River bank failure
If tectonic failure is at fault, research into its effects may aid in the understanding of alluvial systems and their responses to different stresses.
[2] The bank zone is above the ordinary high water level, but can still be effected periodically by currents, and gets the most human and animal traffic.
The overbank area is inland of both the toe and bank zones, and can be classified as either a floodplain or a bluff, depending on its slope.
A cohesive bank is highly susceptible to erosion in times of lowering water levels due to its low permeability.
[6] Hydraulic toe erosion occurs when flow is in the direction of a bank at the bend of the river and the highest velocity is at the outer edge and in the center depth of the water.
Pore water pressure in the saturated bank reduces the frictional shear strength of the soil and increases sliding forces.
[2] Piping failure may occur when high groundwater seepage pressure increases, as well as the rate of flow.
Shallow failure occurs where a layer of material moves along planes parallel to bank surfaces.
[5] Small to medium-sized blocks are forced out at or near the base of the river bank due to excessive pore water pressure and overburden.
The slab of material in the lower half of the bank will fall out, leaving an alcove shaped cavity.
[6] Popout failure is when small to medium-sized blocks are forced out at or near the base of the river bank due to excessive pore water pressure and overburden.
The slab of material in the lower half of the bank will fall out, leaving an alcove shaped cavity.
Small to medium-sized blocks are forced out at, or near the base of the river bank due to excessive pore water pressure and overburden.
[6] Failure caused by dry granular flow occurs typically on non-cohesive banks at, or near to, the angle of repose, which are undercut.
[9] The NMSZ is the result of a failed rift system which remains weak today, and thus is prone to faulting and earthquake activity.
Failure occurs in this area because river banks are composed of clay, due to glacial and lake deposition, as opposed to more resistant sediments such as sand or gravel.
[10] River bank failure is dependent on many solutions, the most common of which are lime stabilization and retaining walls, riprap and sheet piling, maintaining deep vegetation, windrows and trenches, sacks and blocks, gabions and mattresses, soil-cement, and avoiding the construction of structures near the banks of the river.
[12] Riprap made of rocks and other materials, arranged in a way as to inhibit erosional processes on a river bank.
[13] This allows for the need of minimal design work, in that installation is simple on high banks, although other methods could lead to failure.
Results of this method have been seen to be inconsistent, as the steep slope of the bank leads to increased velocity of the river.
This method requires increased man labor and larger amounts of filler material, as all sacks and blocks should be of the same size.
They are expensive and labor-intensive, as well as require periodical inspection for damage and subsequent maintenance, though they have been seen to demonstrate positive performance.
[13] Unfortunately, the mattresses themselves don't fit well in sharp curves, and it may be costly to remove the vegetation on the bank, which is required for placement.
[12] Three main types of vegetation exist to prevent bank failure: Trees, shrubs, and grasses.
Trees will provide for deep and dense root systems, increasing the stresses a river bank will accommodate.
Shrubs are staked into the river bank in order to provide a protective covering against erosion, creating good plant coverage and soil stability.
[12] Typically, willows and cottonwood poles are the most useful materials, however, fiber products may also be used[13][15] are then partially buried and staked in place.
