The City of Cape Town has announced plans focused on prioritising upgrading the Trappies sewer in Strand as a result of its current condition.
The infrastructure will be reassessed before 2025 according to a City statement, which added the capacity issue will be addressed with the construction of the new Gordon’s Bay Sewer Pump Station at the Gordon’s Bay Wastewater Treatment Works. The latter is currently in the final stage of design and the planned start date for the project is in 2023-’24.
Since May, council has repaired six collapsed sewer pipes in Strand, the statement read. While there are various reasons for sewer collapses, the City is working hard to reduce the chances of sewer collapses in Strand. This is being done through a capacity assessment of the pipes, observation of previous incidents in the area, and the municpaility’s annual sewer replacement programme.
Over recent months collapsed sewers in De Villiers and De Ruyter streets have been repaired.
According to a 2015 condition and capacity assessment study completed on the Trappies sewer – about 900 mm diameter section in De Villiers Drive – the infrastructure does not have sufficient capacity for expected future flows. The study assessed the sewer’s existing and future capacity based on the City’s Sewer Master Plan.
“Based on the available information and observations on site, it is clear the sewer has corroded more rapidly than anticipated,” said Siseko Mbandezi, acting Mayoral Committee member for Water and Sanitation. “The City is monitoring the condition of the Trappies sewer, as any internal pressure or increased external pressure can cause a collapse.
He further confirmed, the 450 mm diameter portion of the sewer will be replaced via three projects at R22 million during the current financial year. “Preparatory work is currently under way and lining commencement is expected in January, as the liner needs to be imported,” explained Mbandezi.
During the 2021-’22 financial year, a total of 16 159 m of new sewer mains were installed and 4 089 m sewer mains replaced, the statement further read.
This was an increase in comparison to 2020-’21, when 967 m sewer mains were installed and 3 376 m replaced.
According to a local engineer tracking the City’s progress in addressing the challenges of sewer collapses, the frequent surcharging in the pipeline and problems at the Lourens River Pump Station have resulted in the fragile condition of the pipeline condition. He further said the numerous makeshift attempts to fix sinkholes and pipe breakages since 2019 further exasperated the problem.
“I understand the City plans slip lining of the Trappies sewer, which entails pulling a smaller diameter pipe through the existing pipeline. This might be problematic due to the makeshift repairs on the collapses all along this pipeline from Main Road to the Lourens River as well as the constant full flow conditions in the pipeline,” the engineer pointed out.
“This option would also further reduce the Trappies sewer flow capacity. The option of realigning the pipeline on a new alignment seems better.”
The Strand resident explained the new pipe could be installed in dry conditions and be connected onto the existing Trappies system once complete. “The old pipe could then be rehabilitated under much lower flow conditions and the diameter could then safely be reduced.”
The engineer added constructing the Gordon’s Bay Sewer Pump Station will remove the flows added to the Trappies system in 2009, but he believes the damage to the old pipeline, due to frequent overloading, has already been done.
“None of the works listed by the City relates to the section of the Trappies sewer where all the collapses are occurring,” he related. “When will the work to this section of the Trappies sewer be done, as this should be the priority due to all the collapses that are occurring?”
