Raising of La Gogue dam to boost water supply on Mahé


18-February-2013

La Gogue dam spilling

The first major reservoir and associated treatment works were successfully constructed at Rochon in 1969 but with the opening of the International Airport, the demand for water was expected to rise sharply, this due to the rapid increase in tourist traffic.

Another study was commissioned immediately after to assess potential sites to construct other storage reservoirs to provide extra storage capacity to meet the required water demand. The La Gogue site was chosen due to its flat valley providing a considerable volume of storage and ease of construction of a dam.

La Gogue Scheme

The existing La Gogue Dam which was completed in 1979 is one of the main water reservoirs for the North-East Mahé water supply system. The reservoir, in addition to getting water from its own catchment (25% in total), also receives water through a 450mm diameter ductile iron pipeline from the Rochon reservoir.  Water is drawn from La Gogue reservoir through the same pipeline to the Hermitage Treatment works. The system operates entirely by gravity. The useful storage of the dam at full storage level (FSL) is about 850,000m3.

La Gogue Dam at FSL

The seepages in the dam which is normal occurrence in any dams are measured at three (3) locations at the bottom of the dam. These measurements are one on a weekly basis.
The designed withdrawal capacity (yield) at present is 14,000m3/day. However due to the high withdrawal from the dam as a direct result of increased demand for water, the level of the dam falls to very low values during the dry season.

Enhancing the storage capacity of the dam

The idea of raising the La Gogue Dam to increase the storage capacity has been in discussion since 1990 and was again an issue in the recently completed Water Development Plan (WDP) for Seychelles. This project is widely seen as a potential scheme that will contribute to addressing the water shortage. The existing condition of the dam, the topography and the geotechnical conditions at the site are believed to permit increasing the height of the dam by 5 to 6 m without involving major abutments hill cuttings.

Justifications for increasing the La Gogue Dam storage capacity

During wet years, the records kept shows that the dam overflows an average of two (2) weeks on a continuous basis. This represents a significant loss of water which could have been kept as storage for the drier months.

Rochon Dam – the main source of water for the scheme

Moreover, the PUC is currently building additional transfer schemes directly or through the Hermitage system. The transfer schemes under construction (Salazie and Mt Simpson/Le Niol) will undoubtedly contribute to the increase of the yield from the dam and will therefore necessitate the increase in storage of the dam.

Furthermore the future plan of the PUC to construct a water treatment plant at La Gogue and the proposed improvement to the La Gogue water distribution network to serve about 4,500 inhabitants in the area will bring multiple benefits. These being (1) a more reliable & efficient water distribution system for the area (2) reduced operating & maintenance costs as less pumping stations (6 instead of 12) will be required when compared to the water being supplied from Hermitage, and (3) better pressure management.
The raised dam and proposed treatment plant once completed will bring clear benefit to the water supply situation on Mahé.

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