Geoscience Reference
In-Depth Information
(a) Fowlers Creek
(b) Sandy Creek
10 m
10 m
1 m
1 m
2.5 km
0.0 km
6.5 km
2.0 km
9.2 km
5.0 km
11.5 km
12 km
6.8 km
7.25 km
Figure 13.12
Cross-sections of two ephemeral channels at specified distances beyond the reentrant they make in the scarp
of the Barrier Range of New South Wales and as they flow towards a flood basin. Transmission losses ensure rapid reduction in
channel capacity and width (after Dunkerley, 1992).
In fact, the beds of single-thread streams are often
near-horizontal and planar; any bar forms are also often
flat-topped and rise only 10-20 cm above the thalweg
(Leopold, Emmett and Myrick, 1966; Frostick and Reid,
1977, 1979) (Figure 13.11). It is likely that channel
width and bed flatness are related through flow depth,
since great width spreads the flow, so ensuring shallow
depths. This in turn suppresses the secondary current
cells that might otherwise encourage the building of
bars, but it also maintains sediment transport efficiency
by maintaining relative grain exposure (water depth
and bed grain diameter) throughout flood events. As a
result, temporary 'highs' are planed off and 'lows' are
filled in.
This does not mean that all ephemeral stream channels
are featureless. Where the bed material is sand and the
last bed-forming flow was of appropriate strength, dunes
and megadunes may be found (Williams, 1970). However,
exaggerated bar forms tend to occur only at pronounced
bends, as might be expected from analogy with perennial
rivers, or they occur where the channel is diffused in a
braid plan (e.g. Cooper Creek in the Lake Eyre basin of
Australia; see Rust and Nanson, 1986). There is a sug-
gestion that single-thread ephemeral streams tend to low
sinuosity (Schumm, 1961). This straightness may be the
reason why flat channel beds are so characteristic. How-
ever, the number of examples that have been documented
and that can be categorised is too small to make definitive
comments.
1000
100
10
1.0
0.1
1.0
10
100
Weighted mean percent silt-clay (M)
Figure 13.13
Width-to-depth ratio of stable cross-sections
of four ephemeral streams in four western states of the USA as
a function of weighted mean percentage silt-clay content of
