Geoscience Reference
In-Depth Information
12.4.1 Downstream flow decreases and localised
flood patterns
parts of the catchment and may produce compound rather
than single floodwaves (Cooke, Warren and Goudie, 1993;
Knighton and Nanson, 2001). Hence, some tributaries
may be activated while neighbouring tributaries remain
dormant, and these active tributaries may flow before the
ephemeral trunk channel. Where such localised flood pat-
terns occur, transmission losses along the trunk channel
(and thus the flow survival length) will vary from flood to
flood according to the variable pre-wetting of the channel
bed by direct rainfall and tributary inflows prior to passage
of the flood hydrograph.
Such localised flood patterns also have various impli-
cations for sediment transport and channel morphology
in ephemeral rivers. At the catchment scale, Schumm and
Hadley (1957) have described how tributaries in semi-arid
catchments in eastern Wyoming and northern New Mex-
ico can be found in all stages of integration with the trunk
channel, with each tributary having its own history of allu-
viation and dissection. At the reach scale, in some upland
and piedmont settings, asynchronous tributary flows can
lead to aggradation, gradient steepening or increases in
width or capacity of the trunk channel, and in more ex-
treme cases to partial obstruction or damming (e.g. Wool-
ley, 1946; Schick and Lekach, 1987; Patton, Pickup and
Price, 1993; Macklin et al. , 2010), thus disrupting the
continuity of downstream sediment transfer. By contrast,
in the lower energy environments of the Northern Plains,
central Australia, asynchronous tributary inflows and sed-
iment supply promote the formation of anabranches that
contribute to maintaining downstream sediment transfer
(see Box 12.3). Hence, while the influences are varied,
the localised flood patterns common to many drylands are
quite different to many catchments in other climatic set-
tings, which tend to be characterised by better integration
between tributary and trunk river behaviour.
One distinctive aspect of dryland rivers - both exogenous
and endogenous - is a widespread tendency for down-
stream decreases in flow volume. In most instances, de-
creasing flow volumes are caused principally by trans-
mission losses that result from floodwater infiltration into
the unconsolidated alluvium forming channel boundaries,
with further losses resulting from overbank flooding and
from evaporative and transpirative losses (Babcock and
Cushing, 1942; Keppel and Renard, 1962; Sharp and
Saxton, 1962; Thornes, 1977; Walters, 1990; McCarthy
and Ellery, 1998). Combined with hydrograph attenua-
tion and a common absence of appreciable tributary in-
flows in the lower parts of many dryland catchments, sig-
nificant downstream decreases in total flow volumes are
associated with decreases in flood peaks and flow fre-
quencies, and in larger ephemeral dryland rivers many
flows will fail to travel along the full length of the chan-
nels (Vanney, 1960; Keppel and Renard, 1962; Mabbutt,
1977; Walters, 1989; Kotwicki and Isdale, 1991; Hughes
and Sami, 1992; Lekach, Schick and Schlesinger, 1992;
Knighton and Nanson, 1994b, 2001; Sharma, Murthy and
Dhir, 1994; Sharma and Murthy, 1996; Enzel and Wells,
1997). Along some rivers in central Australia and south-
ern Africa, the reduced flow volumes and flow frequen-
cies lead to downstream decreases in channel size, and
ultimately defined channels cannot be maintained (Mc-
Carthy and Ellery, 1998; Tooth, 1999; Tooth et al. , 2002b).
In these instances, irregular large floods spill beyond the
channel termini and spread slowly across floodouts or
through floodplain wetlands (Figures 12.8(a) and (b) and
12.11(b)).
As addressed in further detail in Chapter 13, even in a
given ephemeral river, transmission losses can be highly
variable as they depend on many interrelated factors, in-
cluding the characteristics of the storm (e.g. size, position
of the storm track, location in relation to the drainage
net), the hydrograph (e.g. flow volume and duration) and
the channel (e.g. size of the wetted perimeter, porosity
and initial moisture content of the perimeter sediments,
stratigraphy of the channel fill) (e.g. Sorman and Abdul-
razzak, 1993; Knighton and Nanson, 1994b, 1997, 2001;
Sharma, Murthy and Dhir, 1994; Dick, Anderson and
Sampson, 1997; Greenbaum et al. , 1998; Lekach et al. ,
1998; Dunkerley and Brown, 1999; Lange, 2005; Dunker-
ley, 2008b). Storm characteristics and resulting hydro-
graph events are important in very large catchments or
in catchments subject to discrete convective storms, for
individual storms are unlikely to wet the entire catch-
12.4.2
Induration of alluvial sediments
Along many dryland rivers, the downstream flow de-
creases and localised flood patterns also have implications
for the nature of channel and floodplain alluvium. Strati-
graphic studies of alluvial deposits in Israeli ephemeral
catchments have revealed the common existence of a cal-
cic 'soil' developed 50-100 cm beneath the surface of
the channel alluvial fill and in Late Quaternary terraces
(Figure 12.11(a)). In active channels, this 'fluvial pedo-
genic unit' occurs at the lower limit of scour and fill
processes and is associated with the cumulative influence
of persistent differences in floodwater availability to var-
ious parts of the channel bed. During floods, the infil-
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