6.4.5 Dikes (Floodplain)
A floodplain dike is an impermeable linear structure for the control or containment of overbank
flow. Most dikes are on floodplains (Figure 6.2) but in some situations (as on wide braided
rivers or on alluvial fans) they may be within channels (see Section 6.4.6). Floodplain dikes are
used to prevent flood water from bypassing a bridge, or to confine channel width and maintain
channel alinement. Some dikes extend upstream from one or both sides of the bridge
opening. These are similar in function to guide banks, but are usually much longer and may
extend to the valley side. Such dikes are commonly called "training dikes." The use of fence-
type structures for training dikes is mostly restricted to arid and semi-arid regions (Brice et al.
1978).
6.4.6 Dikes (Channel)
Both permeable and impermeable dikes are installed in channels. Permeable dikes are those
which permit flow through the dike but at reduced velocities, thereby preventing further erosion
of the banks and causing deposition of suspended sediment from the flow.
Timber or steel pile dikes (also retards) may consist of closely-spaced single, double, or
multiple rows. There are a number of variations to this scheme. For example, wire fence may
be used in conjunction with pile dikes to collect debris and thereby cause effective reduction of
velocity. Double rows of piles can be placed together to form cribs, and rocks may be used to
fill the space between the piles. Pile dikes are vulnerable to failure through scour. This can be
overcome if the piles can be driven to a large depth to achieve safety from scour, or the base
of the piles can be protected from scour with dumped rock in sufficient quantities. The various
forms of pile dikes are illustrated in Figure 6.6.
The arrangement of piles depends upon the velocity of flow, quantity of suspended sediment
transport, and depth and width of the river. If the velocity of flow is large, pile dikes are not
likely to be very effective. Stabilization of the bank by other methods should be considered.
On the other hand, in moderate flow velocities with high concentrations of suspended
sediments, these dikes can be quite effective. Deposition of suspended sediments in the pile
dike field is a necessary consequence of reduced velocities. If there is not sufficient
concentration of suspended sediment in the flow, or the velocities in the dike fields are too
large for deposition, the permeable pile dikes will only partially be effective in training the river
and protecting the bends.
The length of each dike depends on channel width, position relative to other dikes, flow depth
and available pile lengths. Generally, pile dikes are not used in large rivers where depths are
great, although timber pile dikes have been used in the Columbia River. On the other hand,
banks of wide shallow rivers can be successfully protected with dikes. The spacing between
dikes varies from 3 to 20 times the length of the upstream dike, with closer spacing favored for
best results.
Vane dikes are low-elevation structures designed to guide the flow away from an eroding bank
line (Figure 6.7). The structures can be constructed of rock or other erosion-resistant material,
the tops of which are constructed below the design water surface elevation and would not
connect to the high bank. Water would be free to pass over or around the structure with the
main thread of flow directed away from the eroding bank. The structures will discourage high
erosive velocities next to an unprotected bank line, encourage diversity of various channel
depths, and protect existing natural bottomland characteristics. The findings from a model
investigation of these structures include the effects of various vane dike orientation, vane dike
length, and gap length U.S. Army Corps of Engineers, 1981.
6.14
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