2. CAUSES OF BANK FAILURE The riverbank undergoes erosion by hydraulic and geotechnical instability. Hydraulic instability is caused by scouring at the toe of a marginally stable bank, flood propagation and flood recession, debris and vegetation, removal of bank vegetation, detachment of coarse sediment by wave action, secondary current etc. Besides, constricted bridge crossings or other encroachments that involve acceleration and concentration of flood flows tend to cause ‘back eddies’ or reverse circulation downstream, which can sometimes erode huge embankments into river bends. Local bank protection and river training work designed to protect against bank erosion at one point or reach of a river often provoke accelerated bank erosion elsewhere. The shearing of bank material by hydraulic action at high discharges is a most effective process, especially on non-cohesive banks and against bank projections. Large scale eddying induced by bank irregularities can enlarge existing embayment and increase the amplitude of projections, which become more susceptible to subsequent damage. Geo-technical instability is caused by detachment of more coarse-grained layers in any given alluvial bank, by water flowing out of the bank face, termed as ‘piping’ or ‘sapping’ (Hagerty and Hamel, 1989). Cohesive banks are particularly susceptible to seepage force and piping mechanisms that may so lower the internal resistance of the material as to induce failure. Whereas the lower bank is eroded by hydraulic action, the upper bank is less affected by flow forces but fails because of undercutting which produces different types of cantilever action in the cohesive material. River stage drawdown following floodplain inundation contributes to riverward creep and/or sliding of alluvium as does riverward seepage and consequent piping.
3. BANK EROSION MECHANISMS The traditional perception is that banks fail by basal scour. The shear stress associated with the water flow along the bank corrodes the toe of the bank, which steepens the bank, makes the slope unstable, and, eventually, gives rise to structural failure of subaqueous and upper banks. This perception is the basis of several mathematical models of bank erosion in which the rate of bank retreat is assumed to be directly related to near-bank flow velocity and shear stress (Darby and Thorne, 1996; Throne and Osman, 1988). Several studies suggest that there are other hydraulic factors than shear stress exerted on banks, which may also significantly affect rate of bank retreat. Among them, piping due to seepage is an important one.
4. FACTORS TO CONSIDER IN BANK EROSION ASSESSMENT The following factors should consider when bank erosion is assessed for planning protection works (i) Hydraulics (stage-discharge, flow structures, flow resistance, maximum near-bank velocity, distribution of shear stress, secondary currents and turbulence, water level variations). (ii) Morphology (Riverbed deformation by computing bed shear stress, bed topography, channel plan form, migration of bar and bed shear stress). (iii) Sediment transport (suspended sediment, bedload, wash load). (iv) Stability of banks and riparian structures (vegetation, bank angle, critical bank height is a function of bank angle, tension crack depth). (v) Soil properties (size, gradation, stratification of bank sediment, bulk density, friction angle, cohesion, etc.).
5. BANK EROSION PREDICTIONS The prediction of future rates and direction of bank erosion along a river is a difficult problem that arises in many engineering applications. In natural rivers, the best guide to future patterns of bank erosion is a local study of past patterns. Topographic maps and satellite images of several years, supplemented by local witnesses, are usually the best sources of information. However, the satellite images, due to their scale and resolution limitations, give qualitative results to some extent. Although bank erosion is quite a complicated process, over the years a number of methods were developed to predict the bank erosion rates. One of the methods is related to 2D mathematical model to compare bank erosion on the basis of local geometry, flow and sediment processes (Mosselman, 1992 and DHI, 1996 as described in DELFT/DHI, 1996). Another method estimates the yearly bank erosion rate on the basis of (1) overall channel parameters (discharge, bank material characteristics) and (2) local geometry (Hickin and Nanson, 1984). In general, bank erosion (E) is the function of bed and bank material properties, the geometry of the river, flow characteristics.