Incipient motion in cobble/boulder bed rivers

Gazendam, Menno (2005-12)

Thesis (MScEng)--University of Stellenbosch, 2005.

Thesis

ENGLISH ABSTRACT: This study sets out to describe the incipient motion process in cobble/boulder bed rivers in terms of the unit applied power approach. This objective has been met through the collection of data on stone movement from a total of thirteen flood events observed in two undisturbed rivers in the Western Cape, namely the Molenaars and Berg Rivers. The data were plotted on the original modified Liu diagram for incipient motion and it was found that the threshold of movement for the smaller stones did not conform with the portion of the Liu diagram which represents fully turbulent flow (i.e. where ~ gDs tv: = 0.12 for Re. >13). It was concluded that the only reason that could explain this deviation is the fact that the original modified Liu diagram had been derived for uniform particle size beds while the data of the Molenaars and Berg Rivers represent non-uniform particle size beds. This was proved through re-deriving the y-axis function of the original modified Liu diagram to include a factor that makes provision for the roughness of a non-uniform particle size bed. It was found that the average absolute roughness of the non-uniform particle size beds in the Molenaars and Berg Rivers is reasonably well approximated by a value of k=da. Design curves (in terms of the original modified Liu diagram parameters) for intensity of motion were also produced. Although it is not possible to read off accurate percentages of movement values directly from these curves, it should be possible to deduce reasonably accurate values in practical situations.

AFRIKAANSE OPSOMMING: Die studie poog om die begin van beweging proses in klip/rotsbed riviere te beskryf in terme van die eenheids aangewende drywing metode. Hierdie doel is bereik deur die insameling van klipbewegingdata in 'n totaal van dertien vloedgebeurtenisse in twee ongerepte riviere in die Wes-Kaap, naamlik die Molenaars en Bergriviere. Die data was geplot op die oorspronklike aangepaste Liu diagram vir begin van beweging en dit was gevind dat die grens van beweging vir die kleiner klippe nie ooreenstem met die gedeelte van die Liu diagram wat volle turbulente vloei verteenwoordig nie (waar ~ gDs tv; = 0.12 vir Re. >13). Daar was tot die gevolgtrekking gekom dat die enigste rede wat die afwyking kan beskryf is die feit dat die oorspronklike aangepaste Liu diagram afgelei was vir uniforme partikelgrootte beddens terwyl die data van die Molenaars and Bergriviere nie-uniforme partikelgrootte beddens verteenwoordig. Dit was bewys deur die heratleiding van die y-as funksie van die oorspronklike aangepaste Liu diagram om 'n faktor in te sluit wat voorsiening maak vir die ruheid van 'n nie-uniforme partikelgrootte bed. Dit was gevind dat die gemiddelde absolute ruheid van die nieuniforme partikelgrootte beddens in die Molenaars en Bergriviere word redelik goed benaderd met 'n waarde van k=ds4• Ontwerpkurwes (in terme van die oorspronklike aangepaste Liu diagram parameters) vir intensiteit van beweging was ook ontwikkel. Alhoewel dit nie moontlik is om baie akkurate persentasies van beweging af te lees van die kurwes nie, is dit moontlik om akkuraat genoeg waardes te verkry in praktiese situasies.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50368
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