The influence of wave action on coastal erosion along Monwabisi Beach, Cape Town

Abstract

The coastline to the east of the Monwabisi Beach tidal pool has been subject to substantial visible coastal erosion. Monwabisi is located near the City of Cape Town and is situated along the northern coastline of False Bay. The erosion that has occurred has raised concern as it is damaging local infrastructure. The coastline retreat to the east of Monwabisi was investigated through analysis of aerial photographs and wave data to establish whether there is a relationship between dynamic wave action that this bay is subjected to, and the observed erosion within the study area. The maximum lateral coastal erosion at the Monwabisi study area from 2003 until 2014 was approximately 30m in a landward direction. Based on the correlation of the results between the rate of coastline retreat and the wave data, the study has concluded that the extreme rates of coastline retreat experienced within the study area at Monwabisi is most likely a direct result of a combination of influences including the number and height of big wave events, waves coming from a more southerly direction, the underlying geological substrate of the study area, and the impact that the local infrastructure has had on the geological substrate. Three time periods of maximum monthly erosion rates occurred from June 2008 to November 2009, March 2010 to March 2011 and April 2011 to June 2012.