FLOOD06421

• County: Statewide
• Basin: Statewide
• Title: Highways in the River Environment
• Date: 2/1/1990
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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1.4 THE 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 of hiqhway encroachments on
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 possible or 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 cutoffs can be constructed
by man. 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 across the gooseneck to shorten the flow line of the
river, reduce the flood stage and generally improve poor hydraulic conditions in that
location. Several interesting results developed.
In the vicinity of the cutoff, the bank line eroded and degradation was initiated.
Within the bendway, the small tributaries continued to discharge their water and
sediment. Because of the flat gradient in the bend, this channel section could