a good mechanical status. The SWB structures implemented on Alpine rivers had mainly achieved their goal of bank stabilization. Tracing the sites' past hydrology allowed us to estimate the stresses experienced by the different structures and how the vegetation contributed to stability. This adds valuable information to the known resistance stresses used in SWB design, which have been partially documented in the literature (Baird et al., 2015; Evette et al., 2018; Leblois et al., 2016; Peeters et al., 2018), though data for structures more than three years old is limited. The structures in our study that withstood the highest stresses were cribwalls and brush mattresses. The highest stress was 500 N/m² experienced by a cribwall, similar to previous observations (Evette et al., 2018). The brush mattresses with a

riprap toe experienced lower stresses than those recorded in the literature: a maximum of 200 N/m² compared to 450 N/m². This latter result does not call into question the mechanical resistance of the techniques, but simply states that they were not exposed to major floods during our study period. To our knowledge, no previous studies have published resistance values for brush mattresses alone. The brush layers with a riprap toe experienced stresses equivalent to known values, around 200 N/m². The wattle fence failed at a stress of 100 N/m². The literature estimates a maximal resistance for wattle fences around this value, so we assumed the stress threshold to be similar (Evette et al., 2018).

The failed structures were not always those that had undergone the greatest stress, indicating the influence of other mechanisms. Although stress values are useful for structure design, they do not guarantee success. The average stress calculated is representative of a section, but does not accurately reflect the stresses applied locally to the structure. Furthermore, stress values do not take into account essential processes such as progressive erosion or undermining, nor can they account for poorly planned work or limited vegetation development, which are often causes of failure (Leblois et al., 2024; Oplatka, 1998). Shear stress should therefore not be the only design criterion. The observation of mechanical failures revealed a lower frequency of failure for mixed techniques, which were clearly less prone to scouring. Alpine rivers in valley bottoms are highly mobile, implying channel bed changes (Ruiz-Villanueva et al., 2023; Scorpio et al., 2024), while significant bed level changes occur in the steep headwaters (Theule et al., 2015). This hydromorphology justifies the use of riprap toe reserve, embedded below the channel bed and/or riprap at the bank toe. Alternatively to riprap toe reserve - or to complement it - some guidelines recommend implementing bed sills, i.e., structures perpendicular to the channel aiming to prevent bed incision (Bonin et al., 2013; Piton et al., 2017). Surprisingly, only four of the sites we visited during our study were equipped with such structures (Sites 9, 26, 27 and 53), which were probably unnecessary on the mixed-bioengineering and riprap with vegetation sites because they were already equipped with a riprap toe reserve. However, some pure bioengineering structures were damaged by scouring at the bank toe and could have benefitted from reinforcement. Depending on the context, alternatives to riprap, such as large logs or stumps, can be used to fill the scour hole (Baird et al., 2015; Didier et al., 2023). High sediment waves sometimes lead to scouring and bank undermining but can also cause transient bed aggradation and its associated erosion at the top of structures if they are not properly anchored at the top of the bank. It appears crucial to adopt a robust top-of-bank technique and to pay special attention to the interface between the toe and the bank top (Fig. 6B2). Connections with the bank within the structure may become weak points and deserve special attention (Adam, 2008).

Finally, great care must be taken with the structure itself to enable strong vegetation development to increase roughness and reduce shear stress. We found that some structures remained in good condition even if they were subjected to floods close to or even above Q2 values in the first year of installation, while others failed or were severely damaged in the first year. It is crucial to ensure that the initial structure is well-designed for Alpine environments, where vegetation grows more slowly and takes longer to fulfill its stabilizing role.

## 4.3. Lessons learned about SWB vegetation development

Successful resprouting and vegetation development are essential to the longevity of the structure (Rauch et al., 2022). Incomplete woody cover reduces soil cohesion, which can promote local erosion. In our study, the sites with less vegetation cover had more mechanical failures. Even though the structure itself has an important stabilizing role, for example, the logs in a cribwall (Fig. 6B1), vegetation cover plays a key role in stabilizing banks (Simon and Collison, 2002). The failures we observed due to lack of vegetation development were caused by inadequate implementation and maintenance. Drought failure presents
challenges for managers, and the high likelihood of drought conditions emphasizes the critical importance of proper watering operations (Leblois et al., 2022). The impact of drought on vegetation survival could be reduced by carrying out construction work during the autumn, and maintenance the first few years to ensure adequate watering and to repair minor erosion events affecting plant contact with the soil substrate. Erosion at the bank toe can expose cuttings and significantly hinder vegetation recovery. Beginning with the design phase, it is essential to consider and monitor maintenance strategies to ensure effective recovery. For instance, road maintenance and frequent mowing can hinder vegetation regrowth (Fig. 6A, Site 5). Maintenance is beneficial for the proper evolution of the structure over time and contributes to greater resistance. For example, maintain smaller and flexible stems by regular cutting can enhance the carpet effect during floods, whereas large-diameter stems or trunks may create turbulence that increases the risk of erosion (Weitzer et al., 1998). A paradigm shift would be necessary to better recognize the role of vegetation as a building material, and to include adaptive maintenance rather than a one-shot intervention (Moreau et al., 2024).

Vegetation development in SWB structures can control invasive exotic species and limit their spread (Dommanget et al., 2019; Martin et al., 2021). However, at site 26, in close proximity to a Japanese knotweed grove, we observed that knotweed was beginning to invade the top of the structure. We noted that a single layer of branches had been installed and the flat bank behind it simply seeded. In cases of potential invasion and to control the invasive's spread, it is important to plant more densely and in multiple layers (Dommanget et al., 2019).

Plant diversity was not systematically high in sites where vegetation has been used; sometimes only one or two woody species were planted. Our study did not assess biodiversity in the restored riparian habitats since SWB's objective is to stabilize banks and initiate an ecological rehabilitation process stimulated by pioneer engineering species (Polster, 2016). Nevertheless, it is important to emphasize that plant diversity plays a crucial role not only in initiating this process (Tisserant et al., 2021) but also in ensuring greater resilience of the structure (Yachi and Loreau, 1999) and increasing bank stability (Pohl et al., 2009; Stokes et al., 2009).

The use of native species adapted to the local ecological conditions can be an important success factor for SWB structures (Bischoff et al., 2010). In alpine environments, this requirement is even more crucial due to particularly harsh environmental conditions (Körner, 2003). However, a lack of knowledge concerning certain local species, notably high-elevation willows, may hinder the development of these techniques in high mountain areas (Rousset et al., 2025a). We hypothesize that this lack of information could partly explain why no SWB techniques were implemented above 1300 m a.s.l.

## 5. Conclusion

In this study, we examined 48 sites to enhance our understanding of SWB practices in the Alps and to offer insights for their more effective use in these challenging environments. It is crucial to evaluate the effectiveness of the stabilization techniques used in order to identify the most successful approaches and anticipate failures that may occur. Sharing experience is a key lever for developing these techniques and encouraging more integrated, long-term, sustainable approaches. The key takeaways from this study are:

* The SWB projects studied were implemented in different bio-geomorphic contexts including rivers with high morphological diversity, mostly small tributaries, mobile valley bottoms, steep bed and bank slopes and pronounced sediment dynamics.

* Technical choices were strongly influenced by managers' knowledge more than biogeomorphic context; for example, use of riprap at the bank toe was not always proportionate to the size of the surrounding natural embankments or the shear stress.

* A good initial design and maintenance over time are crucial to ensuring the resilience and durability of the SWB structures under the specific constraints of these alpine environments.

* New mechanical thresholds are now available for various SWB techniques within a montane hydromorphological context.

Finally, most of the SWB works were located in the montane belt, whereas a large number of subalpine streambanks are subject to anthropogenic pressures that require stabilization designs to protect against erosion processes. At subalpine elevations, some rivers display hydromorphological dynamics similar to those found in the montane belt. This suggests that selecting suitable vegetation constitutes a more significant technical constraint than sizing issues when developing SWB projects at subalpine elevations. Furthermore, our study has qualitatively highlighted certain sociological levers that influence the technical choices involved in implementing these techniques. To improve and expand the use of SWB techniques in the montane belt and further develop them in the subalpine belt, a more in-depth analysis of the role of local stakeholders is needed.

## CRediT authorship contribution statement

**Juliette Rousset:** Writing – original draft, Visualization, Validation, Project administration, Methodology, Investigation, Formal analysis, Conceptualization.
**Guillaume Piton:** Writing – review & editing, Validation, Supervision, Methodology, Conceptualization.
**Marie Lou Hirschy:** Writing – review & editing, Validation, Methodology, Investigation, Formal analysis.
**Delphine Jaymond:** Writing – review & editing, Methodology, Investigation.
**André Evette:** Writing – review & editing, Validation, Supervision, Methodology, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: (Andre Evette reports financial support was provided by French Biodiversity Office. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.)

## Acknowledgments

The authors would like to thank the French Biodiversity Agency for its financial support and all the river managers for sharing their experience and enthusiasm. We also thank all those who accompanied us in the field: Marie Didier, Sarah Menoli, Adeline François, Marie-Anne Dusz, Taïna Lemoine, Jeanne Desgranges, Clément Lallemand and Salomé Joulin. Many thanks to Fédéric Bray for his help in designing such a clear map. We would also like to thank Vicki Moore for her help with spelling corrections and improving reading fluency.

## Appendix. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.ecoleng.2026.107910.

## Data availability

We have shared the link of our data in the data statement document. Extensive feedback on soil and water bioengineering projects in the Alps (Original data) (recherche data gouv)
## References

Adam, P. (Ed.), 2008. Le génie végétal : Un manuel technique au service de l'aménagement et de la restauration des milieux aquatiques (la Documentation française).

AMEC, 2012. Design Guidelines for Erosion and Flood Control Projects for Streambank and Riparian Stability Restoration (the City of Calgary, Water Resources) [Technical Report]. The City of Calgary, Water Resources.

Arnaud, P., Cantet, P., Odry, J., 2016. SHYREG, a National Database of Flood Frequency Estimation. 7. Scopus. https://doi.org/10.1051/e3sconf/20160701004.

Baird, D., Fotherby, L., Klumpp, C., Scullock, M., 2015. Bank Stabilization Design guidelines (Technical Report no. SRH-2015-25). Bureau of Reclamation. https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/A-BankStab-final6-25-2015.pdf.

Bettler, M., Herzog, B., 2014. Cours d'eau en région alpine : Possibilité et limite du génie biologique (No. 2), 2 septembre. (Article 2).

Bischoff, A., Steinger, T., Müller-Schärer, H., 2010. The Importance of Plant Provenance and Genotypic Diversity of seed Material used for Plant Provenance Restoration. Ecol. 18 (3), 338–348. https://doi.org/10.1111/j.1526-100X.2008.00454.x.

Boncourt, E., Bergès, L., Alp, M., Dupont, B., Herviault, T., Evette, A., 2024. Riparian habitat connectivity restoration in an anthropized landscape : A multi-species approach based on landscape graph and soil bioengineering structures. Environ. Manag. https://doi.org/10.1007/s00267-024-01959-5.

Bonin, L., Evette, A., Frossard, P.A., Prunier, P., Roman, D., Valé, N., 2013. Génie végétal en rivière de montagne—Connaissances et retours d'expériences sur l'utilisation d'espèces et de techniques végétales : Végétalisation des berges et ouvrages bois.: Vol. hal-02598614.

Cavaillé, P., Dommanget, F., Daumergue, N., Loucougaray, G., Spiegelberger, T., Tabacchi, E., Evette, A., 2013. Biodiversity assessment following a naturality gradient of riverbank protection structures in French prealps rivers. Ecol. Eng. 53, 23–30. https://doi.org/10.1016/j.ecoleng.2012.12.105.

Cohen-Shacham, E., Walters, G., Janzen, C., Maginnis, S., 2016. Nature-Based Solutions to Address Global Societal Challenges. IUCN International Union for Conservation of Nature.

Comiti, F., 2012. How natural are Alpine mountain rivers? Evidence from the Italian Alps. Earth Surf. Process. Landf. 37 (7). https://doi.org/10.1002/esp.2267. Article 7.

D'Agostino, V., Michelini, T., 2015. On kinematics and flow velocity prediction in step-pool channels. Water Resour. Res. 51 (6), 4650–4667. https://doi.org/10.1002/2014WR016631.

Didier, M., Evette, A., Schmitt, E., Leblois, S., Jaymond, D., Evette, J.-B., Mira, E., Raymond, P., Frossard, P.-A., Vivier, A., 2023. Les fascines de ligneux en génie végétal. Guide Technique. OFB-INRAE 74. https://genibiodiv.inrae.fr/wp-content/uploads/Les_fascines_de_ligneux_INRAE_OFB.pdf.

Dommanget, F., Evette, A., Breton, V., Daumergue, N., Forestier, O., Poupart, P., Martin, F.-M., Navas, M.-L., 2019. Fast-growing willows significantly reduce invasive knotted spread. J. Environ. Manag. 231, 1–9. https://doi.org/10.1016/j.jenvman.2018.10.004.

European Commission, 2000. Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for Community action in the field of water policy [Legal document]. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32000L0060.

Evette, A., Labonne, S., Rey, F., Liebault, F., Jancke, O., Girel, J., 2009. History of bioengineering techniques for erosion control in rivers in Western Europe. Environ. Manag. 43 (6), 972–984. https://doi.org/10.1007/s00267-009-9275-y.

Evette, A., Recking, A., Piton, G., Rauch, H.P., Frossard, P.-A., Jaymond, D., 2018. The Limits of Mechanical Resistance in Bioengineering for Riverbank Protection, p. 6.

Ferguson, R.I., 2007. Flow resistance equations for gravel-and boulder-bed streams. Water Resour. Res. 43 (5). https://doi.org/10.1029/2006WR005422. Article 5.

Florineth, F., 1982. Experience Gained with Bio-Engineering Measures Performed on Running Waters in Mountains. https://www.osti.gov/etdeweb/biblio/8395792.

Florsheim, J.L., Mount, J.F., Chin, A., 2008. Bank erosion as a desirable attribute of rivers. BioScience 58 (6). https://doi.org/10.1641/B580608. Article 6.

Fox, G.A., Purvis, R.A., Penn, C.J., 2016. Streambanks : A net source of sediment and phosphorus to streams and rivers. J. Environ. Manag. 181, 602–614. https://doi.org/10.1016/j.jenvman.2016.06.071.

Gray, D.H., Sotir, R.B., 1996. Biotechnical and Soil Bioengineering Slope Stabilization : A Practical Guide for Erosion Control. John Wiley & Sons.

Halbritter, A.H., De Boeck, H.J., Eycott, A.E., Reinsch, S., Robinson, D.A., Vicca, S., Berauer, B., Christiansen, C.T., Estiarte, M., Grünzweig, J.M., Gya, R., Hansen, K., Jentsch, A., Lee, H., Linder, S., Marshall, J., Peñuelas, J., Kappel Schmidt, I., Stuart-Haëntjens, E., Vandvik, V., 2020. The handbook for standardized field and laboratory measurements in terrestrial climate change experiments and observational studies (ClimEx). Methods Ecol. Evol. 11 (1), 22–37. https://doi.org/10.1111/2041-210X.13331.

Institut national de l'information géographique et forestière, 2024. Site officiel de l'IGN. IGN. https://www.ign.fr.

Janssen, P., Cavaillé, P., Bray, F., Evette, A., 2019. Soil bioengineering techniques enhance riparian habitat quality and multi-taxonomic diversity in the foothills of the Alps and Jura Mountains. Ecol. Eng. 133, 1–9. https://doi.org/10.1016/j.ecoleng.2019.04.017.

Körner, C., 2003. Alpine Plant Life : Functional Plant Ecology of High Mountain Ecosystems (Softcover Reprint of the Hardcover 2nd Edition). Springer.

Körner, C., Paulsen, J., 2004. A world-wide study of high altitude treecline temperatures. J. Biogeogr. 31 (5). https://doi.org/10.1111/j.1365-2699.2003.01043.x. Article 5.

Körner, C., Möhl, P., Hiltbrunner, E., 2023. Four ways to define the growing season. Ecol. Lett. 26 (8). https://doi.org/10.1111/ele.14260. Article 8.

Krymer, V., Robert, A., 2014. Stream restoration and cribwall performance : A case study of cribwall monitoring in southern Ontario. River Res. Appl. 30 (7). https://doi.org/10.1002/rra.2684. Article 7.

Lachat, B., 1994. Guide de protection des berges de cours d'eau en techniques végétales (en collaboration avec P. Adam, P.-A. Frossard, R. Marcaud). Ministère de l'Environnement. DIREN Rhône-Alpes, Paris.

Leblois, S., Evette, A., Favier, G., Recking, A., 2016. Improvement of the sizing methods of riverbank bioengineering following an empirical approach. Sci. Eaux Territoires 27. https://doi.org/10.14758/SET-REVUE.2016.HS.05. Article 27.

Leblois, S., Evette, A., Jaymond, D., Piton, G., Recking, A., 2022. Processus et causes de défaillance du génie végétal pour la stabilisation des berges de rivière : Retour d'expérience sur un large jeu de données issues de la BD GeniVeg. Géomorphologie : relief, processus, environnement 28 (2). https://doi.org/10.4000/geomorphologie.16954. Article 2.

Leblois, S., Piton, G., Recking, A., Jaymond, D., Buffet, A., Evette, A., 2024. Riverbank fascines mostly fail due to scouring : Consistent evidence from field and flume observations. River Res. Appl. https://doi.org/10.1002/rra.4356 rra.4356.

Leighly, J.B., 1932. Toward a Theory of the Morphologic Significance of Turbulence in the Flow of Water in Streams. University of California Press.

Li, M.-H., Eddleman, K.E., 2002. Biotechnical engineering as an alternative to traditional engineering methods. Landsc. Urban Plan. 60 (4), 225–242. https://doi.org/10.1016/S0169-2046(02)00057-9.

MacKenzie, L.G., Eaton, B.C., Church, M., 2018. Breaking from the average : why large grains matter in gravel-bed streams. Earth Surf. Process. Landf. 43 (15), 3190–3196. https://doi.org/10.1002/esp.4465.

Martin, F.-M., Janssen, P., Bergès, L., Dupont, B., Evette, A., 2021. Higher structural connectivity and resistance against invasions of soil bioengineering over hard-engineering for riverbank stabilisation. Wetl. Ecol. Manag. 29 (1). https://doi.org/10.1007/s11273-020-09765-6. Article 1.

Montgomery, D.R., Buffington, J.M., 1997. Channel-Reach Morphology in Mountain Drainage Basins. Geological Society of America Bulletin (Article 5).

Moreau, C., Evette, A., Cottet, M., Frdette, C., Grioire, G., Poulin, M., 2014. Exploring Barriers to Nature-Based Solutions (NBS) and Mechanisms for Implementation. https://doi.org/10.13140/RG.2.2.14219.09746.

Newson Malcolm, D., Large, A.R.G., 2006. 'Natural' rivers, 'hydromorphological quality' and river restoration : A challenging new agenda for applied fluvial geomorphology. Earth Surf. Process. Landf. 31 (13), 1606–1624. https://doi.org/10.1002/esp.1430.

Oksanen, J., 2013. Vegan : Ecological Diversity.

Oplatka, M., 1998. Stabilität von Weidenverbaugkeiten auf Fusslufern [Doctoral Thesis, ETH Zurich]. https://doi.org/10.3929/ethz-a-001920916.

Oudin, L., Andréassian, V., Perrin, C., Michel, C., Le Moine, N., 2008. Spatial proximity, physical similarity, regression and ungaged catchments : A comparison of regionalization approaches based on 913 French catchments. Water Resour. Res. 44 (3). https://doi.org/10.1029/2007WR006240.

Peeters, A., Houbrechts, G., Hallot, E., Campenhout, J.V., Verniers, G., Petit, F., 2018. Efficacité et résistance de techniques de protection de berges en génie végétal. Géomorphologie 24 (2), Article 2. https://doi.org/10.4000/geomorphologie.11927.

Peeters, A., Verniers, G., Houbrechts, G., Hallot, E., Petit, F., 2020. Protections de berges en génie végétal : Conception, application, suivi et recommandations. https://orbi.uliege.be/handle/2268/248736.

Petts, G.E., 1989. Historical change of large alluvial rivers : Western Europe. In: Historical Analysis of Fluvial Hydrosystems. Wiley, pp. 1–18.

Piton, G., Carladous, S., Recking, A., Liebault, F., Tacnet, J., Kuss, D., Quefféléan, Y., Marco, O., 2017. Why do we build check dams in Alpine streams ? An historical perspective from the French experience. Earth Surf. Process. Landf. 42 (1), 91–108. https://doi.org/10.1002/esp.3967.

Pohl, M., Alig, D., Körner, C., Rixen, C., 2009. Higher plant diversity enhances soil stability in disturbed alpine ecosystems. Plant Soil 324 (1), 91–102. https://doi.org/10.1007/s11104-009-9906-3.

Polster, D.F., 2016. Natural processes for the restoration of drastically disturbed sites. J. Am. Soc. Mining Reclam. 2016 (2), Article 2. https://doi.org/10.21000/JASMR16020077.

Preti, F., Capobianco, V., Sangalli, P., 2022. Soil and Water Bioengineering (SWB) is and has always been a nature-based solution (NBS) : A reasoned comparison of terms and definitions. Ecol. Eng. 181, 106687. https://doi.org/10.1016/j.ecoleng.2022.106687.

R core team., 2022. R: A Language and Environment for Statistical [Logiciel]. R Foundation for Statistical Computing, Vienna, Austria https://www.R-project.org/.

Rauch, H.P., von der Thannen, M., Raymond, P., Mira, E., Evette, A., 2022. Ecological challenges* for the use of soil and water bioengineering techniques in river and coastal engineering projects. Ecol. Eng. 176, 106539. https://doi.org/10.1016/j.ecoleng.2021.106539.

Recking, A., 2009. Theoretical development on the effects of changing flow hydraulics on incipient bed load motion. Water Resour. Res. 45 (4). https://doi.org/10.1029/2008WR006826.

Recking, A., Piton, G., Montabonnet, L., Posi, S., Evette, A., 2019. Design of fascines for riverbank protection in alpine rivers : Insight from flume experiments. Ecol. Eng. 138, 323–333. https://doi.org/10.1016/j.ecoleng.2019.07.019.

Rickenmann, D., Recking, A., 2011. Evaluation of flow resistance in gravel-bed rivers through a large field data set. Water Resour. Res. 47 (7). https://doi.org/10.1029/2010WR009793.

Rousset, J., Menoli, S., François, A., Gaucherand, S., Evette, A., 2025a. Developing Nature-based Solutions in the Alps : an Ex-situ Experiment to Select Willows for Subalpine Soil and Water Bioengineering Structures. Environ. Manag. https://doi.org/10.1007/s00267-025-02211-4.
Rousset, J., Piton, G., François, A., Didier, M., Evette, A., 2025b. Biogeomorphic assessment of natural streambank structures in the Alps : A step to improve soil and water bioengineering techniques. Earth Surf. Process. Landf. 50 (9), e70126. https://doi.org/10.1002/esp.70126.

Ruiz-Villanueva, V., Piégay, H., Scorpio, V., Bachmann, A., Brousse, G., Cavalli, M., Comiti, F., Crema, S., Fernández, E., Furdada, G., Hajdukiewicz, H., Hunzinger, L., Lucía, A., Marchi, L., Moraru, A., Piton, G., Rickenmann, D., Righini, M., Surian, N., Wyżga, B., 2023. River widening in mountain and foothills areas during floods : Insights from a meta-analysis of 51 central European Rivers. Sci. Total Environ. 903, 166103. https://doi.org/10.1016/j.scitotenv.2023.166103.

Schiechl, H.M., 1992. Weiden in der Praxis : Die Weiden Mitteleuropas, ihre Verwendung und ihre Bestimmung. Patzer.

Schiechl, H.M., 1997. Water Bioengineering Techniques for Watercourse, Bank and Shoreline Protection | Titel. https://library.wur.nl/WebQuery/titel/939300.

Schiechl, H.M., Horstmann, N.K., 1980. Bioengineering for Land Reclamation and Conservation. University of Alberta Press.

Schiechl, H.M., Stern, R., 1997. Water Bioengineering Techniques for Watercourse, Bank and Shoreline Protection. Blackwell Science, Oxford.

Scorpio, V., Comiti, F., Liébault, F., Piegay, H., Rinaldi, M., Surian, N., 2024. Channel changes over the last 200 years : A meta data analysis on European rivers. Earth Surf. Process. Landf. https://doi.org/10.1002/esp.5848 esp.5848.

Simon, A., Collison, A.J.C., 2002. Quantifying the mechanical and hydrologic effects of riparian vegetation on streambank stability. Earth Surf. Process. Landf. 27 (5), 527-546. https://doi.org/10.1002/esp.325.

Simon, A., Pollen-Bankhead, N., Thomas, R.E., 2013. Development and application of a deterministic bank stability and toe erosion model for stream restoration. In: Simon, A., Bennett, S.J., Castro, J.M. (Eds.), Geophysical Monograph Series. American Geophysical Union, pp. 453-474. https://doi.org/10.1029/2010GM001006.

Simpson, E.H., 1949. Measurement of diversity. Nature 163 (4148). https://doi.org/10.1038/163688a0. Article 4148.

Stokes, A., Atger, C., Bengough, A., Fourcaud, T., Sidle, R., 2009. Desirable Plant root traits for protecting natural and engineered slopes against landslides. Plant Soil 324, 1-30. https://doi.org/10.1007/s11104-009-0159-y.

Symmank, L., Natho, S., Scholz, M., Schröder, U., Raupach, K., Schulz-Zunkel, C., 2020. The impact of bioengineering techniques for riverbank protection on ecosystem services of riparian zones. Ecol. Eng. 158, 106040. https://doi.org/10.1016/j.ecoleng.2020.106040.

Theule, J., Liébault, F., Laigle, D., Loye, A., Jaboyedoff, M., 2015. Channel scour and fill by debris flows and bedload transport. Geomorphology 243, 92-105. https://doi.org/10.1016/j geomorph.2015.05.003.

Tisserant, M., Bourgeois, B., González, E., Evette, A., Poulin, M., 2021. Controlling erosion while fostering plant biodiversity : A comparison of riverbank stabilization techniques. Ecol. Eng. 172, 106387. https://doi.org/10.1016/j.ecoleng.2021.106387.

Weitzer, C., Doppler, F., Florineth, F., 1998. Untersuchungen über die Wirksamkeit von Pflanzen in Einzugsgebieten des Forsttechnischen Dienstes der Wildbach- und Lawinenverbauung. Universität für Bodenkultur.

Wohl, E.E., 2010. Mountain rivers revisited. Am. Geophys. Union/Geopress. https://doi.org/10.1029/WM019.

Wolman, M.G., 1954. A method of sampling coarse river-bed material. Trans. Am. Geophys. Union 35 (6), 951. https://doi.org/10.1029/TR035i006p00951.

Wulliman, J., Johns, D., 2011. Demonstration Projects Illustrating Void-Filled Riprap Applications in Stream Restoration (Urban Drainage and Flood Control District, p. 33).

Yachi, S., Loreau, M., 1999. Biodiversity and ecosystem productivity in a fluctuating environment : the insurance hypothesis. Proc. Natl. Acad. Sci. 96 (4) (Article 4. Doi: 10.10).