6.2.2 Bed and Bank Material
Resistance of a river bank to erosion is closely related to several characteristics of the bank
material. Bank material deposited in the river can be broadly classified as cohesive,
noncohesive, and composite. Failure of banks for various situations is shown in Figure 6.1.
Cohesive bank material is more resistant to surface erosion and has low permeability which
reduces the effects of seepage, piping, frost heaving, and subsurface flow on the stability of
the banks. However, such banks when undercut and/or saturated are more likely to fail due to
mass wasting processes such as sliding.
Noncohesive bank material tends to be removed grain by grain from the bank line. The rate of
particle removal, and hence the rate of bank erosion, is affected by factors such as the
direction and magnitude of the velocity adjacent to the bank, the turbulent fluctuations, the
magnitude and fluctuations in the shear stress exerted on the banks, seepage force, piping
and wave forces, many of which may act concurrently.
Composite or stratified banks are very common on alluvial rivers and generally are the product
of past transport and deposition of sediment by the river. More specifically, these types of
banks consist of layers of materials of various sizes, permeability, and cohesion. The layers of
noncohesive material are subject to surface erosion, but may be partly protected by adjacent
layers of cohesive material. This type of bank is also vulnerable to erosion and sliding as a
consequence of subsurface flows and piping.
6.2.3 Subsurface Flow
With flow of water from the river into the adjacent banks, a stabilizing seepage force is
generated. Rivers that continuously seep water into the banks tend to have smaller widths and
larger depths for a particular discharge. The reverse is true of the rivers that continuously gain
water by an inflow through their banks. The inflowing water creates a seepage force that
makes the banks less stable. The movement of water through the bank material can be
attributed to various factors.
If the water table is higher than river stage, flow will be from the banks into the river. The high
water table may result from: (1) a wet period during which water draining from tributary
watersheds saturates the floodplain to a higher level; (2) poor drainage conditions resulting
from changes in land use causing an increase in water level; (4) irrigated floodplains; and (5)
development of the adjacent floodplain for homes and businesses that utilize septic tanks and
leach fields to dispose of waste water and sewage.
With a rise in river stage an outward gradient is developed that induces flow into the banks.
This can be caused by: (1) the storage and release of water for pumped storage hydropower
generation which causes numerous fluctuations in river stage; (2) boat and wind waves which
cause local variations in stage that introduce inflow and outflow of water from the banks; (3)
predominately dry and semi-arid channels subject to intermittent floods. However, because the
duration of the change in stage is small, the inflow and outflow phenomena are usually
concentrated locally in the surface of the banks; and (3) the formation and loss of backwater
caused by ice flows and ice jams which lead to both seepage into and out of the banks.
Frequent stage fluctuations, such as may occur with hydropower operations, may exacerbate
the bank erosion process.