Incipient motion in cobble/boulder bed rivers

[摘要] ENGLISH ABSTRACT:This study sets out to describe the incipient motion process in cobble/boulder bed riversin terms of the unit applied power approach. This objective has been met through thecollection of data on stone movement from a total of thirteen flood events observed intwo 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 itwas found that the threshold of movement for the smaller stones did not conform with theportion 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 couldexplain this deviation is the fact that the original modified Liu diagram had been derivedfor uniform particle size beds while the data of the Molenaars and Berg Rivers representnon-uniform particle size beds. This was proved through re-deriving the y-axis functionof the original modified Liu diagram to include a factor that makes provision for theroughness of a non-uniform particle size bed. It was found that the average absoluteroughness of the non-uniform particle size beds in the Molenaars and Berg Rivers isreasonably well approximated by a value of k=da.Design curves (in terms of the original modified Liu diagram parameters) for intensity ofmotion were also produced. Although it is not possible to read off accurate percentages ofmovement values directly from these curves, it should be possible to deduce reasonablyaccurate values in practical situations.

[发布机构] Stellenbosch University