The ideas below have been presented
by many others, but the basic framework of the discussion borrows most heavily
from the Streambank Investigation and Stabilization Handbook. (US Army Engineer Research and Development
Center, Vicksburg, MS. 1998. CD-ROM Veri-Tech, Inc., Vicksburg, MS. www.veritechinc.com.)
System-wide erosion has serious
implications for success or failure of bed or bank protection works. Fluvial systems are complex collections of
physical bodies subject to a variety of physical influences. Due to the laws of physics, these systems
respond to perturbation by seeking a state of approximate equilibrium where
channel geometry (average width, depth, slope, bed material size and average
meander radius, wavelength and amplitude) fluctuate about some set of values
termed “dynamic equilibrium.” When the
major forcing variables (inputs of water or sediment) change, all of the other
variables in the system adjust. System
wide erosion typically propagates through a system in a fashion analogous to
fire moving through a forest. Thus
sites that are apparently stable may suddenly become unstable when a systemic
disturbance propagates through the reach.
System-wide erosion can be triggered by adjustments to three
categories of perturbation: upstream
influences, downstream influences, and basin-wide factors. Upstream influences include the effects of
dams or flow diversions, downstream influences include base level lowering due
to channel straightening or flood height reduction, and basin-wide factors
include land use changes due to deforestation, reforestation, surface mining,
urbanization, grazing, etc. Perturbations
may be triggered by natural events (earthquakes, landslides, volcanic
eruptions), and while these can be quite spectacular, most system-wide erosion
is driven by perturbations caused by human activities. Examples of each category are provided below.
When a dam is constructed and closed, sediments are trapped
behind the structure, creating a deficit in downstream sediment loads that must
be satisfied. Often this produces a
wave of bed lowering (degradation) which progresses downstream from the dam for
many miles. Bed sediments may grow
coarser (armoring) if gravel or cobble is present in the bed. Although this is
the classical response associated with dam closure, many other types of
response have been observed. For
example, flood control reservoirs may reduce downstream high flows so much that
the river can no longer transport sediments supplied by tributaries, leading to
channel aggradation. Diversions of flow
into a reach generally produce erosion as the channel enlarges to accommodate
the additional water, but diversions of sediment-laden waters may have the
opposite impact.
Base-level lowering occurs when the stage of high flows in
downstream reaches is lowered due to channel enlargement and straightening or
closure of an upstream flood control reservoir. Unless the channel bed contains outcrops of bedrock or other
geological controls, the system will respond with general bed degradation.
Land use changes that occur when forests are converted to
grazing lands or when agricultural lands are converted to suburban uses
typically elevate high flows and depress low flows. Channel erosion can be quite rapid, threatening infrastructure
and producing many of the symptoms of general bed degradation.