A unit stream power model for the prediction of local scour

Armitage, Neil Philip (2002-03)

Thesis (PhD)--Stellenbosch University, 2002.

Thesis

ENGLISH ABSTRACT: Local scour is the erosion of a riverbed resulting from the flow of the river around an obstacle. It is a principal cause of failure of bridges and other hydraulic structures. Current design practice relies on the use of empirical formulae that are often extremely inaccurate, or on the use of physical models that are very expensive. Recent advances in the power of microcomputers have however made numerical simulation increasingly attractive. One obstacle to numerical simulation though is that there is no general agreement on the concept of incipient motion, that critical point at which motion - and hence scour - begins. In this dissertation, the unit stream power model developed by Rooseboom (1992) is extended to handle the complex three-dimensional flow conditions that pertain close to the riverbed in the vicinity of an obstacle. The relationship between unit stream power (the dissipation function) and the Movability Number (the ratio of the shear velocity to the terminal settling velocity of the critical sediment particles) is clearly indicated. Since incipient motion is probabilistic in nature, a relationship was established between the Movability Number and the intensity of motion with allowance for bed-slope and relative depth. An extension of this work resulted in a new bed-load transportation equation that could be used to determine the rate of scour development. Physical modelling in a laboratory flume aided the selection of suitable critical conditions for the onset of scour. The usefulness of the above-mentioned relationships was then demonstrated through the construction of a simple mathematical model of scour and deposition around a structure. This model was used in conjunction with commercially available computational fluid dynamics (CFD) software to predict the scour potential around typical engineering structures. Physical model data was obtained for four situations, and the measured scour was compared with that predicted by the numerical model. There was reasonable agreement between the different models and such differences as there were could be readily attributed to constraints on the numerical model, in particular the lack of a free-surface routine and the coarseness of the grid. This dissertation has opened up a new method for the prediction of local scour that could be readily extended to include all types of scour. With the advent of increasingly fast computers, it could become a useful engineering tool that would assist engineers in the design of safe and cost-effective foundations for hydraulic structures.

AFRIKAANSE OPSOMMING: Plaaslike uitskuring is die erosie van 'n rivierbed as gevolg van vloei verby 'n obstruksie. Dit is 'n belangrike oorsaak van die swigting van brfïe en ander hidrouliese strukture. Bestaande ontwerppraktyk berus op empiriese vergelykings wat dikwels hoogs onakkuraat is, of op fisiese modelle, wat baie duur is. Numeriese simulasie het die afgelope tyd 'n al hoe meer aantreklike opsie geword danksy die snelle toename in die kapasiteit van mikro-rekenaars. 'n Struikelblok met numeriese simulasies is die gebrek aan konsensus oor die konsep van begin-van-beweging, daardie kritieke toestand waarby beweging en derhalwe uitskuring begin. In hierdie proefskrif is die eenheidstroomdrywing model, ontwikkel deur Rooseboom (1992), uitgebrei om die komplekse drie-dimensionele vloeitoestande, wat teenaan die rivierbodem verby 'n obstruksie heers,te hanteer. Die verwantskap tussen Eenheid Stroomdrywing (Dissipasiefunksie) en die Beweeglikheidsgetal (verhouding tussen sleursnelheid en die ewewigvalsnelheid van die kritieke sedimentpartikels ) is duidelik uitgewys. Aangesien begin van beweging probabilisties van aard is, is die verwantskap bepaal tussen die Beweeglikheidsgetal en die Intensiteit van Beweging, met voorsiening vir bodernhelling en relatiewe diepte. Verdere uitbreiding het gelei tot 'n nuwe bedvrag vervoervergelyking wat gebruik kan word om die tempo van uitskuring te bepaal. Kritieke toestande, waarby uitskuring begin, is met fisiese modelle in die laboratorium gekwantifiseer. Die bruikbaarheid van bogenoemde verbande is gedemonstreer deur die ontwikkeling van 'n eenvoudige wiskundige model van uitskuring en afsetting rondom 'n struktuur. Hierdie model is saam met bestaande kommersiële sagteware vir vloeidinamika berekenings (CFD) ingespan om uitskuringspotensiaal rondom tipiese ingenieurstrukture te voorspel. Fisiese modelmetings van uitskuring vanaf vier uitlegte is vergelyk met die numeries voorspelde waardes. Bevredigende ooreenkoms is gevind en verskille kon geredelik gewyt word aan beperkings van die numeriese model, veral die gebrek aan' n vryvlakroetine en die growwe maas. Die proefskrif stel 'n nuwe metode vir die voorspelling van uitskuring daar wat geredelik uitgebrei kan word na ander vorms van uitskuring. Met die ontwikkeling van al vinniger rekenaars kan dit 'n nuttige hulpmiddel vir ingenieurs word om veilige en koste-doeltreffende fondamente in waterlope te ontwerp.

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