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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form and side channels fill at such rapid rates, but island formation and side-channel
filling appear to be the normal course of events in any river transporting moderate
or high sediment loads regardless of the river size.
In summary, archaeological, botanical, geological, and geomorphic evidence
supports the conclusion that. most rivers are subject to constant change as a normal
part of their morphologic evolution. Therefore, stable or static channels are the
exception in nature.

1.2.2 Introduction To Hiver Hydraulics And River Response
In the previous section it was established that rivers are dynamic and respond to
changing environmental conditions. The direction and extent of the change depends
on the forces acting on the system. The mechanics of flow in rivers is a complex
subject that requires spedal study which is unfortunately not included in basic
courses of fluid mechanics. 1 he major complicating factors in river mechanics are:
(a) the large number of interrelated variables that can simultaneously respond to
natural or imposed changes in a river system and (b) the continual evolution of river
channel patterns, channel geometry, bars and forms of bed roughness with changing
water and sediment dischar'Je. In order to understand the responses of a river to the
actions of man and nature, a few simple hydraulic and geomorphic concepts are
presented here.
River forms are broadly classified as straiqht, meanderinq, braided or some
gombination of these classifications, but any changes that are imposed on a river
may change its form. The dependence of river sinuosity on the slope which may be
imposed independent of the other river characteristics is illustrated schematically in
Fig. 1.2.2. By changing the slope, it is possible to change the river from a
meandering one that is relatively tranquil and easy to control to a braided one that
varies rapidly with time, has hilJh velocities, is subdivided by sandbars and carries
relatively large quantities of sediment. Such a change could bc caused by a natural
or artificiill cutoff. Conversely, it is possible that a slight decrease in slope could
change an unstable braided river into a meandering one.
The siqnificantly different channel dimensions, shapes, and patterns associateq
~ith different quantities of discharqe and amounts of sediment load indicate that as
these independent vilriables chanqe, major adjustments of channel morpholoqy can
be anticipated. Further, if changes in sinuosity and meander wavelen'Jth as well as
in width and depth are required to cornpensate for a hydrologic change, then a long
period of channel instabilil:y can be envisioned with considerable bank erosion and