STREAMBANK EROSION PROCESSES AND TYPES
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This section describes streambank erosion processes, common types and causes of erosion and guidelines for evaluating streambank and streambed erosion problems.

Streambank Erosion Processes
Streambank erosion is a natural process that occurs when the forces of flowing water exceed the ability of the soil and vegetation to hold the banks in place.  Natural rates of streambank erosion vary with stream size, the amount of vegetative cover, and the type of soil in the streambank.  Under well-vegetated conditions, the smallest streams (those without any tributaries) may show little evidence of erosion over periods of several decades unless subjected to extreme flood events.  Large streams, on the other hand, often show evidence of noticeable erosion, especially on outside bends.  The forces that cause erosion increase during flood events, and most erosion occurs at these times.  Human disturbances to watersheds that increase frequency and magnitude of runoff events also increase streambank erosion.  Human disturbances include logging, mining, agriculture, and urbanization.  Typical urban or suburban developments which may impact a stream include houses, garages, parking lots, and walkways, including areas cleared of forest and replaced by tailored lawns.

Loss of streambank and streamside vegetation reduces the resisting forces and makes streambanks more susceptible to erosion.  This is often the single greatest contributing factor to harmful or accelerated erosion on small and medium-size streams.  Streambank vegetation may be removed intentionally for various reasons, or its loss may be inadvertent due to trampling by animals or humans.

The erosive ability of a stream is a function of

velocity, flow depth, and slope.  Therefore, on small streams, erosion is largely a function of the size and frequency of storm flows.  Small watersheds may have a tendency for very  "flashy" flows that move stormwater quickly through the channel.  Watershed changes that increase the size of floods and frequency of flooding, such as deforestation, agriculture and urbanization, are major contributors of streambank and streambed erosion.  Woody bank vegetation helps reduce flow velocity in the vicinity of the bank.  Thus, loss of woody vegetation increases the potential for streambank erosion. Channel modifications for flood control, drainage, or other purposes often increase stream energy enough to cause widespread erosion problems, especially if soils are easily erodible.

The resistance to erosion provided by soil depends on its cohesiveness and texture.  Sandy soils have low cohesion, and particles are small enough to be moved by flows with velocities of one or two feet per second.  Lenses or layers of erodible soil material are frequently susceptible to erosion problems.  Fines are selectively removed from cohesionless soils that are heter-ogeneous mixtures of sand and gravel, leaving behind an armor of gravel that protects the streambed against further erosion.  Deeply rooted bank vegetation, especially woody vegetation, develops a network, which increases resistance to erosion by adding strength to the bank materials.

If your stream has been straightened and widened (channelized), the stream channel will probably be making changes to recreate the shape and slope it used to have before it was changed (modified).  It is not uncommon for the upper or middle reaches of these

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