Quantification of risks to coastal areas and development: wave run-up and erosion

Abstract

In support of the effective implementation of the Integrated Coastal Management Act (Act No 24 of 2008), a review is presented of coastal hazard assessment methods. In particular the ICM Act legislates the establishment or change of coastal setback lines to protect coastal public property and private property, amongst others. In this paper a practical method applicable to Southern African conditions, and the available data, was identified and further adapted to include additional forcing factors considered to be most relevant under Southern African conditions. In South Africa, the most important drivers of risk to coastal infrastructure from erosion and flooding are waves, tides and future sea level rise. It is the combination of extreme events (sea storms occurring during high tides in conjunction with sea level rise) that will have the greatest impacts and will be the events that increasingly overwhelm existing infrastructure. Appropriate extreme values of wave conditions and tidal levels were combined with reasonable scenarios of sea level rise. Practical methods were developed to model wave run-up based on these inputs and combined with simple methods of estimating erosion due to sea level rise. Further interpretation of these outputs enabled mapping of vulnerable areas and a local test case was conducted to demonstrate the outcomes. The results were incorporated into a Geographic Information System (GIS) database and mapped using fine scale elevation data to spatially depict the results. An output of this study is thus a methodology for assessing, and predicting, wave run-up lines which include the effects of long-term erosion due to sea level rise. Coastal areas within Table Bay near Cape Town were selected to illustrate how such run-up calculations can be used to identify present and future vulnerable areas. It is believed that this approach will be useful in assessing and mapping vulnerable coastal areas in South Africa and to contribute to the determination of future coastal development setback lines as defined in the ICM Act.