Geoscience Reference
In-Depth Information
250 - 2000 coarse/medium sand
15
63 - 250 fine sand
2 - 63 silt
<2
µ
m clay
100
10
50
Mean water velocity
1 <1 m s
-1
2 1-2 m s
-1
3 >2 m s
-1
5
0
0
0
1.0
1.0
10
100
1000
Time, h
Particle size,
µ
m
Figure 13.21
(a) Sedigraph of suspended sediment by grain size class during one flood in the Il Kimere, northern Kenya. (b)
Size distribution of suspended sediment sampled at various flow velocities during one flood in the Il Kimere (after Frostick, Reid
and Layman, 1983; Reid and Frostick, 1987).
rainfall intensities, commonly of 2 mm/h, produce con-
centrations that range, typically, up to 25 000 mg/L. This is
in stark contrast to convectively enhanced, cellular storms
of autumn and spring, which have 5-minute intensities
typically of 20-40 mm/h, with maxima approaching 160
mm/h. In these cases, the average concentration of sus-
pended sediment is 67 300 mg/L and maxima reach nearly
300 000 mg/L.
There is hysteresis in the relation between suspended
sediment concentration and flow, as in perennial rivers.
However, from the limited number of detailed studies that
yield sufficient information to identify the intraflood pat-
tern, the phenomenon appears to vary, depending on the
availability of sediment by size. For a sand-bed ephemeral,
the Il Kimere of northern Kenya, it appears to be almost
nonexistent. This might be because particles of all sizes
are available for transport and transport is dictated by
changes in the hydraulic environment rather than by ei-
ther limitations on sediment supply or the superfluity of
finer material of clay and silt size. In this case, it has been
shown that the size distribution of suspended sediment
varies systematically with water velocity or discharge
(Reid and Frostick, 1987) (Figure 13.21). In contrast, the
Nahal Eshtemoa, a gravel-bed ephemeral set in a land-
scape clothed with Holocene loess, shows strong hystere-
sis between suspended sediment concentration and water
discharge. Again, as with levels of concentration, the sense
in the direction of hysteresis often depends on storm type.
Convectively enhanced storms show a clockwise relation,
reflecting the flushing of sediment from proximal hill-
slopes and the channel bed, while low-intensity frontal
storms show an anticlockwise relation, reflecting gentler
processes of splash and runoff generation and delayed ar-
on distal hillslopes (Alexandrov, Laronne and Reid, 2003)
(Figure 13.22).
13.4.3
Sediment transport along the stream bed
In keeping with river sediment studies in all environ-
ments, much less is known about bedload transport in
desert streams than about transport in suspension. Ironi-
cally, in relative terms there is probably more information
about bedload in desert streams because of recent field
monitoring programmes, mainly in Israel. However, this
'anomaly' only reflects the paucity of studies of any sort
of fluvial sediment transport in the world's drylands!
Leopold, Emmett and Myrick (1966) were able to show
that large clasts of pebble to cobble size can be moved by
as much as 3 km during a single flood event by overpass-
ing a predominantly sand-bed channel. Schick, Lekach
and Hassan (1987) and Hassan (1990b) have extended
this work to cover gravel-bed streams. They have traced
tagged clasts as they move downstream in the Nahel He-
bron in the northern Negev. The use of a two-coil metal
detector allowed them to locate buried clasts as well as
those exposed on the surface of the stream bed. As a result,
they have been able to highlight two important processes:
for the first time, scour and fill has been shown to oper-
ate in a gravel-bed channel; besides this, there is a clear
pattern of exchange between buried and exposed clasts,
flood by flood, which Schick
et al
. relate to the depth
of scour in each event (Figure 13.23). In addition, using
some of the same data, Church and Hassan (1992) have
indicated the effects of clast interlock and exposure on
transport distance as conditioned by clast size and flood
