1.5.1 Variables Affecting River Behavior
Variables affecting alluvial river channels are numerous and interrelated. Their nature is such
that, unlike rigid boundary hydraulic problems, it is extremely difficult to study the role of any
Major factors affecting alluvial stream channel forms are: (1) stream discharge, viscosity,
temperature; (2) sediment discharge; (3) longitudinal slope; (4) bank and bed resistance to
flow; (5) vegetation; (6) geology, including types of sediments; and (7) human activity.
The fluvial processes involved are very complicated and the variables of importance are
difficult to isolate. Many laboratory and field studies have been carried out in an attempt to
relate these and other variables. The problem has been more amenable to an empirical
solution than an analytical one.
In an analysis of flow in alluvial rivers, the flow field is complicated by the constantly changing
discharge. Significant variables are, therefore, quite difficult to relate mathematically. One
approach is to list measurable or computable variables which effectively describe the
processes occurring and then to reduce the list by making simplifying assumptions and
examining relative magnitudes of variables. When this is done, the basic equations of fluid
motion may be simplified (on the basis of valid assumptions) to describe the physical
processes within an acceptable balance between accuracy and limitations of obtaining data.
It is the role of the succeeding chapters to present these variables, define them, show how they
interrelate, quantify their interrelations where feasible, and show how they can be applied to
achieve the successful design of river crossings and encroachments.
1.5.2 Basic Knowledge Required
In order for engineers to cope successfully with river engineering problems, it is necessary to
erosion and sedimentation, river mechanics, soil mechanics, structures, economics, the
environment and related subjects. In all aspects of bridge inspection, maintenance, design and
construction an interdisciplinary approach is needed. In fact, as the public demands more
comprehensive treatment of river development problems, the highway engineer must further
improve his or her knowledge, and the application of it, by soliciting the cooperative efforts of
the hydraulic engineer, hydrologist, geologist, geomorphologist, meteorologist, mathematician,
statistician, computer programmer, systems engineer, soil physicist, soil chemist, biologist,
water management staff and economist. In addition, professional organizations representing
these disciplines should be encouraged to work cooperatively to achieve long-range research
needs and goals relative to river development.
1.5.3 Data Requirements
Large amounts of data pertaining to understanding the behavior of rivers have been acquired
over a long period of time. Nevertheless, some data collection efforts have been sporadic and
unfocused. Agencies should take a careful look at present data requirements needed to solve