normally considerably steeper than sandbed channels and in general have narrower river
valleys. In the extreme are torrential rivers, the beds of which are comprised of large rocks.
This type of river usually exists in a youthful or canyon type environment near the upper end of
large river systems where the slopes are relatively steep.
9.2.2 Location of the Crossing or Encroachment
In selecting the site of a crossing or a longitudinal encroachment, several considerations are
necessary. First, the crossing or encroachment must mesh with the transportation system in
the area. Second, environmental factors must be considered. In fact, unless appropriate
weight is given to the environmental impacts it may not be possible to obtain permission to
proceed with the project at all. Economic considerations are equally important. Depending
upon the characteristics of the river and the environmental considerations, the cost of a
particular crossing or encroachment can be significantly affected by its location. The length
of the approaches versus the length of the crossing, the cost of real estate that must be
acquired to accomplish the crossing, the maintenance cost required to keep the crossing
functional over its estimated life, and the method of construction are some of the specific
aspects that should be considered in locating the crossing. The cost of protective measures
should also be considered in locating an encroachment.
9.2.3 River Characteristics
The subclassifications of river form can be utilized to identify the range of conditions within
which the particular river operates. It is necessary to determine if a river is relatively stable in
form or is likely to be unstable. In Figure 1.3 of Chapter 1, it was pointed out that rivers can be
essentially poised so that a small change in discharge characteristics can change a river
from meandering to braided or vice versa. It is important to know the sensitivity of any river
system to change. Criteria given in Chapter 5, for example Figure 5.18, or Chapter 3, Figure
3.13, can be used to predict this sensitivity. A meandering stream whose slope and
discharge plot close to the braided river line in Figure 5.18 may change to a braided stream
with a small increase in discharge or slope (see also Section 5.5.3).
In addition to river form, it is important to determine other characteristics of the channel: that
is, the channel may have a sand bed and cohesive banks; it may be formed in cobbles or it
may be formed in other combinations of these materials. Each of these river systems
behaves differently depending upon the characteristics of the floodplain material, the bank
material, and the bed material of the river both over the short term and the long term. Hence,
a detailed survey of the characteristics of the bed and bank material coupled with river form
plus other pertinent information are essential to design.
9.2.4 River Geometry
For planning a river crossing or an encroachment it is important to know the river geometry
and its variation with discharge and time. It is essential to know the slope of the channel and,
preferably, the energy gradient through the reach. In Chapter 5, relations were presented that
illustrate how width and depth vary with stage at-a-section as well as along the length of a
9.2
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