In this article we will discuss about the erosional forces and modes of river bank failure.
Erosional Forces for River Bank Failure:
It is not quite easy to identify the processes responsible for the bank erosion.Parallel flow erosion is the detachment and removal of intact grains or aggregates of grains from the bank face by flow along the bank. Evidence includes observation of high flow velocities close to the bank; near bank scouring of the bed; under-cutting of the toe/lower bank relative to the bank top; a fresh, ragged appearance to the bank face; absence of surficial bank vegetation.
Impinging flow erosion is detachment and removal of grains or aggregates of grains by flow attacking the bank at a steep angle to the long-stream direction. Impinging flow occurs in braided channels where braid-bars direct the flow strongly against the bank, in tight meander bends where the radius of curvature of the outer bank is less than that of the channel centerline, and at other locations where an in-stream obstruction deflects and disrupts the orderly flow of water.
Evidence includes observation of high flow velocities approaching the bank at an acute angle to the bank; near-bank scouring of the bed; under-cutting of the toe/lower bank relative to the bank top; a fresh, ragged appearance to the bank face; absence of surficial bank vegetation.
Piping is caused by groundwater seeping out of the bank face. Grains are detached and entrained by the seepage flow (also termed sapping) and may be transported away from the bank face by surface runoff generated by the seepage, if there is sufficient volume of flow. Piping is especially likely in high banks backed by the valley side, a terrace, or some other high ground.
In these locations the high head of water can cause large seepage pressures to occur. Evidence includes: Pronounced seep lines, especially along sand layers or lenses in the bank; pipe shaped cavities in the bank; notches in the bank associated with seepage zones and layers; run-out deposits of eroded material on the lower bank.
Freeze/thaw is caused by sub-zero temperatures which promote freezing of the bank material. Ice wedging cleaves apart blocks of soil. Needle-ice formation loosens and detaches grains and cleaves apart blocks of soil. Needle-ice formation loosens and detaches grains and crumbs at the bank face. Freeze/thaw activity seriously weakens the bank and increases its erodibility. Evidence includes: periods of below freezing temperatures in the river valley; a loose, crumbling surface layer of soil on the bank; loosened crumbs accumulated at the foot of the bank after a frost event; jumbled blocks of loosened bank material.
Sheet erosion is the removal of a surface layer of soil by non-channelized surface run-off. It results from surface water draining over the bank edge, especially where the riparian and bank vegetation has been destroyed by encroachment of human activities. Evidence includes: Surface water drainage down the bank; lack of vegetation cover, fresh appearance to the soil surface; eroded debris accumulated on the lower bank/toe area.
Rilling and gullying occurs when there is sufficient uncontrolled surface runoff over the bank to utilize channelized erosion. This is especially likely where flood plain drainage has been concentrated (often unintentionally) by human activity. Typical locations might be near buildings and parking lots, stock access points and along stream-side paths.
Evidence includes: a corrugated appearance to the bank surface due to closely spaced rills; larger gullied channels incised into the bank face; headward erosion of small tributary gullies into the flood plain surface; and eroded material accumulated on the lower bank/toe in the form of alluvial cones and fans.
Wind waves cause velocity and shear stresses to increase and generate rapid water level fluctuations at the bank. They cause measurable erosion only on large rivers with long fetches which allow the buildup of significant waves. Evidence includes: a large channel width or a long, straight channel with an acute angle between eroding bank and long-stream direction; a wave-cut notch just above normal low water plane; a wave-cut platform of run-up beach around normal low-water plane.
Vessel forces can generate bank erosion in a number of ways. The most obvious way is through the generation of surface waves at the bow and stern which run up against the bank in a similar fashion to wind waves. In the case of large vessels and/ or high speeds these waves may be very damaging. If the size of the vessel is large compared to the dimensions of the channel hydrodynamic effects produce surges and drawdown in the flow.
These rapid changes in water level can loosen and erode material on the banks through generating rapid pore water pressure fluctuations. If the vessels are relatively close to the bank, propeller wash can erode material and re- suspend sediments on the bank below the water surface. Finally, mooring vessels along the bank may involve mechanical damage by the hull.
Evidence includes use of river for navigation; large vessels moving close to the bank; high speeds and observation of significant vessel-induced waves and surges; a wave-cut notch just above the normal low-water plane; a wave-cut platform or “spending” beach around normal low-water plane.
Retreat of river bank often involves geotechnical bank failures as well as direct erosion by the flow. Such failures are often termed as “bank sloughing” or “caving”. Examples of different modes of geotechnical stream bank failure are explained in the next section.
Different Modes of River Bank Failure:
This section summarizes the diff rent ways by which a river bank collapses by classifying them using geotechnical terminologies.
Soil/rock fall occurs only on a steep bank where grains, grain assemblages or blocks fall into the channel. Such failures are found on steep, eroding banks of low operational cohesion. Soil and rock falls often occur when a stream undercuts the toe of a sand, gravel or deeply weathered rock bank. Evidence includes very steep banks; debris falling into the channel; failure masses broken into small blocks; no rotation or sliding failures.
Shallow slide is a shallow seated failure along a plane somewhat parallel to the ground surface. Such failure are common on bank of low cohesion. Shallow slides often occur as secondary failures are common on banks of low cohesion. Shallow slides often occur as secondary failures following rotational slips and/or slab failures. Evidence includes: weakly cohesive bank materials; thin slide layers relative to their area; planar failure surface; no rotation or toppling of failure mass.
Rotational slip is the most widely recognised type of mass failure mode. A deep seated failure along a curved surface results in bank-tilting of the failed mass toward the bank. Such failures are common in high, strongly cohesive banks with slope angles below about 60°. Evidence includes: banks formed in cohesive soils; high, but not especially steep, banks; deep seated, curved failures scars; back-tilting of the top of failure blocks towards intact bank; arcuate shape to intact bank line behind failure mass.
Slab-type block failure is sliding and forward toppling of a deep seated mass into the channel. Often there are deep tension cracks in the bank behind the failure block. Slab failures occur in cohesive banks with steep bank angles greater than about 60°. Such banks are often the result of toe scour and under-cutting of the bank by parallel and impinging flow erosion.
Evidence Includes:
Cohesive bank materials; steep bank angles; deep seated failure surface with a planar lower slope and nearly vertical upper slope; deep tension cracks behind the bank-line; forward tilting of failure mass into channel; planner shape to intact bank-line behind failure mass.
Cantilever failure is the collapse of an overhanging block into the channel. Such failures occur in composite and layered banks where a strongly cohesive layer is underlain by a less resistance one. Under-mining by flow erosion, piping, wave action and/or pop-out failure leaves an overhang which collapses by a beam, shear or tensile failure. Often the upper layer is held together by plant roots. Evidence includes: composite or layered bank stratigraphy; cohesive layer underlain by less resistant layer; under-mining; overhanging bank blocks; failed blocks on the lower bank and at the bank toe.
Pop-out failure results from saturation and strong seepage in the lower half of a steep, cohesive bank. A slab of material in the lower half of the steep bank face falls out, leaving an alcove-shaped cavity. The over-hanging roof of the alcove subsequently collapses as a cantilever failure. Evidence includes cohesive bank materials; steep bank face with seepage area low in the bank; alcove shaped cavities in bank face.
Piping failure is the collapse of part of the bank due to high groundwater seepage pressures and rates of flow. Such are an extension of the piping erosion process, to the point that there is complete loss of strength in the seepage layer. Sections of bank disintegrate and are entrained by the seepage flow (sapping). They may be transported away from the bank face by surface run-off generated by the seepage, if there is sufficient volume of flow.
Evidence includes pronounced seep lines, especially along sand layers or lenses in the bank; pipe shaped cavities in the bank; notches in the bank associated with seepage zones; run-out deposits of eroded material on the lower bank or beach. Note that the effects of piping failure can easily be mistaken for those of wave vessel force erosion.
Dry granular flow describes the flow-type failure of a dry, granular bank material. When a non-cohesive bank at close to the angle of repose is undercut, increasing the local bank angle above the friction angle. A carpet of grains rolls, slides and bounces down the bank in a layer up to a few grains thick. Evidence includes non-cohesive bank materials; bank angle close to the angle of repose; undercutting; toe accumulation of loose grains in cones and fans.
Wet earth flow failure is the loss of strength of a section of bank due to saturation. Such failures occur when water-logging of the bank increases its weight and decreases its strength to the point that the soil flows as a highly viscous liquid. This may occur following heavy and prolonged precipitation, snow-melt or rapid drawdown in the channel. Evidence includes sections of bank which have failed at very low angles; areas of formerly flowing soil that have been preserved when the soil dried out; basal accumulation of soil showing delta-like patterns and structures.