Civil Engineering January-February 2022 | Vol 30 No 1

Civil Engineering  January/February 2022 39 DESIGN EXPLAINED The project increased the existing storage capacity of the Garden Route Dam by raising its full supply level by 2.5 m via the installation of a new spillway, enlarging the storage volume by 2.5 million m ³ . The solution centred on a sophisticated hydraulic design in the form of a novel, state-of-the-art, non-linear spillway in the shape of a duckbill. Although duckbill or bathtub spillways are not unique, they are rare worldwide. The Zutari team used its industry expertise to amend the hydraulic design to ensure that the weir would behave in a safe and predictable manner. The shape of the duckbill causes flow on opposite ends of the overflow to collide within the duckbill, causing an upwelling, also known as flow bulking, which breaks the flow’s momentum and causes it to lose its energy. The flow from the rounded upstream end of the spillway is not so opposed and causes this upwelling of flow to move toward the exit of the spillway at rapid velocity. This increase in velocity reduces the water level to a lower elevation such that it can safely pass under the bridge over the spillway. The final duckbill spillway design resulted in a total spillway length of 80 m, with a maximum discharge capacity of 570 m ³ /s at a freeboard of 4.1 m. The flood event on 22 November 2021 was the first time the new spillway overflowed since its completion in December 2019. Prior to the flood event, the water level in the dam was roughly 1 m below the crest of the spillway. This volume was rapidly filled up in the course of the early morning and the spillway started overflowing at roughly 08:30. The peak of the flood occurred at 13:00 according to the Department of Water and Sanitation’s water level-data record. The water level recorder logged a maximum overflow depth of 0.509 m over the crest, which equates to about 50 m ³ /s in discharge. It thus appears to have been a relatively minor event, with the dam designed to cater for much higher flows. That said, the incoming flood was partially attenuated or absorbed by the storage volume in the dam basin. The flows at the nearby flow logger on the Malgas River likewise recorded a minor flood.  The duckbill shaped spillway is designed to slow down water flow using basic physics principles (Credit: Wessel van Heerden) The new spillway overflowed on 22 November 2021 (Credit: Wessel van Heerden)