Matsoku Weir – Lesotho

Project overview


  • Purpose: to desilt the tunnel forebay
  • Type: FDS automatic scour gate
  • Scour tunnel: 3 m wide x 2 m high
  • Max operating head: 11 m
  • Scour discharge: 70 m3/sec
  • Manual override: yes
  • Material: Stainless Steel Gr 304 L

Location: Lesotho

Client: Lesotho Highlands Development Association (LHDA)

Consultants: JV between Ninham Shand (now Aurecon) and GOBA

Associated firms: Concor Engineering (Pty) Ltd, Fluid Dynamic Services (Pty) Ltd

Scope of services: Design, Fabrication, Installation, Commissioning

Completion: 1999



In order to supplement water supply to the large Katse Dam, Lesotho, flow from the adjacent Matsoku river was diverted into the Katse dam through an 8km diversion tunnel.  A diversion weir was built to transfer water into the tunnel.  A considerable amount of sediment is carried by the Matsoku River in floods and in order to minimize sediment transfer into the Katse Dam, an automatic scour gate was installed in the tunnel forebay.  Because of the remote site in the mountains, the gate has to be totally automatic and not reliant on electrical power.  The consultants selected the FDS automatic scour gate for this purpose.


The FDS scour gate closes off a 3 m wide x 2 m high scour tunnel and operates under 11 m of water head.  The inlets to the gate chamber are specially designed to minimize debris.

A 300 mm NB diaphragm valve was fitted to the gate chamber to manually open and close the scour gate by flooding the gate chamber.

The gate seals are double stem hump and music note seals with PTFE lining to give a smooth opening and closing.

The gate and components were fabricated in stainless steel grade 304L in order to provide a maintenance free installation.

This large scour gate discharges in the order of 70 m3/sec when fully open and the gate works frequently to automatically open and close whenever it rains heavily in the mountainous catchment.