The upcoming six-month closure of the Lesotho Highlands tunnel, from October 1, 2024, to March 31, 2025, for scheduled maintenance is not expected to have a significant impact or disruption on Gauteng’s water supply.

The shutdown will result in an 80-million-cubic-metre shortfall during 2024 in the water supply from the critical Lesotho Highlands Water Project (LHWP) to the Integrated Vaal River System (IVRS).

“This much-needed maintenance is critical to maintain the integrity of the delivery tunnels as a tunnel failure will risk the transfer of the 780-million cubic metres a year of water to the IVRS, from which Rand Water draws water to supply its customers,” Department of Water and Sanitation (DWS) director-general Dr Sean Phillips said on Friday.

He warned that the six-month period required to conduct maintenance was critical to avoid any catastrophic event which may result from a lack of maintenance.

However, while the DWS assured that there were plans in place to mitigate the closure of the transfer tunnel, it was urging all water users affected by the planned tunnel maintenance to use water sparingly during this period, as a means of managing any unforeseen risks, and a water restrictions notice was being prepared in this regard.

After the shutdown period, the water transfers will be increased to enable the shortfall in transfers to be recovered.

Phase 1 of the LHWP, which was completed in 2003 and comprises 82 km of tunnels, the Katse and Mohale dams and the Muela power station, currently transfers 780-million cubic metres of water each year into the IVRS.

The tunnel system, which includes a transfer tunnel linking Katse dam with Muela power station and Muela dam, and a delivery tunnel linking Muela dam with the Ash River Outfall Works between Clarens and Bethlehem, requires inspections and maintenance at intervals of between five and ten years respectively.

During the last maintenance and shutdown undertaken in 2019, it was found that the steel liners in the tunnel urgently needed extensive maintenance on both the South African and Lesotho side, which will take six months to complete.

The work required includes grit-blasting the steel-lined section around the entire circumference and re-applying corrosion protection on the tunnel lining, as well as other maintenance and repair work identified during the 2019 maintenance shutdown.

This work, which is expected to protect the infrastructure for another 20 to 30 years, is being overseen by the Lesotho Highlands Water Commission and will be jointly undertaken by the Lesotho Highlands Development Agency and the Trans-Caledon Tunnel Authority.

An analysis, which was undertaken in May 2023 in preparation for the maintenance to assess the closure’s risks and impacts, indicated that the impact of the outage on the overall IVRS would be insignificant considering that dams in the IVRS, such as the Sterkfontein dam and others, were relatively full, which meant that they could provide a reserve supply of water to top up the Vaal dam as needed.

Water from the Sterkfontein dam is released to the Vaal dam when the Vaal dam reaches a minimum operating level of 18%, which the DWS said was unlikely to occur at any probability level in the 2023/24 operating year.

“This means that the closure of the tunnel for maintenance will not result in any disruption of water supply to Rand Water, and to the municipalities in Gauteng and other provinces which are customers of Rand Water," Phillips assured.

The DWS will undertake further analysis in May 2024 to ensure that there are also no likely risks to water supply from the IVRS in the 2024/25 operating year.

According to the DWS, the tunnel’s closure will also not have any significant water supply implications for domestic users along the Liebenbergsvlei river and its tributaries in the Free State. The Liebenbergsvlei river normally receives outflows from the tunnel.

However, while the tunnel is shut down, additional water will be supplied from the Saulspoort dam, which will be filled with water before the start of the shutdown to ensure that the water requirements of towns along the river, including Bethlehem, Reitz and Tweeling, have sufficient water.

Licensed irrigators along the Liebenbergsvlei river and its tributaries will be issued notices to restrict their abstraction during the shutdown period to certain days of the week, so that they abstract water from rivers fed by the Saulspoort dam in a sustainable way during the tunnel closure.

Abstraction of water from the river will be closely monitored by the department during the closure to ensure that there is no illegal abstraction which might affect the ability of licensed irrigators to abstract the water which they require.

The planned closure of the tunnel for maintenance will also not impact the ongoing construction of Phase II of the LHWP.

Phase 2 of the project, which is currently under construction, includes a 165-m-high concrete-faced rockfill dam at Polihali and a 38-km-long concrete-lined gravity tunnel connecting the Polihali dam reservoir to the Katse dam.

The main contracts for the new dam and the tunnel were awarded in October 2022, the contractors are on site and the project is on track to start delivering additional water into the IVRS in 2028.

Once completed, LHWP 2 will transfer an additional 490-million cubic metres of water a year from the Orange (Sengu) river in Lesotho into the IVRS in South Africa.