[Wikipedia: “weir is a barrier across a river designed to alter its flow characteristics.”]
Speaking at the official handover ceremony of the weir to the Knysna Municipality, Knysna’s Mayor said the weir was an innovative approach to a range of water supply and environmental issues.
“The weir was primarily built to keep salt water out at the abstraction point of our fresh drinking water to reduce contamination, but its design also perfectly addresses other issues such as the need for more fresh water storage capacity and reduced risk of flooding at the nearby low water bridge.
“The brilliance of this solution lies with the stainless steel gates that will open and close automatically depending on the water levels and salinity. The ‘open’ weir gates will float when the river is flowing normally and in flood conditions, but will close automatically when the river stops flowing or when the level of the saline in the water in the Swartvlei rises to a certain level. The risk to human life is also reduced as no manual operation is needed during flood conditions.”
Situated about 10km outside Sedgefield near a low water bridge also known as Jan Sak se Drif, the weir is an integral part of the long-term supply plan of potable water for Sedgefield and surrounds – an area which serves an estimated 11 200 permanent residents and up to an additional 14 000 visitors in the peak summer season.
Knysna’s Municipal Manager said that issues surrounding potable water supply had become increasingly prominent with the economic boom of the Garden Route from 2002 onwards. “The sources of water in the Knysna municipal area are all rivers, which are always vulnerable, especially in times of drought. On-going growth of our towns and the region placed additional strain on our resources and investigations into various options had been initiated.”
The matter came to a serious head when the region’s longest drought in human memory eventually led to Sedgefield running out of potable water in January 2008. The town had water trucked in and made several emergency adjustments to resolve the matter in the interim. Another major concern was the high levels of salinity in the Karatara River and Swartvlei as reduced flow from the rivers could not sufficiently dilute salty water that pushed up from the sea. High salt content in raw water does not only pose bacterial health risks and has the potential to damage water treatment equipment, it is also harmful to fauna and flora in and around rivers.
The municipality initially considered the building of a dam in the Karatara River, but the financial implications (R130-million estimate in 2008) and environmental impacts rendered it not viable. “A more integrated solution for Sedgefield, including several boreholes and a desalination plant, was submitted to the Department of Water Affairs in 2010. It included a specially designed weir that would keep salt water out but also trap enough fresh water to supply the entire Sedgefield community for 10 days – and with it sufficient time to activate the town’s other conjunctive water sources should it become necessary.”
The mayor said it took nearly three years of planning, environmental investigation, consultation and public participation to finally open the weir. “It sounds like a long time to get 80 metres of cement in a line but it actually signifies the dedication and determination with which a complex set of issues were addressed. This project was exceptionally well-managed and is a true testimony to how government, community and business can work together towards sustainable infrastructure solutions. The result is an environmentally sensitive, water collecting and protecting structure that will keep more water fresher for longer.
“Just as a small, fun example, one of the requirements for this design was that provision be made for fish that swim upstream to spawn, and hence a fish ladder was subsequently implemented for just this purpose,” said Ms Wolmarans.
Seventy-five percent of the R7,1-million weir was paid for by a Municipal Infrastructure Grant (MIG), which means the cost to local ratepayers was relatively little and in exchange for infrastructure that will significantly contribute towards a secure, permanent, uncontaminated supply of water for the community of Sedgefield and surrounds until at least 2030.
The Mayor said there were also indirect economic benefits that may come from the establishing of the weir. “The issues surrounding water supply and flooding have influenced the property market in Sedgefield in particular. We believe that in the long term, the on-going successful rollout of the Comprehensive Water Plan for Sedgefield will address this sufficiently and set prospective property owners at ease.”
The mayor thanked the wide range of rolepayers including :
Local community members who had assisted in the selection and provision of labour during the construction phase of this project.
- Interested and affected parties: The Departments of Water Affairs, Environmental Affairs, Public Works (Roads), SANParks and PG Bison (Forestry).
- Tuiniqua Consulting Engineers for the innovative design and efficient management of this project.
- Cape EA Prac for undertaking the EIA process and on-site monitoring of environmental compliance.
- Brian Dalton OHS for monitoring the Occupational Health and Safety on site.
- Amanziflow Products for the specialist design of the estuarine gates.
- B & V Construction for the neat and timeous construction.
- Outeniqua Labs for the geotechnical investigations.
- Western Cape Government for providing the MIG funding.
- Knysna Municipality Officials in the Technical Services Departments in Sedgefield and Knysna.
- Knysna Municipality Councillor Louise Hart, the Portfolio Committee Chairman Planning Development and Infrastructure, and Ward 2 Councillor for Sedgefield and Karatara.
- Knysna Municipality Special Projects Manager Rodney Nay