Garden Route Dam overflows for the first time

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GEORGE NEWS - George Municipality and the engineering community celebrated the success of the award-winning Garden Route Dam spillway last week as the dam overflowed for the first time since the spillway and dam wall were raised in 2019/20.

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, 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," said Daniels. "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.

He said the design is relatively new in the engineering world. "As far as we know is not being used in this way [anywhere else] 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."

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.

Photo gallery: Garden Route dam overflowing after heavy rain

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.

Photo: MC Lamprecht

Project details

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 rock-fill 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 2,5m, due to the local terrain the tallest portion of the new spillway wall is 4,9m high. A total of 1 780m³ of concrete was used (750m³ mass concrete, 300m³ for the walls, 390m³ for the wall footing, and 340m³ for the channel).

The main dam wall was also raised by 1,76m to prevent over-topping by placing earth fill on top of the existing embankment. Selected material was placed as follows: general fill (12 500m³), rip-rap (2 500m³), filter sand (750m³), and topsoil (2 050m³).

Photo: MC Lamprecht