9.2. Flow prevention and protection

Techniques of prevention

Flows processes analysis is complex because it concerns both geotechnics, geomorphology domains (as for landslides in general) and hydraulics domain. Thus, prevention and protection techniques must also concern these different domains.
Flow triggering is mainly caused by heavy rainfalls; prevision of rain falls events is possible thanks to predictive models, however they concern large areas and not small watersheds in which flows occur. Moreover, flows events are characterized by the rapidity with which they propagate in response to a rain fall event. Thus, prediction of flow events is difficult or even impossible.
The best way for flow prevention as for landslides in general is to control urbanization by a zoning map which defines terrains which can be built or not, according to the degree of flow hazard and vulnerability (see more information on landslide mapping).

Moreover, prevention techniques consist in regularly maintaining the main channel and the banks in order to avoid blocking of the channel by trees for example, liable to aggravate the consequences of the flow.

Techniques of protection
A flow is generated because of a great quantity of material is available for sediment transport on the slopes of the watershed. According to this, techniques of protection can be classified in two distinct categories:
– “Active techniques”: they avoid the triggering of the flow. They mainly consist in reducing sediment transport by acting on source areas, by reducing slope erosion processes.
– “Passive techniques”: they seek to control the consequences of the flow. They mainly consist in slowing down the flow by acting on the stream channel.

A. Active techniques

– Role of vegetation

It is admitted by scientific community that vegetation acts on slopes by reducing runoff and erosion processes. Planting is considered as a long-term and efficient strategy for preventing sediment production. From the end of the 19th century large reforestation works have been carried out in French mountainous areas in order to reduce the frequency of mud flows and debris flows. For example, 1000 ha have been planted on highly erodable slopes of the Riou-Bourdoux catchment, in the Barcelonnette Basin. Even if the torrential activity did nit disappear, the frequency of flows rapidly decreased (figure 1).

Fig. 1: Evolution of the forests surfaces between 1896 (a) and 2000 (b) in the Riou-Bourdoux catchment (from Delsigne and al., 2001)

A technique is to build at different levels in the slope banks with wire mesh in order to reduce runoff and improve planting of herbaceous species or shrubs. These banks can also be made off wooden posts (figure 2).

Fig. 2: Slope planting with banks in Isère, France. Evolution of the slope between 1898 (a), 1912 (b), 1988 (c) (from Besson, 2005)

– Correction works in gullies
In order to prevent gullying and erosion processes, small dams can be built across the gullies in combination with planting. These dams can be made of concrete, gabions, grids, geosynthetics or wire mesh anchored by steel or wooden posts (figure 3).

Figure 3: Gullies correction with wire mesh small dams (from Besson, 2005)

B. Passive techniques

While active techniques rather correspond to bio-engineering (planting techniques), passive techniques rather correspond to civil engineering.

– Stabilization dams in the stream channel
These works aim at stabilizing the river profile. Indeed a stream tends to erode its banks and bed in steep slopes and to deposit material in gentle slopes in order to reach an equilibrium profile. Therefore, the upstream part of the channel tends to recede.
Torrential correction consists in building dams in the cross section of the channel in order to attenuate energy of the flow and provoke deposit of material upstream each dam. The purposes are to reduce erosion of the channel and quantity of material reaching the alluvial cone and liable to damage the human infrastructures. The dams also allow stabilization of the banks (figure 4).
Sizing of the dams requires for the planner to consider hydraulic behaviour of the stream, natural equilibrium slope of the channel and sediment transport processes. Dams must be frequently maintained because they can be damaged by major flows (figures 5 and 6).

Fig. 4: Correction of a stream profile with dams. a: natural profile which tends to retreat by erosion process (1 to 4). b: stabilized profile with dams (from Besson, 2005)

Fig. 5: Dams in the Faucon stream, Barcelonnette Basin, France (from Remaître, 2006)

Fig. 6: Damaged dam in the Faucon stream (from Remaître, 2006)

– Deposit area/sediment trap

It aims at slowing down the flow and provoking deposit of the materials.

– Dykes

It aims at protecting the banks from erosion and preventing the flow from overflowing. It usually consists of reinforced concrete walls or gabions walls. Anchor of the wall must be carefully designed in order to prevent erosion at the bottom of the wall.


BESSON L., 2005. Les risques naturels: de la connaissance pratique à la gestion administrative. Editions Techni. Cités, Voiron, 60 p.
DELSIGNE F., LAHOUSSE P., FLEZ C., GUITER G., 2001. Le Riou Bourdoux :un « monstre » alpin sous haute surveillance. Revue Forestière Française, p.527-540
REMAITRE A., 2006. Morphologie et dynamique des laves torrentielles : Applications aux torrents des Terres Noires du bassin de Barcelonnette (Alpes du Sud). PhD Thesis: Laboratoire Geophen, Université de Caen/Basse-Normandie, 487 p.