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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1.2 DYNAMICS OF NATURAL RIVERS AND THEIR TRIBUTARIES

Frequently, environmentalists, river engineers, and those involved in
transportation, navigation, and flood control mistakenly consider a river to be
static; that is, unchanging in shape, dimensions, and pattern. However, an alluvial
river _qenerally is continually chanqinq its position and shape as a consequence of
hydraulic forces actinq on its bed and banks. These chanqes may be slow or rapid
and may result from natural environmental chanqes or from chanqes by man's
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 man-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, that
man-induced change frequently sets in motion a response that can be propagated for
long distances, and that in spite of their complexity all rivers are governed by the
same basic forces. The highway engil'ieer must understand and work with these
natural forces. It is absolutely necesS<\ry for the design engineer to have at hand
competent knowledge about: (I) geological factors, including soil conditions; (2)
hydrologic factors, including possible changes in flows, runoff, and the hydrologic
effects of changes in land use; (3) geometric characteristics of the stream, including
the probable geometric alterations that will be activated by the changes his project
and future projects will impose on the channel; and (4) 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 as long as man's lifetime, components of
the landscape may be relatively stable. Nevertheless stability cannot be
automatically assumed. Rivers are. in fact. the most actively chanqinq of all
qeomorphic forms.

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