The Lane relation QS ~ Qs D50 (Equation 5.28) is one method for determining qualitative river
response. Having identified the qualitative response that can be anticipated, other river
mechanics techniques can be used to predict the possibility of change in river form and to
estimate the magnitude of local, upstream and downstream river response.
The initial river conditions in Table 9.1 include storage dams or water diversions. These
examples are used as illustrations relating to common experience. In general, the effect of a
storage reservoir is to cause a sudden increase of base level for the upstream section of the
river. The result is aggradation of the channel upstream, degradation downstream and a
modification of the flow hydrograph. Similar changes in the channel result if the base level is
raised by some other mechanism, say a tectonic uplift. The effect of diversions is to
decrease the river discharge downstream of the diversion with or without an overall reduction
of the sediment concentration. Similarly, changes in water and sediment input to a river often
occur due to river development projects upstream from the proposed crossings or due to
natural causes.
Case (1) of Figure 9.1 involves the construction of a bridge across a tributary stream
downstream of where the steeper tributary stream has reached the floodplain of the parent
stream. In most cases, the change in gradient of the tributary stream causes significant
deposition. In the case illustrated for Case (1), an alluvial fan develops which in time can
divert the river around the bridge, or, if the water continues to flow under the bridge, the
waterway is significantly reduced, endangering the usefulness and stability of the structure.
Usually, streams on alluvial fans shift laterally so that the future direction of the approach flow
to the bridge is uncertain.
Case (2) sketched in Table 9.1 illustrates a situation where a bridge is constructed across a
tributary stream. The average water surface elevation in the main channel acts as the base
level for the tributary. It is assumed that at some point in time after the construction of the
bridge the base level in the main channel has been lowered. Under the new condition, the
local gradient of the tributary stream is significantly increased. This increased energy
gradient induces head cutting and causes a significant increase in water velocities in the
tributary stream. The result is bank instability, possible major changes in the geomorphic
characteristics of the tributary stream and increased local scour. When the base level is
raised the gradient in the tributary is decreased, resulting in deposition, lateral channel
instability, increased flood levels, and a decrease in flow area under the bridge, similar to
Case (1).
Case (3) illustrates a situation where a bridge supported by piers and footings is constructed
across a channel that is subjected to long periods of low stage. When a river is subject to
long periods of low flows, there is a tendency for the low flow to develop a new low-water
thalweg in the main channel. If the low-water channel aligns itself with a given set of piers, it
is possible that the depth of local scour resulting from this flow condition may be greater than
the depth of local scour at high stage. There are several documented failures where bridges
have been safe in terms of local scour at high stage, but have failed as a consequence of the
development of greater local scour during low-flow periods.
9.9
Previous Page
