Geoscience Reference
In-Depth Information
Flood
Event
1
Flood
Event
2
Flood
Event
3
revealed patterns of behaviour that, as with other
characteristics, set desert streams apart from those of
humid environments. First, bedload transport rates are
much higher over the same range of shear stress and when
allowance is made for differences in bed material grain
size. The differences might be as high as a millionfold
at the threshold of entrainment and tenfold at moderate
levels of shear stress (Figure 13.24) (Laronne and Reid,
1993; Reid and Laronne, 1995). Second, the relation
between bedload transport rate and flow appears to be
simpler and, therefore, more predictable (Figure 13.25)
(Reid, Powell and Laronne, 1996). These differences have
been attributed to a lack of armour development, which
is thought to arise because the sparsely vegetated slopes
of the desert are capable of supplying large quantities
of sediment. Indeed, Pickup (1991) draws attention to
the effect of reducing vegetation cover through drought
or grazing in the Todd River basin, central Australia,
pointing to a tenfold increase in sediment transport over
those rates measured during marginally wetter periods
when the sward is better developed. The ready supply of
sediment to the channel system counteracts the tendency
for selective entrainment of finer clasts on the channel
Exposed
35
14
16
Buried
65
9m
3
s
-1
33m
3
s
-1
18m
3
s
-1
Figure 13.23
Pattern of burial and exhumation of tagged
gravel clasts during downstream transport by three successive
floods of increasing magnitude in the Nahal Hebron, northern
Negev, Israel. The tagged clasts were seeded initially on the
surface of the gravel bed (after Schick, Lekach and Hassan,
1987).
10
1
10
0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
Nahal Yatir
Oak Ck
Virginio Ck
10
-7
East Fork
Turkey Brook
Torlesse
Goodwin Ck
10
-8
0.01
0.1
1
10
100
0.01
0.1
1
10
100
0.01
0.1
1
10
100
ρ
QS
W
Unit stream power,
(kg m
-1
s
-1
)
Figure 13.24
Comparison of bedload transport rates measured by slot samplers in the ephemeral Nahal Yatir, Israel, the seasonal
Goodwin Creek, Mississippi, the perennial East Fork River, Wyoming, Oak Creek, Oregon, Torlesse Stream, New Zealand, Turkey
Brook, England, and Virginio Creek, Tuscany. East Fork River has a gravelly sand bed. The other streams have gravel beds.
E
b
is
