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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carried downstream. It is possible, however, that point
velocities in a natural channel will exceed critical veloc-
ity when the average state of flow is subcritical.

(4) Prior to performing hydraulic calculations, such
as determining water surface profiles, engineers must
determine the state of flow for the range of discharges
and depths being evaluated. When the state of flow is
subcritical (F < I), the water surface proftle is controlled
by channel characteristics at the downstream end of the
river reach. Therefore, steady flow water surface profile
computations proceed from the downstream control point
upstream (referred to as a backwater calculation). If
supercritical flow exists, calculations go from upstream 10
downstream. If the direction of the computation does not
correspond 10 the prevailing state of flow, the computed
water surface profile can diverge from the true profile
and lead 10 erroneous results. If computations proceed in
the proper direction for the state of flow, the calculated
water surface profile converges to the true profile even if
the estimated starting water surface is in error.

2-5. Regimes of Flow

There are four regimes of open channel flow, depending
on the combined effects of viscosity and gravity:
(I) subcritical-Iaminar, (2) subcritical-turbulent,
(3) supercritical-laminar, and (4) supercritical-twbulent.
The two laminar regimes are not relevant 10 natural riv-
ers because fully turbulent flow is always the case.
Therefore, determination of the flow regime for most
open channel and river hydraulics situations involves
verifying that the state of the flow is either subcritical
(F < I) or supercritical (F > I).

a. Subcr/tical flow. In rivers and channels, if the
flow is subcritical (F < I) and the bed immobile, water
will accelerate over shallow humps and obstructions on
the bottom and decelerate over deeper areas and troughs.
This is illustrated in Figure 2-3. In sand bed channels
flow separation often occurs just downstream of the crest
of the sand waves. Surface boils may appear on the
water surface just downstream from the flow separation
locations. In natural alluvial channels, the occurrence of
separation zones and increased flow turbulence leads to
increases in flow resistance and energy losses.

b. Supercritical flow. If the flow is supercritical
(F > I), water flowing over obstructions and humps will
decelerate while accelerating in the pools and troughs as
shown in Figure 2-3.(c) and (d), respectively. The

EM 1110-2-1416
15 Oct 93

interaction and effects of the flow with a mobile alluvial
bed are presented in Chapter 7.

2-6. Types of Flow

The following flow classifications are based on how the
flow velocity varies with respect 10 space and time.
Figure 2-4 shows some of the possible types of open
channel flow that occur in rivers. Each type of flow
must be analyzed using methods that are appropriate for
that flow.

a. Steady flow. A flow is steady if the velocity at a
specific location does not change in magnitude or direc-
tion with time. (Turbulent fluctuations are neglected in
these definitinns.)

b. Unsteady flow. If the velocity at a point changes
with time, the flow is unsteady. Methods for analyzing
unsteady flow problems account for time explicitly as a
variable, while steady flow methods neglect time all
Iogether.

c. Uniform flow. Uniform flow rarely occurs in
natural rivers because, by definition, uniform flow
implies that the depth, water area, velocity, and discharge
do not change with distance along the channel. This also
implies that the energy grade line, water surface, and
channel botlom are all parallel for uniform flow. The
depth associated with uniform flow is termed "normal
depth." Uniform flow is considered to be steady flow
only, since unsteady uniform flow is practically nonexis-
tent (Chow 1959). Only in a long reach of prismatic
channel of uniform roughness carrying a flow that has
been undisturbed at the reach boundaries for a long time
will the flow be uniform.

d. Nonuniform flow. Most flow in natural rivers and
channels is nonuniform, or spatially varied flow. Here,
the term "spatially varied" is to be taken in the one-
dimensional sense; i.e. hydraulic variables vary only
along the length of the river. Even if the flow is steady,
spatial variation can result from changes occurring along
the channel boundaries (e.g., channel geometry changes),
from lateral inflows to the channel, or both.

(I) Rapidly varied. If spatial changes 10 the flow
(depth and/or velocity) occur abruptly and the pressure
distribution is not hydrostatic, the flow is classified as
rapidly varied. Rapidly varied flow is usually a local

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