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

velocities have been eXlI':lpolated and interpolated from
the curves presented in Colby (1964a) for use in flood
conlrOl channel design. Corrections for water temperatUre
and concenlration of fme sediment (Colby 1964a) are not
included because of their small influence. The curves in
Plate 27 should be applica!lle for estimating bed-load dis-
charge in channels having geologic and hydraulic charac-
teristics similar to those in the channels from which the
basic data were obtained. The curves in this plate can
also be used to estimate the relative effects of a change in
channel characteristics on bed-load movemenL For exam.
pie, the effect of a series of check dams or drop sO"Uctures
that are provided to decre:lSC channel slope would be
reflected in the hydraulic characteristics by decre:1sing the
channel velocity. The curves could then be used to esti-
mate the decrease in sediment load. The curves can also
be used to approximate the equilibrium sediment dis-
charge. If the supply of sediment from upstream sources
is less than the sediment discharge computed by the rating
curves. the approximate amount of streambed scour can
be estimated from the curves. Similarly, deposition will
occur if the sediment supply is greater than the sediment
discharge indicated by the rating curves. An example of
this is a large sediment load from a small side channel
that =s deposition in a major flood channel. If the
location of sediment deposition is to be conlrOUed. the
estimated size of a sediment detention facility can be
approximated using the curves. An example of the use of
a sediment discharge equation in channel design is given
in USAED. Los Angeles (1963).

2-7. Stable Channels.

a. General.

(I) The design of stable channels requires that the
channel be in material or lined with material capable of
resisting the scouring forces of the tlow. Channel armor-
ing is required if these forces are greater than those that
the bed and bank material can resist. The basic principles
of stable channel design have been presented by Lane
(1955) and expanded and modified by Terrell and Borland
(1958) and Carlson and Miller (1956). An outline of the
method of channel design to resist scouring forces has
been given in Simons (1957). The most common type of
channel instability encountered in flood conlrOl design is
scouring of the bed and banks. This results from rela-
tively large discharges. steep channel slopes, and nonnally
limited channel right-of-way widths. These factors fre-
quently require the use of protective reveunent to prevent
scouring.

EM 1'10-2-1601
, Jul 91

(2) \\-nile clay and silt are fairly resistant to scour.
especially if covered with vegetation. it is necessary to
provide channel revetment when tractive forces are suffi-
ciently high to cause erosion of channels in line material.
Little is known about the resistance of clay and silt to
erosion as particles in this sm: range are influenced to a
large extent by cohesive forces. A summary of some of
the effects is given by the Task Committee on Preparation
of Sedimentation Manual (1966). Suggested maximum
limiting average channel velocities for noncohesive
materials are listed in c below and plotted in Plate 28.

b. Prevention 0{ scour. Scour and deposition occur
most commonly when pllrticle sizes range from fme sand
to gravel. Le.. from about 0.1 mm through 50 mm
(plate 28). Erosion of sands in the lower range of sizes is
especially critical as the sand particle weight is small.
there is no cohesion between grains, and there is usually
little vegetation along the channel. This pllrticle size
range comprises the majority of the bed and suspended
load in many stre:lms. Paragraph 2-6 above discusses
sediment movement and presents a sediment rating curve
as a guide to predicting channel stability.

c. Pennissible velocity and shear. The permissible
velocity and shear for a nonerodible channel should be
somewhat less than the critical velocity or shear that will
erode the channel. The adoption of maximum permissible
velocities that are used in the design of channels has been
widely accepted since publication of a table of values by
Fortier and Scobey (1926). The latest information on
critical scour velocities is given by the Task Committee
on Preparation of Sedimentation Manual (1966). Table 2-
5 gives a set of permissible velocities that can be used as
a guide to design nonscouring flood cona-ol channels.
Lane (1955) presents curves showing permissible channel
shear Slressto be used for design. and the Soil Conserva-
tion Service (1954) presents information on grass-lined
channels. Departures from suggested permissible velocity
or shear values should be based on reliable field experi-
ence or laboratory tests. Channels whose velocities and/or
shear exceed permissible values will require paving or
bank revea-nenL The permissible values of velocity
and/or shear should be detennined so that damage exceed-
ing normal maintenance will not result from any flood
that could be reasonably expected to occur during the
service life of the channel.

2-15