The presence of water in the banks of rivers and its movement toward or away from the river
affect bank stability and bank erosion in various ways. The related erosion of banks is a
consequence of seepage forces, piping, and mass wasting.
6.2.4 Piping of River Banks
Piping is another phenomenon common to the alluvial banks of rivers. With stratified banks,
i.e., lenses of sand and coarser material sandwiched between a layer of finer cohesive
materials, flow is induced in more permeable layers by changes in river stage and by wind-and
boat-generated waves. If the flow through the permeable lenses is capable of dislodging and
transporting particles from the permeable lenses, the material is slowly removed, undermining
portions of the bank. Without this foundation material to support the overlying layers, a block
of bank material drops down and results in the development of tension cracks sketched in
Figure 6.1c. These cracks allow surface flows to enter, further reducing the stability of the
affected block of bank material. Bank erosion may continue on a grain-by-grain basis or the
block of bank material may ultimately slide downward and outward into the channel, causing
bank failure as a result of a combination of seepage forces, piping, and mass wasting.
6.2.5 Mass Wasting
An alternative form of bank erosion is caused by local mass wasting. If the bank becomes
saturated and possibly undercut by flowing water, blocks of the bank may slump or slide into
the channel. Mass wasting may be further aggravated by construction of homes on river
banks, operation of equipment on the floodplain adjacent to the banks, added gravitational
force resulting from tree growth, location of roads that cause unfavorable drainage conditions,
saturation of banks by leach fields from septic tanks, and increased infiltration of water into the
floodplain as a result of changing land-use practices.
Landslides, the downslope movement of earth and organic materials, result from an imbalance
of forces. Various forces are involved in mass wasting. These forces are associated with the
downslope gravity component of the slope mass. Resisting these downslope forces are the
shear strength of the earth's materials and any additional contributions from vegetation via root
strength or engineered slope reinforcement activities. When a slope is acted upon by a stream
or river, an additional set of forces is added. These forces are associated with removal of
material from the toe of the slope, fluctuations in groundwater levels, and vibration of the slope.
A slope may fail if stable material is removed from the toe. When the toe of a slope is
removed, the slope loses more resistance by buttressing than it does by downslope
gravitational forces. The slope materials may then tend to move downward into the void in
order to establish a new balance of forces or equilibrium. Often this equilibrium is a slope
configuration with less than original surface gradient. The toe of the failed mass can provide a
new buttress against further movements. However, if this buttress is removed by stream
erosion, the force equilibrium may again be upset. For slope toes acted upon by erosive
stream water, the continual removal of toe material can upset the force balance.