FLOOD01888

• County: Statewide
• Title: River Hydraulics
• Date: 10/15/1993
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2.1416
15 Oct 93

requirements as well as the various kinds and forms of
hydraulic routing models that are available.

f. Multidimensional flow. Flow in a river channel is
often considered 10 be one-dimensional in the direction
of flow. As previously discussed, this assumption allows
a simplified mathematical analysis of the flow. Multi-
dimensional flows require accounting for the physics
(mass and momentum conservation) of the flow in two,
and sometimes three, directions. Detailed discussions of
multidimensional flow analysis methods are presented in
Chapter 4 and in the texts by Abbott (1919), Cunge et al.
(1980), and Fischer et al. (1979).

g. Movable boundary analysis. Alluvial rivers often
exhibit significant bed and bank mobility during and after
floods. For erodible channels, use of alternative compu-
tational procedures that account for sediment transport
characteristics may be necessary 10 accurately describe
project performance with respect to channel boundary
reactions and flow characteristics. Methods and proce-
dures for evaluating alluvial channel (mobile boundary)
hydraulics are presented in Chapter 7 and in EM 1110-2-
4000.

h. River channel geomorphology. Natural streams
acquired their present forms from long-term processes
involving land surface erosion, stream channel incise-
ment, streamflow variation, human activities, and land
use changes. The study of these processes associated
with land form development is referred 10 as geomor-
phology. In a natural river, there is a continuous
exchange of sediment particles between the channel bed
and the entraining fluid. If, within a given river reach,
approximately the same amount of sediment is trans-
ported by the flow as is provided by the inflow, the reach

2-14

is said to be in equilibrium. In natural rivers, a primary
design problem is to improve, modify, or maintain the
channel while also maintaining equilibrium. If a new
channel is 10 be constructed, or an existing channel is to
be altered, the primary problem is determining the stable
channel dimensions.

(I) Channels may be straight, braided, or meander-
ing depending upon the hydrology and geology of the
region. The characteristics of an existing channel are a
good indication of the potential success or failure of a
proposed channelization project. River engineers must
have some knowledge of river channel geomorphology in
order to properly identify existing channel problems and
to anticipate potential project-induced responses by the
channel following channel modification or changing flow
regulation. Texts by Leopold et al. (1964), Schumm
(1977), and Petersen (1986) are excellent references.
EM 1110-2-4000 also provides guidance for evaluating
geomorphologic changes that can occur in rivers natur-
ally, or as a result of human actions. .

(2) The most important principle of river geomor-
phology that river engineers must consider is that, once
disturbed, an alluvial stream or channel begins an 3010-
malic and unrelenting process that proceeds towards a
new equilibrium condition. The new equilibrium charac-
teristics (channel shape, size, depth, slope, and bed
material size) mayor may not be similar to the stream's
original characteristics. Failure to recognize important
sediment transport characteristics of an alluvial stream
can lead to a situation in which a project does not per-
form as designed, if that design is based solely on rigid
boundary hydraulics.

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