Abstract
The estimation of design flood events, i.e., floods characterised by a specific magnitude-frequency relationship, at a particular site in a specific region is necessary for the planning, design and operation of hydraulic structures. Both the occurrence and frequency of flood events, along with the uncertainty involved in the estimation thereof, contribute to the practising engineers’ dilemma to make a single, justifiable decision based on the results obtained from the plethora of ‘outdated’ design flood estimation methods available in South Africa. The objectives of this study were: (i) to review the methods currently used for design flood estimation in South Africa for single-site analysis, (ii) to develop a customised, user-friendly Design Flood Estimation Tool (DFET) containing the latest design rainfall information and recognised estimation methods used in South African flood hydrology, and (iii) to demonstrate the use and functionality of the developed DFET by comparing and assessing the performance of the various design flood estimation methods in gauged catchments with areas ranging from 100 km² to 10 000 km² in the C5 secondary drainage region, South Africa. The results showed that the developed DFET will provide designers with an easy-to-use software tool for the rapid estimation and evaluation of alternative design flood estimation methods currently available in South Africa for applications at a site-specific scale in both gauged/ungauged and small/large catchments. In applying the developed DFET to gauged catchments, the simplified ‘small catchment’ (A ≤ 15 km²) deterministic flood estimation methods provided acceptable results when compared to the probabilistic analyses applicable to all of the catchment sizes and return periods, except for the 2-year return period. Less acceptable results were demonstrated by the ‘medium catchment’ (15 km² < A ≤ 5 000 km²) deterministic and ‘large catchment’ (>5 000 km²) empirical flood estimation methods. It can be concluded that there is no single design flood estimation method that is superior to all other methods used to address the wide variety of flood magnitude-frequency problems that are encountered in practice. Practising engineers’ still have to apply their own experience and knowledge to these particular problems until the gap between flood research and practice in South Africa is narrowed by improving existing (outdated) design flood estimation methods and/or evaluating methods used internationally and developing new methods for application in South Africa.Keywords: Design flood, design rainfall, estimation, ungauged catchments, flood magnitude-frequency
Highlights
The estimation of design flood events, i.e., floods characterised by a specific magnitude-frequency relationship, at a particular site in a specific region is necessary for the planning, design and operation of hydraulic structures (Pegram and Parak, 2004)
The objectives of this study were: (i) to review the methods currently used for design flood estimation in South Africa for single-site analysis, (ii) to develop a customised, user-friendly Design Flood Estimation Tool (DFET) containing the latest design rainfall information and recognised estimation methods used in South African flood hydrology, while taking cognisance of the practising engineer’s dilemma to make a single, justifiable decision using the plethora of ‘outdated’ design flood estimation methods locally available, and (iii) to demonstrate the use and functionality of the developed DFET, by comparing and assessing the performance of the various design flood estimation methods in gauged catchments with areas ranging from 100 km2 to 10 000 km2 in the C5 secondary drainage region, South Africa
This paper’s literature review focuses on the use of design flood estimation methods at a single site, with the anticipated focus user group for the developed DFET comprising of general civil engineering technicians, engineering technologists and engineers employed at consultancies, who are not necessarily specialists in the field of flood hydrology who would be more likely to follow a regional approach
Summary
The estimation of design flood events, i.e., floods characterised by a specific magnitude-frequency relationship, at a particular site in a specific region is necessary for the planning, design and operation of hydraulic structures (Pegram and Parak, 2004). Design flood estimations are likely to display relatively wide magnitudefrequency bands of uncertainty (Alexander, 2002) Both the occurrence and the frequency of flood events, along with the uncertainty involved in the estimation thereof, contribute to the practising engineers’ dilemma to make a single, justifiable decision based on the results obtained from the plethora of ‘outdated’ design flood estimation methods available in South Africa. Most of these ‘outdated’ design flood estimation methods were developed in the 1970s, with some still reliant on graphical procedures.
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