Review: Continuous simulation modelling for design flood estimation – a South African perspective and recommendations

Tj Rowe,Jc Smithers

doi:10.4314/wsa.v44i4.18

Abstract

A number of severe flooding events have occurred both in South Africa and internationally in recent years. Furthermore, changes in both the intensity and frequency of extreme rainfall events have been documented, both locally and internationally, associated with climate change. The recent loss of life, destruction of infrastructure, and associated economic losses caused by flooding, compounded by the probability of increased rainfall variability in the future, highlight that design flood estimation (DFE) techniques within South Africa are outdated and in need of revision. A National Flood Studies Programme (NFSP) has recently been initiated to overhaul DFE procedures in South Africa. One of the recommendations in the NFSP is the further development of a continuous simulation modelling (CSM) system for DFE in South Africa. The focus of this paper is a review of CSM techniques for DFE, to guide further development for application in South Africa. An introduction to DFE, and particularly the CSM approach, is presented, followed by a brief overview of DFE techniques used in South Africa, leading into a more detailed summary of CSM for DFE within South Africa to date. This is followed by a review of the development and application of CSM methods for DFE internationally, with a focus on the United Kingdom and Australia, where methods have been developed with the intention of national scale implementation. It is important to highlight that there is a plethora of CSM methods available internationally and this review is not exhaustive; it focuses on and identifies some of the strengths and weaknesses of several popular methods, particularly those intended for national scale application, as the intended outcome from this review is to identify a path towards the development of a usable national scale CSM system for DFE in South Africa. Emphasis on a usable method is important, considering the reality that, despite promising results, numerous benefits, and national scale methods being developed, it appears that the CSM method for DFE is rarely used in practice.

Highlights

The assessment of flood risk by associating a flood event with a probability of exceedance or return period is the standard approach to design flood estimation (DFE) in most countries (Smithers, 2012; Kang et al, 2013)
In the past few years there has been a high prevalence of flooding both in South Africa and internationally, that has caused extensive damage and resulted in the loss of life (Alexander, 2002; Smithers, 2012; UNISDR, 2015; FloodList, 2016)
A continuous simulation modelling (CSM) approach that accounts for spatial differences in rainfall would be beneficial and appropriate in South Africa

Summary

Introduction

The assessment of flood risk by associating a flood event with a probability of exceedance or return period is the standard approach to design flood estimation (DFE) in most countries (Smithers, 2012; Kang et al, 2013). A recent review of flooding events reported in FloodList (2016) highlighted several large flood events across the globe in 2016 including, inter alia, Germany, Romania, China, Paris – France, the Ukraine, the United States, Belgium and Russia. Several of these floods at specific locations exceeded previous records (FloodList, 2016). During the same period the Western Cape experienced floods that affected more than 10 000 people, as reported by local disaster management officials (FloodList, 2016)

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Conclusion