6.4.9 Fencing
Fencing can be used as a low-cost bank protection technique on small to medium size
streams. Special structural design considerations are required in areas subject to ice and
floating debris. Both longitudinal (parallel to stream) fence retards and transverse
(perpendicular to stream) fences have been used in the prototype with varying degrees of
success. A model investigation and literature review of longitudinal fence retards with tiebacks
were conducted to identify the following important design considerations:
Channel gradient must be stable and not be steep (tranquil flow)
Toe scour protection can be provided by extending the support posts well below the
maximum scour expected or by placing loose rock at the base of the fence to launch
downward if scour occurs at the toe
Tiebacks to the bank are important to prevent flanking of the fence and to promote
deposition behind the fence
Fence retards generally reduce attack on the bank so that vegetation can establish
Metal or concrete fences are preferred due to ice damage and fire loss of wooden fences
6.4.10 Guide Banks
Guide banks are placed at or near the ends of approach embankments to guide the stream
through the bridge opening. Constructed properly, flow disturbances, such as eddies and
cross-flow, will be minimized to make a more efficient waterway under the bridge. They are
also used to protect the highway embankment and reduce or eliminate local scour at the
embankment and adjacent piers. The effectiveness of guidebanks is a function of river
geometry, quantity of flow on the floodplain, and size of bridge opening. A typical guidebank at
the end of an embankment is shown in Figure 6.10.
Figure 6.10. Typical guidebank.
6.18
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