Geoscience Reference
In-Depth Information
13.4
Fluvial sediment transport
Tooth and Nanson, in a series of papers (1999, 2000,
2004), have explored some remarkable differences in
channel geometry and pattern that arise from differences
of in-channel tree growth in two ephemeral rivers that fol-
low subparallel paths less than 2.5 km apart on the North-
ern Plains of central Australia. The Plenty River is single
thread, its channel is largely devoid of trees and it has a
bed of sand, while the Marshall River, running the same
longitudinal gradient, has multiple, anastomosed, nar-
row channels separated by tree-induced sediment ridges
and islands and has a bed of coarse sand and granules
(Figure 13.15). Tooth and Nanson rationalise that, in con-
trast with the neighbouring Plenty, ephemeral tributaries
of the Marshall provide both a source of local perched
groundwater in the channel-fill that sustains in-channel
tree growth and coarser bedload sediment, which accu-
mulates as shadow deposits in the lee of the trees. The
growth and consolidation of these deposits has split the
channel into multiple, subparallel threads, in stark contrast
to the single-thread Plenty.
Positive feedback between vegetation growth and chan-
nel sedimentation, with consequences for channel geome-
try, also arises where land-use change brings a reduction in
runoff and sediment yield. Rozin and Schick (1996) show
that soil and water conservation measures implemented in
the catchment of the Nahal Hoga, Israel, have produced a
dramatic shift in river character over a period of 50 years
(Figure 13.16). What was, at one time, a wide, single-
thread channel devoid of vegetation became, first, mul-
tithreaded and, subsequently, has become an entrenched
single-thread channel as vegetation progressively invaded
and sediment accumulated.
Ephemeral streams move vast quantities of sediment dur-
ing each flood event, both as bedload and as suspended
load. In fact, they have produced the highest recorded val-
ues of suspended sediment concentration, reaching 68 %
solids (by weight) in the Rio Puerco, New Mexico (Bon-
durant, 1951). Material is readily available for transport
not only from the thinly vegetated or bare slopes of a
catchment but also from the bed of the stream. Even in
hyper-arid regions, where rainfall may be
50 mm per
year, annual specific sediment yield has been shown to be
high (Schick and Lekach, 1993). However, it is in semi-
arid environments, where annual rainfall is 250-350 mm,
that specific sediment yield reaches its highest levels in
the absence of significant human degradation of soil and
vegetation (Langbein and Schumm, 1958). From a 15-
year study of a 112 km
2
catchment in the northern Negev,
Alexandrov
et al
. (2009) provided an average annual spe-
cific yield of both suspended sediment and bedload of
290 Mg/km
2
, to which bedload contributes 15 Mg/km
2
,
or 5 %. However, weather is capricious in all drylands and
annual total yield ranged wildly from 0 to 1720 Mg/km
2
during the period of monitoring.
Before considering the entrainment and transport of
sediment, attention should be drawn to a characteristic
of some ephemeral streams that distinguishes them from
their perennial counterparts and has a considerable ef-
fect on the flow. The perched groundwater that is held
in the channel-fill makes the channel bank a favoured,
and sometimes only, location for the growth of large trees
(Frostick and Reid, 1979). Because flows of magnitude
sufficient to sweep the channel in perennial streams are
infrequent in ephemerals, trees are also able to establish
themselves on the channel bed, or trees that were once
bankside may find themselves within-channel as the bank
is eroded. Graeme and Dunkerley (1993) (Figure 13.14)
indicate that flow resistance can be greatly increased by
the presence of the trunks. Indeed, because the flow may
encounter more and more vegetative 'roughness' in the
form of branches as stage discharge increases, they show
that, instead of the 'usual' inverse relation between the
roughness parameter and water depth, roughness values
may increase with increasing depth. Where this is signif-
icant, it will obviously have an impact on the amount of
energy available for sediment transport. The importance
of riparian and in-channel tree growth for flow resistance
and channel geometry has also been examined by Graf
(1979) for ephemeral streams of the American southwest.
Here, where valley bottom biomass is low, arroyos are
entrenched, but where it is high - implying an increase in
<
13.4.1
Scour and fill
The fact that ephemeral streams allow complete access to
their bed sediments between floods without the need for
pumping equipment, coffer dams and so on means that
they were the first river type to reveal the process of scour
and fill.
By emplacing scour chains in the small but elongate
Arroyo de Los Frijoles in New Mexico, Leopold, Emmett
and Myrick (1966) were able to show that a single flood
might incise the river bed by as much as 0.3 m. However,
just as significant was the fact that this scour would be
matched by a more or less equal amount of deposition
during flood recession (Figure 13.17). Over and above
this single flood pattern, it was also revealed that the en-
tire channel was in grade, that is to say, in equilibrium,
since the bed was shown to be restored to more or less the
