Reservoir sedimentation mitigation measures to deal with a severe drought at Graaff-Reinet, South Africa

The Nqweba Dam is one of the oldest dams in South Africa and was commissioned in 1925 for irrigation. The original reservoir depth at the dam wall was 31 m, but over the years sedimentation has reduced the water depth to 9.8 m. The original storage capacity of 79 million m3 has decreased to 46 million m3 by 2020. The dam supplies the town of Graaff-Reinet and due to the growing population at 1.9% pa it was realised in 1995 that the water use should change from irrigation to 100% potable use. For the past 25 years the town has been supplied from the dam when it has water and from ground water when the dam runs empty during droughts. The current water requirement of the town is 3.3 million m3/a and during 2019 the dam ran dry, while the ground water resource only supplied less than 50% of the demand. Urgent short and medium term measures were sought to solve the water crisis. Dam raising was found not to be beneficial, but options to increase storage capacity such as dredging or a new off-channel dam, and reduce evaporation such as floating balls/solar panels on part of the reservoir will restore the dam’s firm yield