Evaluation of the SDF method using a customised design flood estimation tool

[摘要] ENGLISH ABSTRACT: The primary aim of this study was to evaluate, calibrate and verify the SDF run-offcoefficients at a quaternary catchment level in the C5 secondary drainage region(SDF basin 9) and other selected SDF basins in South Africa by establishing thecatchment parameters and SDF/probability distribution-ratios. The probabilitydistribution-ratios were based on the comparison between the flood peaksestimated by the SDF method and statistical analyses of observed flow data.These quaternary run-off coefficients were then compared with the existingregional SDF run-off coefficients, whilst the run-off coefficient adjustment factorsas proposed by Van Bladeren (2005) were also evaluated.It was evident from this study that the calibrated run-off coefficients obtained arespread around those of Alexander (2003), but were generally lower in magnitude.The adjusted run-off coefficients (Van Bladeren, 2005) had a tendency todecrease in magnitude with increasing recurrence interval, whilst some of theadjusted run-off coefficients exceeded unity.The extent to which the original SDF method overestimated the magnitude andfrequency of flood peaks varied form basin to basin, with the SDF/probabilitydistribution-ratios the highest in the Highveld and southern coastal regions withsummer convective precipitation. In these regions the flood peak-ratios wereoccasionally different by up to a factor of 3 or even more. The southern coastalregions with winter orographic/frontal precipitation demonstrated the best floodpeak-ratios, varying from 0.78 to 1.63.The adjusted SDF method results (Van Bladeren, 2005) were only better in 26%of all the basins under consideration when compared to those estimated by theoriginal SDF method. On average, the adjusted SDF/probability distribution-ratiosvaried between 0.30 and 6.58, which is unacceptable.The calibrated version of the SDF method proved to be the most accurate in allthe basins under consideration. On average, the calibrated SDF/probability distribution-ratios varied between 0.85and 1.15, whilst at some basins and individual return periods, less accurateresults were evident.Verification tests were conducted in catchments not considered during thecalibration process with a view to establish whether the calibrated run-offcoefficients are predictable and to confirm that the method is reliable. Theverification results showed that the calibrated/verified SDF method is the mostaccurate and similar trends were evident in all the basins under consideration. Onaverage, the verified SDF/probability distribution-ratios varied between 0.82 and1.19, except in SDF basins 6 and 21 where the 5 to 20-year return period floodpeaks were overestimated by 41% and 56% respectively, which is stillconservative.The secondary aim of this study was to develop a customised, user-friendlyDesign Flood Estimation Tool (DFET) in a Microsoft Office Excel/Visual Basicfor Applications environment in order to assess the use and applicability of thevarious design flood estimation methods.The developed DFET will provide designers with a software tool for the rapidinvestigation and evaluation of alternative design flood estimation methods eitherat a regional or site specific scale. The focus user group of the application willcomprises of engineering technicians, engineering technologist and engineersemployed at civil engineering consultants, not necessarily specialists in the field offlood hydrology. The DFET processed all the catchment, meteorological(precipitation) and hydrological (observed flows) data used as input for the variousdesign flood estimation methods.

[发布机构] Stellenbosch University