# Investigating the influence of weirs and small dams on coarse sediment connectivity in low-energy rivers: A case study of the orne and vire rivers in Normandy, France

A.J. Rollet, R. Reulier, G. Fantino, A. Glais

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A.J. Rollet, R. Reulier, G. Fantino, A. Glais. Investigating the influence of weirs and small dams on coarse sediment connectivity in low-energy rivers: A case study of the orne and vire rivers in Normandy, France. Geomorphology, 2024, 465, pp.109356. 10.1016/j geomorph.2024.109356. hal-04717685

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# Investigating the influence of weirs and small dams on coarse sediment connectivity in low-energy rivers: A case study of the orne and vire rivers in Normandy, France

A.J. Rollet<sup>a,*</sup>, R. Reulier<sup>b</sup>, G. Fantino<sup>c</sup>, A. Glais<sup>d</sup>

## ARTICLE INFO

**Keywords:**

Bedload transport

Weirs

Bedload connectivity

Morphological adjustment

Low energy river

## ABSTRACT

The removal of weirs has become emblematic of efforts to restore the good ecological status of rivers in France (Water Framework Directive -WFD- in 2006; the French Eel national management plan in 2007); however, it is increasingly facing opposition for the recreational and cultural considerations it brings (Germaine and Lespez, 2017). Previous studies have highlighted that bedload can pass through or over these weirs, although they acknowledged the difficulty in applying their conclusions to all geographic contexts. In this context, we aimed to determine the influence of these structures on the bedload connectivity in low-energy, highly human modified rivers. In this way, the objective of this study is threefold: generate knowledge about the mobility of a potential bedload in low-energy, highly anthropized contexts, better understand the influence of weirs and dams on these bedload dynamics, and identify the associated morphological changes. We monitored the bedload fraction of the Orne and Vire rivers (France) for 4 years using 848 pebbles equipped with passive transponders injected upstream of 2 dams and 4 weirs of different size and 2 reference sites where a weir or dam had been removed. While tracers' mobility was low (~3–5 m yr⁻¹ for T < 2 yrs), it was highly influenced by the presence of transverse structure (shorter distances and frequency of mobilization), even small ones. After 4 years of monitoring, no tracers crossed any of the weirs. This reduction in mobility was also due to decrease of shear stress within the reservoir as well as the physical barrier created by the weir. This interruption in bedload continuity resulted in a downstream coarsening of bed material downstream of the weirs but not at sites that had been restored for several years. Ultimately, the results of this study contribute to the scientific debate by focusing on two river reaches that have been greatly influenced by human activities for centuries.

## 1. Introduction

Since the 2000s, an increasing number of national and international regulations (e.g., Water Framework Directive -WFD-2006; the French Eel national management plan 2007) have focused on the concept of connectivity in an effort to improve the quality of European rivers. It encompasses several dimensions (i.e., ecology, hydrology, and sediment) and has prompted the removal of transverse structures in Europe. Thus, bedload continuity is crucial in gravel bed rivers, since it shapes much of the fluvial landscape (Andrews and Nankervis, 1995; Whiting et al., 1999). The current European Union (EU) Biodiversity Strategy aims to reconnect 25,000 km of European rivers by 2030 (Belletti et al., 2020).

Reconnection usually involves the removal of small run-of-river structures (i.e., low-head dams, weirs, or overflow dams) as they represented 68 % of the listed structures in the EU in 2020 and 81 % of the listed structures in France in 2014 (Dépret et al., 2019; Belletti et al., 2020). These structures are no higher than the river banks and occupy the full width of the river without entirely obstructing water discharge (Csiki and Rhoads, 2010; Gargan et al., 2011; Casserly et al., 2020). In most Western countries, the number of removals is increasing, and it has become in France one of the most emblematic ways to reach a good ecological status of water bodies, especially in low-energy rivers (< 25–30 W/m²) (Germaine and Barraud, 2013; Lespez et al., 2015). Such actions have since become one of the most visible and controversial

<footnote><sup>a</sup> Université Rennes 2, UMR 6554-LETG, France</footnote>

<footnote><sup>b</sup> Université Caen Normandy, UMR 6266 IDEES, France</footnote>

<footnote><sup>c</sup> GeoPeka consulting firm, France</footnote>

<footnote><sup>d</sup> Service Départemental d'Archéologie du Calvados, France</footnote>

<footnote>* Corresponding author at: Université Rennes 2 Place du recteur Henri Le MoalCS, 24307-35043 Rennes cedex, France.</footnote>

<footnote>E-mail address: anne-julia.rollet@univ-rennes2.fr (A.J. Rollet).</footnote>
aspects of national river-restoration projects (Germaine and Lespez, 2017). These structures are often associated with former water mills dating back several hundred years and can thus have a form of heritage interest, which unites particularly active opposition groups (Sneddon and Barraud, 2017).

From a physical functioning point of view, actions to remove transverse structures only make sense if not only the channel is actually experiencing bedload transport, but also if this is interrupted by the structures. Unlike the morphological effects of dams, which have been widely documented since the first publications of Williams and Wolman (1984) (e.g., Kondolf, 1997; Brandt, 2000; Graf, 2006; Major et al., 2017; Rollet et al., 2014), those of weirs remain much less documented (Csiki and Rhoads, 2014; Poeppl et al., 2015). They have been addressed by comparing morphological parameters upstream and downstream of weirs (Fencl et al., 2015), monitoring bathymetry (Rollet et al., 2023) and, more recently, monitoring tracers injected upstream of structures (Peeters et al., 2020; Casserly et al., 2020, 2021; Magilligan et al., 2021). These case studies covered relatively small geographical areas, primarily in North America as noticed by Casserly et al. (2020), and in contexts with bedload transport sufficiently active so that, reservoirs were partially filled up or contained a sediment ramp. These states promote the recovery of bedload connectivity, even for the coarsest particles (Kibler et al., 2011; Pearson and Pizzuto, 2015). Thus, Pearson and Pizzuto (2015) suggested that all particle-size fractions in the bed material supplied from upstream could have been transported through the reservoir they studied, up the sediment ramp, and over the top of the 2.5 m dam. When this occurs, the weirs act as leaky barriers, trapping only sediment fractions larger than the median (Peeters et al., 2020), or trapping only the coarsest particles (Magilligan et al., 2021). Since bedload connectivity changes little, only localized morphological changes are observed, which are related to sedimentation in the reservoir and bed degradation due to hydraulic overtopping (Skalak et al., 2009; Csiki and Rhoads, 2010). The materials eroded in this manner contribute to the formation of stable median banks immediately downstream of weirs (Shaw, 2012). However, there is no definitive adjustment of the larger-scale geometries associated with the interruption of bedload, except for changes in particle-size distribution, with an increase in median particle size and a slightly lower percentage of fine particles (< 2 mm) downstream of weirs (Skalak et al., 2009; Csiki and Rhoads, 2014; Magilligan et al., 2021). The bedload trapping effect varies over time. Thus Casserly et al. (2020) demonstrated transient storage and thus temporal variability in bedload trapping and documented pulsating bedload behavior at the sites they studied. They observed periods of reservoir filling during which the particle sizes downstream of structures changed, but this effect disappeared once the reservoir was partially filled and bedload connectivity was temporarily restored. Thus, the variability in spatial patterns of channel morphology and sediment characteristics among sites suggest that local site-specific factors strongly influence geomorphological responses (Csiki and Rhoads, 2014). This influence also depends on operations at structures during flood events and the condition of structures. For example, bedload trapping can be mitigated when the structure has opened or collapsed flushing gates that facilitate bedload transport (Peeters et al., 2020), but it is difficult to extrapolate these conclusions to all contexts. The importance of reservoirs as trapping zones indicates that backwater zones created by run-of-river weirs differ in their ability to trap sediment. These differences could be related to regional differences in the watershed and stream characteristics or to differences in dam characteristics (Csiki and Rhoads, 2010; Rollet et al., 2023). The response of rivers to the long-term existence of weirs varies greatly (Csiki and Rhoads, 2010) and depends mainly on the characteristics of the structure (e.g., form, height of the crest, presence or absence of sluicegate systems, level of maintenance), the river (e.g., occurrence of large flood events, bedload characteristics, ability of river hydraulics to transport sediment over the weir crest), and general characteristics of the watershed (e.g., upstream weir density, available sediment supply) (Pearson

and Pizzuto, 2015; Rollet et al., 2023).

Thus, existing studies have reported limited effect of weirs on sediment trapping and consequently on morphological adjustments. Unlike dams, the effect of weirs is both weaker in intensity and less abrupt over time. Their trapping capacities are also more variable (controlled by e. g., level of sediment filling, sluice gate operations, nature of bedload transport as pulses or continuous load). These findings partially challenge the relevance of removing weir to facilitate bedload connectivity (Dufour et al., 2017).

Nevertheless, these small structures are mostly set along extensive river lengths characterised by highly diverse geographic settings. This makes it challenging to generalize these initial results. In France, these contexts often include low-energy rivers that have been heavily anthropised for a long time (Lespez et al., 2015) and which are specific in many ways. First of all, they have low slopes and therefore low stream power. This naturally limits the morpho-sedimentary dynamics of these systems. They are recognized as being low energy and generally close to functioning thresholds (Ferguson, 1981; Dépret et al., 2017). In addition, their riverbeds are made of coarse sediment, mostly inherited, whose effective mobility is unknown. (Beauchamp et al., 2017). There is a probable gap between the hydrological (e.g., flood intensity, flow variation) and sedimentary (e.g., sediment supply) conditions that existed when these sediments were deposited and current conditions (natural and anthropogenic). This gap raises questions about the current capacity of these rivers to mobilise the sediments that compose their bed. Lastly, these systems result from a strong hybridization between natural processes and multi-century human interventions. This hybridization may be responsible for a reduction in the capacity for morphological adjustments in these systems, where human activities have largely shaped the geometry (Lespez et al., 2015).

In systems that are already close to boundary conditions due to natural factors (current and inherited) and artificial factors (modification of geometry, protection of banks, etc.), it is interesting to establish whether the influence of small weirs on the bedload transport becomes significant. From a scientific point of view, this question involves looking at the sensitivity of these systems, considered here as the ratio between disturbing (i.e., weirs or small dams) and resisting forces. (Downs and Gregory, 1995; Fryirs, 2017). On the one hand, such systems experiencing strong constraints (natural or human-induced), they experience limited to no bedload transport and planform adjustments and their behavioural (adjustement) regime will be narrower: they will then be considered *insensitive* to adjustment (Fryirs, 2017). On the other, if a system is closed to boundary conditions, even relatively small events can reconfigure the river system (Bull, 1979; Schumm, 1979; Church, 2002) which can be considered as *sensitive*. Thus, in low-energy contexts, relatively small perturbation (e.g., weirs) can theoretically either have no influence or lead to a reconfiguration of the river towards a different state. To answer these questions, it is important to increase the number of studies documenting the morphological and sedimentary functioning of these particular rivers, which are largely under-studied, particularly in France (Dépret et al., 2017).

In this context, the objectives of this study are threefold: (I) generate knowledge about the mobility of bedload in low-energy, highly anthropized contexts, (II) better understand the influence of weirs and dams on these bedload dynamics, and (III) identify the associated morphological changes. In a context of controversy over weir removal, this fundamental knowledge is also intended to help river managers in their current practices.

To reach these objectives, we adopted a comparative approach between morpho-sedimentary functioning with and without the influence of transverse structures. We first compared the mobility of the coarse sediments (> 32 mm) that constitute the riverbed in different configurations, on sections with transverse structures (weirs and small dams) that differed in size and type, as well as on two recently restored sites (whose structures had been removed). These comparisons rely on sediment tracing based on a radio frequency identification (RFID) system.
We then described and compared geomorphological parameters (i.e., geometry and particle size) of upstream or restored section considered as reference and downstream sections directly influenced by the structures.

## 2. Geographical context

The studied sites were located in the Orne and Vire rivers in Normandy (northwestern France), both of which flow into the English Channel. These rivers have a thermo-pluvial hydrological regime with high winter flows and sometimes severely low summer flows (Fig. 1). The Orne is a sixth-order river 170 km long that drains a 2932 km² watershed. It has generally low slopes (≈ 0.11 %), but they are irregular. The mean annual discharge and biennial flood are respectively 27.5 m³/s (specific discharge of 9.4 l/s/km²) and 153 m³/s (Thury-Harcourt gauging station). The Vire is a smaller fifth-order river 128 km long that drains a 1969 km² watershed and originates at an elevation of 308 m on the granodiorites of the Massif de Vire (plutonic rock) with a mean slope of 0.26 %. The mean annual discharge and biennial flood are respectively of 12.6 m³/s (specific discharge of 6.4 l/s/km²) and 121 m³/s (Saint-Lô gauging station). The two rivers are located at the interface between the sedimentary formations of the Paris Basin, which forms the Plain of Caen, and the siliceous and granitic formations of the Armorican Massif. The particle size of the surface of the channel bed is mainly pebble at all study sites.

To ensure that the study sites had similar morphological contexts, we focused on the Armorican part of the rivers, which is a morphologically uniform area referred to as “Armorican Rivers of the Normandy Bocage” by the Agence de l’Eau Seine-Normandie. In this area, both rivers have

relatively consistent valley bottom slopes: 0.09 % for the Orne and 0.12 % for the Vire. They can be considered as low-energy rivers, with a specific power at bankfull stage of 22–25 W/m².

Like most Norman rivers, they have been extensively and historically engineered for their hydraulic potential (Beauchamp et al., 2017; Cador and Reulier, 2020). In recent years, these rivers have been the focus of restoration efforts to remove transverse structures. As a result, the density of structures on the Orne and Vire rivers has decreased from 0.72 and 0.50 structures per km, respectively, in the mid-20th century to 0.50 and 0.40 structures per km, respectively, in 2018 (Michler et al., 2020). The densities before the removal period are similar to those observed in other Norman systems such as the Seulles (1.09 structures per km; Lespez et al., 2015) or to the average densities in English watercourses (0.75 structures per km, Jones et al., 2019).

This study focused on eight sites along the Orne and Vire rivers, of which six still had structures, while the other two had had weirs completely removed (Fig. 2). The existing structures represented the diversity of transverse structures in rivers in northwestern France, having a variety of heights, slopes, lengths of reservoir, and hydraulic equipment (Table 1). The structures included two small hydroelectric power station dams (O1, V3), each >2 m high; three mill weirs, two of which were equipped with a fish pass (V1, V2), while one (O2) was not; and an artificially breached factory weir (O3). Dam O1 was the only structure still associated with hydroelectric activity and was thus regularly operated. These sites had no recent history of dredging. Establishing the exact age of these structures was challenging, since they often existed in various forms since the medieval period. The length of the reservoir at each site (500–3005 m) depended on the local slope, on the

<figure><img src="image_3.png" /><figcaption>Fig. 1. Location maps of the study sites at a regional (A) and national scale (B) and geological context of the study reach on the Vire River (C) and the Orne River (D).</figcaption></figure>
<figure><img src="image_4.png" /></figure>

Pictures credits: R.Reulier, 2018

<caption>**Fig. 2.** Photographs of the study sites: (A) small dam of La Courbe (O1) and (B) small dam of Chapelle-sur-Vire (V3), both equipped with a hydroelectric power station; (C) mill weir of Moulin à Papier (O2); (D) mill weir of Moulin Rose (V1) and (E) mill weir of Pleine Oeuvres (V2), the latter two equipped with a fish pass; (F) restored site of l'Enfernay (O4) and (G) restored site of Candol (V4), whose weirs had been removed in 2013 and 2017, respectively; and (H) partially restored site of Bô (O3), whose factory weir had been artificially breached at the center (color figures available online).</caption>

type of structure (mill/weir or hydroelectric power station dam) and the head (up to 4.4 m). However, it was also significantly constrained by the influence of upstream weirs. Slopes upstream of the structures ranged from 0.005 to 0.069 %. The two restored sites (O4 and V4) had their weirs removed in 2013 and 2017, respectively. Since the flow was completely free at these sites, we considered them as reference sites.

# 3. Methods

## 3.1. Bedload tracing

Bedload tracking using passive RFID tags is already widely deployed in many studies of bedload transport in coarse-grained rivers (e.g., Lamarre, and mac Vicar, B., Roy, A.G., 2005; Houbrechts et al., 2012; Arnaud et al., 2017; Liébault et al., 2023) and more recently to highlight the effect of weirs on bedload transport (Casserly et al., 2020; Magilligan et al., 2021; Peeters et al., 2020). This technique is recognized to be particularly suited for studying sediment dynamics in low to moderate energy systems (Liébault et al., 2023). We injected 848 pebbles equipped with RFID PIT tags at the eight study sites (i.e., 100–110 per site).

To this end, pebbles were collected along transverse profiles in the reservoirs at each site to ensure that they adequately represented the local coarse sediment fraction composing the riverbed. Due to the size of the transponders (23 mm), however, no particles smaller than 32 mm (b-axis) were equipped. Nonetheless, the median particle size ($D_{50}$) of the tracers and their remaining distribution accurately represented those of the riverbed (Fig. 3).

Once equipped, the tracers were injected into the riverbed in July 2018 along the transverse profiles ($n = 43$) from which they had been initially sampled. These profiles were distributed along 2–3 injection sectors per site chosen based on site configurations (restored or unre-stored) (Fig. 4).

At each of the five unrestored sites and the partially leveled Bô site (O3), the equipped particles were injected upstream of the reservoir, within the reservoir itself, and, when possible, on sediment ramps located immediately upstream ($< 20$ m) of the structure. At each of the two restored sites, the tracers were injected along transects upstream and downstream of the location of the former structure. We used a detection system to monitor the tracers that combined a Reader + Datamax recorder, detection antennae, and a geolocation system (GPS Trimble GeoXH6000 - with decimetric accuracy). The size of the antenna depended on the prospecting technique: a rectangular antenna

(1.4 m × 0.5 m) was used for onboard surveys in deep water sectors, while a square antenna (0.5 m × 0.5 m) was used for surveys performed on foot in wadable sectors.

The distance covered by each tracer was calculated using linear referencing, in which the tracer's location was projected perpendicular to the median axis of the channel (river centerline), which was obtained using GIS processing. The displacement distance between 2 surveys represented the movement along the river centerline. This method allowed us to calculate the actual displacement distance in sinuous sections and detect negative displacement distances (associated with geopositioning errors) (Arnaud et al., 2017). For each survey at each site, we calculated the maximum mean margin of error for geopositioning the tracers (due to the prospection conditions and DGPS limitations) based on recorded duplicates (i.e., mean of the distances between the duplicates) (Cassel et al., 2017). We added to these positioning errors the estimated detection error margins of 0.70 m (Chapuis et al., 2014; Arnaud et al., 2017). Individual particles were considered mobile if their displacement distance exceeded this total error margin, varying from 2.5 m to 3.84 m depending on the site and the survey period.

Two tracers' surveys were conducted on both rivers in august 2019 and july 2020 (S1 and S2, respectively). Due to hydrological conditions, survey in september 2021 was limited to the Vire (S3a), which resulted in additional survey of only the Orne in june 2022 (S3b) (Fig. 5). Given the low flows in the Orne from 2021 to 2022 (Qmax <0.64 of biennial flow) and the observation that the mean displacement distances recorded from 2019 to 2020 and 2020–2022 were similar (9.29 and 9.01 m, respectively), we considered that the displacement distances recorded from 2020 to 2022 occurred primarily from 2020 to 2021. For each tracking period, we calculated the virtual velocity, defined as the total distance traveled (potentially incorporating multiple steps) by individual mobile grains between 2 surveys divided by the measurement interval expressed in meter per year (Haschenburger and Church, 1998; Liébault et al., 2023). In this study the measurement interval incorporates multiple flood events (Hassan et al., 1992; Haschenburger and Church, 1998; Casserly et al., 2020). Tracers not recovered or considered as non mobile (distance < margin of error) were excluded from the calculation to distinguish the analysis of the behavior of mobile particles from the question of whether they were set in motion. To address the latter we also calculated the mobility rate per injection transect between each survey in order to distinguish the behavior of the tracers within the different configurations studied (upstream of
<table><caption>**Table 1**
Structural and hydrological characteristics of the selected transverse structures and their reservoirs.</caption><thead><tr><th>Site</th><th>Style</th><th>Current status (Year built)</th><th>Drainage area (km²)</th><th>O₂ flow (m³/s)</th><th>Fall down height (m)</th><th>Reservoir width (m)</th><th>Reservoir length (m)</th><th>Location of sluice gates</th><th>Upstream slope (%)</th><th>Upstream D₅₀ (mm)</th><th>Downstream D₅₀ (mm)</th></tr></thead><tbody><tr><td rowspan="4">Orne River</td><td>La Courbe (O1)</td><td>SD</td><td>In use (1959)</td><td>2037</td><td>142</td><td>4.40</td><td>92</td><td>3005</td><td>Bottom</td><td>0.005</td><td>88</td><td>78</td></tr><tr><td>Moulin à Papier (O2)</td><td>FO</td><td>In place (ACM)</td><td>2044</td><td>143</td><td>1.30</td><td>35</td><td>1050</td><td>On the side (but still open)</td><td>0.040</td><td>62</td><td>142</td></tr><tr><td>Le Bô (O3)</td><td>FO</td><td>Artificially breached (ACM)</td><td>2047</td><td>144</td><td>0.48</td><td>39</td><td>790</td><td>On the side (but still open)</td><td>0.062</td><td>50</td><td>64</td></tr><tr><td>l'Enfernay (O4)</td><td>1</td><td>Removed in 2013 (ACM)</td><td>2085</td><td>145</td><td>0 (2.45)</td><td>NA</td><td>1200</td><td>No sluice gates</td><td>0.065</td><td>77</td><td>66</td></tr><tr><td rowspan="4">Vire River</td><td>Moulin Rose (V1)</td><td>FO</td><td>In place (Unknown)</td><td>475</td><td>54</td><td>1.19</td><td>16</td><td>1200</td><td>On the side (on a tail race)</td><td>0.054</td><td>36</td><td>50</td></tr><tr><td>Pleines-Euvres (V2)</td><td>FO</td><td>In place (Attested in 1832)</td><td>491</td><td>55</td><td>1.58</td><td>23</td><td>900</td><td>On the side</td><td>0.069</td><td>89</td><td>60</td></tr><tr><td>Chapelle-sur-Vire (V3)</td><td>SD</td><td>Removed in 2020 (1850)</td><td>739</td><td>88</td><td>2.70</td><td>30</td><td>3640</td><td>On the side (but not operational)</td><td>0.020</td><td>69</td><td>149</td></tr><tr><td>Candol (V4)</td><td>FO</td><td>Removed in 2017 (ACM)</td><td>1230</td><td>132</td><td>0 (2.70)</td><td>NA</td><td>2190</td><td>No sluice gates</td><td>0.065</td><td>71</td><td>71</td></tr></tbody></table>

NA: not applicable. ACM: shown on the Cassini Map of France (ca. 1756 to ca. 1815). Values in italics and parentheses represent parameter values before the given structure was removed. For weirs styles, SD corresponds to Small Dam and FO to Flow over. Slopes were measured in the reservoir at bankfull discharge.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Grain-size distribution of injected tracers and Orne and Vire riverbeds. Note the log scale of the x-axis.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Measurement and monitoring system implemented at (a) each site with a structure (n = 6) and (b) each restored site (n = 2) (diagrams not to scale).</figcaption></figure>

reversoir, within reservoir and at proximity of the structures). Tracers that were not recovered were excluded from the calculation. Between-groups (e.g., restored/unrestored, upstream reservoir/within reservoir/upstream structures) differences were tested using Mann-Whitney non-parametric tests for nonnormal data. The threshold for statistical significance for each test was specified at *p* < 0.05.

During the four years of monitoring, flows differed only slightly between successive monitoring periods. Flood events were slightly more frequent in the Orne, exceeding the biennial flow on 3, 12, and 10 days during S1, S2, and S3b, respectively (Fig. 5A). In comparison, they always remained slightly below the biennial flow in the Vire (Fig. 5B) but did exceed 0.80 of biennial flow, which is the theoretical flow rate at which river bedload begins to move (Houbrechts et al., 2006). Recovery percentages decreased progressively over the monitoring period, starting at 91 % in S1 (2019) and decreasing to 63 % in S3a and S3b (2021–2022), which reflected the gradual dispersal of tracers and hydrological conditions during the surveys.

The site V3 was restored during the study, in September 2020 after the monitoring S2. Because of the river bed reworking during the dam removal operation, all tracers near the dam were lost (extraction by mechanical equipment, or buried). The remaining tracers injected in the reservoir were considered as belonging to a restored site for S3a monitoring.

## 3.2. Comparing morpho-sedimentary characteristics upstream and downstream

Bed geometry and particle-size parameters were measured in 2018 at
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Maximum daily flow (2018–2022) and tracer survey dates (S1, S2, and S3a/b at the end of each of the 3 monitoring periods, respectively) (A) in the Orne River (Thury-Harcourt gauging station, no. I352 1020 01) and (B) the Vire River (Saint-Lô gauging station, no. I522 1010 01). Italic values in gray correspond to the percent of recovery.</figcaption></figure>

stations located at the ends of each study section, both upstream (i.e., hydraulic head) and downstream (i.e., hydraulic jump) of the hydraulic influence zones of each structure. This approach ensured that the morphological and sediment differences observed between the upstream and downstream stations were due to the supposed deficit caused by bedload blockage within the reservoir rather than local changes in hydraulic conditions (lower slope within the reservoir vs. hydraulic jump downstream) (Skalak et al., 2009). The distance between stations and structures varied among sites, ranging from 250 to 3400 m, especially for the upstream stations, and depended on the length of the eddy. For example, at the Chapelle-sur-Vire site (V3), the eddy extended upstream for 1900 m, all the way to the next structure upstream (Table 1). Consequently, we placed the station 3400 m from the structure to ensure that it was located outside the hydraulic influence of any structure. For these configurations, we verified that no tributaries lay between the station and the structure.

The length of each station was 14 times the bankfull width (i.e.,

150–300 m), following recommendations of the CarHyCE protocol (Gob et al., 2014). In each station, we performed topographic profiles along 7–8 transects spaced one bankfull width apart using a theodolite with centimetric precision, and recorded their locations using differential GPS. In addition, particle size was measured (100 particles per station) at regular intervals along these transects using the Wolman (1954) protocol (Skalak et al., 2009).

# 4. Results

## 4.1. Bedload mobility with or without structures

The tracer's mobility in free-flowing sectors was relatively low (Fig. 6). On average, only 37 % of the particles found on each transect had a significant virtual velocity (i.e., larger than the margin of error). The distances traveled between two consecutive surveys are limited (mean of 5.8 m) and did not significantly exceed the margin of error

<figure><img src="image_8.png" /><figcaption>**Fig. 6.** (A) Mobility rate (n = injection transects) and (B) virtual velocity (m yr⁻¹) (n = mobile tracers). \* *p* < 0.05 according to Mann-Whitney test.</figcaption></figure>
(ranging from 2.5 to 3.84 m). Once mobilized, these particles traveled relatively slowly (median virtual velocity of 5.6 m yr⁻¹), and only 25 % of tracers exceeded a virtual velocity of 10 m yr⁻¹, reaching a maximum of 61 m yr⁻¹ in the Orne River and 176 m yr⁻¹ in the Vire River.

No linear correlation was found between the size and the distances traveled by the mobilized tracers, but rather a relationship between the bed's $D_{50}$ and the distances achieved. No tracers larger than the $D_{75}$ of the bed were mobilized and 72 % of the tracers considered mobile are smaller than the bed's $D_{50}$. The mean distances traveled by tracers in grain size classes finer than the bed's $D_{50}$ (32 to 64 mm) are invariant (12.6 to 12 m) and decrease to 10.9 m for the 64–90 mm class and 7.03 m for the 90–128 mm class.

Most of the particles mobilized were found in the center of the channel (Fig. 7). In the Vire, 66 % of mobilized particles were found in a central strip 6 m wide (28 % of the bed width), while in the Orne, 88 % of mobilized particles were found in a central strip 20 m wide (44 % of the bed width).

Despite these low values, restored sites experienced significantly higher tracer mobility than unrestored sites (Fig. 6). On 75 % of the transects monitored, mobility rates of the tracers is 0.5 on restored site compared to <0.2 on unrestored site. >50 % of the tracers on restored sites presented a virtual velocity higher than 6 m yr⁻¹, whereas only <25 % of the tracers exceeded this velocity on non-restored sites.

## 4.2. Bedload mobility and type of structure

The type of structure influenced the mobility rate and the virtual velocity upstream of the highest structures (V3 and O1) was significantly lower (5 %) than that upstream of small structures (18 %) (Fig. 8A). Virtual velocity was also significantly lower upstream of large structures (3.4 m yr⁻¹) than that upstream of small structures (5.7 m yr⁻¹) (Fig. 8B). Tracers upstream of the partially restored structure behaved the same as those upstream of restored sites, since their mobility rates and displacement distances did not differ significantly.

## 4.3. Bedload mobility and longitudinal location in the reservoir

The tracers' locations (e.g., upstream of the reservoir, in the reservoir, near the structure) also influenced their mobility (Fig. 9). Tracers injected upstream of the reservoir that had a flow equivalent to the mean annual flow behaved like those at restored and partially restored sites, showing significantly higher median mobility rates and virtual velocities (0.29 and 6.6 m yr⁻¹, respectively) than those injected in the reservoirs (0.06 and 3.8 m yr⁻¹, respectively).

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Virtual velocity of tracers as a function of their lateral location relative to the river centerline (survey S-1) in the (A) Orne River and (B) Vire River.</figcaption></figure>

Tracers in the reservoir and upstream of structures had significantly lower mobility rates and virtual velocities than the other tracers. Tracers in the reservoir differed the most, having the lowest mobility rates (median at 0.08) and lowest virtual velocities (all <25 m yr⁻¹, and 75 % of them <6 m yr⁻¹). Tracers upstream of structures displayed more ambiguous dynamics: their mobility rate differed significantly from that of tracers in free-flowing sectors, but not from that of tracers upstream of reservoirs or the partially restored structure, although median values were lower (16 % vs. 29 % and 40 %, respectively). Nonetheless, tracers injected upstream of structures generally moved more slowly than those injected in reservoirs.

The median virtual velocity was not influenced strongly by the longitudinal location of the tracers, given the low velocities observed in all configurations. In contrast, the highest velocities were strongly influenced: the closer to the structure, the greater the decrease in extreme values (Figs. 9 and 10).

These observations differed among sites (Fig. 10). At sites O1, O2, V2, and V3, the reservoir had as much influence on slowing the movement of mobile tracers as the weirs or small dams did. Tracers located >500 m upstream of a structure behaved like tracers located on free-flowing reaches. In contrast, virtual velocities were low at site V1, regardless of the location of the tracers. We observed no gradual slowing of tracers as they entered the reservoirs, since, of the 432 tracers identified as having been mobile at least once during the study, 23 % were mobile during two monitoring periods and 9 % were mobile during all three periods. Thus, 68 % of the particles were mobile during only one period.

## 4.4. Morphological response

The width/depth ratios measured upstream and downstream of the structures did not differ significantly between the Orne and Vire sites nor between the types of structure (Fig. 11A and B). Comparing the geometry at each station revealed contrasting dynamics among sites, with significant differences for five of them (O2, O4 along the Orne River and V2, V3 and V4 along the Vire River). For three of these sites (O2, V3 and V4), downstream sections were narrower and deeper than upstream (Fig. 11C and D).

Particles on most unrestored sites (O2, V1 and V3) were coarser downstream of the structures than upstream (Fig. 12). This difference in particle size corresponded to over-representation of the 16–32 mm class upstream and more particles in coarse fractions (< 180 mm) downstream; in contrast, the finest fractions (< 16 mm) did not differ.

At the restored sites, particle-size distribution differed less between upstream and downstream stations overall, particularly for coarse fractions, which were less over-represented downstream of the stations (Fig. 12B). Only site V4 was similar to the non-restored sites, but it had been restored only one year before the measurements were taken. At site O3, like at unrestored sites, the differences in particle size concerned mainly the 16–32 mm class, which occurred less downstream.

# 5. Discussion

## 5.1. Moderate but existing bedload mobility

Tracers under free-flow conditions (at restored sites and upstream of reservoirs) showed that the sediments (> 32 mm) composing the riverbed were mobile, but that this mobility was lowalthough biennial flows occurred at least once in the Orne River. <50 % of traced particles were mobilized when flows exceeded 0.8 of biennial flow, and only 32 % of particles were mobile during at least two monitoring periods. The tracers' virtual velocities were also relatively slow (median virtual velocity of 5.6 m yr⁻¹). These values are consistent with those observed in other low-energy systems. For example, on the Cher River and on Belgian rivers (Eau Blanche River and Bocq River), the bedload displacement velocity has been estimated respectively at 30 to 300 m
<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Tracer mobility by type of structure: (a) mobility rates (n = injection transects) and (b) virtual velocity (n = mobile tracers). Letters a, b, and c indicate groups significantly different according to the Mann Whitney test (*p* < 0.05).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Tracer mobility as a function of location along the reach: (A) mobility rates (n = injection transects) and (B) annual displacement distance (m) (n = mobile tracers). Letters a and b indicate groups significantly different according to the Mann-Whitney test (*p* < 0.05).</figcaption></figure>

per decade (Dépret, 2021) and 3 m yr⁻¹ (Peeters and Houbrechts, 2021). Many tracers’ displacements were slightly under the margin of error (ranging from 2.5 to 3.84 m) integrating both antennas detection ranges and positioning errors. This forced us to exclude many recovered tracers from our calculations. We thus consider ourselves to be at the limit of the conditions of use of RFID tags, and our results may only integrate the most mobile fractions, neglecting for example movements <3.85 m for the last survey.

The measured bedload transport corresponds to near critical conditions. On both the Orne and the Vire rivers, mean travel distances between each survey (5.9 m) are considerably shorter than the mean bankfull width of the study sites (33 m). In gravel-bed mountain streams mean travel distances have been oftent recorded at orders of two to four channel widths during low transport events (Bradley and Tucker, 2012; Schneider et al., 2014). Tracers’ travel distances remaining within one channel width is an indication of near critical conditions (Papangelakis

and Hassan, 2016). This mobility characteristic is specific to gravel-bed streams with low sediment transport rates and stable morphology (Parker, 1978; Church et al., 1998).

Under free flow conditions, we did not observe a linear relationship either between the size of the tracers and the distance they traveled. The fact that nearly all particles smaller than D₇₅ were displaced cannot be used to conclude that the entire bed has been fully entrained (Wilcock and McArdell, 1997; Haschenburger and Wilcock, 2003; Papangelakis and Hassan, 2016). In fact, the mobility we observed on the two rivers concerned only a fraction (<45 %) of the channel width. Furthermore, travel distances observed were correlated with riverbed grain size. We noted a decrease in mean travel distance when the size of the tracers exceeded the D₅₀ of the riverbed. These observations bring us closer to a situation of partial mobility, which is defined as the condition in which some surface grains remain static over the duration of a transport event (Wilcock and McArdell, 1993, 1997). This also suggests that bedload
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Virtual velocities of tracers at unrestored sites during monitoring period between S and S + 1 as a function of the distance to a structure at survey S for (A) the whole study site scale and (B) in the 70 m reach upstream of each structure.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Comparison of channel geometry upstream (white boxes) and downstream (gray boxes) reaches as a function of (A) river, (B) type of structure, (C) site along the Orne River, and (D) site along the Vire River. \* *p* < 0.05 according to the Mann-Whitney test.</figcaption></figure>

transport incorporates smaller particles, although this could not be tested with the range of sizes used as tracers. This assumption is supported by estimates based on shear stress and critical shear stress calculation (Michler et al., 2020). These calculations showed the finest fractions (D₅₀ of 20–30 mm) theoretically pass over the coarse river bed and could contribute to solid transport. This fits the definition of a “traveling bedload” as described by Recking et al. (2016), where the riverbed’s particles are too coarse for bed scouring to occur and to

constitute an active layer contributing to total sediment transport (MacKenzie et al., 2018; Vázquez-Tarrío et al., 2021).

Thus, the bedload mobility observed in this study corresponds to near-critical mobilization conditions, which are attributed both to the characteristics of the studied sites (e.g., low slope) and to the low flows recorded during the study period. Numerous studies demonstrated that substrate entrainment in gravel-bedded rivers typically begins at or near bankfull stage, which occurs approximately every 1–2 years (Parker,
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Channel bed particle-size distribution along upstream and downstream reaches for (A) unrestored sites and (B) restored sites of the Orne (O) and Vire (V) rivers. See Table 1 for the site names.</figcaption></figure>

1979; Andrews, 1983). On the Orne River, the biennial flow was only slightly exceeded (1.2 to 1.3 x Q₂) for 3 to 10 days between surveys, while it was not attained on the Vire River (0.6 to 0.8 x Q₂). These observed hydrological events are quite representative of the very calm hydrological period recorded by the two rivers over the past 20 years. The gauging stations have recorded only a very short flood (2 to 3 h) reaching Q₅ on the Orne River and two on the Vire River since 2000. It would therefore be necessary to observe episods with more contrasting hydrology to study sediment transport when threshold discharges are widely exceeded.

## 5.2. Considerable interruption of bedload connectivity by structures

Even though bedload was not actively transported in these rivers during the study period, the structures significantly modified the conditions that set coarse particles in motion and the distances they covered. These effects depend on the size of the structure: weirs significantly reduce this mobility, whereas small dams cut it to almost zero. Structures influence mobility not only because of the physical barriers they form, but also because of the reservoir they create. We observe a first decrease in mobility rates for tracers injected inside the impoundment areas (on average 60 m upstream of the structures). This decrease then becomes more pronounced in the vicinity of structures. The maximum virtual velocities of tracers placed near the structures were lower than those placed inside the reservoirs, since the structures rapidly blocked the mobilized tracers. In addition, regardless of the type of structure (except for breached weir O3), no tracers were found downstream four years after being injected, despite meticulous prospecting

over at least 250 m downstream. If sluice gates are not opened during floods, the structures on both rivers act as physical barriers that cannot be crossed during biennial flows, as observed on other rivers (Peeters et al., 2020).

These results contrast with observations of previous studies for systems with slightly steeper slopes, which showed that weirs do not block coarse particles larger than the D₅₀ or even larger than the D₉₀ (Csiki and Rhoads, 2014; Pearson and Pizzuto, 2015; Magilligan et al., 2021; Casserly et al., 2020). The contexts differ because bedload dynamics in these studies were more active due to often having more energy and the availability of coarse sediment. As a result, the reservoir structures are partially or completely filled, and the formation of sediment ramps facilitates the transfer of bedload, sometimes in the form of sediment pulses (Csiki and Rhoads, 2010; Casserly et al., 2020). The bathymetric profiles performed upstream of the four weirs in the present study showed that the reservoirs did not fill with sediment. The sediment ramps upstream of the structures were small and never reached the top of the structures remaining at least 40 cm below them. The tracers injected on the ramps were not more mobile than those injected in the reservoirs, nor did they have lower percentage of recovery. In contrast, these tracers recoveries were the highest (92 %), and even higher than those of free-flow tracers (82 % upstream of reservoirs or 71 % in restored sectors), which were more widely scattered. All of these observations indicate that even uncovered tracers are unlikely to have overpasse the structure.

The retention effect of bedload by the structures is inconsistent with the low sediment storage observed upstream of the structures or in reservoirs (Michler et al., 2020). This can be attributed to several
factors. First, we face a lack of information regarding potential maintenance dredging operations or undocumented gate openings. Based on the low sediment mobility observed, even if these operations were carried out several decades ago, their effects may be long-lasting. Furthermore, discharges during the study period remained modest. It is possible that the observed trapping effect diminishes or disappears with increased discharges, allowing bedload to overpass the structures. Finally, it is possible that bedload is transported as sediment pulses, as detailed by Casserly et al. (2020). In such case, our observations were conducted after a period of sediment release (scour phase). The systems would then be entering in a consecutive transient storage phase which could be particularly long given the low sediment dynamic observed and the weirs density upstream. As a consequence, the interruption effect of sediment connectivity could last longer than in more dynamic or less anthropized systems. In any case, it appears that the low intensity of sediment transport makes these low energy rivers more sensitive to anthropogenic disturbances than more dynamic rivers, notably due to the slow sediment filling upstream of the structures. However, we lack observations during more significant floods (Q₅ or Q₁₀) that would allow us to definitively conclude on the non-transparency of the studied structures. These floods have already been recognized as particularly important for the activation of 'scour phases' and the establishment of a state of dynamic disconnectivity (Wohl et al., 2019; Casserly et al., 2020). It is therefore necessary to continue monitoring and complete these preliminary results.

### 5.3. Influence of artificial bedload interruption on morphological parameters

Despite the influence of structures on the slowing of bedload transport, we observed no significant differences in geometry between reaches upstream or downstream of structures. Due to the absence of an important bedload and the low energy of these systems, morphological changes related to bedload trapping upstream of the structures were weak or non-existent. It was also difficult to identify sediment deficit downstream of the structures since there was little bedload transport, and the bed pavement was generally not pronounced. Pavement indices measured at three of the six sites ranged from 2.5 to 2.7 in both rivers (Michler et al., 2020), which were similar to those observed by Vázquez-Tarrío et al. (2020) for rivers with a low-to-moderate supply of coarse particles. The lack of upstream-downstream differences could also have been due to the high degree of human modification of these rivers over centuries (Lespez et al., 2015), which makes it difficult to distinguish effects of the structures themselves.

As in previous studies, bed particle size was the parameter most sensitive to the presence of weirs, even in contexts in which weirs have little influence on bedload transport (Casserly et al., 2020). Thus, bed particle size was coarser downstream than upstream of some of the weirs or small dams studied. This was reflected in downstream sectors by under-representation of 16–22 mm fractions and over-representation of the coarsest fractions (> 181 mm). The slight gravel deficit observed downstream of the structures confirms that the tracers likely provided information on dynamics of a sediment fraction coarser than that actually mobilized and thus influenced by the structures. The particle-size fraction traced, which was representative of the bed structure, was not highly mobile and thus had little deficit downstream. It would be interesting to use 12 mm tracers to monitor the dynamics of coarse gravel in detail. Although these tracers are more difficult to detect (Chapuis et al., 2014), their use will be facilitated by the low dispersal of pebbles and the absence of an active layer bed.

### 5.4. Implications for river restoration projects

Our study demonstrates that the removal (or breaching of weirs) appears as effective operations to restore the sediment connectivity in low energy rivers, which seem to be more sensitive to the effects of these

structures than rivers with abundant bedload. The tracers injected at the restored sites behaved in the same way as those injected upstream of the reservoirs, regardless of the type of restoration performed (completely removed or breached). We also observed that at sites restored several years previously (O3 and O4), differences in particle size between upstream and downstream were not visible, which demonstrates (i) the effectiveness of breaching in restoring bedload connectivity and (ii) the re-establishment of bedload transport composed mainly of gravel after restoration. Site O3 (with breached weir) was the only one where tracers passed through the structure, with three RFID tags found downstream. As demonstrated by Peeters et al. (2020), these initial results confirm that in certain contexts, partial breaching can remobilize sediment in a manner similar to that of free-flow conditions, even though the distances covered by the particles remained slightly shorter than those at sites where the structure had been removed completely. However, precautions must be taken when carrying out this type of operation. For example, we observed that tracers could remain trapped in the breach due to the excessive roughness of the structure. In the light of these operations, opening the gates appears as an alternative to weir removal. Studies in Belgium already demonstrated the effectiveness of such measure (Peeters et al., 2020). The fact that many of the studied sites are secondary properties and that the gates are rarely operated (or even degraded) makes it difficult to implement such measures at our sites. Regarding the only dam remaining in place at the end of our study (Barrage de la Courbe, site O1), we observed a decrease in mobility far upstream of the structure (> 1 km). Considering the low virtual velocity that we observed upstream of the reservoir, it is unlikely that simply opening the gates will be an effective short-term solution.

Although the observed bedload transport dynamics are modest in the free-flow sectors, the widespread removal of structures is likely to significantly accelerate a bedload that has been artificially limited for at least several decades. In a longer-term restoration perspective, it seems important for managers and scientists to also focus on inputs of coarse sediments into these systems. In these contexts, lateral mobility is low, riverbanks are primarily composed of fine particles, and watersheds produce little gravel or pebbles. This type of system corresponds to the “stable bed aggrading bank” model developed by Brown and Keough (1992) and highlighted by Beauchamp et al. (2017) for similar rivers in Normandy, such as the Seulles River. Regionally, the banks of these rivers result from sediment filling the valley floor following the production of fine particles after agricultural development of watersheds during the medieval period (Beauchamp, 2018). In addition, these banks are not subject to significant erosion due to the low energy content of these rivers. We only observed traces of erosion on <10 % of the banks along the two sections studied (Michler et al., 2020). We identified two sediment sources that should be investigated in greater depth, and which illustrate the hybrid nature of this type of river: (i) small tributaries, where small sediment fans, composed mainly of fine particles, are formed at their confluence; and (ii) farmers, who cultivate riverside fields and spontaneously discharge coarse particles from the soil into the river. During the surveys of the Orne and Vire rivers, we observed many of these injection points, which would be interesting to quantify.

## 6. Conclusions

This study highlighted the existence of partial transport of solid bedload under conditions similar to the biennial flood in two low energy rivers. Mobile particles were transported on a fixed bed consisting of coarse sediment, which produced more a traveling bedload than a bedload transport with a true active layer. Virtual velocities were moderate, and few particles were mobile during the four years of monitoring. Because of the near critical conditions of bedload transport in the low-energy context of Normandy, bedload transport in Orne and Vire rivers is particularly sensitive to structure (dams or weirs) under moderate hydrological conditions (+/− Q₂). Unlike previous studies, small (1–2 m) or larger (> 2 m) structures significantly influenced
bedload transport by reducing mobility rates (50 % lower) and the vital velocity (86 % lower). This was due to the lower slope of the water line in the reservoir and the physical presence of the structures, which act as a physical barrier in these low-energy systems (at least during moderate floods). After four years of monitoring, no tracers were found downstream of the structures, except for the breached weir (O3). This bedload interruption did not, however, degrade the bed's geometry but did significantly change its particle-size distribution, with less gravel and more coarse pebbles downstream.

Complete removal or breaching of transverse structures helps restore bedload mobility and riverbed sediment distribution similar to those in free-flowing reaches. However, these initial results need to be put into perspective, since no flood exceeded the biennial flow during the study, and only one breached structure was monitored. It would therefore be necessary to record events with more contrasting hydrology to assess whether our conclusions are still valid when threshold discharges are widely exceeded.

### CRediT authorship contribution statement

A.J. Rollet: Writing - review & editing, Writing - original draft, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. R. Reulier: Validation, Investigation. G. Fantino: Validation, Methodology, Investigation. A. Glais: Investigation.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Rollet reports financial support was provided by Regional Water Agency Seine-Normandy. 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.

### Data availability

Data will be made available on request.

### Acknowledgements

We thank the Agence de l'eau Seine Normandie for financial support, as well as the students of Licence 3 de Géographie parcours Environnement at Rennes 2 University and students of Master 1 TELENI - Rennes 2 University who took field measurements during the summers of 2018 and 2019. Thanks also go to Laurence David for producing maps, Simon Dufour and Baptiste Marteau for proofreading. The comments and suggestions from two anonymous reviewers and the guest editor substantially improved the quality of this submission.

### Funding

This study was supported by the Regional water agency of Seine Normandy.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j geomorph.2024.109356.

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