Garden Route Dam spillway and embankment crest raising
Raising of the Garden Route Dam spillway and embankment crest
The Garden Route Dam is the major source of drinking water in George. In 2018, Zutari was appointed by the George Local Municipality to raise the dam, and had to devise a novel spillway to overcome unique technical challenges.
The solution that was developed included a duckbill spillway, so called due to its shape, which not only allowed for the dam’s existing storage capacity to be increased by 25% but also significantly increased the discharge capacity of the spillway, thus improving the dam’s safety.
A duckbill spillway is a type of non-linear spillway. The main goal is to increase the overflow length in such a way that the spillway can pass more flow for a given overflow depth. This generally involves the construction of very long spillways, typically four to five times longer than a linear spillway, in a limited area. The existing Garden Route Dam spillway was only 25 m wide, and this distance was extended to 80 m 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).
Expanding an existing water-supply resource is also preferable to the development of new sites, as it limits the extent of the environmental impact to an already impacted site
The main dam wall was also raised by 1.76 m to prevent overtopping by placing earth fill on top of the existing embankment. This task had to be achieved in confined spaces, atop high slopes, mostly using available material from the dam basin. 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 2.5 m raising of the FSL of the dam equates to an increase in the storage capacity of 2.5 million m3 to a total gross storage capacity of 12.5 million m3.
“This additional storage will add much-needed drought resilience to the water-supply system. Expanding an existing water-supply resource is also preferable to the development of new sites, as it limits the extent of the environmental impact to an already impacted site,” comments Frank Denys, Zutari Water Engineer.
*The Aurecon Africa business has been officially renamed Zutari as at 21 July 2020. Zutari acquired Aurecon Middle East on 20 November 2020.