To address the Seychelles islands water supply challenge, owing to irregular rainfall and longer dry periods, South African consulting engineering firm Gibb, in partnership with Gibb Mauritius, is con-ducting a feasibility study to increase the storage capacity of the existing La Gogue dam reservior on Mahé Island, Mauritius.

The La Gogue Water Supply Study, which will be undertaken in two phases, will be followed by the tender design phase and also aims to improving the La Gogue distribution system. The study is expected to be completed by the end of 2013.

Continuous population growth, tourism activities and other commercial developments will result in an increase of 6% to 8% a year in the estimated demand for potable water in the Seychelles and, at that rate, the current water supply will soon be exceeded, says Gibb.

The Seychelles islands had an estimated population of 83 000 in 2010, with 90% of people living on Mahé Island.

In light of this, the Seychelles Public Utilities Corporation (PUC) commissioned the raising of the La Gogue dam, which is one of the main water supply reservoirs for the North East Mahé Water Supply System. The project forms part of the Seychelles Water Development Plan (WDP), but the suggestion to raise the dam wall to increase storage capacity has been under discussion since the 1990s.

Gibb notes that, at the current project stage, costs are preliminary and are for the PUC’s internal use only and will be made available during the tender stage. However, the corporation says the project will be funded through a financial institute with which PUC is negotiating.

“Water restrictions are common on the three main Seychelles islands of Mahé, Praslin and La Digue, and the conditions will worsen if appropriate measures are not taken soon,” says Gibb technical executive and project director of dams, hydropower and underground works Colin Logan.

He adds that the islands are experiencing water shortages, with only 60% of the current demand being met on average. The water shortage crisis is worse during the dry season (June to November), with water restrictions and rationing being more frequent and consistent in recent years.

“Because of the topography of the Seychelles islands, it is difficult to create suf- ficient storage capacity at reasonable cost and, up to now, the traditional source of fresh water has been abstraction from streams,” Logan notes.

The La Gogue dam, which was constructed in 1979, is a homogeneous earth fill dam, with a maximum height of 35 m from the excavated foundation level and a length of 152 m at the crest. The present total storage capacity is 970 000 m³, with useful storage at 860 000 m³ and the withdrawal designed at 14 000 m³/d.

The current demand of about 44 000 m³/d is expected to increase to 55 000 m³/d by 2020.

“The raising of the dam is widely regarded as a potential scheme that will contribute to addressing water shortages in the Seychelles. “With the existing conditions of the dam, the topography and the geotechnical condi-tions at the site, it is technically feasible to increase the height of the dam by 6 m without involving major hill cuttings,” says Logan.

Previous studies and the WDP estimate that raising the height of the La Gogue dam by about 6 m will increase the reliable yield by an average of 3 600 m³/d. This additional resource is equivalent to about 50 days of water for the islands, representing 14% of water currently produced.

In terms of improving the La Gogue distribution system, the WDP has proposed the construction of a new treatment plant downstream of the dam, as well as the construction of distribution mains and connections to existing pipelines, where needed.

A high percentage of water – about 44% – is unaccounted for and, therefore, lost, owing to an ageing reticulation system, which is complex because of the hilly topography of the island.

“For the Seychelles, this project brings improved security of water supply for economic and domestic use and will undoubtedly help meet the increasing water demand, while providing much-needed employment opportunities in the area by increasing water availability for industrial use.

“Water supply generally comes with a level of economic upliftment,” Logan explains.

“To date, it is our understanding that the locals are generally supportive of the project. Considering that it is a slight enlargement of an existing dam, there should not be any major objections, as the additions are minimal,” he says.

Logan notes that, currently, the project does not require local involvement, but will offer employment opportunities in the construction phase. “With construction activity comes the need for construction workers, and additional water supply will hopefully improve the economy to the extent that it provides further employment opportunities,” he adds.

Meanwhile, the company notes that water is distributed unequally across Africa. “Robust regional management plans are important to ensure that water resources are effectively distributed to areas of potential economic growth for the benefit of the whole continent,” says Logan.

He points out that the abundant water sources in Africa are often remote from areas of significant economic growth and, hence, there is a need for regional cooperation as far as the use of the resource is concerned. “It is no longer sustainable for African countries to concern themselves only with their own needs – regional solutions should rather be developed,” he adds.

Gibb highlights that the latest trends involve water engineering projects largely being driven by environmental due diligence processes and responsible engineering solu-tions to ensure that water resources are used sustainably.

“The availability of water in nature is poten- tially being affected by climate change, where the intensity of floods appears to be increasing. “To maintain high water-supply security for growing economies while taking the spatial spread of precipitation into account, responsible development of water storage capacity and interbasin and intercountry water transfer schemes will have to be devel-oped,” says Logan, highlighting that these schemes will have to consider that water is a finite resource that must be managed at various levels and preserved and conserved at all costs.