Fundamentals of Fluvial Geomorphology and Channel Processes
3.1.1.4 Thresholds
Geomorphic thresholds may be thought of as the straw that broke the camel's back. In the fluvial
system this means that progressive change in one variable may eventually result in an abrupt change in the
system. If a river erodes a few grains of soil from the toe of the river bank, no particular response will be
noticed. If that continues with no deposition to balance the loss, the bank may eventually fail abruptly and
dramatically due to undermining. The amount of flow impinging along a bank may vary considerably with
no apparent effect on the stabilization; however, at some critical point the bank material will begin to move
and disastrous consequences can result.
In this example the change was a gradual erosion of a few grains of soil and a variability of stream
velocity, both which could be considered to be within the natural system. This type of threshold would
be called an intrinsic threshold. Perhaps the threshold was exceeded due to an earthquake or caused by
an ill-planned bank stabilization project. These would be called extrinsic thresholds. The planner must be
aware of geomorphic thresholds, and the effect that their project may have in causing the system to exceed
the threshold.
Channel systems have a measure of elasticity that enables change to be absorbed by a shift in
equilibrium. The amount of change a system can absorb before that natural equilibrium is disturbed
depends on the sensitivity of the system, and if the system is near a threshold condition, a minor change may
result in a dramatic response.
3.1.1.5 Time
We all have been exposed to the geologists view of time. The Paleozoic Era ended only 248
million years ago, the Mesozoic Era ended only 65 million years ago, and so on. Fortunately, we do not
have to concern ourselves with that terminology. An aquatic biologist may be concerned with the duration
of an insect life stage, only a few hours or days. What we should be aware of is that the geologist temporal
perspective is much broader than the temporal perspective of the engineer, and the biologist perspective
may be a narrowly focused time scale. Neither profession is good nor bad because of the temporal
perspective; just remember the background of people or the literature with which you are working.
Geomorphologists usually refer to three time scales in working with rivers: 1) geologic time, 2)
modern time, and 3) present time. Geologic time is usually expressed in thousands or millions of years, and
in this time scale only major geologic activity would be significant. Formation of mountain ranges, changes
in sea level, and climate change would be significant in this time scale. The modern time scale describes
a period of tens of years to several hundred years, and has been called the graded time scale (Schumm and
Lichty, 1965). During this period a river may adjust to a balanced condition, adjusting to watershed water
and sediment discharge. The present time is considered a shorter period, perhaps one year to ten years.
No fixed rules govern these definitions. Design of a major project may require less than ten years, and
numerous minor projects are designed and built within the limitations of present time. Project life often
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