Case (5) illustrates the situation where a bridge is constructed across a river immediately
downstream of the confluence with a steep tributary. The tributary introduces relatively large
quantities of bed material into the main channel. As a result, an island has formed in the main
channel and divided flow exists. In order to reduce the cost of the bridge structure, the bridge
is built across one subchannel to the island or bar formed by deposition, closing the
secondary channel. Such a procedure forces all of the water and sediment to pass through a
reduced width. This contraction of the river in general increases the local velocity, increases
general and local scour, and may increase bank instability. In addition, the contraction can
change the alignment of the flow in the vicinity of the bridge and affect the downstream
channel for a considerable distance. A chute channel can develop across the second point
bar downstream, adversely affecting several meander loops downstream. Upstream of the
bridge, there is aggradation. The amount depends on the magnitude of water and sediment
being introduced from the tributary. Also, there is an increase in the backwater upstream of
the bridge at high flows which in turn affects other tributaries farther upstream of the crossing,
see Case (2).
Case (6) illustrates a situation where the main channel is realigned in the vicinity of the bridge
crossing. A cutoff is made to straighten the main channel through the selected bridge site.
As discussed in Case (4), increased local gradient, local velocities, local bed material
transport, and possible changes in the characteristics of the channel are expected due to the
new conditions. As a result the channel may braid. A short cut off section (1 or 2 bends) can
be designed to transport the same sediment loads that the river is capable of carrying
upstream and downstream of the straightened reach; however, it may be difficult to achieve
stability when multiple bends in a long reach are cut off.
It is possible to build modified reaches of main channels that do not introduce major adverse
responses due to local steepening of the main channel. In order to design a straightened
channel so t at it behaves essentially as the natural channel in terms of velocities and
h
magnitude of bed material transport, it is usually necessary to build a wider, shallower
section.
Case (7) illustrates a bridge constructed across a main channel. Subsequently, the base level
for the channel is raised by the construction of a dam downstream. Whenever the base level
of a channel is raised, a pool is created extending a considerable distance upstream
depending on the size of dam and slope of the channel. As the water and sediment being
transported by the river encounter this pool, most of the sediments drop out, forming a
delta-like structure at the mouth. If the bridge lies within the effects imposed by the new base
level, the following response at the crossing will be expected: aggradation of the bed, a loss of
waterway at the bridge site, significant changes in river geometry, and increased flood stages
and lateral channel instability. This is similar to Case (1).
Case (8) considers the situation where the sediment load is reduced in the channel after a
bridge has been constructed. This may happen due to the construction of a storage dam
upstream of the crossing. As stated in the preceding case, the raising of the base level of a
river, as in the development of storage by constructing a dam on a river, provides a
sedimentation basin for the water flowing in the system. In most instances, all of the
sediment coming into a reservoir drops out within the reservoir. Water released from the
reservoir is mostly clear. With sediment-free flow, the channel is too steep and bed
sediments are entrained from the bed and the banks causing significant degradation. If the
bridge is sufficiently close to the reservoir to be affected by the degradation in the channel, the
depth due to general and local scour at the bridge may be significantly increased. Also, the
channel banks may become unstable due to degradation, and there is a possibility that the
river, as its profile flattens, may change its form. In the extreme case, it is possible that the
degradation may cause failure of the dam and the release of a flood wave.
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