FLOOD03351

• County: Statewide
• Basin: Statewide
• Title: Engineering and Design Hydraulic Design of Flood Control Channels
• Date: 7/1/1991
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

EM 1110-2.1601
1 Jul 91

4-5 Hydraulic Model Studies

a. General. The use of hydraulic models has
become a standard procedure in the design of cQmplex
open channels not subject to analytical analyses or for
which existing design criteria based on available model
and field tests are inadequate. Hydraulic models afford a
means of checking peIformance and devising modifICa-
tions to obtain the best possible design at minimum cost.
Model tests should be used to supplement but not replace
theoretical knowledge, good judgment. and experience of
the design engineer. They often indicate design changes
that save substantial amounts in consa-uction costs as well
as effect improvements in operation. Model tests of large
flood conlrOl channels are generally desirable where sup-
ercritical flow results in standing waves and other major
disturbances in channels containing junctions, transition
SlrUClUreS, alignment curvature, multiple bridge piers, or
stilling basins.

b. Model design.

(I) The theory of model design is lre:lted in
EM 1110-2-1602 and other publications (Rouse 1950,
Davis and Sorenson (1969), American Sociery of Civil
Engineers (ASCE) 1942). For open channel models, the
gravity force will dominate the now and similitude will
require equality of Froude number in the model and pro-
totype. The Froudian scale relations (model-to-prototype)
in Table 4-1 apply to undistorted models. The length ratio
Lr is the model-to- prototype ratio Ln/Lp. These
lransfer relations are based on equal force of gravity and
density of fluid in model and prototype. The procedure
for initiation of model studies is discussed in
EM 1110-2.1602.

(2) Model scale ratios for nood conlrOl channels
have ranged from 1:15 to 1:70. depending on the type of
problem being studied. the relative roughness of the
model and prototype, and the size of the prototype SlrOC-
ture. Scale ratios of 1:15 to 1:30 are usually employed

where supercritical flow wave problems are involved.
They are also used for sectional models of drop SlrUC-
lUreS, spillways, etc. The smailer scale ratios (1:30 to
1:70) are used for general model studies where long chan-
nel lengths are reproduced. The accuracy of possible
model consa-uction and flow measurements may conlrOl
the permissible scale rntios. Most models of channels are
generally built to give depths of flow about 0..5 ft or more
and channel widths of about I to 2 ft. The most common
scale ratios ased by the USAED, Los Angeles. Hydraulic
Labol3lOry for channel model studies are from 1:25 to
1:40.

c. Model roughness. Turbulent flow will prevail
with model channel velocities and depths commonly used
in testing. In most cases, the channel now is
rough-turbulent or nearly so: therefore, hydraulic resis-
tance is determined primarily by the relative size of the
roughness elements. However, the model Reynolds num-
ber will always be smaller than the prototype. and this
will to some extent c:luse scale distortion of certain phe-
nomena such as zones of separation. wave dissipation.
flow instability, and turbulence in the model. Particular
care should be taken in interpreting those effects that are
known to be slrOngly dependent on viscous forces.

d. Slope distorrion. An empirical equation of the
Manning type may be used to give the required model
roughness (Rouse 1950) for large-scale models where
fully rough-turbulent flow prevails. This condition is
expressed by the equation

n = L 1/6
r r

(4-9)

If this roughness criterion cannot be fulfilled. slope ad-
justment or distortion must be applied to the model so
that prototype flow conditions can be simulated in the
model The amount of additional slope required is given
by the equation (Rouse 1950)

Table 4-1
Scale R.lelione
MannIng'S
Langm Area Volume Tune Velocity Discharge n
L L2 L3 L1/2 L1/2 L5/2 Ll15
r. r r r r r r

4-8