Extensive erosion has occurred at Big Bay in Bloubergstrand, Cape Town over the last decade. During 2017 the City of Cape Town implemented a managed retreat of the dunes and walkways at Big Bay. However, ongoing erosion has led to further loss of beach amenity and is putting infrastructure such as the lifesaving building at severe risk. PRDW was appointed by the City of Cape Town to undertake a coastal processes study to investigate the erosion at Big Bay.
The primary objectives of the study were the following:
- Investigate the causes of the erosion at Big Bay, specifically whether it is due to a change in wave and/or wind climate which is found to cause either continuous erosion or alternating cycles of erosion/accretion
- Based on the results of the above analysis and the observed historical shoreline trends, predict the likely future shoreline erosion/accretion trends at Big Bay and Small Bay over the short (2019), medium (2025) and long term (2030), e.g. identify whether the existing erosional trend is most likely to continue, accelerate, stabilise or reverse
The MIKE by DHI suite of numerical models was used to simulate the waves, water levels, currents, sediment transport and erosion at Big Bay over the 39-year period from 1979 to 2018.
The results of the modelling indicated the following key conclusions in relation to the sediment transport dynamics in Big Bay:
- The inshore area of Big Bay typically loses sand northwards during summer due to more southerly waves in this season. Sand tends to return to Big Bay during winter when the waves are typically more westerly
- Some winters have less westerly waves which leads to an annual imbalance, resulting in a net erosion
- Historically, the duration of the erosion cycle has been approximately four years and is followed by a stabilisation or recovery of the beach
- In addition to the four-year cycles, there has been a longer-term erosional trend over the last 20 years
- The data suggests a correlation between erosion at Big Bay and drought in Cape Town, which is consistent with both being linked to years with a below-average occurrence of north-westerly winter storms
S2001/101
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