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Abstract. Open check dams are strategic structures to control sediment and large-wood
transport during extreme flood events in steep streams and piedmont rivers.
Large wood (LW) tends to accumulate at such structures, obstruct their
openings and increase energy head losses, thus increasing flow levels. The
extent and variability to which the stage–discharge relationship of a check dam is modified by LW presence has so far not been clear. In addition, sufficiently high flows may trigger a sudden release of the trapped LW with eventual dramatic consequences downstream. This paper provides experimental quantification of LW-related energy head loss and simple ways to compute the related increase in water depth at dams of various shapes: trapezoidal, slit, slot and sabo (i.e. made of piles), with consideration of the flow capacity through their open bodies and atop their spillways. In addition, it was observed that LW is often released over the structure when the overflowing depth, i.e. total depth minus spillway elevation, is about 3–5 times the mean log diameter. Two regimes of LW accumulations were observed. Dams with low permeability generate low velocity upstream, and LW then accumulates as floating carpets, i.e. as a single floating layer. Conversely, dams with high permeability maintain high velocities immediately upstream of the dams and LW tends to accumulate in dense complex 3D patterns. This is because the drag forces are stronger than the buoyancy, allowing the logs to be sucked below the flow surface. In such cases, LW releases occur for higher overflowing depth and the LW-related head losses are higher. A new dimensionless number, namely the buoyancy-to-drag-force ratio, can be used to compute whether (or not) flows stay in the floating-carpet domain where buoyancy prevails over drag force.

Abstract. This paper presents a methodology to analyse debris flows focusing at the surge scale rather than the full scale of the debris-flow event, as well as its application to a French site. Providing bulk surge features like volume, peak discharge, front height, front velocity and Froude numbers allows for numerical and experimental debris-flow investigations to be designed with narrower physical ranges and thus for deeper scientific questions to be explored. We suggest a method to access such features at the surge scale that can be applied to a wide variety of monitoring stations. Requirements for monitoring stations for the methodology to be applicable include (i) flow height measurements, (ii) a cross-section assumption and (iii) a velocity estimation. Raw data from three monitoring stations on the Réal torrent (drainage area: 2 km2, southeastern France) are used to illustrate an application to 34 surges measured from 2011 to 2020 at three monitoring stations. Volumes of debris-flow surges on the Réal torrent are typically sized at a few thousand cubic metres. The peak flow height of surges ranges from 1 to 2 m. The peak discharge range is around a few dozen cubic metres per second. Finally, we show that Froude numbers of such surges are near critical.

Abstract. The ability to understand and predict coarse-sediment transport in torrent catchments is a key element for the protection against and prevention of the associated hazards. In this study, we collected data describing sediment supply at 99 torrential catchments in the northern French Alps. The sample covers a wide range of geomorphic activity: from torrents experiencing debris flows every few years to fully forested catchments exporting small bed load volumes every decade. These catchments have long records of past events and sediment supply to debris basins. The mean annual, the 10-year return period and the reference volume (i.e., the 100-year return level or the largest observed volume) of sediment supply were derived for the studied torrents. We examined the relationships between specific sediment supply volumes and many explanatory variables using linear regression and random forest approaches. Results showed that the ratio of sediment-contributing area (bare soil or rock) to catchment area is the most important predictor of the specific sediment production volumes (m3 km−2). Other variables such as the Melton index or the indices of sediment connectivity also have an influence. Several predictive models were developed in order to estimate the sediment supply in torrents that are not equipped with debris basins.

International audience ; To protect against natural hazards in mountainous areas, the French government has implemented a number of forestry and civil engineering works such as check dams in public forests since the 19th century. Specifying each dam's objective and protective functions is the first requirement for their continued maintenance. The potential technical functions of check dams have been clarified in a recent publication (Piton et al. 2016). In the first part of a series of papers on the analysis of the context and objectives, this paper focuses on how they were implemented in public areas by the Water and Forestry Administration from the end of the 19th century to the 1960s. We detail the objectives over time and the geographical locations. This national overview will help managers consider their present local protection structure management problems within an historical perspective.

International audience ; To protect against natural hazards in mountainous areas, the French government has implemented a number of forestry and civil engineering works such as check dams in public forests since the 19th century. Specifying each dam's objective and protective functions is the first requirement for their continued maintenance. The potential technical functions of check dams have been clarified in a recent publication (Piton et al. 2016). In the first part of a series of papers on the analysis of the context and objectives, this paper focuses on how they were implemented in public areas by the Water and Forestry Administration from the end of the 19th century to the 1960s. We detail the objectives over time and the geographical locations. This national overview will help managers consider their present local protection structure management problems within an historical perspective.

International audience ; To protect against natural hazards in mountainous areas, the French government has implemented a number of forestry and civil engineering works such as check dams in public forests since the 19th century. Specifying each dam's objective and protective functions is the first requirement for their continued maintenance. The potential technical functions of check dams have been clarified in a recent publication (Piton et al. 2016). In the first part of a series of papers on the analysis of the context and objectives, this paper focuses on how they were implemented in public areas by the Water and Forestry Administration from the end of the 19th century to the 1960s. We detail the objectives over time and the geographical locations. This national overview will help managers consider their present local protection structure management problems within an historical perspective.

International audience ; To protect against natural hazards in mountainous areas, the French government has implemented a number of forestry and civil engineering works such as check dams in public forests since the 19th century. Specifying each dam's objective and protective functions is the first requirement for their continued maintenance. The potential technical functions of check dams have been clarified in a recent publication (Piton et al. 2016). In the first part of a series of papers on the analysis of the context and objectives, this paper focuses on how they were implemented in public areas by the Water and Forestry Administration from the end of the 19th century to the 1960s. We detail the objectives over time and the geographical locations. This national overview will help managers consider their present local protection structure management problems within an historical perspective.

International audience ; Large operations of torrent control works in the most active catchments have been launched in France by the 1882's mountain areas restoration policy, based on older pioneer works. It first implemented headwater soil and erosion control structures and plantations, although protection structures closer from assets were built on alluvial fans to trap sediment and channelize flows. Soil erosion and the related torrential hazard mitigation continue to be financed by public grants to maintain existing systems and, if necessary, build new ones. The environmental, socio-economic and technical contexts of risk management have changed. The current use of check dams and open check dams in protection strategies may consequently need an update. This paper firstly reviews the historical emergence of check dams and open check dams. It secondly lists their function, i.e., the qualitative effect we can expect they have on hazards in the light of 150 years of tests and trials. The last section gives some perspectives on the use of these structures: why, although complicated and expensive, it worth maintaining some headwater structures? When is it justified to abandon or even dismantled some structures? And finally what seems to be the future way of designing and planning these structures, especially the open check dams? ; La correction torrentielle des torrents les plus actifs a été lancée en France par la politique de restauration des terrains en montagne de 1882 sur la base de travaux pionniers plus anciens. Elle a d'abord mis à profit la correction active des hauts bassins par la lutte contre l'érosion à la source mais a aussi accompagné le développement de la protection passive des enjeux par le piégeage et la canalisation des écoulements dans les zones aval. Cette lutte contre l'érosion et les risques torrentiels induits fait toujours l'objet d'investissements publics pour entretenir les systèmes de protection existants et, si nécessaire, en construire de nouveaux. Les contextes environnementaux, ...