Geoscience Reference
In-Depth Information
Sediment concentration by weight (%)
99
91
80
50 40
20
9.1
5.0
0.99
2.8
Hyperconcentrated Extreme
High
2.6
Streamflow
2.4
2.2
2.0
1.8
1.6
Debris flow
1.4
Debris flow
Ephemeral streamflow
Perennial streamflow
Wet concrete
1.2
1.0
1
10
100
1000
10,000
Water content (%)
Fig. 5.11
The continuous spectrum of sediment concentrations from sediment-rich ephemeral rivers to debris flows. (Simplified from
Hutchinson 1988.)
within the arid environment, unless they cross
or terminate in an area of relative tectonic sub-
sidence. In addition, during periods of no flow
they may also act as a sediment source for
aeolian processes. Ephemeral streams are domin-
ated by large amounts of scour and fill within
single flood events (Leopold et al. 1966). The
period of subcritical flow during a flood event
in ephemeral streams is short-lived (Reid &
Frostick 1987). Thus the mechanism for the scour
is also short lived and may relate to plane beds
(Frostick & Reid 1987) and antidune migration
(Foley 1978).
The bed scouring, together with abundant
sediment supply, means that ephemeral rivers
carry very high sediment loads (described as
'too thin to plough and too thick to drink'
by Colorado farmers; Beveridge & Culbertson
1964). These flows can be extremely hyper-
concentrated in sediment (Fig. 5.11). As the
flood discharge increases, so does the sediment
load. Sediment supply exhaustion is unlikely in
arid catchments, and ephemeral streams carry-
ing 35 to 1700 times the sediment concentration
of perennial rivers are not uncommon (Frostick
et al. 1983). In addition, suspended sediment
concentrations tend to remain more in-phase
with flow characteristics than is observed in
perennial rivers. Bedload transport in ephemeral
rivers is also much greater than in perennial
rivers. These differences may be 10
6
times larger
at the threshold of entrainment and 10
1
at mod-
erate shear-stress levels (Laronne & Reid 1993;
Reid & Laronne 1995). In addition this trans-
