Development of a customised design flood estimation tool to estimate floods in gauged and ungauged catchments

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engineers' dilemma to make a single, justifiable decision based on the results obtained from the plethora of 'outdated' design flood estimation methods available in South Africa.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, userfriendly 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.

Review of methods used to estimate catchment response time for the purpose of peak discharge estimation

“…The critical storm durations as required to estimate T P were obtained from Gericke (2010) and Gericke and Du Plessis (2013), who applied the SUH method in all the catchments under consideration. In each case, user-defined critical storm durations based on a trial-and-error approach were used to establish the critical storm duration which results in the highest peak discharge.…”

“…Both the weighted runoff curve numbers (CN), as required by equations (2), (8) and (A32), and weighted runoff coefficients, as required by equation (A4), were obtained from the analyses performed by Gericke and Du Plessis (2013). The catchment storage coefficients, as applicable to the HRU T L estimation method (equation (7)), were obtained from Gericke (2010), while the catchment storage coefficients applicable to the T L estimation methods of Snyder (1938) (equation (A16)), USACE (1958) (equation (A18)) and Bell and Kar (1969) (equation (A21)), were based on the default values as proposed by the original authors.…”

Development and Assessment of a Single-Event Design Flood Estimation Tool: Case Study in Four Climatological Regions of South Africa

“…The original DFET [2] was developed and automated using Microsoft Visual Basic for Applications (VBA). The integral part of automation is dependent on the development of the VBA project comprising of a set of modules which contains macros consisting each of a set of declarations followed by procedures or methods acting on toolbar controls.…”

“…The estimation of design floods characterised by a specific magnitude-frequency relationship at a particular site is necessary for the planning, design and operation of hydraulic structures [1]. Both the occurrence and frequency of floods, along with the uncertainty involved in the estimation thereof, contribute to the practitioners' dilemma to make a single, justifiable decision based on the results obtained from the various single-event design flood estimation methods available internationally [2].…”

“…Hence, in order to assess the uncertainty in using the latter design flood estimation methods, all approaches should be included in any specific design situation and not be limited to using only the more simplified methods, e.g. the +160-year old Rational method [2]. In order to overcome some of the inherent limitations of the current available methods and to enhance the practitioners' decision-making process, in conjunction with the overall objective of the National Flood Studies Programme (NFSP) in South Africa [5] to update and enhance design flood estimation methodologies, the Utility Programs for Drainage (UPD) software [6], [7] and Design Flood Estimation Tool (DFET) [2] were developed and are frequently updated.…”