Dykes, groynes, dredging and channels also work in the mountains

Dykes are installed to protect against floods by containing them in a dedicated channel. Unfortunately, history shows that they are sometimes not very effective against heavy floods and debris flows. These protective dykes can be made of earth, masonry or concrete. In torrents, they are regularly attacked and eroded by the violence of the flow. Sometimes they are simply filled in and covered by deposits.

Longitudinal dykes have also been built in ravines to correct sinuosities that persist after silting. They are also useful for supporting banks whose bottom is at risk of being severely eroded (scoured).

Some torrents also have dykes, groynes and channels upstream, associated with dams. In these cases, they serve to extend the flow-guiding effect associated with the latter. They can also be used to create a basin in which deposits can form during heavy floods: these are known as sediment traps.

Groynes are structures that are generally positioned obliquely to the course of the torrent and pointing upstream. They do not cross the entire width of the watercourse. They are used to straighten out sinuosities in the bed or to protect a bank from scouring. They are also known as spurs. If they are placed on one bank only, it is to send the current to the opposite "un-scourable" bank. During a flood, deposits of material partially cover the spur, creating a silting zone that pushes the water towards the middle of the bed.

Dredging is the removal or relocation of materials that have settled in the riverbed and risk diverting the flow in an undesirable way (back towards an unstable, scourable slope, overflow towards an area at risk). The operation requires the use of human and material resources capable of mobilising large boulders and large volumes of material, in a hostile environment of steep slopes and unstable materials. Dredging operations are not trivial and must be carefully orchestrated: carried out on a recurrent basis, they generate sediment deficits downstream and increase erosion, particularly at the bottom of the beds.

Channels enable the bed to be rectified in order to increase the gradient and thus facilitate the evacuation of materials carried by the water. The arrival of the channel in the main river must be oblique to avoid the formation of deposits blocking the main river. An apron is often built at the bottom of the channel. This limits scouring by forming a masonry pavement that is more stable than the natural bed of the torrent.

It is, of course, the combination of these systems and their maintenance after the most devastating floods or debris flows that makes it possible to reduce the risk to the inhabitants of the catchments of these torrents.

INRAE  teams in Aix-en-Provence and Grenoble work with the RTM (mountain land restoration) services managed by the ONF (Office national des forêts, the French National Forests Office). They study and analyse the dangers posed to protective dykes. The different types of dyke failure and possible failure scenarios are studied as part of risk analyses. Research focuses in particular on case studies, historical and geomorphological analyses and expert opinions.

Guides on dams and dykes for public decision-makers and engineering and design offices are also produced to guide and inform the choices that need to be made to protect homes, infrastructure and businesses in our regions.

P. Demontzey, 1882. Traité pratique du reboisement et du gazonnement des montagnes.(Practical treatise on mountain reforestation and turfing. In French).
P. Demontzey, 1882. Le reboisement des montagnes. La nature n°496. (Reforestation of the mountains. In French) p.151-155, p. 182-186, p.215-218, p.260-263.
F. Bénardeau ; E. Cuny, 1889. La science forestière illustrée. (Illustrated forestry science. In French) Fascicule 1, Partie 4.
E. Thiery, 1891. Restauration des montagnes, correction des torrents, reboisement. (Mountain restoration, torrent correction, reforestation. In French).  
Cl. Bernard 1927. Cours de restauration des montagnes.(Mountain restoration course. In French). ENEF. 1er fascicule, texte et planches. (Document disponible hiver 2024-2025)
Cl. Bernard 1927. Cours de restauration des montagnes. (Mountain restoration course. In French). ENEF. 2e fascicule, planches. (Document disponible hiver 2024-2025)
Clave A. Rapport VI.3. Le rôle de la restauration des terrains en montagne dans la protection contre les inondations. (The role of mountain land restoration in flood protection. In French) In: La prévision des crues et la protection contre les inondations. Dixièmes journées de l'hydraulique. Paris, 5, 6 et 7 juin 1968. Tome 6, 1969.
S. Labonne, F. Rey, J. Girel, A. Evette. Historique des techniques de génie biologique appliquées aux cours d’eau. (History of environmental engineering techniques applied to watercourses. In French) Ingénieries eau-agriculture-territoires, 2007, 52, pp.37-48.  hal-00602456
Guillaume Piton, Simon Carladous, Alain Recking, Jean-Marc Tacnet, Frédéric Liebault, Damien Kuss, Yann Queffelean et Olivier Marco, « Fonctions des barrages de correction torrentielle » (Functions of check dams in steep creeks. In French), Cybergeo: European Journal of Geography [En ligne], Environnement, Nature, Paysage, document 896, mis en ligne le 15 mai 2019, consulté le 19 juillet 2023. URL : http://journals.openedition.org/cybergeo/32190 ; DOI : https://doi.org/10.4000/cybergeo.32190 
Guillaume Piton, Simon Carladous, Olivier Marco, Didier Richard, Frédéric Liébault, et al.. Usage des ouvrages de correction torrentielle et plages de dépôt : origine, état des lieux, perspectives. (Using check dams and open check dams in torrent control: origin, state of the art and perspectives. In French) La Houille Blanche - Revue internationale de l’eau, 2019, 2019 (1), pp.56-67. hal-02129862
Guillaume, P. (2018, février 1). Check dams on torrents, why? Encyclopaedia of the Environment. https://www.encyclopedie-environnement.org/en/water/dams-on-torrents-why/
Quefféléan Y, Carladous S, Piton G, Kuss D, Fouquet M, Tourment R. 2019. Spécificités des systèmes d’endiguement et de l’analyse de risque en contexte torrentiel. (Specificities of embankment systems and risk analysis in torrential context. In French) 1–11 pp. [online] Available from: https://doi.org/10.5281/zenodo.2531313
Tourment, R., Beullac, B., Berthelier, E., Boulay, A., Maurin, J., & Queffelean, Y. (2019). Inondations-Analyse de risque des systèmes de protection-Application aux études de dangers (Floods: Risk analysis of protection systems - application to conducting hazard studies. In French) (p. 356). Lavoisier. 

Text written by Pascale Hénaut (INRAE-DipSO).

Thanks to Guillaume Piton (Univ. Grenoble Alpes, INRAE, IGE) for his proofreading.

How to cite: Focus Agate: Pascale Hénaut (INRAE-DipSO), Dykes, groynes, dredging and channels also work in the mountains, january 2025, https://agate.inrae.fr/agate/en/content/highlights

The banner was made from a photography "73-2355 : Digue sur la rive droite de la Valloirette, travaux en régie. Anchierri. 1962"

The thumbnail was made from a photography "73-2352 : orifice aval de la galerie : construction de seuils, perrés de l’amorce du canal d’écoulement. Anchierri. 1959"