System-wide erosion

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.