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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As a consequence of construction, many areas become hiqhly 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 flowing from the construction site is
usually a consequence of rain. The surface runoff and the accompanying erosion can
significantly increase the sediment yield to the river channel unless careful control
is exercised. The large sediment particles transported to the main channel may
reside in the vicinity of the construction site for a long period of time or may be
slowly moved away. On the other hand, the fine sediments are easily transported
and generally pollute the whole cross section of the river. The fine sediments are
transported downstream to the nearest reservoir or to the sea. As will be discussed
later, the sudden injection of the larger sediments into the channel may cause local
aggradation, thereby steepening the channel, increasing the flow velocities and
pOSSibly causing instability in the river at that site.
The suspended fine sediments can have very siqnificant effects on the biomass
of the stream. Certain species of fish can only tolerate large quantities of
suspended sediment for relatively short periods of time. This is particularly true of
the eggs and fry. This type of biological response to development normally falls
outside of the competence of the engineer. Yet his work may be responsible for the
discharge of these sediments into the system and if he is unable to cope with the
problem, the engineer should utilize adequate technical assistance from experts in
fisheries, biology, and other related areas to overcome the consequences of
sediment pollution in a river. Only with such knowledge can he develop the
necessary arguments to sell. his case that erosion control measures must be
exercised to avoid significant deterioration of the stream environment not only in
the immediate vicinity of the bridge but in many instances for great distances
downstream.
Another possible immediate response of the river system to construction is the
loss of the recreational use of the river. In many streams, there may be an
immediate drop in the quality of the fishing due to the increase of sediment load, or
other changed hydraulic characteristics within the channel. Some natural rivers
consist of a series of pools and riffles. Both form an important part of the
environment from the viewpoint of fisheries. The introduction of larger quantities
of sediment into the channel and changes made in the geometry of the channel may
result in the loss of these pools and riffles. Along the same lines, construction work
within the river may cause 3 loss of food essential to fish life and often it is
difficult to get the food chain reestablished in the system.