chapter, consideration is given to some of the more obvious effects that can be induced on a
river system over a long time period by highway construction.
Sometimes it is necessary to employ training works in connection with highway encroachments
to align the flow with bridge or culvert openings. When such training works are used, they
generally straighten the channel, shorten the flow line, and increase the local velocity within the
channel. Any such changes made in the system that cause an increase in the gradient may
cause an increase in local velocities. The increase in velocity increases local and contraction
scour with subsequent deposition downstream where the channel takes on its normal
characteristics. If significant lengths of the river are trained and straightened, there can be a
noticeable decrease in the elevation of the water surface profile for a given discharge in the
main channel. Tributaries emptying into the main channel in such reaches are significantly
affected. Having a lower water level in the main channel for a given discharge means that the
tributary streams entering in that vicinity are subjected to a steeper gradient and higher
velocities which cause degradation in the tributary streams. In extreme cases, degradation can
be induced of such magnitude as to cause failure of structures such as bridges, culverts or
other encroachments on the tributary systems. In general, any increase in transported
materials from the tributaries to the main channel causes a reduction in the quality of the
environment within the river. More specifically, as degradation occurs in the tributaries, bank
instabilities are induced and the sediment loads are greatly increased. Increased sediment
loads usually result in a deterioration of the environment.
1.4 EFFECTS OF RIVER DEVELOPMENT ON HIGHWAY ENCROACHMENTS
Some of the possible immediate and delayed responses of rivers and river systems to the
construction of bridges, approaches, culverts, channel stabilization, longitudinal
encroachments, and the utilization of training works have been mentioned. It is necessary also
to consider the effects on highway encroachments of river development works. These works
may include, for example, water diversions to and from the river system, construction of
reservoirs, flood control works, cutoffs, levees, navigation works, and the mining of sand and
gravel. It is essential to consider the probable long-term plans of all agencies and groups as
they pertain to a river when designing crossings or when dealing with the river in any way. Let
us consider a few typical responses of a crossing to different types of water resources
development.
Cutoffs may develop naturally in the river system or they may be the result of human activity.
The general consequence of cutoffs is to shorten the flow path and steepen the gradient of the
channel. The local steepening can significantly increase the velocities and sediment transport.
Also, this action can induce significant instability such as bank erosion and degradation in the
reach. The material scoured in the reach affected by the cutoff is probably carried only to an
adjacent downstream reach where the gradient is flatter. In this region of slower velocities, the
sediment drops out rapidly. Deposition can have significant detrimental effect on the
downstream reach of river, increasing the flood stage in the river itself and increasing the base
level for the tributary stream, thereby causing aggradation in the tributaries.
Consider a classic example of a cutoff that was constructed on a large bend in one of the
tributaries to the Mississippi. Along this bend, small towns had developed and small tributary
streams entered the main channel within the bendway. It was decided to develop a cutoff
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