consequence of hydraulic forces acting on its bed and banks. These changes may be slow or
rapid and may result from natural environmental changes or from changes by human activities.
When an engineer modifies a river channel locally, this local change frequently causes
modification of channel characteristics both up and down the stream. The response of a river
to human-induced changes often occurs in spite of attempts by engineers to keep the
anticipated response under control. The points that must be stressed are that a river through
time is dynamic and that human-induced change frequently sets in motion a response that can
be propagated upstream or downstream for long distances.
Figure 1.1. Geometric properties of bridge crossings.
In spite of their complexity, all rivers are governed by the same basic forces. The design
engineer must understand, and work with these natural forces:
Geological factors, including soil conditions
Hydrologic factors, including possible changes in flows, runoff, and the hydrologic effects of
changes in land use
Geometric characteristics of the stream, including the probable geometric alterations that
will be activated by the changes a project and future projects will impose on the channel
Hydraulic characteristics such as depths, slopes, and velocity of streams and what
changes may be expected in these characteristics in space and time.
1.2.1 Historical Evidence of the Natural Instability of Fluvial Systems
In order to emphasize the inherent dynamic qualities of river channels, evidence is cited below
to demonstrate that most alluvial rivers are not static in their natural state. Indeed, scientists
concerned with the history of landforms (geomorphologists), vegetation (botanists), and the
past activities of man (archaeologists), rarely consider the landscape as unchanging. Rivers,
glaciers, sand dunes, and seacoasts are highly susceptible to change with time. Over a
relatively short period of time, perhaps in some cases up to 100 years, components of the