[Article by Nane Zietsman]
The Faculty of Engineering would like to congratulate Prof Gerrit Basson and Dr Jeanine Vonkeman whose work in the Hydraulics Laboratory at Stellenbosch University has contributed to a SAICE Commendation Award for Most Outstanding Civil Engineering Project: Xhora Off Channel Dam, Eastern Cape, Hatch Africa. This accolade forms part of the annual SANRAL SAICE National Awards that celebrate and reward outstanding projects that advance innovation and good practice in the development of civil engineering. Prof Basson and Dr Vonkeman designed the spillway of the Xhora Off-Channel Dam and optimised the design in a physical model study.
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. The key hydraulic components of the dam are a side channel spillway that is 25 m in length, a discharge carrier that consists of a 5 m wide trapezoidal concrete-lined chute, a stilling basin that is excavated in rock to dissipate energy, and an unlined river channel with flows from the stilling basin turning 90 degrees before entering the river downstream. Apart from these components, Prof Basson and Dr Vonkeman had to consider challenges such as finding a way to minimise the environmental impact on the surrounding rivers and streams, seeking innovative solutions for the shortage of suitable embankment materials, and ensuring minimum leakage. (Video: Hydraulic Design and Construction of the Spillway and Stilling Basin of Xhora Dam.)
Prof Basson and Dr Vonkeman performed their work in the Stellenbosch University Hydraulics Laboratory. This facility forms part of the Water Division in the Department of Civil Engineering and enables research in specialised hydraulic engineering fields. It is currently the largest hydraulics laboratory in South Africa and permits physical model studies such as the one conducted in the Xhora Off-Channel Storage Dam Project. To achieve their objective of designing a partially lined stilling basin that will dissipate energy at the dam’s spillway, Prof Basson and Dr Vonkeman performed a physical model study with an undistorted scale of 1:30.
The Xhora Off-Channel Dam Project is unique because it required the design of a non-standard stilling basin that would shorten the length of the hydraulic jump to fit into the limited space before the flow turns 90 degrees and enters the river downstream. Ultimately, Prof Basson and Dr Vonkeman succeeded to improve the energy dissipation by diverging the 5 m wide chute to 9.3 m at the stilling basin starting 52 m upstream and modifying the lower chute with a vertical curve for uniform supercritical flow. They used custom chute and baffle blocks in the stilling basin to shorten the hydraulic jump length and narrowed the stilling basin width from 27.5 m to 14 m. Additionally, they set a concrete lining 19.3 m from the chute that extends to the last row of baffle blocks and provides erosion protection in the form of riprap that they positioned above the terrace with a 1m high concrete kicker wall.
The final model design was implemented by Hatch Africa, who contracted Prof Basson and Dr Vonkeman as specialist sub-consultants for the larger Xhora Off-Channel Storage Dam project. This is not the first time that Hatch has employed engineers from the Faculty as specialist consultants – demonstrating the highly sought-after capabilities of our students and staff. We applaud Prof. Basson and Dr Vonkeman for their work and the impact it has made on the field and the communities of the Amathole district who now have access to portable water for the first time.
Photograph: The completed Xhora Off-Channel Dam.