Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 286

Prolonged occupation of an area by a river leads to the
production of a channel belt occupied by active and aban-
doned reaches. The relatively sudden movement of a
whole channel belt (not just a single reach or bend cutoff)
to another position on the floodplain is termed avulsion .
The process is recorded by abandoned channel belts pre-
served on floodplains or buried partly or wholly beneath
them. Avulsion leaves a very characteristic imprint on the
fluvial landscape. This is best illustrated by the
Saskatchewan wetlands, where an avulsion in the 1870s
led to the production of a vast complex of splays, wetlands,
and channels in the Cumberland Marshes (Fig. 6.68a).
A river may adjust the following variables in response to
independently imposed climatic or tectonic changes to
runoff/discharge and slope over which the river itself has
absolutely no control: cross-sectional size ( wh ), cross-
sectional ratio ( w/h ), bed configuration, bed material grain
size, plan-form shape (sinuosity) and size (meander
wavelength), and channel bed slope. The equilibrium
graded stream is defined as “... one in which, over a period
of years, slope, velocity, depth, width, roughness and channel
morphology mutually adjust to provide the power and effi-
ciency necessary to transport the load supplied from the
drainage basin without aggradation or degradation of the
channel .” Channels are extremely sensitive to perturbations
in slope, sediment load, and water discharge. These per-
turbations may be imposed by climate change, base-level
change, and tectonics. For example, the hydraulic geome-
try equations imply that the magnitude of water discharge
and the nature of sediment load should radically affect
channel sinuosity. Many river systems around the world
record major changes in channel magnitude and geometry
since the last glacial maximum, commonly exhibiting a
trend from large, braided, aggrading channels to large and
then smaller, meandering, incised channels. These changes
have occurred due to large decreases in sediment supply in
response to a general decrease in runoff and increase in
vegetation in the past 15,000 years. Increased temperature
and humidity after the last Ice Age caused vegetation
growth and substantially reduced the amount of coarse
sediment liberated from drainage basins.

The transport of sand modifies the surface morphology in
marked ways due to the formation and migration of bed-
forms . These range in size over more than four orders of
magnitude, from the centimetric-scale ripples familiar to
many from the action of breezes on dry beach sand, to
gigantic dunehills of sand hundreds of meters high cap-
tured by aerial and satellite images of arid-zone deserts.
Ripples and ridges form a continuous series with wave-
lengths 0.02-2.0 m and heights from a few millimeters to
1 m (Figs 6.69 and 6.70). Wavelength increases linearly
with increasing grain size and with flow strength. Ripple
migration and growth from random irregularities occurs
by segregation of coarser grains that are bumped along by
collisions from saltating grains in bedload transport. A sta-
ble ripple regime is reached when the crests of adjacent
patches of coarser grains align and when sand transport
between ripples relates to approximately the equilibrium
jump length of the transported grains, itself adjusted to
the magnitude of momentum flux at the bed.
Flow-transverse dunes (Figs 6.71-6.73) occur where the
predominant seasonal winds of importance for sand trans-
port are unidirectional. There is a continuum of flow-
transverse forms related to the availability of sand cover. It
is possible that an analogy with subaqueous dunes is appo-
site. This would require the wavelength of large aeolian
dunes (up to several hundreds of meters) to be of the
order of boundary-layer thickness, a correlation in line
with known values of these parameters (Section 6.2). Dunes
are frequently organized into a hierarchy of forms. Draas
are composite duneforms of two types. In one (Fig. 6.74)
the relationship is rather like the dispersive behavior of pro-
gressive deep-water gravity wave groups (Section 4.9) so
that dunes pass through the larger form and emerge

microripples

6.7.4 Sediment transport in the atmospheric
boundary layer (ABL) over land

0.1 m

For the most part the land surface itself is “solid” and
therefore immune to alteration by wind shear. Momentum
transfer is manifest most obviously in the transport of par-
ticles from soils and/or loose sediment; we have already
discussed dust storms generated in the ABL (Section 6.3).

Fig. 6.69 Wind or ballistic ripples reflect the role that impacting
saltating sand grains have upon their development. The coarser
asymmetric crests are subject to most energetic bombardment.