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
remain 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 disperse into
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 significant 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,
engineering works may be responsible for the discharge of these sediments into the system.
In this regard, the engineer should utilize adequate technical assistance from experts in
fisheries, biology, and other related areas to assess the consequences of sediment in a river.
Only with such knowledge can one develop the necessary arguments to "sell" the 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
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 for fish. 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 a loss of
food essential to fish life and often it is difficult to get the food chain reestablished in the
system. In contrast, construction of barbs, spurs, and other river control works to stabilize the
river channel and protect a bridge crossing can decrease erosion, provide a better habitat for
fish and in general improve the environment.
Construction and operation of highways in water supply watersheds present very real problems
and may require special precautionary designs to protect the water supplies from highway
runoff or accidental spills. Runoff from highway construction may increase sediment and
turbidity of the water.
The preceding discussion is related to only a few immediate responses to construction along a
river. However, they are responses that illustrate the importance of considering the
environment in the design of highway encroachments.
1.3.2 Delayed Response of Rivers to Encroachment
In addition to the examples of possible immediate response discussed above, there are
important delayed responses of rivers to highway development. As part of this introductory