George Municipality and the engineering community this week celebrated the success of the award-winning Garden Route Dam spillway as the dam overflowed for the first time since the spillway and dam wall was raised in 2019/20.
George Municipality Civil Engineering Services Acting Director Lionel Daniels said the heavy rainfall and flash floods that occurred on Monday had understandably overshadowed the dam’s overflowing on the same day (22 November 2021), but it remained a significant milestone in the city’s long-term water security. “There are many reasons we are celebrating this project including an increased capacity of 25% by raising the full supply level of the dam by only 2.5 metres. The ingenious duckbill-shaped spillway design improved the dam’s safety in a practical and cost-effective way that earned the designers, Zutari (formerly Aurecon), a win in the Technical Excellence category at the South African Institute of Civil Engineering (SAICE) Awards in November 2020.
“The design is relatively new in the engineering world, and as far as we know is not being used in this way in South Africa. It was extensively researched and tested at the University of Tshwane and its shape is designed to slow down water flow using basic physics principles. The design is brilliant in its simplicity and it was a real victory to see the dam overflow and the spillway working so effectively,” said Mr Daniels.
The Garden Route Dam, situated just outside the north-east urban edge of George, is the main source of potable water for the city and its capacity had become a concern as the municipal area continued to grow exponentially since the early 2000s.
In addition, the Southern Cape had experienced two major droughts in seven years, which had been unheard of in this prolific all-year rainfall area. The dam recorded its lowest ever level of 16.9% in February 2010 and again dropped to 41,2% in September 2017.
The Garden Route Dam spillway and wall project was completed in January 2020 and increased the dam storage capacity by 25% from 10 million cubic metres to an estimated new capacity of 12,5 million cubic metres.
The previous Garden Route spillway was 25m wide, which was extended to 80m by curving the spillway in the upstream direction. The new spillway consists of a reinforced concrete cantilever structure, which is unusual for hydraulic structures of this type, as these are normally self-stable by their mass. To enhance the stability, the structure is provided with rockfill on the upstream side of the wall footing, in addition to rock anchors.
Although the full supply level (FSL) of the dam was raised by 2.5 m, due to the local terrain the tallest portion of the new spillway wall is 4.9 m tall. A total of 1,780 m3 of concrete was used (750 m3 mass concrete, 300 m3 for the walls, 390 m3 for the wall footing, and 340 m3 for the channel).
The main dam wall was also raised by 1.76m to prevent overtopping by placing earth fill on top of the existing embankment. Selected material was placed as follows: general fill (12,500 m3), rip-rap (2,500 m3), filter sand (750 m3), and topsoil (2,050 m3).
The Garden Route Dam overflowed for the first time since its spillway had been raised in 2019. The award-winning bill-shaped design increased the dam’s overall capacity by 25% by raising the spillway by only 2.5m.