User:Kinshh92/Bank erosion

Bank erosion is the wearing away of the banks of a stream or river. This is distinguished from erosion of the bed of the watercourse, which is referred to as scour. Bank erosion occurs when concentrated flow removes sediment from the stream bank (cite Ward, Envi. Hydrology, pg 309). Such erosion leads to sediment transport through streams and rivers.

The roots of trees growing by a stream are undercut by such erosion. As the roots bind the soil tightly, they form abutments which jut out over the water. These have a significant effect upon the rate and progress of the erosion.

Measurement
Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times. This simple measurement technique can be enhanced with the use of a data logger attached to a rod of photoreceptors; the logger records the voltage, which is an indication of how much of the rod is exposed.

Permissible Velocity
Permissible velocity is the maximum velocity in a stream that can occur before erosion will happen. A table of permissible velocities for common types of channels has been established by the United State Army Corps of Engineers.

Include table – can be found in Chaudry, Open Channel Flow, pg 287 Table 9-3 These permissible velocities can be calculated using the Manning formula:


 * $$V = \frac{k}{n} {R}^{2/3} \, S^{1/2}$$

where:
 * V is the average velocity across the cross-section (length/time, ft/s, m/s),
 * k is a conversion factor equal to 1 for SI units or 1.49 for U.S. customary units,
 * n is the Manning coefficient (unitless),
 * R is the hydraulic radius (length, ft, m),
 * S is the stream slope (length/length, ft/ft, m/m)

Permissible Velocity Example
For example, consider a man-made channel with grassy earthen lining that was recently completed. The rectangular channel was constructed to have a bottom width of 15 ft and carry a maximum depth of 5 ft. The channel bed slope was built with an average of 0.01 ft/ft. When the flow depth is at a baseline of 2.75 ft, what is the maximum permissible velocity?
 * n = 0.018 (from Manning’s n table: http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Mannings_n_Tables.htm, for Excavated or Dredged Channels > Earth, straight, and uniform > clean, recently completed)
 * k = 1.49 for SI units
 * Area = (bottom width)x(flow depth) = (15 ft)x(2.75 ft) = 41.25 ft^2
 * Wetted Perimeter = (bottom width) + 2x(flow depth) = 15 ft + 2x(2.75 ft) = 20.5 ft
 * Hydraulic Radius = Area / Wetted Perimeter = 41.25 ft^2 / 20.5 ft = 2.01 ft
 * S = 0.01 ft/ft

Therefore,
 * $$V = \frac{1.49}{0.018} {2.01 ft}^{2/3} \, (0.01 ft/ft)^{1/2}$$


 * $$V = 13.2 ft/s$$

The maximum permissible velocity in the stream channel before bank erosion will occur is 13.2 ft/s.

Shear Stress
Shear stress with regards to open channel flow is the force parallel to the boundary of the stream channel and fluid. Shear stress varies over a stream cross section. This variation can be approximated (cite Chaudry, Open Channel Flow, pg 289) for a trapezoidal channel as:

where:
 * $$\tau = \gamma {y} {S0} $$ at the bottom of the channel
 * $$\tau = {0.76} \gamma {y} {S 0} $$ at the sides of the channel
 * $$\gamma$$ = [Specific_weight] of the fluid (weight/volume, N/m^3),
 * $$y$$ is the depth (length, m),
 * $$S 0$$ is the channel slope (length/length, m/m)

In bank erosion, the shear stress can be used to identify the point when erosion will occur, in which case the shear stress is known as the critical shear stress.

Control
Bank erosion is a natural process: without it, rivers would not meander and change course. However, land management patterns that change the hydrograph and/or vegetation cover can act to increase or decrease channel migration rates. In many places, whether or not the banks are unstable due to human activities, people try to keep a river in a single place. This can be done for environmental reclamation or to prevent a river from changing course into land that is being used by people. There are ways to try to prevent or slow these processes via stream restoration and erosion control measures. One way that this is done is by placing riprap or gabions along the bank.