The calibration of compound crump and sharp-crested gauging weirs in South Africa

[摘要] ENGLISH ABSTRACT:The present network of now gauging stations in South Africa has grown from isolatedobservations on an ad extensive is to an extensive network of stations across the country.Standardised gauging stations to suit local conditions have been developed which includepurposely desigll4:d compound weirs.Nearly all. compound gauging weirs In South Africa have. for practical reasons. beenconstructed without dividing walls. thus deviating lr',:.n the standards set by the BritishStandards Institution (1981). Uncertainty about the accuracy of calibration of such structureshad to be darifi,ed and. where ~cessary. adjustments had to be made to existing calibrationformulae in order to compensate for the de,,·iations. It was also necessary to determinewhether the accuracies that could be attained were adequate in terms of the potential financialimplications of inaccuracies. It has thus become necessary to re-evaluate the calibration ofthese structures which consist of mainly compound Crump .md sharp-crested weirs.Selected flow lt'Cords were analysed and the data wa.\ used to determine the impact of errorson the required capacities of reservoirs. This wa., done in an anempt to provide guidelines forthe accuracy required in flow records.An31ysing a single application of a flow record cannot provide guidelines for the requiredaccuracy of a flow record and thus the gauging of flow. Although no general conclusions canbe drawn. it appears thal the benefits arising from an improvement in the accuracy of a flowrecord are proportionally greater than the percentage improvement in accuracy.Three-dimensional flow conditions exist either upstream or downstream of the point of stagemeasurement depending on the presence or absence of diViding walls at a compound gaugingweir. The existing calibro1tion theory does not account for the influences of three-dimensionalflow conditions and a.-'isociatcd energy losses in the determination of the upstream total energyhead. Hydraulic model tests were thus undenaken to detemline the magnitude of the resultingenergy losses. New techniques were developed to compensate for these energy losses in thecalibration theory of compound gauging weirs.Application of the new calculation techniques to rate compound weirs using a single point ofstage measurement results in improvements in accuracy. It was found that compound weirs without dividing walls can be rated to greater levels of accuracy than weirs with dividing walls, where stage measurements are taken at a single point

[发布机构] Stellenbosch University