Avis dam – Windhoek, Namibia

Project overview


  • Purpose: to retain F.S.L. for a deepened side channel spillway
  • Type: TOPS automatic spillway gates
  • Size: 11 m long by 3,5 m high
  • № off: 2
  • Operating system: telescope arm from ballast tank attached to a level controlled diaphragm valve
  • Manual override: yes
  • Material: Mild steel with polyamine coating. Concrete skin to upstream face.

Location: Windhoek, Namibia

Client: Windhoek City Council

Consultant: K Lund and Associates

Contractor: Concor Engineering (Pty) Ltd

Scope of services: Design, Fabrication, Installation, Commissioning

Completion: 2002



The Avis dam was built in the German colonial time and the spillway is now inadequate to meet current dam safety requirements.

The existing side spillway had to have a channel 25 m wide by 4 m deep cut into the existing channel to meet the increase discharge capacity required.

The local authority could not permit a reduction in the F.S.L. and consequently spillway gates were required. Electrically operated gates were not an option because of the remoteness of the site and the unreliability of electrical supply.

After investigating different gated systems the consultant chose TOPS automatic spillway gates to retain the F.S.L.


Two TOPS gates were fitted to the spillway, each 11 m long by 3,5 m high.  The requirement was for the first gate to open for a 150 mm rise in water level above F.S.L. and the second to open with a further 150 mm rise in water level.

To achieve this, the gates were fitted with telescope arms each attached to the ballast tank to discharge through a float level controlled diaphragm valve.

These gates opened automatically in February 2004 to pass an estimated 1:50 year flood through the dam and closed automatically to retain the F.S.L.

The top plate is wide enough to accommodate a walkway over the gates which was an additional benefit for the community to gain access across the deepened spillway.