6.2 STREAM BANK EROSION
The erosion, instability, and/or retreat of a stream bank is dependent on the processes
responsible for the erosion of material from the bank and the mechanisms of failure resulting
from the instability created by those processes. Bank retreat is often a combination of these
processes and mechanisms varying at seasonal and sub-seasonal time scales. Changes in
channel geometry with time are particularly significant during periods when alluvial channels
are subjected to high flows. The converse situation exists during relatively dry periods. Erosive
forces during high flow periods may have a capacity approximately 100 times greater than
those forces acting during periods of intermediate and low flow. In most instances when
considering the instability of alluvial rivers, it can be shown that approximately 90 percent of all
river changes occur during the small percentage of the time when the discharge exceeds the
dominant discharge.
Regardless of the fact that the majority of bank changes occur during comparatively short time
periods, there may also be regions within a river in which some degree of instability is exhibited
for all flow conditions. Raw banks may develop on the outside of bends as a consequence of
direct impingement of the flowing water. Sloughing banks may occur as a result of seepage
and other secondary forces created by water draining back through the banks into the river.
Continuous wave action, generated either naturally or by human activities, may also precipitate
erosion problems.
6.2.1 Causes of Streambank Failure
Bank retreat processes may be grouped into three categories: weakening and weathering
processes, direct fluvial entrainment, and mass failure. HEC-20 (Lagasse et al. 2001) provides
more detail on these processes. The impact of these processes on bank retreat is dependent
on site characteristics, especially near-bank hydraulic fields, bank height, and the geotechnical
properties of the bank material. Table 1 lists factors affecting bank erosion.
Table 6.1. Factors Affecting Erosion of River Banks.
A. Hydraulic
B. Geomorphic
C. Human
D. Biological
E. Climatic
F. Other
Factors
Factors
Factors
Factors
Factors
Fluid Properties
River Planform
Agricultural Activities
Vegetation
Freezing
Subsurface Flows
Specific Weight
Meandering
Mining
Trees
Ice Thickness
Seepage Forces
Viscosity (Temp)
Straight
Dams
Shrubs
Duration
Piping
Flow Characteristics
Braided
Navigation
Grass
Waves
Discharge
Anabranched
Transportation
Animals
Thawing
Wind
Bed and Bank Material
Urbanization
Domestic
Permafrost
Boats
Duration
Size
Wild
Floodplain Develop.
Velocity
Shape
Recreational Boating
Velocity Distrib.
Specific Weight
Commercial Boating
Turbulence
Fall Velocity
Shear Stress
Bank Characteristics
Drag/Lift Forces
Cohesive
Momentum
Noncohesive
Energy
Stratified
Height
6.2
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