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

In swiftly flowing streams, the superelevation of the
water surface on the outside of a bend, required to accel-
erate the water Iowards the inside in making the turn,
needs not disrupt the one-dimensionality of the flow from
the computational standpoint. The superelevation is
predictable from the one-dimensional computed velocity
and the bend radius, and can be added 10 the water sur-
face elevation at the stream axis after this has been com-
puted. For a third example, a strong cross wind in a
wide shallow estuary can generate water surface eleva-
tinns considerably greater on the downwind bank than on
the main axis of the channel.

e. Determination of flow dimensionality. It is not
possible to state with theoretical certainty that a given
reach can be assumed one-dimensional unless multi-
dimensional studies on the reach have been carried out
and compared to the results of a one-dimensional
approach. As a practical rule of thumb, however, if the
reach length is more than twenty times the reach width,
and if transverse flow and stage variations are not specif-
ically of interest, the assumption of one dimensionality
will likely prove adequate. Events of record in wide
reaches can yield indications of susceptibility 10 strong
cross winds or large transverse differences in atmospheric
pressures. The hislory of flooding in the reach should be
studied for potential sources of significant transverse
disturbance. As an extreme example, it was the massive
failure of the left bank, which fell inlo the reservoir, that
produced the catastrophic overtopping of Viaont Dam in
Italy in 1963, and it was the ride up of the resulting
wave from the dammed tributary which crossed the chan-
nel of the main stream, the Piave River, and obliterated
the town of Longarone. In most cases departures from
strictly one-dimensional flow are confmed to regions in
the vicinity of local disturbances. Expansions and con-
tractions in cross sections lead to transverse nonuniform
velocity distributions and, if severe enough, in water
surface elevations as well. These local effects are
usually accounted for in a one-dimensional analysis by
adjusting coefficients for head loss.

f Composite channels. The concept of a composite
channel is typically used to account for retardation of
flow by very rough floodplains in a one-dimensional
analysis. It is assumed that, with a horizontal water
surface and energy slope common 10 main channel and
overbank flows, the total discharge can be distributed
among the main channel and overbanks in proportion to
their individual conveyances. The different length trav-
eled by the portion of the flow in the floodplains can, in
principle, be accommodated by computing three

2-2

contiguous one-dimensional flows, the main channel, and
the right and left floodplains (Smith 1978, U.S. Army
Corps of Engineers I99Ob).

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g. Floodplains. A river rtSmg rapidly and going
overbank may take significant time 10 inundate the flood-
plain. The transverse water surface will then not be
horizontal and will slope downward (laterally outward
from the main channel) to provide the force for the flood
proceeding up the floodplain. The cross-sectional area
for carrying the streamwise flow will then be less than
that under a horizontal line at the elevation of the water
surface in the main channel. In the absence of two-
dimensional computations, information from past records
of the timing of floodplain inundation should be com-
pared 10 rise time in the main channel to determine the
importance of this effect.

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h. Networks. While a network of interconnected
streams is surely two-dimensional, lite individual chan-
nels comprising each reach of the network can usually be
treated as one-dimensional. In some cases of multiple
flow paths, such as through bridges crossing wide flood-
plains with multiple asymmetric openings, the flow dis-
tribution may be difficult to determine and the water
surface elevation substantially non-horizontal; in such
cases, two-dimensional modeling may be preferable
(U.S. Department of Transportation 1989).

.

2-3. Water Waves

a. General. Water flowing (or standing) with a free
surface open to the atmosphere is always susceptible to
wave motion. The essence of wave motion exists in the
concept of the propagation of disturbances. If a given
flow is perturbed by something somewhere within its
boundaries, some manifestation of that perturbation is
transmitted at some velocity of propagation 10 other
portions of the water body. There are different catego-
ries of water waves, many of which are not pertinent 10
river hydraulics studies. A pebble cast into a body of
water generates waves which radiate from the point of
entry in all directions at speeds, relative to the bank,
dependent upon the water velocity and depth. In still
water they radiate as concentric circles. The concept of
wave propagation depending upon wave celerity and
water velocity is common 10 the analysis of all water
waves. The waves generated by a dropped pebble are
usually capillary waves, whose celerity is strongly depen-
dent upon the surface tension at the air-water interface.
They are unrelated 10 river hydraulics except that they

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