Ngounié River – GABON

« Chutes de l’Impératrice » Hydro Power Plan

Project Summary Data


HPP Company :

Owner engineer :

Type :
Low-head hydroelectric power station run off river type, with a water level regulated by two movable weirs with valves and a free Craeger-type weir.


Site :
Ngounié River, 5 km from the town of Fougamou, Gabon

Average flow :
700 m3/s

Installed power :
5 x 24 MW (120 MW) expandable to 7 x 24 MW (168 MW)

Turbines :
Kaplan turbines with horizontal axis
Turbine flow 80 m3/s per turbine


Drop height :
24 m

Average annual producible :
730 GWh (910 GWh after extension)

Injection of production :
225 kV HT transmission line

Duration of works :
2 years for phase N°1

Provisional commissioning date :
2nd half of 2025


The project consists of carrying out a low-head, medium-power hydroelectric development at the Chutes de l’Impératrice on the Ngounié River in Gabon, about 5 km downstream from the town of Fougamou. This run off water scheme is of very simple design, without dam or underground work. The project will not generate any population displacement due to the absence of the creation of a reservoir resulting in the submersion of the land.

The development is sized to accommodate 7 groups of 24 MW of the horizontal Kaplan type, i.e. 168 MW installed, for an average yearly producible of 910 GWh. Phase 1 concerns the installation of the first 5 groups for a total installed power of 120 MW and a producible of 730 GWh/year.

The facilities also include a free weir (Craeger type) and two gated weirs whose purpose is to create a regulating water level upstream of the intake canal supplying the hydroelectric plant.

The duration of construction of the works is two years from the establishment of the financing.

The technologies selected for the various structures and equipment have been widely tested and mastered for several decades.

The risks in terms of design reliability and construction contingencies are extremely low.

The works will be designed and built in accordance with Gabonese regulations, in particular on the environmental aspect.

As such, a detailed impact study is carried out as part of this preliminary project, with the aim of providing designers with the environmental constraints linked to the context, the potential impacts of the project and the provisions and compensatory measures to be implemented.

In addition to the expected benefits in energy and environmental terms, the project will generate significant economic benefits in Gabon since a significant part of the amount of the investment will be intended to pay local companies for earthworks, roads, structural work and civil engineering. .

The renewable energy produced at a much lower cost will replace the energy currently produced by the thermal power stations of Bifoun, Lambaréné, Fougamou, Mandji, Mouila and Tchibanga. The rest of the energy will be evacuated to Libreville.

This project enables rural electrification in the provinces of Moyen Ogooué, Ngounié and Nianga. 85 villages representing more than 12,000 inhabitants are concerned in the first phase.

GWh generated per year

Jobs generated by construction

Villages connected to electricity

Environmental impacts

The project will make it possible to produce 730 GWh each year for phase 1, then 910 GWh once the 7 turbines have been installed. As part of the project, various accesses will be made to initially isolated areas through the creation of tracks and roads.

The realization of the canal and the construction of the power station also require a modification of the natural terrain. This work consists in particular of producing large volumes of cuttings and consolidating the embankments.

The most significant effects will be present mainly during the construction phase. In the operation phase, the impacts will be non-existent since no more modifications will be made, the whole will be stabilized and for a large part re-vegetated.

In operation, these weirs, which will also serve as crossing structures, will have no impact on the environment since their behavior is comparable to that of a bridge. The modification of the hydraulic behavior of the river in the area concerned has been the subject of an in-depth study.

The main effects of the project are on the one hand the flooding of a limited area upstream of the weirs and the power plant at elevation +72 NGG initially emerged, and on the other hand the modification of the natural flow of the course of the river. ‘water. The changes made by the project remain moderate since the watercourse will not exceed the major bed of the river. The structure is of the “run-of-the-river” type, without any storage, and does not bring any modification to the level of the flow of the river either upstream or downstream of the installations.

During the works phase, temporary cofferdams will be installed and will make it possible to work in dry areas, in particular at the level of the two arms of the river where the gated weirs will be installed. These cofferdams do not constitute a major impact since they generate a drying out of the area they protect but do not modify the overall behavior of the river downstream: No diversion of the watercourse will be carried out.

The flow of the river is largely in excess of the flow necessary for the planned energy production, and will enable a high flow to be maintained downstream of the weirs. Downstream from the release point at the foot of the powerhouse, the flow will remain unchanged. Whatever the flow of the river, an instream flow will be maintained downstream of the weirs.

Finally, the project will endeavor to maintain the living conditions of the aquatic fauna and flora, in particular by providing for the possibility of exchange and passage between the upstream and downstream of the installations: A ladder-type pass fish tank will be designed and produced by a company specializing in the field. Adapted to the ecosystem and the site’s operating conditions, this structure will notably allow the passage of aquatic species requiring the ascent or descent of the river in their life cycles.

Energy impacts

This project represents the electrical opening up of agglomerations throughout the region: Indeed, the global program to which the project is attached will make it possible to connect the various networks currently not interconnected:
Fougamou, Sindara, Lambaréné, Mouila, Libreville. This global network will eventually make it possible to connect any new hydroelectric production site, including that of Sindara located 10 km downstream.

It will supply the niobium production and processing site, but also the possibility of supplying new energy-consuming sites such as forestry or mining sites.

The electrical opening up of the area will result in certain socio-economic development through new activities that will be able to benefit from energy at a competitive cost. This energy opening up is the first stage in the development of this region.

Socio-economic impacts

The construction of the structures will generate the creation of 500 full-time jobs over a period of three years (400,000 man/days in total).

On the other hand, upon commissioning, 30 full-time jobs will be created to operate the hydroelectric power plant and the THT transmission line. Simultaneously with the opening up of electricity, the river crossing structures will also open up the right bank from Ngounié to Fougamou.

The impact of the project is twofold: It generates direct benefits in terms of activity in the construction sector during construction, and it initiates development of the Fougamou area by opening up this town.

The impact of the project is twofold: It generates direct benefits in terms of activity in the construction sector during construction, and it initiates development of the Fougamou area by opening up this town.