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

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Figure 5-1. Looped reting curve induced by beckwat...

profIles from a steady flow model are suspected. In gen-
eral, for large rivers and low lying coastal areas, steady
flow analysis is not appropriate.

5-3. Conditions that Require Unsteady Flow
Analysis

Unsteady flow analysis should be used under the follow-
ing conditions:

a. Rapid changes in flow and stage. If the inflow or
the stage at a boundary is changing rapidly, the accelera-
tion terms in the momentum equation (see Section 5-12)
become important The leading example is dam break
analysis; rapid gate openings and closures are another
example. Regardless of bed slope, unsteady flow analy-
sis should be used for all rapidly changing hydrographs.
Any information on events of record, high water marks,

5-2

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eyewilness accounts, and so on can be useful in identify-
ing such conditions. Eyewimess accounts of the J ohns-
town dam-break flood. for example, describe seiching in
a major tributary valley. Occupants of floating houses
made the trip up and down the valley several times as
the currents reversed direction. Only an unsteady flow
model with all acceieratinn terms intact is capable of
modeling such an effect on downstream hydrographs and
water levels.

b. Mild. channel slope. Unsteady flow analysis
should be used for all streams where the slope is less
than 2 feet per mile. On these streams, the loop effect is
predominant and peak stage does not coincide with peak
flow. Backwater affects the outflow from tributaries and
storage or flow dynamics may strongly attenuate flow;
thus, the profile of maximum flow may be difficult 10

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