Flow behaviour, suspended sediment transport and transmission losses in a small (sub-bank-full) flow event in an Australian desert stream
✍ Scribed by David Dunkerley; Kate Brown
- Publisher
- John Wiley and Sons
- Year
- 1999
- Tongue
- English
- Weight
- 398 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0885-6087
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✦ Synopsis
The behaviour of a discrete sub-bank-full ¯ow event in a small desert stream in western NSW, Australia, is analysed from direct observation and sediment sampling during the ¯ow event and from later channel surveys. The ¯ow event, the result of an isolated afternoon thunderstorm, had a peak discharge of 9 m 3 /s at an upstream station. Transmission loss totally consumed the ¯ow over the following 7 . 6 km. Suspended sediment concentration was highest at the ¯ow front (not the discharge peak) and declined linearly with the log of time since passage of the ¯ow front, regardless of discharge variation. The transmission loss responsible for the waning and eventual cessation of ¯ow occurred at a mean rate of 13.2% per km. This is quite rapid, and is more than twice the corresponding ®gure for bank-full ¯ows estimated by Dunkerley (1992) on the same stream system. It is proposed that transmission losses in ephemeral streams of the kind studied may be minimized in ¯ows near bank-full stage, and be higher in both sub-bank-full and overbank ¯ows. Factors contributing to enhanced ¯ow loss in the sub-bank-full ¯ow studied included abstractions of ¯ow to pools, scour holes and other low points along the channel, and over¯ow abstractions into channel ®laments that did not rejoin the main ¯ow. On the other hand, losses were curtailed by the shallow depth of banks wetted and by extensive mud drapes that were set down over sand bars and other porous channel materials during the ¯ow. Thus, in contrast with the relatively regular pattern of transmission loss inferred from large ¯oods, losses from low ¯ows exhibit marked spatial variability and depend to a considerable extent on streamwise variations in channel geometry, in addition to the depth and porosity of channel perimeter sediments.