The raising of Hazelmere dam, South Africa

Hazelmere Dam, located on the Mdloti River, Kwazulu Natal Province, South Africa was constructed as a concrete gravity dam in 1975 to 1976.

The intended Phase II of the works was to raise the Full Supply Level (FSL) of the dam by 7m, by way of introducing radial gates in the spillway, in order to increase the yield of the dam. However, due to the unreliable maintenance of mechanical gates, it was decided to raise the FSL by way of installing a Piano Key Weir (PKW) structure in the existing spillway, thereby allowing for a free overflow spillway which required far less maintenance than mechanically operated spillway gates.

INGEROP South Africa (ISA)’s expertise in dam inspection and rehabilitation made them win the open tender and ISA were appointed as the Consulting Engineers for the design and the supervision of the works.

Because of the modern trend in dam design shifting from designing to the Probable Maximum Flood (PMF) as the Safety Evaluation Flood (SEF) instead of the Regional Maximum Flood (RMF) for Category 3 dams, a check on the stability of the dam wall in its raised state was carried out. The results of the stability analysis revealed less than satisfactory results for the factors of safety against sliding and overturning, which prompted further investigations into possible methods to bring the factors of safety into line with modern dam design practice. The use of either mass concrete or post-tensioned anchor cables was initially considered, but due mainly to the cost effectiveness of the post-tensioned anchors in comparison to the cost of mass concrete, the use of post-tensioned anchors was given priority, investigated further and finally settled upon as a design solution to the stability concerns.

Hazelmere Dam, in its raised state is of great importance to the region, as it forms the major water supply of the existing 2 systems in the area. The dam’s importance has been highlighted by inspections from the President’s Office, as well as the National Portfolio Committee for water from the Director General’s office of the Department of Water Affairs The raising of Hazelmere Dam will increase the assured yield of the dam by about 50% so that the current demand in the ever increasing development area north of Durban can be satisfactorily met.   




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