Check Dams (drop structures). In general, check dams are effective in preventing channel
degradation. The potential for erosion at a check dam depends on its design and construction,
its height and the use of revetment on adjoining banks. A series of low check dams, less than
about 0.5 m (2 ft) in height, is probably preferable to a single higher structure, because the
potential for erosion and failure is reduced. By simulating rapids, low check dams may add
visual interest to the flow in a channel. One critical problem arising with check dams relates to
improper design for large flows. Higher flows have worked around the ends of many
installations to produce failure.
Maintenance. The following problems that can be controlled by maintenance were observed
along relocated channels: (1) growth of annual vegetation in channel; (2) reduction of channel
conveyance by overhanging trees; (3) local bank cutting; and (4) bank slumping. The expense
of routine maintenance or inspection of relocated channels beyond the highway right-of-way is
probably prohibitive. However, most of the serious problems could be detected by periodic
inspection, perhaps by aerial photography, during the first 5 or 10 years after construction.
Relationship Between Sinuosity And Stability. This relationship is summarized as follows:
(1) Meandering does not necessarily indicate instability; an unstable stream will not remain
highly sinuous for very long, because the sinuosity will be reduced by frequent meander
cutoffs; (2) Where instability is present along a reach, it occurs mainly at bends; straight
segments may remain stable for decades; and (3) The highest instability is for reaches whose
sinuosity is in the range of 1.2 to 2 and whose type is either wide bend or braided point bar.
5.8.4 Estimation of Future Channel Stability and Behavior
One objective of stability assessment is to anticipate the migration of bends and the
development of new bends. Lateral erosion is probably more frequently involved in hydraulic
problems at bridges than any other stream process. Problems caused by general scour, local
scour, channel degradation and accumulation of debris are somewhat less common.
The lateral stability of a stream can be measured from records of its position at two or more
different times where the available records are usually maps or aerial photographs. Historic
surveyed cross-sections are extremely useful and can frequently be located in bridge
inspection files. It is recognized that some progress is being made on the numerical prediction
of loop deformation and bend migration (see Section 5.8.5). At present, however, the best
available estimates are based on past rates of lateral migration at a particular reach. However,
erosion rates may fluctuate substantially from one period of years to the next.
Measurements of bank erosion on two time-sequential aerial photographs (or maps) require
the identification of reference points which are common to both. Useful reference points
include roads, buildings, irrigation canals, bridges and fence corners. This analysis of lateral
stability is greatly facilitated by a drawing of time changes in bankline position. To prepare
such a drawing, aerial photographs are matched in scale and the photographs are
superimposed holding the reference points fixed. For further discussion see HEC-20 (Lagasse
et al. 2001).
Bank erosion rates increase with the stream size as shown in Figure 5.26. Sinuous canaliform
streams can then be expected to have the lowest erosion rates and the sinuous braided
streams, the highest.
5.60
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