Update on Garden Route Dam capacity
In a press statement released on Monday 11 September, Municipal Communications Chief Chantel Edwards-Klose said that the raising of the Garden Route Dam wall must be completed by 30 June next year.
She said the reconceptualisation of the project, as previously reported in the
George Herald, involved a further tender process for consultants, which concluded in January this year. Following administrative and further legal processes required by the municipal supply chain policy, the tender for the newly designed project was advertised on 7 September and will close on 9 October.
"The supply chain process still requires some prescribed activities which will require several additional weeks prior to the official start of the construction portion of the project, which time cannot be fixed just yet. However, it must be noted that this is a long project which must by special effort be completed by 30 June next year. The amount budgeted for this project is R12,8-million," said Edwards-Klose.
The redesign of the project required the environmental authorisation being reviewed accordingly. Inputs received during the EA process had to be dealt with. In the final design of the project, a dam safety plan had to be drawn up, which was approved by Water Affairs on 22 August.
The municipality applied for environmental authorisation in 2009 to raise the Garden Route Dam spillway by up to 3m and to install a fuse gate system (FGS) at the point of overflow after it had been determined that, with the hydrology data available then, this was the most favourable solution for increasing the dam's capacity.
"The authorisation for this activity was granted in March 2012. However, the authorised fuse gate alternative was no longer considered to be the most viable option, following a comprehensive review of the catchment hydrology when the Water Affairs permit was issued in December 2014," said Edwards-Klose.
The alternative of constructing a sidechannel was investigated to extend the flood overflow length and it was found to be the most effective in raising the dam capacity, while minimising any future damages risks caused by higher order floods, including those with 1:200-year return periods. For this option, a "duckbill"-shaped concrete spillway wall up to 100m long has to be constructed upstream of the existing spillway, connected to the existing retaining wall.
This design requires the dam wall overflow to be raised by 2,5m and the crest to be raised by 1,8m. This will provide sufficient freeboard to prevent overtopping of the embankment by wave action that may occur during extreme flood events. The material required to raise the dam wall will be sourced from within the dam.
The projected increase to the dam capacity is 25%, while the yield increase to the George water supply system would be about 11%. - Alida de Beer