In the Oratoire and Plan du Pont district, the subject is sensitive. Omnipresent. We can no longer count the episodes of flooding which have affected the sector due to the vagaries of the Gapeau. 1999, 2014, 2019, to name a few. So many painful memories that residents would like to put away in a drawer and never see repeated.
The Gapeau Flood Prevention Action Program (PAPI) supported by the Syndicat Mixte du Bassin-versant and certified at the end of 2020 should make it possible to respond – at least in part – to this problem even if, as Patrick Martinelli, president, recalls of the union, “we cannot prevent the floods”.
Various developments have already been carried out upstream on the watercourse, and major work is underway in its flood expansion zone, a few kilometers from its mouth into the Mediterranean.
An impressive project to divert the Muât channel, but which is not necessarily unanimous among residents who doubt its effectiveness. They did not hesitate to let elected officials know during a site visit (our 8/11 edition).
“The project has evolved over time. It is a district that is regularly flooded, studies have been carried out for a long time to find solutions. The oldest were to develop the flood expansion zone by building dikes and basins. But when we do a PAPI, all the necessary studies are carried out to find the best solutions, and we found a better solution.”replies Patrick Martinelli.
Several options have been considered in recent years to safeguard this residential area. Some have been abandoned, others remain to be studied
Basin and dikes…
This is the first solution that was presented to the residents, the one to which “they stuck”. That of the creation of dikes and a retention basin.
“A solution abandoned because it was less effective than the current project. It made it possible to reduce floods of up to 380 m3/s, i.e. floods occurring in 15 or 20 years. The current project responds to thirty-year floods.”
Would this have been effective in 2014? “We don't know if it would have saved anything. With the rains of the previous days, it might have been full.” Knowing that the planned storage volume was 1.5 million m3 and that a ten-year flood is 10 million m3…
The Oratory bunkered?
Could the solution of a containment system around the neighborhood be the miracle solution? “It is a very complicated project to implement. The dike must be 7m high in relation to the watercourse, or 2m in front of the houses. We are on close protection, which the State does not recommend because we fear technological rather than natural risks. There are very dangerous risks of rupture for homes.”
On the raising of the fence walls requested by certain residents? “We cannot authorize this. A fence wall has never been built like a dike. There are significant safety constraints. And in a flood zone it is not authorized to have a fence wall…”
Buying back houses
No house should have been in this neighborhood. This observation, shared by all, is now clear. Most were built before the Barnier law, and the risk of flooding was not taken into account in town planning…
Why then not buy the homes at risk rather than spending millions on work? “This was a hypothesis when the PAPI was written. We made calculations, which estimated 70M to buy the most exposed houses only. And that didn't save the neighborhood, there would always have been flooding. The most exposed are not necessarily those closest to the watercourse but those against the hill, in a sort of basin. underlines the union.
And to add for the 240 homes affected by the risk that “the purchase of houses by the Barnier fund is based on issues of high speeds and high water levels. In the Oratoire district there are no high speeds.” So impossible to use the millions from the Barnier fund, and far too expensive for communities.
The continuation…
Once the channel diversion project is completed, the PAPI work will not be completed. A new phase will begin with a study on the development possibilities on the downstream part, and the water inlets of the Gapeau.
Studies on the right bank of the river in order to find the “best possible arrangements”: damming, recalibration of the watercourse to avoid water ingress on certain parts of the district, closure of certain water inlets, installation of pumps to facilitate draining, compensation necessary for the probable overflooding of the opposite bank or of the downstream part, purchase of land for flooded areas…
The union will also be mobilized to “encourage residents to equip themselves. We will come back to them so that they can equip themselves because the aid does not last forever”, recalls the union which adds that “the diagnosis is free and the equipment helps 80%”.
Diverting the Muat, the best solution?
The diversion of the Muat channel, work on which is in progress, should resolve the problems until a thirty-year flood. “This solution has been analyzed and modeled by competent people. The project is more effective than the project initially presented”indicates the union. And for larger floods like that of 2014, the volume of water entering the neighborhood will be reduced.
“During the 2014 flood, this system would have made it possible to protect half of the neighborhood from water and 70cm to 80cm of water in certain areas. For this, we are proposing solutions to reduce the vulnerability of buildings with cofferdams. in due form which will protect the houses not all are equipped, especially the most impacted, insists the union And there is aid of up to 80%.
Rock or vegetated banks?
During the visit to the site, residents expressed their skepticism about the plant technique for stabilizing the banks, calling for riprap so as to no longer see their fence sag due to erosion.
“It was integrated and carried out as a priorityrecalls the union which specifies that calculations of stability and erosion were carried out and that the revegetation technique could be used with, according to the calculations carried out, resistance up to a hundred-year flood. The rocks would not bring any more efficiency.”
