NECKARTAL AUTOMATIC SCOUR GATE

BACKGROUND

The Neckartal dam project consists of a 80 m high curved gravity dam constructed from RCC in the Fish river, Namibia and an abstraction weir some 21 km downstream. The abstraction weir is fitted with an automatic scour gate to remove gravel and sediment from the abstraction point. A large pump-house has been constructed to pump water diverted by the weir into a storage dam which will feed into a proposed 5000 ha irrigation project. Controlled hydropower releases from the main dam fill the weir and water is pumped to the holding dam. The Fish river carries a large amount of sediment in this very arid part of Namibia and the gravel trap channel and scour tunnel fitted with the scour gate keep the abstraction point relatively sediment free.

DESCRIPTION

The scour tunnel leading away from the sloping concrete floor in the gravel trap channel is sealed with the scour gate closure plate, 4 m wide  x 2.3m high. During normal operation water will overflow the weir wall into the gravel channel and flow through the trash racks into the sand trap filter channels to the pump station. Sediment will settle on the sloping channel floor and the scour tunnel will remain flooded while the automatic scour gate remains closed. 
During flooding water will pour into the gate chamber in which the fixed buoyancy tank is housed directly above the scour tunnel through baffled openings in the upstream wall. The cantilevered closure plate which seals the scour tunnel opening is connected to this fixed buoyancy tank above which is attached with pivot arms to the cast in axle units in the chamber floor. As the water level rises inside the chamber, the tank will start to float and the closure plate will be lifted vertically out of the scour tunnel. Water will drain continuously from two fixed openings out of the chamber and the gate will continue to rise until it is in its fully open position. The high velocity flow out of the gravel abstraction channel through the scour tunnel will remove deposited sediment, sand and gravel and only once the flood subsides and water no longer flows through the baffled openings into the buoyancy chamber will the outflow exceed the inflow and the tank will drop and the closure plate will once more seal off the scour tunnel opening. 
This operation is automatic and requires not electrical power nor operator intervention. The gate can be manually actuated if required by opening a butterfly valve connected to a submerged pipe in the crump weir on the upstream side of the weir, which will result in flooding of the chamber and opening of the gate as described above. Once the manual weir inlet butterfly valve is closed the outflow will drain the chamber and the tank will drop and the tunnel will once more be sealed.
A maintenance gate has been fitted upstream of the scour tunnel for future maintenance and inspection of the submerged closure plate and seals. A manual filler pipe and tunnel vent pipe have been fitted to the chamber to enable the scour tunnel to be filled after inspection in order to balance the forces on either side of the maintenance gate which can then be lifted after the inspection.