Thesis (M.Eng.)--Stellenbosch University, 2000. / ENGLISH ABSTRACT: This study entailed the investigation and evaluation of a new methodology for measuring high discharges passing through bridges. Pressure differences generated around bridge piers have been measured and related to the discharges. These pressure differences are mainly functions of downstream flow conditions. The pressure differences have been converted into velocities by applying Newton's second law expressed in terms of the laws of conservation of energy; momentum; and of power. The energy principle was re-evaluated following a preVIOUSstudy (Retief, 1999) on a limited number of model pier combinations and flow conditions. Comparison of the energy approach with newly developed theories in terms of the momentum and power laws respectively led to the conclusion that the energy principle gave the best results. The question of applicability of the theory to practical pier/stream width and length ratios as well as its validity under flow conditions commonly found under flood conditions required that additional laboratory tests be done. The energy-based discharge equation was calibrated in terms of newly selected measuring points, different pier width and length ratios, as well as pier rotations for both super and sub-critical downstream conditions. According to the new tests performed at the Hydraulics Laboratory of the University of Stellenbosch on model piers, clear relationships were found between discharges and pressure differences measured against the pier. Calibration curves for practical flow measurement application were derived in terms of principle dimensionless parameters. Application of the energy approach at the prototype level needs further investigation. / AFRIKAANSE OPSOMMING: Hierdie studie het behels die ondersoek en evaluering van 'n nuwe metode om hoë vloeie verby brïïe te meet. Drukverskille wat rondom brug pylers opgewekword is gemeet en omgeskakel na vloeie. Hierdie drukverskille is hoofsaaklik funksies van die stroomaf vloei toestande. Die drukverskille is omgeskakel na vloeisnelhede deur die toepassing van Newton se tweede wet, uitgedruk in terme van die behoud van energie, momentum so wel as drywing. Die energie beginsel is weer geëvalueer sedert 'n vorige studie (Retief, 1999) gedoen is op 'n beperkte aantal model pylerkombinasies en vloeitoestande. Hierdie energie-benadering is met nuwe metodes vergelyk, naamlik die momentum en drywings wette. Die gevolgtrekking is gemaak dat die energie metode die beste resultate gee. Die vraag oor die toepaslikheid van die teorie met praktiese pyler/stroom wydte en lengte verhoudings so wel as die toepaslikheid hiervan onder die vloei toestande wat algemeen onder vloedtoestande voorkom het addisionele laboratoriumtoetse vereis. Die energie gebaseerde vloeivergelyking is gekalibreer in terme van nuut geselekteerde meetpunte, verskillende pyler wydte en lengte verhoudings, asook pyler rotasies vir beide super en sub-kritiese stroomaf toestande. Na aanleiding van die toetse gedoen in die Hidroulika Laboratorium van die Universiteit van Stellenbosch op brugpylers, is duidelike verbande gekry tussen die vloeie en die drukverskille soos gemeet teen die pyler. Kalibrasie-kurwes vir die toepassing van praktiese vloeimeting is ontwikkel in terme van die toepassing van dimensielose parameters. Die toepassing van die energiebenadering op prototipe vlak verg verdere ondersoek.