Geoscience Reference
In-Depth Information
Figure 3.8.
a
,
b
and
c
axes of a boulder.
There is a proportional relationship between the size of the flood, in terms of
discharge and stream power, and the size of clast able to be transported. Very
large boulders can usually only be transported by very large floods and the
characteristics of any such boulders left in a channel, as long as they have not
fallen to their present position from slope processes, can be used to estimate the
size of the past flood. The boulder characteristics that determine the magnitude
of the past flood include the boulder size, particularly the length of the
a
,
b
and
c
axes (Fig. 3.8), the boulder rock density and boulder mass, the boulder shape and
theposition of the boulder prior to transport. This latter variable is difficult to
determine but boulders in an imbricate position (where they are stacked against
each other like shingle tiles), or where they are tightly packed in a group, require
considerably greater forces to initiate motion than a boulder standing in an
isolated position. Boulders are usually assumed to have been standing originally
in an isolated position prior to transportation to their present position.
Aparticle will be entrained in the flow when the drag and lift forces acting
on that particle exceed the force of resistance. Resistance is a function of the
particle mass, shape and pre-transport position minus the buoyancy of the water
covering a submerged particle. This can be expressed by the following equation
drag force (
F
D
)
+
lift force (
F
L
)
>
resistance force (
F
R
)
−
buoyancy
(3.1)
Drag and lift forces are dependent on the difference in pressure in the fluid
flow on opposite sides of the boulder (i.e. upstream and downstream sides). Drag
force increases on the upstream side of a boulder as velocity increases. Likewise,
as velocity increases there is a corresponding drop in pressure immediately above
theboulder (Bernoulli effect) causing the boulder to be lifted upwards into the
flow. Boulders are usually transported by rolling over their shortest axis. Boulders
transported in a stream or in any fluid flow are usually, but not always, deposited
with their long axis perpendicular to the direction of flow. In other words, the
boulder is transported by rolling over its
c
axis and is deposited with its
a
axis
perpendicular to the flow direction. So when a subsequent flow entrains the
boulder the drag force will operate on the upstream face bounded by the
a
and
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