Thesis (PhD)--Stellenbosch University, 2002. / 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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52716 |
| Date | 03 1900 |
| Creators | Armitage, Neil Philip |
| Contributors | Rooseboom, A., Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 1 v. (various pagings) : ill. |
| Rights | Stellenbosch University |
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