Toulon’s flood alert: erosion risk

Ness writes*

Le Gapeau is both the name of a river and a watershed in the south of France, Toulon, dominated by forests around its slopes, and urbanization and agriculture at the bottom of the watershed towards Hyeres (Rollet et al, 2022).

There is currently a sedimentary deficit in le Gapeau mainly due to extractions of granulates and sand that has been taking place since the 19^th century. These “massive extractions” do not consider the dynamic nature of the rivers (SAGE, 2022). Between 1859 and 1986, measures were taken to reduce extractions, but illegal and “authorized” extractions continued. In 1966, the construction of the Hyeres airport used so much sediments that the river’s banks destabilized, reducing water quality and water retention. The government did not acknowledge those impacts, and information on them can only be found by talking with those who live by the river (riverains); there is no information on the internet (Capanni, 2022).

Nowadays, in the Golfe de Giens, a strong deficit in sediment is observed now of around 1 to 3 meters in 73 years, and in the rade d’Hyàeres, a natural zone classifies as a “Special zone of conservation,” erosion is important especially in the seasonal periods due to tourism (SAGE, 2022). Despite all of the above, it must be noted however that climate change exacerbated these issues regardless of human management, since increases in temperature increase the likelihood of storms, leading to even more erosion (Capanni, 2022). This affects the capacity of the water body to store water, leading to potentially water scarcity issues.

Extractions are not the only reason for sedimentary deficits. The presence of dams and weirs is argued to influence fluvial morphology. For instance, a definite issue causing presence was an anti-salt dam constructed in the 1970’s. This dam caused contamination of groundwater as well as siltation, which is the deposition of fine sediments, which now and in the future decreases the storage capacity of the river (Capanni, 2022).

Erosion has a direct impact on an already existing issue; the value of riparian areas. They are essential for the correct workings of the watercourse, they filter pollution, and habitat many species (SAGE, 2022). Farmers and riverains rely on these forests yet they do not care enough for them, which exacerbates issues of sedimentary changes as erosion degrades the health of these areas.

Flooding is especially exacerbated by this issue of deficits in sediment. The flood plain of Cuers drains le Gapeau and le Réal Martin, which are both main rivers of Le Gapeau watershed. There are sedimentary repositories in these plains, and during rain periods, flood plains are more likely to overflow (see image below) since they have a lower water holding capacity due to less sediment, which can also lead to erosion.

Bottom Line: Humans have taken too much sediment and erected too many barriers on Le Gapeau, thereby increasing flooding, lowering water quality, and harming ecosystems. Climate change will only worsen these impacts.

* Please help my Water Scarcity students by commenting on unclear analysis, alternative perspectives, better data sources, or maybe just saying something nice 🙂