Xhora Off-channel Dam Project by Hatch for Amathole District Municipality, Eastern Cape

PROJECT TEAM

Client: Amathole District Municipality Consulting Engineer: Hatch

Contractor:

Stefanutti Stocks/Mfuraa Consortium

The Xhora Water Supply Scheme was identified in a feasibility study that was completed in 2002. The Xhora Off-channel Storage Dam provides assurance of supply to the Xhora Water Supply Scheme, which is intended to provide potable water to 70 000 people in the Elliotdale District of Mbhashe Local Municipality, within the Amathole District Municipality.

The Xhora Off-channel Dam is a 29.4 m high crest, 230 m long, zoned earth-fill embankment dam with a diversion culvert to allow for flood water during construction, outlet pipework, and a 25 m long ogee side-channel spillway on the right flank, with a 5 m wide trapezoidal spillway chute, widening to 9.5 m before entering the stilling basin. The stilling basin was designed and model tested by the University of Stellenbosch.

To minimise the environmental impact of the dam, and the impact of siltation, it was conceived as an off-channel storage dam in a small section of the catchment. Water releases are provided to maintain the aquatic environment within the river channel downstream of the dam.

The dam is categorised as a category 2 dam by the Department of Water and Sanitation Dam Safety Office. The Stefanutti Stocks/Mfuraa Consortium was appointed as the contractor for the project.

Maximum use of the materials from the basin was made, which required a project-specific density test method to be developed. Limited higher PI clay was imported.

The water stored in the dam is expensive, as a substantial portion is pumped from the Xhora River raw water abstraction weir to the dam. It was therefore important to grout the rock below the dam to minimise leakage. In order to allow a higher firststage grouting pressure, grouting was undertaken through the imported clay cut-off. This allowed a significant increase in the pressure that could be specified for the first stage of grouting. On completion of the dam, the river diversion culvert was closed through the installation of two sets of precast reinforced concrete planks, with the void between the planks filled with mass concrete postgrouted through a tube manchette system. The diversion culvert provides passage for the two outlet pipes, a domestic supply to the water treatment works, and for emergency emptying of the dam, as well as making provision for environmental releases. A low-cost solution to abstract water for domestic consumption was implemented utilising a floating intake.