Methods for design flood estimation in South Africa

Smithers, J.

doi:10.4314/wsa.v38i4.19

Cited by 46 publications

(97 citation statements)

References 40 publications

(24 reference statements)

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“…Alexander (2002) and Smithers and Schulze (2003) highlighted the need for alternative approaches to DFE, or the development and improvement of conventional DFE techniques, after severe flooding in Southern Africa in 1999 and 2000. Smithers (2012) supports the comments of Alexander (2002) (Kusangaya et al, 2014), which will impact on the estimation of design floods and reinforce the need for improved approaches to DFE. In addition, after a review of flood frequency estimation techniques and approaches in Europe, The European Cooperation in Science and Technology (COST, 2013) highlighted that in most countries flood frequency estimation is currently being undertaken using models which assume that historical records of flood flows or rainfall are stationary, i.e., the statistics of the distributions do not change with time.…”

Section: Introductionmentioning

confidence: 74%

“…This involves the assessment of flood risk by associating a flood event with a probability of exceedance or return period (Smithers, 2012;Kang et al, 2013). Alexander (2002) and Smithers and Schulze (2003) highlighted the need for alternative approaches to DFE, or the development and improvement of conventional DFE techniques, after severe flooding in Southern Africa in 1999 and 2000.…”

Section: Introductionmentioning

confidence: 99%

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Development and assessment of rules to parameterise the ACRU model for design flood estimation

Smithers

Rowe

Horan

et al. 2018

WSA

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Design flood estimation (DFE) is essential in the planning and design of hydraulic structures. In South Africa, outdated methods are widely applied for DFE. In this paper the potential of a continuous simulation modelling (CSM) approach to DFE in South Africa, using the daily time-step ACRU agrohydrological model, is investigated. The paper focuses on the links and similarities between the SCS-SA and ACRU models and the subsequent preliminary investigations that were undertaken to account for and incorporate the land cover classes, including land management practices and hydrological condition, of the SCS-SA model into the ACRU CSM approach. The approach to this study was to investigate how design volumes simulated by the SCS-SA model for various land management practices or conditions could be simulated by the ACRU model. Since peak discharge estimation in both models is directly dependent on simulated volumes, this preliminary study focused only on design runoff volumes, with subsequent investigations on peak discharge required in future research. In the absence of observed data, design runoff volumes and changes in design runoff volumes, as simulated by the SCS-SA model, were used as a substitute for observed data, i.e., as a reference, to achieve similar design runoff volumes and changes in design volumes in the ACRU model. This was achieved by adjusting relevant input parameters in the ACRU model to represent the change in management practice or hydrological condition, as represented in the SCS-SA model. Following a sensitivity analysis of relevant ACRU parameters, calibration of 2 selected parameters against SCS-SA CN values for selected land cover classes was performed. A strong linear relationship (R 2 = 0.94) between these ACRU parameters and SCS-SA CNs for selected land cover classes was found and consequently specific rules and equations were developed to represent SCS-SA land cover classes in ACRU. Recommendations are made to further validate and verify the approach and to further the development of a CSM system for DFE in South Africa.

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Section: Introductionmentioning

confidence: 74%

Section: Introductionmentioning

confidence: 99%

Development and assessment of rules to parameterise the ACRU model for design flood estimation

Smithers

Rowe

Horan

et al. 2018

WSA

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“…16 The traditional approach to extreme value analysis in South African rainfall analyses has been to use the annual maximum series. 5,17,18 The block maxima approach relies on identifying the highest value for each year and fitting a distribution to the data, while the POT approach fits a distribution to all data points that exceed a defined threshold. 16 The advantage of the block maxima approach is its relative simple application; however, this method results in a substantial loss of information resulting from discarding values that are less than but close to the maximum in each year.…”

Section: Methods and Datamentioning

confidence: 99%

Extreme 1-day rainfall distributions: Analysing change in the Western Cape

Waal¹,

Chapman²,

Kemp³

2017

S. Afr. J. Sci.

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ARTICLE INCLUDES:× Supplementary material × Data set FUNDING:None Severe floods in the Western Cape Province of South Africa have caused significant damage to property and infrastructure over the past decade (2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014). The hydrological design criteria for exposed structures and design flood calculations are based mostly on the implicit assumption of stationarity, which holds that natural systems vary within an envelope of variability that does not change with time. This assumption was tested by examining the changes in extreme 1-day rainfall high percentiles (95th and 98th) and both the 20-and 50-year return period rainfall, comparing the period 1950-1979 against that of 1980-2009 across the province. A generalised Pareto distribution and a peaks-over-threshold sampling approach was applied to 76 rainfall stations across the province. Of these stations, 48 (63%) showed an increase in the 50-year return period 1-day rainfall and 28 (37%) showed a decrease in the 1980-2009 period at the 95th percentile peaks-over-threshold. At the 98th percentile peaks-overthreshold, 49 stations (64%) observed an increase and 27 (36%) a decrease for the later period. The change in the number of 3-day storms from the first to the second period is negligible, evaluated at 0.9% and 0.5% at the 95th and 98th percentile peaks-over-threshold levels, using cluster analysis. While there is no clear spatial coherency to the results, the general trend indicates an increase in frequency of intense rainfalls in the latter half of the 20th and early 21st centuries. These results bring into question assumptions of stationarity commonly used in design rainfall. Significance:• 63% of analysed rainfall stations in the Western Cape display an increase in 20-and 50-year 1-day rainfall extremes.• The results challenge the current assumptions of climate stationarity made in design rainfall estimations.• We propose an alternative methodology to rainfall extremes analysis for design flood estimation.• The methods employed can be replicated by future studies in other regions.

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“…SANRAL (1986) identifies empirical and statistical, as well as the Rational, SCS, run-hydrograph and synthetic unit hydrograph approaches, as appropriate and promising methods for estimating design floods in South Africa. As reviewed by Smithers (2012), more recently developed methods for design flood estimation in South Africa include a probabilistically-based Rational Method, termed the Standard Design Flood (SDF), developed by Alexander (2002a;2002c;2002d), the Joint Peak-Volume (JPV) design flood methodology developed by Görgens (2007), and the Regional Estimation of Extreme Flood Peaks by Selective Statistical Analyses (REFSSA) method developed by Nortje (2010).…”

Section: Introductionmentioning

confidence: 99%

“…Most of the methods used for design flood estimation in South Africa were developed in the late 1960s and early 1970s, as summarised in HRU (1972), and are in need of updating, with more than 40 years of additional data currently available and with new approaches used internationally (Alexander, 2002b;Smithers and Schulze, 2003;Görgens, 2007;Van der Spuy and Rademeyer, 2010;Smithers, 2012). Software to implement design flood procedures currently used in South Africa has been developed (e.g.…”

Section: Introductionmentioning

confidence: 99%

Development and assessment of a daily time-step continuous simulation modelling approach for design flood estimation at ungauged locations: ACRU model and Thukela Catchment case study

Smithers¹,

Chetty²,

Frezghi³

et al. 2013

WSA

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The estimation of design floods is necessary for the design of hydraulic structures. Commonly used event-based approaches to design flood estimation have several limitations, which include the estimation of antecedent soil moisture conditions and the assumption that the exceedance probability of the design flood is the same as the exceedance probability of the design rainfall. Many of the limitations of event-based approaches may be overcome by the use of continuous simulation modelling for design flood estimation. This paper contains a brief summary on the development and assessment of a continuous simulation modelling system for design flood estimation in ungauged catchments. These developments include an investigation into the appropriate spatial scale of model configuration for optimum performance of the system, the temporal disaggregation of daily rainfall for hydrograph generation, flood routing in ungauged catchments and the use of radar information and rain-gauge data to improve the estimation of catchment rainfall. Results from the application of the system for design flood estimation in the Thukela Catchment in South Africa are presented and discussed. The results from the study highlight the challenges of hydrological modelling in an operational catchment and the need for reliable rainfall and runoff data. From the results obtained, it is concluded that reasonable and consistent estimates of design floods in the Thukela Catchment, particularly in smaller sub-catchments, can be obtained using the ACRU model.

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Methods for design flood estimation in South Africa

Cited by 46 publications

References 40 publications

Development and assessment of rules to parameterise the ACRU model for design flood estimation

Development and assessment of rules to parameterise the ACRU model for design flood estimation

Extreme 1-day rainfall distributions: Analysing change in the Western Cape

Development and assessment of a daily time-step continuous simulation modelling approach for design flood estimation at ungauged locations: ACRU model and Thukela Catchment case study

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