Reliability and uncertainty in flow measurement techniques - some current thinking

Abstract Improvements in the quality and availability of flow measurement equipment are undoubtedly capable of enhancing the reliability and accuracy of the hydrometric data that we require. However much of the UK's hydrometric data is acquired by the tried and trusted methods that have remained the mainstay of flow monitoring for many years. Should the results provided by these established techniques always be so readily accepted given the range of assumptions on which they are based? Current meter gauging is the principle technique used for the establishment of stage discharge relationships in the UK. Either directly for the establishment of stage-discharge relationships in open channels, indirectly for calibration of flow measurement equipment (e.g. ultrasonic Doppler velocity meters) or as a means of verification of existing flow measurement structures. Recent projects involving current meter gauging techniques have provoked much thought as to the validity of established techniques and in particular the assumptions on which they are based. The chosen case studies highlight a number of projects where there have been questions regarding the reliability and uncertainty of the flow measurement techniques employed. The alternative approaches required to deal with such problems are also discussed.