Bend diversion to minimise sediment intake

Brink, C.J. (2004-12)

Thesis (MScIng)--University of Stellenbosch, 2004.


ENGLISH ABSTRACT: The primary aim of the research was to determine the optimum diversion location in a curved channel to minimise the abstraction of sediment. The secondary aim was to determine the optimum diversion angle for a diversion channel located on the outside of a bend at the optimum diversion location. The velocity distribution in the curved channel was investigated to try obtain a better understanding of curvilinear flow. The scour patterns in the channel were monitored in order to compare them with the measured velocity distributions. Simulations were carried out with the DELFT 3D (hydrodynamics) and Mike21C (sediment dynamics) modelling programmes and compared with the results obtained from laboratory experiments and with existing empirical formulas. The optimum diversion location was found to be located on the outside of the bend in the downstream section of the bend. Three main scour zones were identified with the third scour zone at the location of the maximum velocity. The location of the maximum velocity was found to be relatively constant with varying Froude numbers, but moving in the downstream direction with increasing radius of curvature-to-width ratio. The velocity distributions in the horizontal and vertical planes are well defined and correspond to descriptions in the literature. It is evident that the diverted discharge ratio increases with an increase in the diversion angle while it decreases with an increase in Froude number. Higher Froude numbers in the curved channel lead to more favourable conditions for the diversion of water. The diversion does not influence the secondary flow patterns (for the range of Diversion Discharge Ratio’s tested) and that the maximum velocity zone stayed in the same location as in the tests without a diversion. The hydrodynamics of the laboratory experiments were well simulated with the DELFT 3D hydrodynamic model, using three-dimensional and two-dimensional formulations. Mike21C was used to simulate the sediment dynamics of some of thelaboratory experiments that gave relatively good agreement with experimental data. A two-dimensional depth averaged model could therefore be used with reliability to simulate field conditions in relatively shallow rivers, and is preferred to empirical methods to predict maximum scour that were calibrated under very specific hydraulic conditions.

AFRIKAANSE OPSOMMING: Die primêre doel van navorsing was om die optimum uitkeer-posisie in ‘n draai te bepaal om sodoende sediment onttrekking te minimiseer. Die sekondêre doel was om die optimum uitkeringshoek vir ‘n uitkeerkanaal te bepaal wat geleë is aan die buitekant van ‘n draai by die voorgestelde optimum uitkeer-posisie. Die snelheidsverspreiding in die draai was ook ondersoek om te probeer om spiraalvloei beter te verstaan. Die uitskuurpatrone in die kanaal is ook gemonitor om dit te kon vergelyk met die gemete snelheidsverspreiding. Numeriese simulasies is gedoen met DELFT 3D (hidrodinamika) en Mike21C (sediment dinamika) modelleringsprogrammatuur en is vergelyk met die resultate van die laboratorium eksperimente asook met die van bestaande empiriese vergelykings. Daar is gevind dat die optimum uitkeer-posisie aan die buitekant van ‘n draai aan die stroomaf-kant van die draai geleë is. Drie hoof uitskurings-areas is gevind terwyl die derde area ooreenstem met die posisie van maksimum snelheid. Die posisie waar die maksimum snelheid voorkom is relatief konstant met ‘n verandering in Froude-getal, maar beweeg in die stroomaf-rigting met ‘n styging in die radius-tot-wydte verhouding. Die vertikale en horisontale snelheidsverspreiding is goed gedefinieer en stem ooreen met soortgelyke beskrywings in die literatuur. Die uitkeer-vloei verhouding styg met ‘n stygende uitkeerhoek terwyl dit daal met ‘n styging in Froude-getal. Daar is ook gevind dat groter Froude-getalle meer gunstige omstandighede skep vir die uitkeer van water. Die uitkeer-kanaal beïnvloed nie die sekondêre vloei-patrone nie (vir die reeks van uitkeer vloei-verhoudings wat getoetsis) en die sone van maksimum snelheid bly in dieselfde omgewing vir hierdie toetse as vir die toetse sonder ‘n uitkeer kanaal. Die hidrodinamika van die laboratorium eksperimente is goed gesimuleer m.b.v die DELFT 3D numeriese program, terwyl Mike21C gebruik is om die sedimentdinamika te simuleer. Die resultate van die Mike21C simulasies vergelyk relatief goed met die eksperimentele data en kan met ‘n redelike graad van vertroue aangewend word om veldkondisies te simuleer in relatiewe vlak riviere. Dit word aanbeveel bo die empiriese vergelykings om maksimum uitskuring te voorspel aangesien die empiriese vergelykings gekalibreer is vir baie spesifieke hidroulise kondisies.

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