City of Cape Town mayoral committee member for transport, Brett Herron, has responded to a letter from Somerset West resident, Jenny Patchett (“Of floods and water”, Bolander November 1), about the flood alleviation work currently under-way along the Lourens River.
The original letter is available at www.bolanderlifestyle.co.za
This is a big project, and funding constraints, as well as the terms of the environmental approval from DEADP, particularly in that the City may only work in the river channel during certain times of the year, have required the project to be implemented in several phases over many years.
We should not confuse the current shortage of water with long-term flooding risk. Despite the fact that we are now in the midst of a 1:1 000 year low rainfall period – the worst drought in recorded history of the Western Cape – the medium- and long-term risk of flooding in certain areas of the city remains. The scarcity of water is a short-term impact associated with the current low rainfall period.
This does not have relevance for the protection of residents from flooding, because climate change shows that extreme weather events (both floods and droughts) are expected to become more frequent over time.
These predictions are borne out in a recent scientific review of our rainfall patterns (http://www.sajs.co.za/sites/default/files/publications/pdf/SAJS-113-7-8_deWaal_ResearchArticle.pdf). This study showed that the 1:20 and 1:50 year rainfall extremes are increasing at the majority of rainfall stations in the Western Cape. Flood alleviation and protection measures must therefore still continue to be implemented where necessary.
The flood that we experienced in 2013 was a 1:8 year flood. Even so, this does not mean that floods of an estimated return interval occur regularly at that precise interval.
Flood risks are expressed as a percentage of its probability per year – so a 1:50 year flood has a 1/50, or 2%, chance of occurring in any year.
This does not, however, mean that if a flood with such a return period occurs, then the next will occur in about 50 years’ time.
Instead, it means that in any given year, there is a 2% chance that it will happen, regardless of when the last similar event was.
The replanting of vegetation follows in strict accordance with the environmental approval and is based on an approved list of plants that are endemic to the river and are sourced and propagated from, among others, the river itself.
While it is true that those trees that are removed in order to increase the capacity of the flow channel are not replaced, this does not necessarily detract from the quality of the re-vegetation that will be done once the earthworks have been completed and the embankments have been reinstated.
Perhaps it is worth mentioning that after the completion of the flood alleviation measures between
Main Road and
Andries Pretorius Street, a study established that that the quality of the water on that section of river had improved dramatically if compared with the pre-construction state, as did the general health of the river from a biodiversity perspective, particularly as it pertains to the variability among living organisms in the aquatic ecosystems of the river channel.
The work in the river is far from done and it is only reasonable and logical
that it should be evaluated once the earthworks have been completed and the embankments have been reinstated.