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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Constru~tion and operatio!}. of hiqhways in water-supply watersh!lds present
very real problems and require special precautionary desiqns to protect the water
supplies from hiqhway residue. These residues may be largely sedimentary and may
increase the turbidity of the water. There have been instances, however, where
other unwelcome materials such as asphalt distillates and deicing salts which have
been traced to highway operations.
The preceding discussion is related to--only a few immediate responses to
construction along a river. However, they are responses that illustrate their
importance to design and the environment.

1.3.2 Delayed Response Of Rivers To Development
In addition to the example of possible immediate responses discussed above,
there are important delayed responses uf rivers to highway development. As par~, of
this introductory 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 favorably 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.

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