Two methods of predicting response are physical and mathematical models. Engineers have
long used small-scale hydraulic models to assist them in anticipating the effect of altering
conditions in a reach of a river. With proper awareness of the large-scale effects that can
exist, the results of hydraulic model testing can be extremely useful for this purpose. An
alternative method of predicting short-term and long-term changes in rivers involves the use of
mathematical computer models. To study a transient phenomenon in natural alluvial channels,
the equations of motion and continuity for sediment laden water and the continuity equation for
sediment can be used as discussed in Chapters 2, 3, 4, and 5.
1.3 EFFECTS OF HIGHWAY CONSTRUCTION ON RIVER SYSTEMS
Highway construction can have significant general and local effects on the geomorphology and
hydraulics of river systems. Hence, it is necessary to consider induced short-term and
long-term responses of the river and its tributaries, the impact on environmental factors, the
aesthetics of the river environment and short-term and long-term effects of erosion and
sedimentation on the surrounding landscape and the river. The biological response of the river
system should also be considered and evaluated.
1.3.1 Immediate Response of Rivers to Encroachment
Let us consider a few of the numerous and immediate responses of rivers to the construction
of bridges, channel stabilization, and countermeasures.
Local changes made in the geometry or the hydraulic properties of the river might be of such a
magnitude as to have an immediate impact upon the entire river system. At bridges,
contraction due to the construction of encroachments usually cause contraction and local
scour, and the sediments removed from this location are usually dropped in the immediate
reach downstream. In the event that the contraction is extended further downstream, the river
may be capable of carrying the increased sediment load an additional distance, but only until a
reduction in gradient and a reduction in transport capability is encountered. The increased
velocities caused by encroachments may also affect the general lateral stability of the river
downstream.
In addition, the development of crossings and the contraction of river sections may have a
significant effect on the water level in the vicinity and upstream of the bridge. Such changes in
water level upstream of the bridge are called backwater effects. The highway engineer must
be in a position to accurately assess the effects of the construction of crossings upon the water
surface profile.
In many instances, to offset increased velocities and to reduce bank instabilities and related
problems, the river is stabilized or channelized to some degree. When it is necessary to do
this, every effort should be made to accomplish the channelization in a manner which does not
degrade the river environment, including the river's aesthetic value.
As a consequence of construction, many areas become highly susceptible to erosion. The
transported sediment is carried from the construction site by surface flow into the minor rills,
which combine within a short distance to form larger channels leading to the river. The water
1.9
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