FLOOD01888

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

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applications, the velocity variations in the vertical are
much less important than those in the transverse and
streamwise directions. The above equations can be aver-
aged in the vertical (i.e., depth averaged) to yield the
two-dimensional equations for flow in the horizontal
plane which adequately describe the flow field for most
rivers with these characteristics. Two-dimensional flow
analysis should be considered for river hydraulics prob-
lems where the direction or distribution of flow is of
importance, either directly or because it affects variables
of interest such as water surface elevation, and cannot be
assumed as is required by a one-dimensional analysis.
Figure 4-1 depicts a situation where the flow could be
adequately modeled by a two-dimensional approach.
Figure 4-2 contrasts the one-dimensional approach 10 the
same problem where one must select cross sections per-
pendicular to the flow direction. While it may be possi-
ble 10 calibrate a one-dimensional model to reproduce the
overall energy loss in this flow field, key components

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15 Oct 93

of the flow field such as flow separatinns and recircula-
tion zones would not be reproduced at all by a one-
dimensional model.

b. Specific situations. Another situation that may
require a two-dimensional analysis is that of a bridge
with multiple openings crossing a broad. flat, floodplain.
In this case the water surface elevation upstream of the
bridge may be strongly dependent upon the distribution
of flow among the bridge openings. This distribution of
flow cannot be directly computed with a one-dimensional
approach. Such situations require that the engineer care-
fully select the level of analysis; physical model, numeri-
cal model, or other analytical technique (refer 10
Chapter 3).

c. Dynamic simulations. Multidimensional flow
analysis can be either unsteady (dynamic) or steady.
Dynamic simulations require substantially more

DNG WATER SUArACE

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SECTION 2
(NO SCALE)

NOTES:
1. F'LOWS ARE NOT NECESSARILY PERPENDICUlAR
TO SECTION uNES.
2. VELOCmES AND DEPTHS vARY F'ROM POINT
TO POWT RESULTING IN VARYING WATER
SURFACE ACROSS A SEcnON.
J. FlOW CONTRACnONS AND EXPANSIONS ARE
SIMUlATED. AS SEEN AT SECTIONS 1 AND 3.

Figure 4-1. Two-dimensionel flow representetion in cache creek settling basin

4-3