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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1.3 EFFECTS OF HIGHWAY CONSTRUCTION ON RIVER SYSTEMS

Highway construction can have siynificant 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
evaluated and considered.

1.3.1 Immediate ResPonses
Let us consider a few of the numerous and immediate responses of rivers to the
construction of bridges, training and channel stabilization works and approaches.
In the preceding paragraphs we indicated that local changes made in the
geometry or the hydraulic properties of the river may be of such a magnitude as to
have an immediate impact upon the entire river system. More specifically,
contractions 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.
To offset increased velocities and to reduce bank instabilities and related
problems, one ends up, in many instances, with stabilizing or channelizing the river
to some degree. When it is necessary to do this, every effort should be made to do
the channelization in a manner which does not degrade the river environment, which
includes the river's aesthetic value.

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