Sade, a subsidiary of NGE, is modernizing the Grand-Galet drainage gallery to secure the drinking water supply for approximately 30.000 residents.

The rehabilitation of the Grand-Galet drainage gallery aims both to secure the structure and to sustainably improve the conditions of water collection in a context of increasing water stress in Reunion Island.

Coping with water stress

Like many municipalities on the island, Saint-Joseph is regularly subject to water conservation measures or restrictions during periods of drought. The Grand-Galet drainage gallery, which collects groundwater from the Langevin area, alone provides nearly a third of the municipality's drinking water supply, serving approximately 30.000 inhabitants. Its restoration is therefore a direct means of combating water stress and strengthening the resilience of the region.

A project of strategic importance welcomed by the authorities

On November 12, 2025, Patrice Latron, Prefect of Réunion, accompanied by Patrick Lebreton, Mayor of Saint-Joseph, and Jacquet Hoarau, President of CASUD, visited the construction site to monitor its progress. Their visit allowed the authorities to speak with the teams and reaffirm the importance of this essential project for securing the municipality's drinking water supply.

Rehabilitate the gallery to better capture the resource

The main structure is a 260m long, 2,70m wide and 2,50m high accessible gallery, with a central drainage channel at its base. Having become outdated, it no longer allowed for the full exploitation of the existing vertical boreholes.

SADE is thus rehabilitating 220 meters of tunnel by installing GRP shells (2100 × 1780 mm) and filling the annular void with gravel to stabilize the structure. This is a new process in France for this type of project. Usually reserved for accessible wastewater treatment plants, these shells are being adapted here for a drinking water facility located in a difficult-to-access site, where traditional shotcrete techniques are not readily available.

Once the gallery has been rehabilitated, FORINTECH will create about fifteen new horizontal boreholes halfway up the hulls and connect the 8 existing vertical boreholes in the vault.

The SADE teams will then install the new collection network, removing the old network, which was deemed to be in poor condition and not functional: 2 new collectors that will carry A1 quality water, directly consumable, from the 16 boreholes to a reservoir.

The works, which began in September 2025, will continue until August 2026.

Better separation, better measurement, better protection

Improving the conditions for water capture also relies on a more precise organization of water flows within the structure:

• The water captured by the 16 boreholes is directed, via the new collectors, to a drinking water reservoir;
• The drainage water from the gallery, unfit for consumption, remains confined in the central gutter and is returned to the natural environment;
• A venturi-type channel, equipped with a probe, will now make it possible to accurately count the volumes of water discharged into the natural environment.

By thoroughly modernizing the Grand-Galet drainage gallery and redesigning the entire water intake chain, from the boreholes to the reservoir, the project provides a structural solution to Saint-Joseph's drinking water needs. It significantly improves intake capacity and operational flexibility, while also offering better flow control.

This rehabilitation will contribute to securing the municipality's water supply in a sustainable way, in a context where preserving the resource is more than ever a collective issue.