Because of the general effectiveness of dumped riprap, a more detailed analysis of the
relatively small number of cases in which it failed has been presented by Brice and Blodgett
(1978). The principal causes of failure and methods of mitigation are given in Table 6.3.
Table 6.3. Causes of Riprap Failure and Solutions (Brice et al. 1978)
Cause
Solution
Inadequate size of riprap
Larger riprap
Impingement of current directly upon riprap
Heavier stones, flatten riprap slopes, redirect
rather than having flow parallel to riprap
flow
Channel degradation
Provide a volume of reserve riprap at the
revetment toe
Reduce the riprap slope angle
Internal slope failure (slump)
Follow gradation specifications
Riprap with high percentage of fines causes
washing out of the fines
6.6 BANK PROTECTION OTHER THAN RIPRAP
There are many methods other than riprap that can be used for bank protection, including:
vegetation, mattresses, baskets, and blocks. This section provides an overview of several of
these methods. HEC-23 (Lagasse et al. 2001) and HEC-11 (Brown and Clyde 1989) provide
detailed design guidance for several of these methods (see Section 6.3.4).
6.6.1 Bioengineering Erosion Control
Vegetation is probably the most natural method for protecting streambanks because it is
relatively easy to establish and maintain, is visually attractive and environmentally more
desirable.
Below a stream's waterline, vegetation can effectively protect a bank in two ways. First, the
root system helps to hold the soil together and increases overall bank stability by forming a
binding network. Second, the exposed stalks, stems, branches and foliage provide resistance
to the streamflow, causing the flow to lose energy by deforming the plants rather than by
removing soil particles. Above the waterline, vegetation prevents surface erosion by absorbing
the impact of falling raindrops and reducing the velocity of overbank drainage flow and rainfall
runoff. Further, vegetation takes water from the soil providing additional capacity for infiltration
and may improve bank stability by water withdrawal.
Vegetation is generally divided into two broad categories: grasses and woody plants (trees
and shrubs). The grasses are less costly to plant on an eroding bank above the toe and
require a shorter period of time to become established. Woody plants offer greater protection
against erosion because of their more extensive root systems; however, under some
conditions the weight of the plant will offset the advantage of the root system. On very high
banks, tree root systems do not always penetrate to the toe of the bank. If the toe becomes
eroded, the weight of the tree and its root mass may cause a bank failure.
6.36
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