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

2
n~
$... <: ---=-
.. L~/,j

(4-10)

Equation 4-10 applies only when the model and prototype
channels are geomelrically similar in cross section. With-
out slope distortion (Sr = I), this equation would reduce
to Equation 4-9.

e. S caie distortion.

(1) Distorted scales are generaUy used in models of
river channels. floodways. harbors. and estuaries.
Movable.bed models are distorted in order to ensure the
movement of particle-size bed material under model flow
conditions. flood conlrOl projects for the improvement of
river channels through urbanized areas often require the
reproduction of long channel lengths and wide floodway
widths. Most such channels have mild slopes and the
nows are tranquil at very low Froude numbers. In order
to fit this type of model in a reasonably economical
space, the horizontal scale ratio has to be limited and
vertical scale distortion selected to give measurable depths
and slopes as well as to ensure turbulent flow in the
model. The use of distorted models should be generally
limited to problems involvin g tranquil nows. A number
of reports (USAEWES 1949a. 1949b. 1953) have been
published that illustrate the application of diSlDrted models
for the solution of complex local flood protection prob-
lems and channel improvements.

(2) The scale relations for distorted models are given
in ASCE (1942). If the bed slope ratio is made equal to
the energy slope ratio. the slope ratio will also be equal to
the amount of model distortion.

Yr
sr =
~

(4-11)

where y r is the vertical scale ratio and Lr is the hori-
lontal scale ratio. model to prototype. The Manning
equation can then be used to obtain a roughness crileria
for model design (Rouse 1950).

EM 1110-2.1601
1 Jul 91

R~/3
n... ': --=--
. L;'-

(4-12)

For a wide channel Equation 4-12 reduces to

'1-
y; .
n = - (4-13)
r ~
L;' ...

The required roughness in the model can be computed by
Equation 4-12 and used as a guide in designing the
model. Distorted models should be verified using mea.
sured field data or computed prototype data prior to
testing of improvement plans. flood conlrOl channel
models should be built to as small a dislDrtion as is eco-
nomically feasible. A distortion of 3 or less is desirable.
but depends to some extent on the type of information
needed from the model study. It may sometimes be
economically feasible to divide a long channel study into
several problem areas and model each one independenlly.
In this marmer different scales could be used as required
by the problem to be studied in each reach.

f Movable-bed models. Open channel studies
involving problems of sediment erosion. lransportalion. or
deposition require a bed of sand or other material that will
move when subjected to flow. Rouse (1950). Davis and
Sorenson (1969), and ASCE (1942) give considerable
det:til on design. construction. verification. and use of
movable.bed models. Qualitative indication of bed
movement has been used in flood conlrOl channel models
for design purposes. For example. the effectiveness of a
hydr:wlic jump to dissipate energy is often obtained
through the relative extent of oownSlre:lm scour. The sta.
bility of riprap protection can also be obtained from
model studies. A typical example of a study to determine
the relative scour and design of riprap protection at inlet
and oullet channels is given in USAED, Los
Angeles (l960a).

4-9