General Principles of Erosion Protection
6.2.5.1 Stage Duration
The period of time that river stages exceed a given elevation is important for two
reasons:
It determines how long the upper bank will be subject to potentially erosive
current.
It determines the lowest elevation at which riparian vegetative growth will not
be impeded by inundation. This is a factor in determining the minimum top
elevation for armor revetment when it is used in combination with vegetation
for upper bank protection.
Unfortunately, there is no published guidance that relates the top elevation of
protection works with stage duration. However, an example of terminating armor protection
below the elevation of the design flood flowline is provided by riprap upper bank protection
on some portions of the Lower Mississippi River. It is routinely terminated at a flowline
elevation exceeded 5 to 10 percent of the time. On small streams in north Mississippi, the top
of the rock in longitudinal stone toe protection (see section 7.1.4) routinely corresponds to
a stage that equaled or exceeded about 1 to 5 percent of the time.
6.2.5.2 Severity of Overbank Flow During Floods
The velocity of flow at the interface of the protection and the unprotected bank during
floods is influenced by channel alignment, local variations in the elevation of the bank, and the
extent of vegetative cover on the upper bank and overbank. Local bankline irregularities
which cause a convergence of streamlines, accompanied by higher velocities, can also play
a role. The most severe conditions occur at the necks of sharp bends with relatively low bank
elevations and little overbank vegetation. Also, the downstream half of sharp bends is where
highest velocities against the bank and in the overbank usually exist. Therefore, a higher
elevation of protection may be prudent there.
6.2.5.3 Erodibility of Upper Bank Material
This factor is best evaluated by on-site observation. The rate of historic bankline
recession alone is not a totally reliable indicator of erodibility of the material in the zone being
considered for the top elevation of protection. The material in the upper bank may be very
erosion resistant, but still fail from toe scour and subsequent mass failure. General guidance
on the erodibility of different bank materials is available, and can be used if experience with
particular site conditions is lacking. The most erosive soils are fine sands and silty sands, and
the least erosive are clay and coarse gravels.
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