Geoscience Reference
In-Depth Information
holding particles in place (
erodibility
). In general terms,
where erosivity exceeds the resistance due to erodibility
parameters, particle dislodgement and erosion will take
place. This simple equation is complicated by a vari-
ety of surface characteristics (such as roughness, slope
and moisture content), which can affect both erosivity
and erodibility. This chapter investigates the key fac-
tors influencing erosivity and erodibility and examines
some of the methods with which we can measure and
characterise them. In particular, there is a focus on our
burgeoning understanding of how both the presence of
vegetation and wind turbulence may affect sediment en-
trainment - both of which have seen considerable ad-
vances in understanding since the previous edition of
Arid
Zone Geomorphology
.
(a)
(b)
40
40
30
30
20
20
10
10
0
0
0
1
2
3
0
1
2
3
Wind velocity (m s
-1
)
Figure 18.1
Near-surface vertical velocity profiles showing
(a) the smaller surface shear stresses in laminar flow (b) when
compared to turbulent flow (after Bagnold, 1941).
where
18.2
The nature of windflow in deserts
ρ
=
fluid density
As air moves over the surface of the Earth it is retarded
by friction at its base and a
velocity profile
develops. The
zone of flow where air is affected by surface friction is
called the
boundary layer
and within this layer there is
a gradation from zero velocity in a very thin layer at
the surface to free stream velocity at a height beyond
the effects of surface friction. The atmospheric boundary
layer (ABL) is approximately 1-2 kilometres thick.
The structure of the velocity profile within the boundary
layer is highly dependent on the type of airflow: laminar or
turbulent. In
laminar
flow, there is little mixing between
the different layers of fluid and faster layers slip over
slower layers, while at the surface itself the air is station-
ary. With this type of flow, momentum transfer between
layers of fluid is accomplished by means of molecular
transfer. Slower moving molecules of air drift into faster
moving layers, thus causing a drag on the faster overlying
airflow. Such a transfer of momentum produces shearing
forces between layers of air.
Turbulent
flow also comprises zero flow velocity at the
surface, but the exchange of momentum in the boundary
layer is achieved through the action of gusts and turbulent
eddies mixing between layers. Such momentum exchange
is far more efficient than the molecular exchange seen in
laminar flow and this is represented by a differing velocity
profile. The greater mixing in turbulent flow results in a
steeper velocity gradient at the surface and hence higher
shearing stresses (Figure 18.1).
Laminar and turbulent flows can be distinguished from
each other by the Reynolds number (Re):
h
=
flow depth
V
=
flow velocity
ν
=
viscosity
An increasing Reynolds number signifies an increasing
turbulence intensity and a shift towards inertial forces
dominating over viscous forces, such that where Re
<
500-2000 laminar flow exists and where Re
≥
2000 tur-
bulent flow predominates.
In the atmosphere, airflow is nearly always turbulent
because air has a low viscosity and boundary layer depths
are normally quite high. Only in very viscous, slow or thin
flows do laminar characteristics develop.
18.2.1
The turbulent velocity profile
Under normal atmospheric conditions on flat, unvegetated
surfaces and in the absence of intense solar heating, the
turbulent velocity profile plots as a straight line on a semi-
logarithmic chart (Figure 18.2). The gradient of the semi-
logarithmic profile is a result of the
surface roughness
producing a drag on the overlying airflow. Hence, if the
gradient of the velocity profile is known, the
shear stress
(drag) at the surface can be determined. In sedimentolog-
ical research, a common method for describing the gradi-
ent of the velocity profile is in terms of the
shear velocity
(
u
∗
). It is the value of
u
∗
that has been traditionally used
in calculations for determining thresholds of erosion and
