Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Author "Armitage, Neil Philip"

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    A unit stream power model for the prediction of local scour
    (Stellenbosch : Stellenbosch University, 2002-03) Armitage, Neil Philip; Rooseboom, A.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    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.