Ngonye Falls hydropower, Zambia

The run-of-the-river hydroelectric plant will help strengthen and stabilise the Zambian power network by reducing flows and losses from the south to the west of the country as well as support government policy to export electricity to neighbouring countries. The facility will connect to the grid through a 110km 330kV (initially energised at 220kV) transmission line to Sesheke on the border with Namibia.

Financial close is expected in 2024, with construction taking about three years. Electricity production is due to commence in 2027. It is the largest private investment in western Zambia.

Unique location

Located about 345km upstream of the Victoria Falls, the Ngonye Falls mark the southern extent of the vast Barotse floodplains on the upper Zambezi River. The floodplains dominate the hydrology of the Zambezi by storing rainfall from the upper catchment and slowly releasing it into the lower river. They also dominate the lives of the Lozi people who live on them and along their edge.

The Zambezi splits into several channels at Ngonye Falls, and water drops over a series of falls and rapids for more than 1km of the river’s width. Below the falls the river flows through a deep, narrow gorge cut into sandstone.

The combination of falls and floodplain make Ngonye Falls a unique location for hydropower as they allow a similar annual flow to that provided by a large reservoir. The project is a low head, high flow run-of-the-river scheme that uses the potential of the Ngonye Falls by passing water from above the falls through hydropower turbines to generate electricity before returning to the river about 6.5km downstream.

Innovating to raise capacity and reduce risk

We became owner’s engineer for the project in 2016 and reviewed the scheme previously developed by another consultant as part of a feasibility study.

The scope of the review included:

• Data quality – assessment of the raw data for hydrology, topography and site investigations
• Hydrology and energy – review of the energy modelling assumptions, calculations and results
• Design – assessment of the proposed scheme layout to identify areas that may benefit from additional optimisation
• Environmental and social impacts – assist the E+S consultant on ways to benefit the local community, design fish passes and support the ecological consultants on environmental water requirements.

2D hydraulic modelling found that the original proposals were not financially viable as they did not make best use of the available natural resource.

Our hydraulic and hydropower engineers used the model to test various alternative options. They identified that a concrete barrage containing fixed and adjustable weirs across the whole river would raise capacity from 45MW to 180MW. Upstream flooding also posed a risk, and we solved this problem by adopting innovative technology that we had developed for the Leeds flood defence project in the UK. Moveable flap weir gates with inflatable rubber bladders, known as Obermeyer gates, were installed at two locations on the River Aire in Leeds and our hydraulic modelling identified that the gates would be ideal for a 3m high, 820m long series of weirs at Ngonye Falls.

Our solution

Our proposal takes water from the left channel of the falls. The proposed headpond or upper-level arrangement combines flood control gates in the long, low weir and environmental flow gates in the barrage to divert as much flow as required into the channel. Fish ladders and a fishway are built into the design. The flow control structures will release both environmental and flood flows upstream. Left and right flood embankments will prevent floods bypassing the headworks. The 3km long concrete lined power canal is 11m deep, with a maximum water depth of 9m and capacity to convey 1100m3/s. At the end of the canal, a forebay is formed from the natural landscape and retaining dams. The forebay holds a buffer of water for the power station to maintain constant flow through the turbines. The powerhouse will be almost 60m high, with most of the structure excavated into the riverbank so it is not visible from the surface.

Our solution transformed the project economics. It will generate 830GWh compared with 312GWh for the original scheme, maximising the natural resource of the river and providing better environmental and social protection. At 3m, the barrage is the optimum height to balance electricity generation and keep potential upstream impacts to within acceptable limits.

We also recommended moving the powerhouse, as the original location was close a deep plunge pool that would have made it extremely difficult for a contractor to install a coffer dam. Our revised powerhouse location provides many advantages, including a better location for a coffer dam, safer work site, more energy, and use of a natural feature to make a forebay.

A new feasibility study and site investigations confirmed the viability of our scheme prior to the project entering a tendering phase. We are now supporting our client through to financial close, including developing the tender design and procurement process.

Supporting local communities and important habitat

Resettlement has been kept to a minimum and a Community Participation Agreement gives the communities of the western Province a 6% shareholding in the project, and they will receive a fixed annual community payment administered by a trust with broad local membership. This is expected to be as much as US$2.5M a year. Construction will create more than 3000 direct jobs, and more in local supply chains.

The need to safeguard the aesthetics of the falls and the aquatic environment is paramount. The headworks – concrete barrage, weirs and embankments – have been designed to meet Environmental Flow Requirements (EFRs) to maintain the ecology and habitat between these structures and the powerhouse. EFRs are the flows that must be constantly allowed to pass, and these been calculated through a comprehensive and systematic analysis of a suite of environmental indicator species by an independent consultant.

Fish passages will be constructed in the weirs and barrage to allow for migration. A rock ramp fishway as well as connecting channels and guide –walls have also been outlined to provide easier passage for fish around the natural falls.