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Is removing weirs always effective at countering the sediment deficit? Case study in a Mediterranean context: the Gapeau River

La suppression des seuils est-elle toujours efficace pour lutter contre le déficit sédimentaire ? Etude de cas en contexte méditerranéen : le Gapeau
Anne-Julia Rollet, Simon Dufour, Romain Capanni et Mireille Lippmann Provansal
p. 187-200

Résumés

Dans les systèmes littoraux et fluviaux en déficit sédimentaire la restauration du transport solide fait aujourd’hui l’objet d’une attention particulière. La suppression d’ouvrages transversaux (seuils, barrages) est parfois préconisée même si l’effet réel des petits seuils sur le transport de la charge de fond n’est pas démontré dans tous types de contexte. Dans ce cadre, notre étude a pour objectif d’apporter des éléments quantifiés pour (i) documenter l’interruption des transferts sédimentaires grossiers (> sables fins) par un petit seuil sur un système fluvial côtier méditerranéen (le Gapeau), et (ii) discuter la pertinence de sa suppression pour la restauration de la continuité sédimentaire. Ces éléments sont produits partir d’approches croisées de suivis de la dynamique sédimentaire du fond du lit (bathymétrie, traçages sédimentaires, chaines d’érosion et suivis topographiques) et de modélisation de capacités de transport. Nos résultats nous permettent de conclure que le seuil étudié ne semble pas constituer d’entrave physique au transfert de la charge de fond dans la mesure où aucune accrétion nette n’a été observée en amont de l’ouvrage malgré des crues importantes enregistrées durant le suivi. Néanmoins, la mesure indirecte du transport solide montre qu’il n’existe pas ou plus de charriage sur ce cours d’eau qui connait un fort déficit sédimentaire. Ainsi, la suppression de seuil sur le Gapeau serait insuffisante pour atténuer le déficit sédimentaire fluvial et/ou littoral. Il conviendrait plutôt de concentrer la réflexion sur la réalité des entrées sédimentaires et l’efficacité des connexions entre les versants et le chenal.

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Notes de la rédaction

Manuscript received on April 27, 2022, revised version received on January 12, 2022. Paper definitively accepted on January 14, 2022

Texte intégral

The authors thank the Rhône Méditerranée Corse Water Agency for financing the project from which the results were derived, as well as Samuel Meulé and Philippe Dussoulier for their help in processing the data.

1. Introduction

1.1. Fluvial sediment input is a challenge for managing the costal sediment deficit

1Many of the world's beaches are retreating (Bird, 1980), mainly due to the rise in sea level over the past 100 years (Warrick and Oerlemans, 1990; Pilkey, 1991; Church et al., 2001; Leatherman et al., 2003) related to global warming (Gornitz et al., 1982; IPCC, 2001). However, the decrease in sediment inputs from rivers contributes to this retreat (Paskoff, 1998; Walling and Fang, 2003; Syvitski et al., 2009; Paskoff and Clus-Auby, 2007; Brunel, 2012), especially in the Mediterranean basin (Guillen and Pallanques, 1992; Innocent and Pranzini, 1993; Poulos et al., 1996; Sabatier et al., 2009; Bergillos et al., 2016; Hadour et al., 2021). Brunel and Sabatier (2009) showed that coastline retreat on Mediterranean pocket beaches was due mainly to the rise in sea level, whereas on beaches open to the swell, it was due to the decrease in fluvial sediment inputs. Although the influence of large fluvial systems on the supply of coastal sediments is known, that of smaller hydrological systems is often underestimated given the volumes involved (Milliman and Syvitski, 1992; Poulos and Collins, 2002; Farnsworth and Milliman, 2003). Delmas et al. (2012) observed that Alpine/Mediterranean mountainous areas contribute 60% of the total sediment export in France but have a drainage area that covers only 19% of the total area. Temporary streams can have higher specific sediment load than perennial streams, and the former are often overlooked, even though they are common in the Mediterranean climate (Reid and Laronne, 1995; Reid, 2002). Thus, understanding sediment dynamics of small Mediterranean rivers is a particularly relevant issue for managing coastal sediment.

1.2. Removing barriers to address sediment deficits

2For several decades, sediment continuity within river systems has undergone great changes, and large hydraulic structures are one of the main causes. Their morpho-sedimentary and ecological effects have been widely documented in the literature for 50 years (Williams and Wolman, 1984; Wasson et al., 1998; Brandt, 2000a, b; Poff and Hart, 2002; Batalla et al., 2004; Petts and Gurnell, 2005; Philipps et al., 2005; Gordon and Meetenmeyer, 2006; Rollet et al., 2013; Csiki and Rhoads, 2010). Based on these findings, restoration measures since the 1990s have progressively included removing barriers to restore sediment and ecological continuity (Graf, 2003; Magilligan et al., 2016), and dam removal has received increasing attention, first in the USA and then in Europe (Bellmore et al., 2017; Sneddon et al., 2017). Thus, in France, removing barriers has become a key measure in programs to restore river ecology (Germaine and Barraud, 2013). A variety of structures (e.g., weirs, dams) have been removed, usually small run-of-river structures that were usually lower than the channel banks associated with former water mills (Sneden et al., 2017). In France, these structures represented 81% of the listed barriers in 2014 that could influence sediment continuity (Depret, 2019). Although the influence of large dams is well known (and thus removing them can restore sediment flows), the influence of weirs remains unclear. Thus, the increase in weir removal clearly preceded the increase in the number of scientific studies of weirs impacts (Bravard and Lévêque, 2020). It is thus necessary to better understand the influence of weirs on sediment transfer.

1.3. Little evidence of the influence of weirs on sediment transport

3Many studies have highlighted the influence of small structures on channel morphology (Fencl et al., 2015), but only a few morphological studies have focused on the influence of weirs on coarse-sediment continuity. The morphological changes that weirs cause often cover a relatively small area and are related either to submergence of the structure upstream (e.g., sedimentation) or a hydraulic surge downstream (e.g., incision downstream of the structure, appearance of median banks). To date, no morphological change downstream of weirs has been explicitly correlated with a sediment deficit that they generated. The few studies that focused on the influence of weirs on sediment continuity suggest that they do not influence it strongly (Csiki and Rhoad, 2010; Pearson and Pizzuto, 2015; Peeters et al., 2020; Casserly et al., 2021). Bedload sediment can leave a reservoir during high-flow events (Pearson et al., 2011; Casserly et al., 2021) or after exceeding the reservoir’s storage capacity (Major et al., 2012). 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 sloping ramp, and over the top of the 2.5 m dam, while Peeters et al. (2020) observed a selective transfer of particles around the median. However, the response of rivers to the long-term existence of weirs varies greatly (Csiki and Rhoad, 2010), and depends mainly on characteristics of the structures (i.e., form, height of the crest, presence or absence of sluicegate systems), the river (i.e., occurrence of large flood events, sediment size, ability of river hydraulics to transport sediments over the weir crest) and general characteristics of the watershed (e.g., upstream weir density, available sediment supply) (Pearson and Pizzuto, 2015). Therefore, understanding the influence of weirs on sediment flows (and thus the relevance of removing them) requires a different and more integrated approach to research and management than the individualistic approach that has been applied to larger dams (Fencl et al., 2015).

4While interruption of sediment continuity in rivers strongly influences sediment supply to the coast, the influence of other factors on sediment deficit should not be minimized, such as the reduction in the capacity of watersheds to produce sediments after changes in land use (Walling and Fang, 2003). This was highlighted in a mountainous northern Mediterranean river system when agricultural practices were abandoned and slopes were reforested (Gautier, 1994; Gaillot and Piégay, 1999; Liebault, 2003; Piégay et al., 2004), as well as in the Lower Rhône basin (Provansal et al., 2014).

5These elements raise questions about the degree to which weirs contribute to the coastal sediment deficit. They also indicate the need for additional studies to determine the capacity of small structures to interrupt sediment transfer significantly and the capacity of their watershed area to produce sufficient sediment. Addressing these two points will help determine whether removing small structures is a relevant and sufficient management practice to mitigate the coastal sediment deficit, particularly in the highly anthropized Mediterranean context, where many factors can have a negative influence on sediment dynamics.

6To address this issue, we use the example of the Gapeau River, which is a small coastal river that may contribute to the sediment supply of the Var coast (Capanni, 2011). The Gapeau River is a relevant example because it has undergone profound human modifications throughout the 20th century (e.g., reforestation, forest fires, sediment extraction, construction of weirs) that likely reduced (or increased during the occasional fires) sediment production and connectivity between the watershed and its mouth. Because a large coastal sediment deficit requires costly recharge operations to maintain beach systems (Courtaud, 2000; Battiau-Queney, 2015), understanding sediment transfer within this river is a crucial management issue. The objective of this study was to identify whether removing cross-sectional structures in this context is an appropriate practice for overall sediment management. To this end, we explored (i) whether weirs hinder fine- and coarse-sediment flows and (ii) whether this sediment flow was sufficient and sustainable in the context of coastal management. We conducted multi-scale approach at two levels: (i) the local (weir) scale, to determine the weir’s ability to alter sediment continuity within the channel; (ii) the river-section scale, via longitudinal analysis of sediment transport indicators, to identify whether the river experiences a sediment deficit all along the channel, and thus to assess the state of connectivity between the watershed and the river.

2. Study area

2.1. General presentation

7The Gapeau River is a 47.5 km long coastal river (slope: 0.7 m.m-1) that drains a 564 km² watershed located between the mountain ridges of Sainte Baume and Morières to the north and west and the mountain ridge of Maures to the east (fig. 1). The river opens to the south on the plain and empties into the harbor of Hyères. The Gapeau watershed contains two sub-watersheds with contrasting geological characteristics, which likely results in the differing sediment supply to the downstream section: (i) the western sub-watershed, where the Gapeau River flows on a limestone substrate, which is permeable and favorable to infiltration, and (ii) the eastern sub-watershed (315 km²) of the Réal-Martin River (the main tributary of the Gapeau River), which flows on metamorphic, impermeable substrates. The Gapeau River has a narrow and deep channel over most of its length as well as high banks (Capanni, 2011). This morphology, more like a rectified and incised channel than a natural channel, is especially suitable for the transit of water and sediment flows.

Fig. 1 – Location map.
Fig. 1 – Carte de localisation.

Fig. 1 – Location map.Fig. 1 – Carte de localisation.

A : Regional scale; B : Local scale. 1. Watercourses; 2. Coast line.
A : Echelle régionale ; B : Echelle locale. 1. Cours d’eau ; 2. Trait de côte.

8Given its watershed area, the Gapeau River has the potential to supply the most sediment (i.e., 90%) to the harbor of Hyères, at an estimated 20,000-30,000 m3.yr-1 (Capanni, 2011). The hydrological regime in the lower Gapeau River is of the uninfluenced Mediterranean rainfall type (Pardé, 1933). Thus, it can vary greatly (i.e., ratios of 1:160 and 1:520 between low-water flow and 2-year and 50-year floods, respectively) and include flash floods. Flow characteristics of the Gapeau River at the Sainte Eulalie gauging station (6 km upstream of the mouth) are 4 m3.s-1 for mean annual discharge, 0.24 m3.s-1 for low-water discharge and 88 m3.s-1 for the 2-year flood (Banque Hydro, 2021). In terms of interannual variability, periods of high hydrological activity (during the 1970s and 1990s) alternate with periods of relative calm (during the 1960s, 1980s and 2000s), which is consistent with the rainfall history. Hydrological activity varies according to the watershed’s geology: water discharge and flood amplitudes are thus higher in the metamorphic Réal-Martin sub-watershed than in the limestone Gapeau sub-watershed (SCP, 1973). On the studied reach, the riverbed slope varies from 0.002 m.m-1 to 0.004 m.m-1 and the D50 of the river bed varies from 32 mm to 77 mm (tab. 1).

9The Gapeau watershed is dominated by forest on its slopes (ca. 66% of the watershed in 2006) and by agriculture and urbanization in its valley bottoms. Unlike the Var department as a whole, the watershed experienced significant reforestation of landscapes (+50 km², or 15% of its area) from 1973-2006 (Capanni, 2011). In addition, the Gapeau River and its tributaries are subjected to substantial human pressure, including sediment extraction and weir construction to provide water for agriculture.

Tab. 1 – Tracer and riverbed sediment characteristics.
Tab. 1 – Caractéristiques des traceurs et des sédiments du lit.

Tab. 1 – Tracer and riverbed sediment characteristics.Tab. 1 – Caractéristiques des traceurs et des sédiments du lit.

2.1. Gravel extraction

10Sediment extraction from the Gapeau River bed is a major issue. Interviews with riverside residents helped to understand this phenomenon. The residents reported many sporadic, low-volume extractions that are impossible to quantify. These extractions intensified during the construction of Toulon airport in the 1960s. According to the residents, for several weeks trucks exported Gapeau River sediments mainly from the reach downstream of the confluence with the Borrel River. However, the extractions were never officially recorded or described in reports or administrative documents.

2.2. Weir density

11The density of river partitioning by weirs differs in the upstream and downstream areas of the watershed. A total of 37 obstacles to fish migration are listed in the Gapeau River, of which nearly 94% are located upstream of the confluence with the Réal-Martin River (ROE, 2020). The Réal-Martin River contains 167 hydraulic structures, including 38 masonry weirs (one weir every 1.2 km) that range from 0.5-2.0 m in height (S.I.E.E., 2004) and 14 structures that are an obstacle to fish migration (ROE, 2020). The exact date when each of them was constructed is unknown, but all of them were listed in the “profile of major hydraulic forces” in 1954. These weirs are almost exclusively intended for water withdrawal for irrigation and are largely abandoned, but are not in poor condition (Barral, 2002).

12In the downstream section of the Gapeau River, the only structure that can trap sediment in transit, the Sainte Eulalie weir, is located 2.2 km downstream of the confluence with the Réal-Martin River (fig. 2). This weir (called the “La Clapière dam” in the ROE database) is a 3.75 m high masonry structure and generates an upstream water body about 700 meters long at low flow conditions. Its exact date of construction is unknown, but it is shown on maps from 1896. During this study, it was not completely filled with sediment, which implied (i) low sediment inputs from upstream, or (ii) that it does not hinder transit of the bedload or (iii) frequent cleaning. According to local residents, however, the weir did not seem to have been cleaned recently.

Fig. 2 – Sainte Eulalie weir.
Fig. 2 – Seuil de Sainte Eulalie.

Fig. 2 – Sainte Eulalie weir.Fig. 2 – Seuil de Sainte Eulalie.

13The river’s sediment dynamics may have changed profoundly during the 20th century. Nevertheless, its channel did not experience major or general morphological changes, either in width or longitudinal profile, which raises questions about the current sediment dynamics. Our study focused on the last section of the Gapeau River, which corresponded to the 8 km between the confluence with the Réal-Martin River and the mouth. Diachronic and synchronic analyses were conducted to describe sediment dynamics. We first analyzed the sediment-trapping capacity of the Sainte Eulalie weir during two flood events and then focused on spatial variability at four sites to quantify sediment-transport dynamics and the sediment deficit (fig. 1).

3. Materials and methods

3.1. Sediment dynamics at the Sainte Eulalie weir

14To determine the influence of weir on the sediment continuity of the river, we monitored changes in the sediment stock upstream of the Saint Eulalie weir along a 700 meters reach (the entire water reservoir), which is the main trapping area. The weir’s sediment-trapping capacity was assessed by analyzing bathymetric differentials before vs. after floods (Rowan et al., 1995; Furnans, 2008). We monitored the bathymetry of the reservoir using an ADCP (Teledyne RD Instruments) Qliner (2 MHz), specifically developed by Nortek for gauging in small and medium-sized rivers (1-30 m wide and 0.3-5.0 m deep). The ADCP was combined with real-time kinematic and differential GPS to take measurements in a context in which a tree canopy could have degraded the satellite signal. Using the bottom tracking mode, we collected four measurements per point to ensure a vertical margin of error of ±0.10 m after post-treatment. Three bathymetric surveys were conducted in September 2007, March 2008 and June 2008. We used these data to describe the sediment mobility generated during floods of three different intensities: instantaneous water discharge of 42 m3.s-1 in January 2008 (Q1 = 40 m3.s-1), and 57 m3.s-1 and 67 m3.s-1 in April and May 2008, respectively (Q2 = 88 m3.s-1) (fig. 3).

Fig. 3 – Hydrological context (mean daily discharge) of the bathymetric, topographic and sediment-tracing surveys.
Fig. 3 – Contexte hydrologique (débits moyens journaliers) des suivis bathymétriques, topographiques et des traçages sédimentaires.

Fig. 3 – Hydrological context (mean daily discharge) of the bathymetric, topographic and sediment-tracing surveys.Fig. 3 – Contexte hydrologique (débits moyens journaliers) des suivis bathymétriques, topographiques et des traçages sédimentaires.

1. Bathymetric surveys; 2. Topographic and tracers surveys.
1. Suivis bathymétriques ; 2. Suivis topographiques et traceurs.

15Matlab was used to process the data. For each survey, a DEM with 50 cm planimetric resolution was generated via interpolation by triangulation (triangulated irregular network), with a margin of error in elevation of ±10 cm.

3.2. Assessment of sediment deficit

16The sediment deficit was assessed by comparing the river’s transport capacities to the coarse-sediment transport measured in situ at bi-annual (July 2007 to April 2009) and flood-event (December 2008 Q5 flood) scales and then by estimating pavement index. The sediment deficit focused on bedload transfer because the washload flow, composed of more than 50% silt and clay, with the rest being medium sand 0.3-0.7 mm in size (Geomer, 1996), is rapidly transported out to sea, which increases the turbidity of the entire harbor of Hyères. The sand fraction is trapped in the mouth of the Gapeau River and then redistributed along the near foreshore, becoming trapped by the seagrass (Posidonia oceanica) meadows without feeding the coastline (Courtaud, 2000; Capanni, 2011). To sample volumes of sediment easily, we selected four sites along the Gapeau River, three within the study reach and one in the Réal-Martin River, that were morphologically homogeneous (bars), with straight, accessible sections and raised pebble bars (fig. 1).

3.2.1. Indirect estimation of bedload

17Sediment transport was estimated at the four sites using the integrated indirect method of Laronne et al. (1992), which is particularly adapted to small-to-medium rivers that have highly variable discharge with strong low water period (Laronne et al., 1994; Boutault 2020). It is based on the theoretical equation of continuity of background transport:

18where Vb = volume of bedload that moves downstream from a reach (m3.s-1), A = active cross-sectional area (m²), L = mean distance of bedload transport (m), and p = porosity.

19Porosity (p) was calculated using the equation developed by Carlin and Reader (1982):

20where Dm = mean sediment diameter (m).

21The active cross-sectional area was measured using scour chains inserted into the bed (Liébault, 2003; Rollet, 2007; Houbrecht et al., 2012; Chapuis et al., 2015; Brenna et al., 2019) and topographic measurements made using differential GPS. The mean distance of bedload transport was measured by tracing pebbles placed along transects reproducing as best as possible the imbrication of the particles composing the river bed (Rollet, 2007; Houbrecht et al. 2012; Hassan and Roy, 2016).

22Scour chains were inserted according to recommendations of Liébault (2003). Along each transect, the mean spacing between chains was 17% of the width of the section studied, which is considered sufficient for robust assessment of bedload transport (Liébault and Laronne, 2008). Unfortunately, municipal services frequently removed the vegetation on the gravel bar at site 4 (Gapeau downstream 2) during the monitoring period to prevent erosion of the right bank, which threatened military land. The scour chains there were quickly lost.

23The topographic surveys of the transects had an estimated margin of error of ±0.05 m due to the measurement tool (tacheometer) and the surface texture (Rollet, 2007). Nevertheless, some vertical movements close to the margin of error were considered significant, especially when the sediment-tracing and scour-chain surveys showed movement of the bedload.

24Transport distances were estimated via tracking by passive radio frequency identification transponders (Lamarre et al., 2005; Rollet et al., 2008; Houbrecht et al., 2012; Hassan and Roy, 2016). At each site, 60 pebbles were randomly sampled, equipped and placed along transects equipped with scour chains. Due to limitations related to transponder size, the particle-size distribution of tracers was slightly coarser than that of the gravel bed (tab. 1). Only site 2 had a particularly coarse particle-size distribution, with a D50 larger than that of the tracers. Tracers were considered as mobile when their observed travel distance exceeded 1 meter.

25Six surveys were conducted using scour chains and tracers for hydrological events of 18-216 m³.s-1 at the Sainte Eulalie station. Mean daily discharge was measured at the hydrological stations of Sollies-Pont (site 1), Crau in the Réal-Martin River (site 2) and Sainte Eulalie (sites 3 and 4) via the hydroportail website. The highest flood recorded (216 m³.s-1, i.e., Q5) lasted nearly four days, from 14-17 December 2008 (fig. 2).

26We calculated the specific power of each flood (ω) to describe the hydraulic conditions of the sites, using the equation of Bagnold (1966):

27where ρ = density of the fluid; g = acceleration of gravity; Q = maximum instantaneous flood flow, w = width of the channel, and S = slope of the water line.

28We were unable to measure the slope of the water line for each event, and the geometry of the banks in the sectors studied had been modified greatly during channelization; thus, the slope of the top of the banks did not completely represent the energy lines in the channel. Thus, we used the default slope of the low-water line measured during the topographic surveys.

3.2.2. Determining the sediment-transport capacity

29Mean annual transport capacity was calculated using two modules of the 1-dimensional SAM (Stable channel Analytical Model) modeling software developed by the US Army Corps of Engineers (Thomas et al., 2002). It includes (i) SAM.hyd, an hydraulic module that uses flows to estimate hydraulic parameters for stable channels in alluvial material, which are used by (ii) the SAM.sed module to estimate the solid-transport capacity of stationary flows. Of the 20 solid-transport equations provided by the software, we selected four whose application was most similar to the hydrogeomorphological characteristics of the Gapeau River: Meyer-Peter and Müller (1948), Einstein (1950), Schoklitsch (1934) and Parker (1990).

30Since the equations are sensitive to their parameters (Gomez and Church, 1989), especially slope and texture (Rollet, 2007), particular attention was paid to defining the parameters. Particle-size data for each site were assessed using the volumetric bulk sampling method to consider the subsurface sediment fraction (Bunt and Abt, 2001). A Manning coefficient of 0.032 was used based on the roughness coefficients determined by CEDRAT in 2001 (CEDRAT, 2001). The slope parameters were the same as those used to calculate the specific stream power.

31We calculated the sediment-transport capacity per 10 m³.s-1 flow block. We also calculated theoretical sediment transport at the flood-event scale (flood of December 2008) for a 2-year period (July 2007-April 2009) by multiplying the transport capacities per flow block by the frequency of the flows block at the flood-event scale and over the two years studied period. We used hydrological data at variable time steps (averaged over 30 min) from the three hydrological stations according to their geographical proximity to the sites (BanqueHydro, 2021).

3.2.3. Armoring ratio

32Since surface armoring is less developed in rivers whose transport capacity equals the amount of sediment that enters the section, we used the armoring index (ip) as an indicator of potential sediment deficit at the four sites:

33where D50sf = median size of the b-axis (mm) of sediments on the river bed, measured in the middle of gravel bars according to the Wolman protocol, and D50sbf = median size of the b-axis (mm) of sediments in the subsurface layer, measured by volumetric sampling on same sediment patches (Bunt and Abt, 2001).

34The mass (ms) of sediment to be treated was estimated for each site using the most frequently used empirical equation (Church et al., 1987):

35where Dmax (m) = mean size of the b-axis of the 30 largest particles in the sampled area.

36Since the distributions estimated by volumetric weight-frequency sampling are the same as those estimated by headcount frequency grid sampling, results of these two techniques did not need to be converted to compare them. (Kellerhals and Bray, 1971; Bunte and Abt, 2001). The volumetric texture of the subsurface layers was considered to estimate the percentages of sand and gravel in the bed sections mobilized during floods.

4. Results

4.1. Influence of the Sainte Eulalie weir on sediment transport

37The sediment balance estimated from the bathymetric differentials of the river bed over the entire monitoring period (September 2007-June 2008) showed that the bed was relatively stable, except for two sectors with slight erosion (80 m3, on a retention area of 15,000 m²) located in sinuous zones and the downstream end of the sector, immediately upstream of the weir (fig. 4). However, the monitoring identified alternating phases of accretion and erosion of the river bed depending on the flow. For example, the bathymetric differential between September 2007 and March 2008, a period that contained a moderate flood of 42 m³.s-1 (fig. 4), revealed an accretion of 10-20 cm, consisting essentially of gravel and sand (observed from Eckman grab samples), over most of the river bed. Locally, accretion was higher (e.g., 0.5-1.0 m on the concave bank of the meander). This accretion corresponded to an accumulation of ca. 500 m³ over the 700 m of linear distance considered.

Fig. 4 - Bathymetric differentials between September 2007, March 2008 and June 2008.
Fig. 4 - Différentiels bathymétriques entre septembre 2007, mars 2008 et juin 2008.

Fig. 4 - Bathymetric differentials between September 2007, March 2008 and June 2008.Fig. 4 - Différentiels bathymétriques entre septembre 2007, mars 2008 et juin 2008.

38In contrast, the period from March-June 2008 was dominated mainly by erosion of the bed. The two floods of 57 and 67 m³.s-1, respectively, caused erosion of 10-30 cm over the entire section. Locally, erosion was higher (e.g., > 1 m in the meander), which indicated a specific hydraulic configuration at the section scale. The accretion measured during this period (< 50 m³) was considered insignificant. Except for the specific configuration in the main meander, we observed no pattern in the spatial distribution of the morphological changes that indicated significant sediment accumulation at the head of the hydraulic eddy generated by the weir or even upstream of it. These results indicate that only a little part of the solid load reached the reservoir and that the reservoir itself did not seem to trap the load that did reach it. Moreover, the erosion observed along the reach 50-100 m immediately upstream of the weir would seem to indicate that the structure is transparent to the grain size encountered along this section.

4.2. Measurement of bed mobility and bedload flux

4.2.1. Estimation of topographic changes in the studied sections

39The mobility of the active layer was too low to be recorded by the scour chains; thus, only topographic monitoring could be used. Overall, the mean thickness of sediments mobilized (erosion and accretion) per section was low (< 5 cm) during most of the floods studied. In detail, the floods did not influence the bed of the Gapeau River upstream of its confluence with the Réal-Martin River, regardless of their magnitude (site 1) (fig. 5). At the other sites, the mean thickness of the active layer (erosion and accretion) per section remained less than 10 cm as long as the flow rate remained less than or close to the 2-year flood. Only the December 2008 event (216 m3.s-1), slightly higher than a Q5 flood, resulted in significant mean topographic changes in the river bed (15-30 cm per section), which reflected movement of the bedload. These changes concerned all the sites, except site 1.

Fig. 5 – Thickness of the active layer of the riverbed.
Fig. 5 – Epaisseur des couches actives du chenal.

Fig. 5 – Thickness of the active layer of the riverbed.Fig. 5 – Epaisseur des couches actives du chenal.

1. Site 1; 2. Site 2; 3. Site 3; 4. Site 4.
1. Site 1 ; 2. Site 2 ; 3. Site 3 ; 4. Site 4.

40Based on the mean thickness of the active layer per section, we calculated the cumulative reworked area (erosion and accretion) as 0.4-7.0 m², depending on the flood and the site.

4.2.2. Travel distances of passive radio tracers

41The recovery rates of the tracers released on the bed varied greatly, from 95% after small floods (40-70 m3.s-1) to 47% after the largest flood (i.e., Q5) (tab. 2). The lowest recovery rates (sites 3 and 4) were due to particularly difficult survey conditions (e.g., deep pools) in the downstream Gapeau River.

42Tab. 2 – Tracer mobility and recovery rates after floods.
Tab. 2 – Mobilité des traceurs et taux de retour après crues.

Qimax = maximum mean daily discharge; NM = not mobilized; pp = painted particules.
Qimax = débit moyen maximum journalier ; NM = pas mobilisé ; pp = particules peintes.

43At all sites, sediment mobility (more than 25% of the tracers) began only when the specific power exceeded 50 W.m-², which occurred at 0.36-0.50 Q2, depending on the site (fig. 6A).

Fig 6 – (A) Percentage of tracers mobilized and (B) mean distance traveled as a function of specific stream power.
Fig 6 - Taux de traceurs mobilisés (A) et distances moyennes parcourues (B) en fonction des puissances spécifiques.

Fig 6 – (A) Percentage of tracers mobilized and (B) mean distance traveled as a function of specific stream power.Fig 6 - Taux de traceurs mobilisés (A) et distances moyennes parcourues (B) en fonction des puissances spécifiques.

1. Site 1; 2. Site 2; 3. Site 3; 4. Site 4.
1. Site 1 ; 2. Site 2 ; 3. Site 3 ; 4. Site 4.

44The sediment tracing showed moderate or low mobility of bed particles (2-6 m), regardless of particle size, during moderate floods (i.e., Q2), even though the specific power was high (50-120 W.m-²) (fig. 6B). No significant difference was observed among the sites during the moderate floods, except for site 1, which had the longest travel distances due to longer period during which the critical flow rate was exceeded (fig. 7). The mean travel distance was 150 m during the December 2008 flood, but only at site 4 and site 1, where the critical discharge was exceeded for a longer period. Sites 2 and 3 had the shortest travel distances (< 20 m), regardless of the flood considered, even though the specific power exceeded 150 W.m-² during the 5-year flood of December 2008 (fig. 6B-7).

Fig 7 – Mean distance traveled as a function of flood duration.
Fig 7 - Distances moyennes parcourues en fonction de la durée des crues.

Fig 7 – Mean distance traveled as a function of flood duration.Fig 7 - Distances moyennes parcourues en fonction de la durée des crues.

1. Site 1; 2. Site 2; 3. Site 3; 4. Site 4.
1. Site 1 ; 2. Site 2 ; 3. Site 3 ; 4. Site 4.

4.2.3. Quantification of mean annual solid transport from July 2007-April 2009

45The volume of bedload mobilized over the monitoring period (July 2007-April 2009) ranged from 26 m³ at site 2 to 464 m³ at site 4 (tab. 3) (i.e., 16 and 278 m3.yr-1, respectively). These volumes contained 4.5-195 m3 of sand over the monitoring period (i.e., 3-117 m3.yr-1).

Tab 3 - Volume of bedload (sand and coarse sediment) mobilized during the monitoring period (July 2007-April 2009).
Tab 3 - Volume de charge de fond (sables et sédiments grossiers) mobilisés pendant la période de suivi (juillet 2007 – Avril 2009).

Tab 3 - Volume of bedload (sand and coarse sediment) mobilized during the monitoring period (July 2007-April 2009).Tab 3 - Volume de charge de fond (sables et sédiments grossiers) mobilisés pendant la période de suivi (juillet 2007 – Avril 2009).

4.3. Transport capacity and pavement index

4.3.1. At the flood scale: example of the December 2008 flood

46The transport capacity estimated for the 5-year flood of December 2008 ranged from 6,800-288,100 m3 depending on the site and the equation used (tab. 4). While the Parker and Schoklitsch equations overestimated the probable capacity of the Gapeau River, the Meyer-Peter and Einstein equations were more consistent.

Tab. 4 – Measured sediment load and transport capacity of the December 2008 flood.
Tab. 4 – Volume de transport solide mesuré et capacités de transport correspondant à la crue de décembre 2008.

Tab. 4 – Measured sediment load and transport capacity of the December 2008 flood.Tab. 4 – Volume de transport solide mesuré et capacités de transport correspondant à la crue de décembre 2008.

47Comparing the volume mobilized by the December 2008 flood to the transport capacity indicated that the capacity greatly exceeded the solid transport (e.g., 50 times as high at site 4, depending on the equation used). Considering the bed particle size at site 4 and the presence of ca. 40% sand and gravel in the total volume of the bedload, the December 2008 flood would have mobilized nearly 180 m³ of material likely to feed the beaches of the harbor of Hyères, compared to a theoretical capacity of 6,400 m³ according to the Einstein equation.

4.3.2. Over the monitoring period (July 2007-April 2009)

48Over the monitoring period, only the Einstein equation provided consistent and the lowest estimates of transport capacity given the size and slope of the river (tab. 5). The estimates were much higher than the volumes of sediment actually transported (100 to 500 times as high, depending on the site). According to this equation, the transport capacity ranged from 7,300-51,300 m3, although this hydrological period was particularly active because it included a relatively long 5-year flood (discharge higher than the threshold discharge for 3-4 consecutive days).

Tab. 5 – Measured sediment load and transport capacity during the monitoring period (July 2007-April 2009).
Tab. 5 – Volume de transport solide mesuré et capacités de transport correspondant à la période de suivi (juillet 2007 à avril 2009).

Tab. 5 – Measured sediment load and transport capacity during the monitoring period (July 2007-April 2009).Tab. 5 – Volume de transport solide mesuré et capacités de transport correspondant à la période de suivi (juillet 2007 à avril 2009).

49Tracer monitoring provided estimates of the threshold discharge for sediment mobility, which was ca. 17 m3.s-1 at site 1 and 42 m3.s-1 at the other sites (tab. 2). However, all of the equations estimated that sediment transport for all sectors began at 2 m3.s-1, which is a large underestimate. Because the equations describe a river in an equilibrium state (Gomez and Church, 1989), they do not consider the development of surface paving related to a sediment load deficit. Moreover, the threshold discharge estimated by the equations corresponds to the mobility of particles in the subsurface layer (whose particle size is used when calculating the transport capacity), which are finer than those in the surface layer, which also had a high pavement index, especially at site 4. Estimates of the Einstein equation also showed that the 5-year flood of December 2008 represented at least half of the estimated transport capacity over the monitoring period, except at site 4, whose paving index was particularly large (10.5) (tab.4). The subsurface particle size there was particularly small (D50 = 4.31 mm, vs. 30.82 mm at site 3), which explained why even moderate floods contributed more than 70% of the capacity, according to the model. Based on our topographic monitoring and sediment tracing, most floods barely managed to mobilize the surface layer. This overestimate of transport capacity for the site 4 indicates that the transport capacity estimated for site 3 (20,200 m3) is probably much closer to the effective capacity of the downstream Gapeau River (tab. 5).

5. Discussion

5.1. Is removing weirs necessary?

50Our results indicate that the Sainte Eulalie weir was not a physical obstacle to the transfer of bedload since no net accretion was observed upstream of it during monitoring. Although the first recorded flood slightly filled the reservoir with sediment (480 m3), we observed erosion (650 m3) of these deposits during the next flood, even though it was smaller than the 2-year flood. Nevertheless, the sediment transports consisted of finer sediments (sand and fine gravel) than those in the river bed (coarse gravel and pebbles). These observations were supported by the sediment transport measured at the four sites, where no significant movement of coarse sediments in the river bed (cobbles and coarse gravel) was observed during the first flood. The sediment movement detected in the weir’s reservoir thus could have consisted only of the finest sediment fractions (i.e., silt, sand and gravel). This silty-sandy load, which varied greatly in quantity depending on the flood, has been estimated to range from 2,000-4,000 m3.yr-1 (Sogreah 1988; Geomer, 1996) and helps stabilize the seagrass meadow in front of the mouth of the Gapeau River and in the entire sublittoral zone of the harbor of Hyères (Courtaud, 2000). While the weir did not influence sand or gravel transfer, we do not know whether it influenced that of sediments coarser than gravel.

51Indirect measurement of sediment transport showed that bedload is not (or no longer) transported along the Gapeau River (except for a little residual transport of sand and gravel we did not manage to quantify). The sediment transport measured, even during a 5-year flood, had an extremely low volume (456 m3) and was much lower than the transport capacity (20,200 m3). This difference indicates that the Gapeau River has a large sediment deficit. We also observed significant paving of the river bed that ranged from 2.1-10.5 downstream of the study area. The Gapeau River bed is thus nearly stable during the most common floods, and only a small sediment flow composed of sand and gravel remains. This flow is too small to be detected using indirect measurements of sediment transport, but we detected it when monitoring the Sainte Eulalie reservoir.

52Additional bathymetric measurements conducted at the mouth of the Gapeau River before and after the December 2008 flood indicated that 1,300-1,400 m3 of sediment (medium-to-coarse sand) was supplied to the beaches (Brunel, 2010; Capanni, 2011). However, these inputs under active hydrological conditions (Q5) do not compensate for the estimated 2,700 m3.yr-1 of coastal erosion (Capanni, 2011). Thus, the fluvial inputs of the Gapeau River appear to contribute little to the coastline. The current volumes of fluvial coarse sediments are not sufficient to maintain the river in a good status, and the Gapeau River system has a severe sediment deficit regardless of the presence of the Sainte Eulalie weir. These elements thus raise questions about the utility of removing the weir to restore sediment continuity, and to maintain the coastline. The weir does not hinder the sand flow, whose volume is much too small to counteract coastal sand deficits. Based on our observations of the Sainte Eulalie weir, we hypothesize that most of the other structures in the watershed do not hinder sediment transfer either because they are already full of sediment or because they have never completely interrupted sediment transfer. The Sainte Eulalie weir, like all weirs in the Gapeau River, was already present in the “profile of major hydraulic forces” in 1954 (and probably much earlier). Before the 1970s, however, topographic data indicates that no incision or retraction occurred (Capanni, 2011). The other structures are located upstream in the watershed area, in contexts with slopes that are often steeper than those of Sainte Eulalie, which is also the highest structure (3.5 m, vs. 2.0 m for the others). Thus, if the Saint Eulalie weir does not store bedload, the other weirs likely do not either. These observations are consistent with results of most studies on the influence of small structures on bedload transport in geomorphologically dynamic contexts. For example, Csiki and Rhoad (2014) demonstrated that stored sediments were remobilized by hydraulic eddies upstream of structures, while other studies observed transfer of finer sediment fractions during high-flow events (Pearson et al., 2011; Casserly et al., 2021).

53Thus, removing weirs does not seem necessary for managing the fluvial and coastal sediment deficit in the context of the Gapeau watershed. Although doing so could be crucial for other compartments of the hydrosystem (e.g., ecological), it would only allow a short-term release of a small volume of sediment without significantly increasing sediment transfer downstream.

5.2. Is removing weirs enough to restore the Gapeau River?

54For more than 60 years, studies have shown the importance of considering the watershed scale to understand and manage sediment dynamics of rivers (Bravard, 1991; Kondolf and al., 2002; Liébault, 2003; Liébault et al., 2005; Goudie 2020). However, most restoration efforts focus on the influence of structures, whereas analysis at the watershed scale to determine whether sediment transfer occurs is equally (or even more) relevant (Piégay and Hicks, 2005). Unfortunately, this scale is rarely addressed in policies that focus on removing small structures (Maggiligan 2016; Dufour et al., 2017).

55For the Gapeau River, historical data on morphological changes indicate accentuation of the incision and retraction of the river at the end of the 1970s, which destabilized the banks and was subsequently the subject of many and varied protections (Capanni, 2011). These changes resulted from major sediment extractions in the minor bed in the 1960s during construction of the Toulon airport, combined with reforestation observed since 1973 that prevented compensation by sediment production from the watershed. Although quantifying these extractions is impossible due to the lack of archival documents, they could have been large enough to create a sediment deficit in the downstream section of the Gapeau River. Since this period, the river no longer seems able to compensate for these extractions with contributions from reforested slopes or lateral erosion, since most of the riverbanks are protected. The strong influence of reforestation and extraction has been demonstrated in other Mediterranean systems, particularly in Corsican rivers (Gaillot and Piégay, 1999). This indicates a context of global and multifactorial deficits at the scale of watershed, within which structures can have only reinforce this underlying trend (Bravard and Lévêque, 2020).

56The focus on structures suggests that removing weirs alone is necessary and sufficient to ensure "a complete and definitive restoration of all the functionalities of the river" (Malavoi et al., 2011). This idea, which corresponds to a “structure by structure” point of view, should be questioned since sediment continuity applies to a scale larger than that of the channel (Csiki and Rhoad, 2014; Fryirs, 2017). Like other rivers, the Gapeau River is a system in the process of adjusting to sediment deficit, whose origin is much more complex than a single interruption in continuity caused by structures (Downs and Piégay, 2019). For the Gapeau River, removing the weirs would not be sufficient to mitigate the sediment deficit. It is more relevant to focus on sediment inputs and the effectiveness of connections between slopes and the channel.

6. Conclusion

57This study highlights the need to understand the importance of small structures in sediment-transfer processes. This is a complex issue since their potential effects depend strongly on the watershed’s context and its capacity to produce sediments. Although it is difficult to develop general rules, this knowledge is crucial for implementing relevant management practices and for effectively removing structures in a context of sediment deficit.

*Corresponding author: Tel: +33 (0)2 99 14 18 44
anne-julia.rollet@univ-rennes2.fr (A.-J. Rollet)

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Bibliographie

Barral M. (2002) - État des Lieux de la Circulation piscicole sur les affluents de rive gauche du Rhône et les fleuves côtiers méditerranéens. Hiérarchisation des Priorités d’Aménagement et Intégration des Résultats dans le Volet B du Plan Migrateurs. Rapport de l’Association Migrateurs Rhône-Méditerranée, 76 p.

Batalla R.J., Kondolf G.M., Gomez C.M. (2004) - Reservoir-induced hydrological changes inthe Ebro River basin, NE Spain. Journal of Hydrology, 290, 117–136.

DOI : 10.1016/j.jhydrol.2003.12.002

Battiau-Queney Y. (2015) - Du grain de sable au système côtier : quel avenir pour nos plages ? Revue Maritime, 504, 100–117.

Bellmore J.R., Duda J.J., Craig L.S., Greene S.L., Torgersen C.E., Collins M. J., Vittum, K. (2017) - Status and trends of dam removal research in the United States. Wiley Interdisciplinary Reviews: Water, 4 (2), e1164.

DOI : 10.1002/wat2.1164

Bergillos R. J., López-Ruiz A., Ortega-Sánchez M., Masselink G., Losada M. A. (2016) - Implications of delta retreat on wave propagation and longshore sediment transport-Guadalfeo case study (southern Spain). Marine Geology, 382, 1–16.

DOI : 10.1016/j.margeo.2016.09.011

Bird, E.C.F. (1980) - Recent change on the world’s sandy shorlines. 24e Congrès international de géographie, Commission de l’environnement côtier, Excursion C-15, Shimoda (Japon), 26 p.

Boutault F. (2020) - Etude de l'impact cumulé des facteurs d'anthropisation sur la Dordogne moyenne et préconisations en vue d'une restauration écologique du cours d'eau. Thèse de doctorat, Université Jean Moulin, 216 p.

Brandt S.A. (2000a) - Classification of geomorphological effects downstream of dams. Catena, 40, 375–401.

DOI : 10.1016/S0341-8162(00)00093-X

Brandt S.A. (2000b) - Prediction of downstream geomorphological changes after dam construction: a stream power approach. Water Resources Development, 16 (3), 343–367.

DOI : 10.1080/713672510

Bravard J. P. (1991) - La dynamique fluviale à l'épreuve des changements environnementaux : Quels enseignements applicables à l'aménagement des rivières ? La Houille Blanche, 7-8, 515–522.

DOI : 10.1051/lhb/1991047

Bravard, J.P., Lévêque C. (2020) - La gestion écologique des rivières française. Regards de scientifiques sur une controverse. L’harmattan, Paris, 364 p.

Brenna A., Surian N., Mao L. (2019) - Virtual velocity approach for estimating bed material transport in gravel‐bed rivers: Key factors and significance. Water Resources Research, 55, 1651–1674.

DOI : 10.1029/2018WR023556

Brunel C. (2010) - Evolution séculaire de l’avant-côte de la Méditerranée française, impact de l’élévation du niveau de la mer et des tempêtes. Thèse de doctorat, Aix-Marseille Université, 470 p.

Brunel C. (2012) - Tempêtes et élévation marine sur les plages françaises de Méditerranées. L’Harmattan, Paris, 285 p.

Brunel C., Sabatier F. (2009) - Potential influence of sea-level rise in controlling shoreline

position on the French Mediterranean Coast. Geomorphology, 107, 57–74.

DOI : 10.1016/j.geomorph.2007.05.024

Bunte K., Abt S. (2001) - Sampling surface and subsurface particle-size distributions in wadable gravel- and cobble-bed streams for analyses in sediment transport, hydraulics and streambed monitoring. Rapport de l’U.S. Department of agriculture, Forest service, Rocky mountain research station, 428 p.

Capanni R. (2011) - Étude et gestion intégrée des transferts sédimentaires dans le système Gapeau/rade d'Hyères. Thèse de doctorat, Aix-Marseille Université, 317p.

Casserly C. M., Turner J. N., O’Sullivan J. J., Bruen M., Magee D., O’Coiléir S., Kelly-Quinn M. (2021) - Coarse sediment dynamics and low-head dams: Monitoring instantaneous bedload transport using a stationary RFID antenna. Journal of Environmental Management, 300, 113671.

DOI : 10.1016/j.jenvman.2021.113671

C.E.D.R.A.T, (2001). Etude hydraulique sur la commune d’Hyères les Palmiers, Commune d’Hyères les Palmiers.

Chapuis M., Dufour S., Provansal M., Couvert B., De Linares M. (2015) - Coupling channel evolution monitoring and RFID tracking in a large, wandering, gravel-bed river: Insights into sediment routing on geomorphic continuity through a riffle–pool sequence. Geomorphology, 231, 258–269.

DOI :10.1016/j.geomorph.2014.12.013

Church M.A., D.G. McLean, Wolcott J.F. (1987) - River Bed gravels: sampling and analysis. In Barthurst J.C., Thorne C.R., Hey R.D. (Eds.): Sediment transport in gravel bed Rivers, Wiley and Sons, 43–88.

Church J.A., Gregory J.M., Huybrechts P., Kuhn M., Lambeck K., Nhuan M.T., Qin D., Woodworth P.L. (2001) - Changes in sea level. In Houghton J.T., Ding Y., Griggs D.J., Noguer M., Van der Linden P.J., Dai X., Maskell, K., Johnson C.A. (Eds.): Climate Change, 2001: the Scientific Basis. Contribution of Working Group 1 to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, 639-693.

Courtaud J. (2000) - Dynamiques géomorphologiques et risques littoraux, cas du tombolo de Giens, (Var, France méridionale), thèse de doctorat, Université Aix-Marseille 1, CEREGE-CNRS, 268 p.

Csiki S., Rhoads B.L. (2010). Hydraulic and geomorphological effects of run-of-river dams. Progress in physical geography, 34 (6), 755–780.

DOI : 10.1177/0309133310369435

Csiki S.J., Rhoads B.L. (2014). Influence of four run-of-river dams on channel morphology and sediment characteristics in Illinois, USA. Geomorphology, 206, 215–229.

DOI : 10.1016/j.geomorph.2013.10.009

Delmas M., Cerdan O., Cheviron B., Mouchel J.M., Eyrolle F. (2012) - Sediment export from French rivers to the sea. Earth Surface Processes and Landforms, 37 (7), 754–762.

DOI :10.1002/esp.3219

Dépret T., Piégay H., Dugué V., Vaudor L., Faure J.B., Le Coz J., Camenen B. (2019) - Estimating and restoring bedload transport through a run-of-river reservoir. Science of the Total Environment, 654, 1146–1157.

DOI : 10.1016/j.scitotenv.2018.11.177

Downs P.W., Piégay H. (2019) - Catchment-scale cumulative impact of human activities on river channels in the late Anthropocene: implications, limitations, prospect. Geomorphology, 338, 88–104.

DOI : 10.1016/j.geomorph.2019.03.021

Dufour S., Rollet A.J., Chapuis M., Provansal M., Capanni R. (2017) - On the political roles of freshwater science in studying dam and weir removal policies: A critical physical geography approach. Water Alternatives, 10 (3), 853–869.

Einstein H.A. (1950) - The bedload function for sediment transport in open channel flows. Technical Bulletin, 1026, U.S. department of agriculture, Soil conservation service, Washington D.C., 71 p.

Farnsworth K.L., Milliman J.D. (2003) - Effects of climatic and anthropogenic change on small mountainous rivers: the Salinas River example. Global and Planetary change, 39 (1-2), 53–64.

DOI : 10.1016/S0921-8181(03)00017-1

Fencl J.S., Mather M.E., Costigan K.H., Daniels M.D. (2015) - How big of an effect do small dams have? Using geomorphological footprints to quantify spatial impact of low-head dams and identify patterns of across-dam variation. PLoS one, 10 (11), e0141210.

DOI : 10.1371/journal.pone.0141210

Fryirs K.A. (2017) - River sensitivity: a lost foundation concept in fluvial geomorphology. Earth Surface Processes and Landforms, 42 (1), 55–70.

DOI : 10.1002/esp.3940

Furnans J., Austin B. (2008) - Hydrographic survey methods for determining reservoir volume. Environmental Modelling and Software, 23, 139–146.

DOI : 10.1016/j.envsoft.2007.05.011

Gaillot S., Piégay H. (1999) - Impact of Gravel-Mining on Stream Channel and Coastal Sediment Supply: Example of the Calvi Bay in Corsica (France). Journal of Coastal Research, 15(3), 774–788.

Gautier E. (1994) - Interférence des facteurs anthropiques et naturels dans le processus d'incision sur une rivière alpine - L'exemple du Buëch (Alpes du sud). Revue de géographie de Lyon, 69 (1), 57–62.

DOI : 10.3406/geoca.1994.4238

GEOMER (1996) - Aménagement du littoral- Etude de faisabilité d'un port intérieur à l'embouchure du Gapeau: Etude courantologique et sédimentologique, Marseille, 2 rapports.

Germaine M.A., Barraud R. (2013) - Les rivières de l’ouest de la France sont-elles seulement des infrastructures naturelles ? Les modèles de gestion à l’épreuve de la directive-cadre sur l’eau. Natures Sciences Sociétés, 21 (4), 373–384.

DOI : 10.1051/nss/2014003

Gomez B., Church M. (1989) - An assessment of bed load sediment transport formulae for gravel bed river. Water Ressources Research, 25 (6), 1161–1186.

DOI : 10.1029/WR025i006p01161

Gordon E., Meentemeyer R.K. (2006) - Effects of dam operation and land use on stream

channel morphology and riparian vegetation. Geomorphology, 82, 412–429.

DOI : 10.1016/j.geomorph.2006.06.001

Gornitz V., Lebedeff S., Hansen J. (1982) - Global Sea Level in the Past Century. Science, 215, 1611–1614.

DOI : 10.1126/science.215.4540.1611

Goudie A. (2020) - The human impact in geomorphology–50 years of change. Geomorphology, 366, 106601.

DOI : 10.1016/j.geomorph.2018.12.002

Graf W.L. (2003) - Dam Removal Research: Status and Prospects. Heinz Center for Science, Economics and the Environment, Washington, DC, 151 p.

Guillen J., Palanques A. (1992) - Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: the Ebro River. Sedimentology, 39, 567–579.

DOI : 10.1111/j.1365-3091.1992.tb02137.x

Hadour A., Mahé G., Meddi M. (2021) - Climatic and anthropogenic impacts on the decrease of sediment discharge to the Mediterranean coast from the largest river of Maghreb. International Journal of Sediment Research, 36 (2), 268–278.

DOI : 10.1016/j.ijsrc.2020.07.001

Hassan M.A., Roy A.G. (2016) - Coarse particle tracing in fluvial geomorphology. In Kondolf G.M., Piégay H. (Eds): Tools in fluvial geomorphology. John Wiley & Sons, Ltd., Chichester, 306–323.

DOI : 10.1002/9781118648551.ch14

Houbrechts G., Van Campenhout J., Levecq Y., Hallot E., Peeters A., Petit F. (2012) - Comparison of methods for quantifying active layer dynamics and bedload discharge in armoured gravel-bed rivers. Earth Surface Processes and Landforms, 37, 1501–1517.

DOI : 10.1002/esp.3258

Innocent L., Pranzini E. (1993) - Geomorphological Evolution and Sedimentology of the Ombrone River Delta, Italy. Journal of Coastal Research, 9 (2), 481–493

Intergovernmental Panel on Climate Change (IPCC) (2001) - Climate change 2001: impacts, adaptation and vulnerability. Contribution of the working group to the third assessment report of the intergovernmental Panel of Climate Change. Rapport du World Meteorological Organization, 124 p.

Kellerhals R., Bray D.I. (1971) - Sampling procedure for fluvial sediments. ASCE Journal of the Hydraulics Division, 97, 1165–1180.

Kondolf G.M., Piégay H., Landon N. (2002) - Channel response to increased and decreased bedload supply from land use change: contrasts between two catchments. Geomorphology, 45, 35–51.

DOI : 10.1016/S0169-555X(01)00188-X

Lamarre H., McVicar B., Roy A. (2005) - Using passive integrated transpondeur (PIT) tags to investigate sediment transport in gravel bed rivers. Journal of Sedimentary Research, 75 (4), 736–741.

DOI : 10.2110/jsr.2005.059

Laronne J.B., Outhet D.N., Duckham J.L., McCabe T.J. (1992) - Determining event bedload volumes for evaluation of potential degradation sites due to gravel extraction, N.S.W., Australia. In IAHS (ed.): Erosion and sediment transport monitoring programmes in river basins, Proceedings of the Oslo Symposium, august 1992, IAHS Publ. n° 210, 87–94.

Laronne J.B., Outhet D.N., Carling P.A., Mc Cabe T.J. (1994) - Scour chain employment in gravel bed rivers. Catena, 22, 299–306.

DOI : 10.1016/0341-8162(94)90040-X

Leatherman S.P., Douglas B.C., LaBrecque J.L. (2003) - Sea level and coastal erosion require large‐scale monitoring. Eos, Transactions American Geophysical Union, 84 (2), 13–16.

DOI : 10.1029/2003EO020001

Liébault F., Laronne J.B. (2008) - Evaluation of bedload yield in gravel-bed rivers using scour chains and painted tracers: the case of the Esconavette Torrent (Southern French Prealps). Geodinamica Acta, 21 (1-2), 23–34.

DOI : 10.3166/ga.21.23-34

Liébault F., Gomez B., Page M., Marden M., Peacock D., Richard D., Trotter C.M. (2005) - Land-use change, sediment production and channel response in upland regions. River Research and Applications, 21, 739–756.

DOI : 10.1002/rra.880.

Liébault F. (2003) - Les rivières torrentielles des montagnes drômoises : évolution contemporaine et fonctionnement géomorphologique actuel (massifs du Diois et des Baronnies). Thèse de Doctorat, Université Lyon 2, 357 p.

Magilligan F.J., Graber B.E.; Nislow K.H., Chipman J.W., Sneddon C.S., Fox C.A. (2016) - River restoration by dam removal:Enhancing connectivity at watershed scales. Elementa: Science of the Anthropocene, 4.

DOI : 10.12952/journal.elementa.000108/

Major J.J., O’Connor J.E., Podolak C.J., Keith M.K., Grant G.E., Spicer K.R., Pittman S., Bragg H.M., Wallick J.R., Tanner D.Q., Rhode A., Wilcock P.R. (2012) - Geomorphic response of the Sandy River, Oregon, to removal of Marmot Dam. Rapport de l’US Geological Survey (Reston), 64 p.

Malavoi J.R., Garnier C.C., Landon N., Recking A., Baran P. (2011) - Eléments de connaissance pour la gestion du transport solide en rivière. Onema-Cemagref Lyon, Toulouse, Grenoble, Ministère chargé de l’écologie, Université Lyon 2, Collection Comprendre pour agir, 216 p.

Meyer-Peter R., Müller R. (1948) - Formulas for bedload transport. Proceeding 2nd meeting international association of hydraulic research (Stockholm), 39–64.

Milliman, J.D., Syvitski J.P. (1992) - Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. The journal of Geology, 100 (5), 525–544.

DOI : 10.1086/629606

Pardé M. (1933) - Fleuves et rivières. Armand Colin, Paris, 241 p.

Parker G. (1990) - The" acronym" series of Pascal programs for computing bedload transport in gravel rivers. St. Anthony Falls Hydraulic Laboratory, University of Minnesota.

Paskoff R. (1998) - La crise des plages : pénurie de sédiments. Mappemonde, 52 (4), 11–15.

Paskoff R., Clus-Auby C. (2007) - L’érosion des plages. Les causes, les remèdes. Institut Océanographique, Paris, 184 p.

Pearson A.J., Pizzuto J. (2015) - Bedload transport over run-of-river dams, Delaware, USA. Geomorphology, 248, 382–395.

DOI : 10.1016/j.geomorph.2015.07.025

Pearson A.J., Snyder N.P., Collins M.J. (2011) - Rates and processes of channel response to dam removal with a sand‐filled impoundment. Water Resources Research, 47 (8), W08504.

DOI : 10.1029/2010WR009733

Peeters A., Houbrechts G., Hallot E., Van Campenhout J., Gob F., Petit F. (2020) - Can coarse bedload pass through weirs? Geomorphology, 359, 107131.

DOI : 10.1016/j.geomorph.2020.107131

Petts G.E., Gurnell A.M. (2005) - Dams and geomorphology: research progress and future directions. Geomorphology, 71 (1-2), 27–47.

DOI : 10.1016/j.geomorph.2004.02.015

Phillips J.D., Slaterry M.C., Musselman Z.A. (2005) - Channel adjustments of the lower Trinity River, Texas, downstream of Livingston dam. Earth Surface Processes and Landforms, 30 (11), 1419–1439.

DOI : 10.1002/esp.1203

Piégay H., Walling D.E, Landon N., He Q., Liébault F., Petiot R. (2004) – Contemporary changes in sediment yield in an alpine mountain basin due to afforestation (the upper Drôme in France). Catena, 55 (2), 183–212.

DOI : 10.1016/S0341-8162(03)00118-8

Piégay H., Hicks D.M. (2005) - Sediment management in river systems: a need to assess changing processes in the long term and at a large scale. River Research and Applications, 21 (7), 689–691.

DOI : 10.1002/rra.877

Poff N.L., Hart D.D. (2002) - How dams vary and why it matters for the emerging science of dam removal: an ecological classification of dams is needed to characterize how the tremendous variation in the size, operational mode, age, and number of dams in a river basin influences the potential for restoring regulated rivers via dam removal. BioScience, 52 (8), 659–668.

DOI : 10.1641/0006-3568(2002)052[0659:HDVAWI]2.0.CO;2

Poulos S.E., Collins M.B. (2002) - Fluviatile sediment fluxes to the Mediterranean Sea: a quantitative approach and the influence of dams. Geological Society, London, Special Publications, 191 (1), 227–245.

DOI : 10.1144/GSL.SP.2002.191.01.16

Poulos S.E., Collins M., Evans G. (1996) - Water-sediment fluxes of Greek rivers, southeastern Alpine Europe: annual yields, seasonal variability, delta formation and human impact. Zeitschrift für Geomorphologie, 243–261.

DOI : 10.1127/zfg/40/1996/243

Provansal M, Dufour S., Sabatier F., Anthony E.J., Raccasi G., Robresco S. (2014) - The geomorphic evolution and sediment balance of the Lower Rhône River (southern France) over the last 130 years: hydropower dams versus other control factors. Geomorphology, 219, 27–41.

DOI : 10.1016/j.geomorph.2014.04.033

Reid I., (2002) - Sediment dynamics of ephemeral channels. In Bull L. J., Kirkby M.J. (Eds.): Dryland Rivers. John Wiley & Sons, Ltd., Chichester, 107–128.

Reid I., Laronne J. (1995) - Bedload sediment transport in an ephemeral stream and a comparison with seasonal and perennial counterparts. Water Resources Research, 31 (3), 773–781.

DOI : 10.1029/94WR02233

Rollet A.J. (2007) - Etude et gestion de la dynamique sédimentaire sur une section fluviale à l’aval d’un barrage : le cas de la basse vallée de l’Ain. Thèse de Doctorat, Université Lyon 3, 305 p.

Rollet A.J., MacVicar B., Piégay H., Roy A. (2008) - L’utilisation de transpondeurs passifs pour l’estimation du transport sédimentaire : premiers retours d’expérience. La Houille Blanche, 94 (4), 110–116.

DOI : 10.1051/lhb:2008047

Rollet A.J., Piégay H., Dufour S., Bornette G., Persat H. (2013) - Assessment of consequences of sediment deficit on a gravel river-bed downstream of dams in restoration perspectives: application of a multicriteria, hierarchical, and spatially explicit diagnosis. River Research and Applications, 30 (8), 939–953.

DOI : 10.1002/rra.2689

Rowan J.S., Goodwill P., Greco M. (1995) - Temporal variability in catchment sediment yield determined from repeated bathymetric surveys: Abbeystead reservoir, U.K. Physics and Chemistry of the Earth, 20 (2), 199–206.

DOI : 10.1016/0079-1946(95)00024-0

S.I.E.E. (2004) - Elaboration d’un programme de restauration, d’entretien et de mise en valeur de la ripisylve du Réal Martin et du Réal Collobrier. Rapport d’étude, Syndicat Mixte du Bassin-versant du Gapeau, 64 p.

Sabatier F., Samat O., Ullmann A., Suanez S. (2009) - Connecting large-scale coastal behaviour with coastal management of the Rhône delta. Geomorphology, 107 (1-2), 79–89.

DOI : 10.1016/j.geomorph.2006.09.026

Schoklitsch A. (1934) - Der geschiebetrieb und die geschiebefracht. Wasserkraft Wasserwirtschaft, 4, 1–7.

S.C.P. (1973) - Protection contre les crues du bas Gapeau. Rapport pour le compte de la D.D.A., Mémoire général + annexes, 137 p.

S.I.E.E. (2004) - Elaboration d’un programme de restauration, d’entretien et de mise en valeur de la ripisylve du Réal Martin et du Réal Collobrier. Rapport d’étude, Syndicat Mixte du Bassin-versant du Gapeau, 64 p.

Sneddon C.S., Barraud R., Germaine M.-A. (2017) - Dam removals and river restoration in international perspective. Water Alternatives, 10 (3), 648–654.

S.O.G.R.E.A.H. (1988) - Etudes sédimentologiques de la rade d’Hyères. Rapport général, 5 tomes.

Syvitski J., Kettner A., Overeem I., Hutton E.W., Hannon M.T., Brakenridge G.R., Day J., Vörösmarty C., Saito Y., Giosan L., Nicholls J.R. (2009) - Sinking deltas due to human activities. Nature Geoscience, 2 (10), 681–686.

DOI : 10.1038/ngeo629

Walling D.E., Fang D. (2003) - Recent trends in the suspended sediment loads of the world's rivers. Global and Planetary Change, 39 (1–2), 111–126.

DOI : 10.1016/S0921-8181(03)00020-1

Warrick R., Oerlemans J. (1990) - Sea level rise. In Houghton J.T., Jenkins G.J., Ephraums J.J., (Eds.): Climate Change. IPCC, WMO/UNEP, Cambridge, 257–281.

Wasson J. G., Malavoi J. R., Maridet, L. (1998) - Impacts écologiques de la chenalisation des rivières, vol. 14. Editions Quae.

Williams G.P., Wolman G.P. (1984) - Downstream effects of dams on alluvial rivers. USGS circular 781, 48 p.

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Annexe

Version abrégée en français

De nombreux systèmes littoraux et fluviaux occidentaux connaissent aujourd’hui des situations de déficit sédimentaire. La suppression des ouvrages transversaux est parfois préconisée pour limiter ce phénomène sans que l’effet réel des seuils sur le transport de la charge de fond ne soit démontré dans tous types de contexte. Dans ce cadre, notre étude a pour objectif d’établir si la restauration de la continuité sédimentaire du Gapeau, fleuve côtier méditerranéen (fig.1) via la suppression du seuil de Sainte Eulalie (fig.2) aurait une répercussion significative permettant de limiter le déficit sédimentaire littoral. Cette étude repose sur des données quantitatives issues d’approches croisées relevant à la fois de mesures in situ (relevés bathymétriques par ADCP, traçages sédimentaires (tab.1), chaînes d’érosion et suivis topographiques) effectuées dans différentes conditions hydrologiques (fig.3) et de la modélisation des capacités de transport modélisées pour ces mêmes évènements.  Nos résultats nous permettent de conclure que le seuil de Sainte Eulalie ne semble pas constituer d’entrave physique au transfert de la charge de fond dans la mesure où aucune accrétion nette n’a été observée en amont de l’ouvrage malgré des crues importantes enregistrées durant le suivi (fig.4). Néanmoins, la mesure indirecte du transport solide montre qu’il n’existe pas ou plus de mobilité du fond du lit (fig.5, tab.2) et de transport par charriage sur ce cours d’eau (tab. 3) quelles que soient les puissances (fig.6) ou durées (fig.7) des crues, et malgré des capacités de transport de transport excédant 7000 m3.an-1 (tab.4) largement supérieures au transport solide observé indépendamment du site ou de l’équation considérés (tab.5). Ainsi, la suppression de seuil sur le Gapeau serait insuffisante pour atténuer le déficit sédimentaire fluvial et/ou littoral. Il conviendrait plutôt de concentrer la réflexion sur la réalité des entrées sédimentaires et l’efficacité des connexions entre les versants et le chenal.

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Table des illustrations

Titre Fig. 1 – Location map.Fig. 1 – Carte de localisation.
Légende A : Regional scale; B : Local scale. 1. Watercourses; 2. Coast line.A : Echelle régionale ; B : Echelle locale. 1. Cours d’eau ; 2. Trait de côte.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-1.jpg
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Titre Tab. 1 – Tracer and riverbed sediment characteristics.Tab. 1 – Caractéristiques des traceurs et des sédiments du lit.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-2.jpg
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Titre Fig. 2 – Sainte Eulalie weir.Fig. 2 – Seuil de Sainte Eulalie.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-3.jpg
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Titre Fig. 3 – Hydrological context (mean daily discharge) of the bathymetric, topographic and sediment-tracing surveys.Fig. 3 – Contexte hydrologique (débits moyens journaliers) des suivis bathymétriques, topographiques et des traçages sédimentaires.
Légende 1. Bathymetric surveys; 2. Topographic and tracers surveys.1. Suivis bathymétriques ; 2. Suivis topographiques et traceurs.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-4.jpg
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Titre Fig. 4 - Bathymetric differentials between September 2007, March 2008 and June 2008.Fig. 4 - Différentiels bathymétriques entre septembre 2007, mars 2008 et juin 2008.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-10.jpg
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Titre Fig. 5 – Thickness of the active layer of the riverbed.Fig. 5 – Epaisseur des couches actives du chenal.
Légende 1. Site 1; 2. Site 2; 3. Site 3; 4. Site 4.1. Site 1 ; 2. Site 2 ; 3. Site 3 ; 4. Site 4.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-11.jpg
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Légende Qimax = maximum mean daily discharge; NM = not mobilized; pp = painted particules.Qimax = débit moyen maximum journalier ; NM = pas mobilisé ; pp = particules peintes.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-12.jpg
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Titre Fig 6 – (A) Percentage of tracers mobilized and (B) mean distance traveled as a function of specific stream power.Fig 6 - Taux de traceurs mobilisés (A) et distances moyennes parcourues (B) en fonction des puissances spécifiques.
Légende 1. Site 1; 2. Site 2; 3. Site 3; 4. Site 4.1. Site 1 ; 2. Site 2 ; 3. Site 3 ; 4. Site 4.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-13.jpg
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Titre Fig 7 – Mean distance traveled as a function of flood duration.Fig 7 - Distances moyennes parcourues en fonction de la durée des crues.
Légende 1. Site 1; 2. Site 2; 3. Site 3; 4. Site 4.1. Site 1 ; 2. Site 2 ; 3. Site 3 ; 4. Site 4.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-14.jpg
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Titre Tab 3 - Volume of bedload (sand and coarse sediment) mobilized during the monitoring period (July 2007-April 2009).Tab 3 - Volume de charge de fond (sables et sédiments grossiers) mobilisés pendant la période de suivi (juillet 2007 – Avril 2009).
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-15.jpg
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Titre Tab. 4 – Measured sediment load and transport capacity of the December 2008 flood.Tab. 4 – Volume de transport solide mesuré et capacités de transport correspondant à la crue de décembre 2008.
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-16.jpg
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Titre Tab. 5 – Measured sediment load and transport capacity during the monitoring period (July 2007-April 2009).Tab. 5 – Volume de transport solide mesuré et capacités de transport correspondant à la période de suivi (juillet 2007 à avril 2009).
URL http://journals.openedition.org/geomorphologie/docannexe/image/17371/img-17.jpg
Fichier image/jpeg, 53k
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Anne-Julia Rollet, Simon Dufour, Romain Capanni et Mireille Lippmann Provansal, « Is removing weirs always effective at countering the sediment deficit? Case study in a Mediterranean context: the Gapeau River »Géomorphologie : relief, processus, environnement, vol. 28 - n° 3 | 2022, 187-200.

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Anne-Julia Rollet, Simon Dufour, Romain Capanni et Mireille Lippmann Provansal, « Is removing weirs always effective at countering the sediment deficit? Case study in a Mediterranean context: the Gapeau River »Géomorphologie : relief, processus, environnement [En ligne], vol. 28 - n° 3 | 2022, mis en ligne le 10 février 2023, consulté le 21 mai 2024. URL : http://journals.openedition.org/geomorphologie/17371 ; DOI : https://doi.org/10.4000/geomorphologie.17371

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Auteurs

Anne-Julia Rollet

LETG UMR 6554 CNRS, Université Rennes 2, Place du recteur Henri Le Moal CS 24307, 35043 Rennes cedex, France

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Simon Dufour

LETG UMR 6554 CNRS, Université Rennes 2, Place du recteur Henri Le Moal CS 24307, 35043 Rennes cedex, France

Articles du même auteur

Romain Capanni

CEREGE UMR 6635 CNRS, Aix-Marseille Université, Technopôle de l’Arbois-Méditerranée, 13100 Aix-en-Provence, France

Mireille Lippmann Provansal

CEREGE UMR 6635 CNRS, Aix-Marseille Université, Technopôle de l’Arbois-Méditerranée, 13100 Aix-en-Provence, France

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