Mechanics of Stream Bank Failure and Its Control | Soil Management

In this article we will discuss about the mechanics of stream bank failure and its control.

The flow of water in the stream generates shear stress on the bed and banks, both, as a result the bank gets retreated in following two ways:

i. Materials get eroded directly from the bank and carried downstream by the flow.

ii. When shear stress acting on the stream bed exceeds the critical shear stress, then bed materials are set in motion. If the potential transport capacity of the flow exceeds the sediment inflow from u/s areas, then bed degradation begins, along with increase in bank angle and height.

However, the rate of entrainment of bank and bed materials is controlled by the size, geometry and structure of these materials. The susceptibility of bank failure is largely dependent on cohesiveness of the bank’s material. The cohesive bank materials consist of large aggregates of strongly bonded conglomerates of clay, silt and sand particles, causing fluvial erosion of cohesive soil, that often takes place by the entrainment of aggregates rather primary particles. In addition, the erosion susceptibility of cohesive bank also depends on the moisture content and degree of weathering. For example, the hard and dry banks are very resistant to erosion than the wet banks.

The stream bed degradation can proceed down-stream as well as upstream sides, depending on the degradation types. The ‘scour’, which usually refers to the local and often temporary lowering of stream bed level over a short distance, while degradation’ implies on extensive and often progressive lowering of the stream bed over fairly long distance. The d/s progressing degradation is generally the result of changes in discharge, u/s sediment supply, or size of sediment.

Mass Failure of Homogeneous Cohesive Banks:

Usually, the failure of stream bank slope is occurred in following two forms:

i. Progressive and continuous failure by creep movement over long periods; and

ii. Catastrophic shear failure.

The catastrophic shear failure in cohesive bank is the most common type of failure. It takes place at rapid speed, when shear strength along a slip surface within the slope exceeds, either due to reduction in shear strength of the bank’s soil or increase in the stress, caused by soil saturation or human activities. In contrast to non-cohesive banks, maintained at the natural angle of repose where stability is independent of height, the stability of cohesive banks is strongly dependent on the bank’s slope angle and height, both.

The followings are the types of failure associated to the cohesive stream banks:

1. Rotational slip failure

2. Plane slip failure; and

3. Cantilever failure.

1. Rotational Slip Failure:

This type of stream bank failure mostly takes place along a circular surface. The slip may be from the base, toe or slope failure, depending on where the slip are intersects the soil surface.

The non-circular slips are associated to the highly fissured materials, non-homogeneous banks and certain types of soil drainage. This type of failures are critical in cohesive banks of greater height with less slope. In this particular case, the orientation of principal stresses gets change with depth, which alters the position of slip surface.

ii. Plane Slip Failure:

The plane slip failures are generally observed in the banks with steep slopes. In nearly vertical cohesive banks, there is little change in the orientation of principal stresses with depth, and the failure surface is almost plane. Behind steep bank, a signi­ficant tensile stress is generated adjacent to the upper part of the bank. This leads to the develop­ment of vertical tension crack, which results into slab failure of the bank by tension cracking and plane slip.

3. Cantilever Failure:

The cantilever type stream bank failure occurs, when bank is subject to undercutting caused by seepage flow, wave action or basal erosion. All these activities wash the soil materials from the lower portion/or from the point of strike of waves and create a type of overhanging soil mass or cantilever.

The cantilever developed so, gets collapsed due to further undermining or weakening by wetting or cracking. This type of failure is mainly by shear beam or tensile failure, depending on the cantilever shape.

The stream bank failure mechanism of cohesive materials, caused by rotational and plane failure technique, can be analysed by considering the balance between disturbing and resisting forces acting on the slip surface. However, the most simple approach is the Column method, in which it is assumed that a plane slip surface passes through the bank’s toe.

Causes and Processes of Mass Movement:

The cause of mass movement is an imbalance between the soil cover, stored water and plant cover, and the friction they exert on the sloping sub-stratum of weathered rock on which they rest (maximum slope of 30° to 40°). This imbalance can manifest itself progressively on one or more slide planes, following wetting or when the soil goes beyond the point of plasticity or liquidity (mudflows).

Such imbalance most often occurs suddenly during earthquakes and very heavy cloudbursts (over 75 mm in 2-3 hours). As water starts flow through fissures or megapores (tunneling) and down to rotten rock, the hydrostatic pressure builds up at a certain distance from the crest (5 to 95 m) or at the confluence of underground trickles of water. This pressure can push away the soil mass, detaching it from a fragile level of rotten rock; as result very frequent plate slides on porous volcanic material deposits over impervious rock.

The imbalance can be created by earthquakes, cracking due to frost and thaw alternate, the desiccation of swelling clay, rock weathering, wetting of the soil to the point of saturation, wetting of the slide bed-plane so that it becomes slippery, the presence of rocks with preferential fracture planes etc. Human based activities can cause more such mass movements by altering the external geometry of slope.

Mass Movement Control:

Mass movement control is very expansive.

However, it can be done by the following ways:

i. Preventing the rainwater from soaking into the soil.

ii. Development of soil cover and rapidly covering the slide bed-plane.

iii. Drainage to evacuate the runoff to less vulnerable zones, generally to the convex sections of a slope. The drained zone over the slide bed-plane prevents interstitial pressure from detaching the soil cover from the stable zone beneath the slide bed-plane.

iv. Drying the land by increasing plant evapotranspiration. However, it is important to prevent such vegetation from becoming overwhelming, the shrubs must be kept on the edges of fields. If trees are introduced they must be coppiced, i.e., the vegetation must be kept young. Very tall trees should not be kept on slopes where risks of sliding are high.

When the slide bed-plane is close to the soil surface, then tree roots can oppose strong mechanical resistance to shearing of the soil cover, whereas when the potential slide surface is too deep for the roots to reach, then such resistance is no longer operative. The overloading slopes with trees increase slide risks. Moreover, the trees get shaking in the wind, transmit vibrations to the soil and produce cracks that favour localized infiltration of runoff water down to the slide bed-plane.

Quick-growing species with taproot systems are preferable, and clear felling is avoided. Trees not only increase resistance to shearing through the mechanical action of their roots, but they also alter the water content of the soil, e.g., the evapotranspiration is high in forest and this keeps the interstitial pressure of water in the soil cover lower in forest area. Preventive measures are always better than the control measures.

In case of mass movement the following preventive measures can be undertaken:

i. Any establishment should not be built on unstable slopes.

ii. If there is no other choice, then cuts and fills that upset the slope equilibrium must be kept to a minimum. For example, if a slope has to be cut for road construction, then embankment must be strengthened by providing the abutment with a rip-rapping mask or supporting wall. Also, there should be constructed a ditch uphill of the road to intercept the runoff and preventing from infiltrating in the soil cover above the cutting.

iii. If cracks are there on the soil surface such as between micro-terraces formed by livestock, then surface tillage can help to spread the infiltrated water over the whole soil cover, and thus delays the advance of wetting front toward slide bed-plane and improves evaporation of the water mass.

iv. If a road is built on steep slope, then stabilizing the road by planting the trees or grass on bank above and below.

v. A drainage wall can also be built with foundation anchored in the rock.

vi. On very steep rocky slopes in mountainous areas, the wire net can be spread, down to break the fall of rocks.