General Principles of Erosion Protection
Erodibility of the upper bank material is also an important factor in selecting the top
elevation of indirect protection structures. If the upper bank is highly erodible, then the
structures should be high enough to reduce near-bank velocities during most flows. If the
upper bank is more erosion-resistant, then the structure needs to be only high enough to
induce deposition in front of the bank, especially if the duration of high flows is short, the
stream carries a large suspended sediment load, and/or if vegetation can be expected to
colonize the area of induced deposition.
6.2.5.4 Type of Protection and Slope of the Bank
These two factors act together to affect the near-bank velocity at the top of armor
protection. The rougher the armor and the flatter the bank slope, the lower the velocity at
the top of the armor will be, and the lower down the bank the armor can be terminated. Also,
flatter slopes are more conducive to vegetative growth, thus upper bank erosion in the form
of "shelving" behind the armor will be more likely to be arrested by subsequent volunteer
vegetative growth, if climate and soils are favorable.
Since rigid armor cannot adjust well to local scour, it should normally be carried to
a higher elevation than adjustable or flexible armor. The alternative to carrying it to a higher
elevation is to use adjustable or flexible armor above the rigid armor as a means of
transitioning to a non-erosive elevation or to the elevation where vegetative protection will
begin.
To some degree, the type of protection determines the amount of design effort that
can reasonably be expended to optimize the top elevation, and the potential savings from
doing so. If the protection material is relatively inexpensive, the potential savings from a
lower elevation will be relatively small, and taking a considered risk in terminating it at a
lower elevation will be less attractive.
6.2.5.5 Consequences of Failure
This may well be the dominant factor in setting the top elevation of your protection
work, especially if the other factors are poorly defined. Since this is an integral component
in determining the overall "safety factor" against failure of the work, it is discussed in 6.6.
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