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 1'10-2-1601
1 Jul 91

d. Bend loss. There has been no complete. sys-
tematic study of head losses in channel bends. Data by
Shulay (1950), Raju (1937), and Bagnold (1960) suggest
that the increased resisrance loss over and above that
attributable to an equivalent straight channel is very small
for values of r/W > 3.0. For very sinuous channels, it
may be necessary to increase friction losses used in de.
sign. Based on tests in the Tiger Creek Flume, Scobey
(1933) recommended that Manning's n be increased by
0.001 for each 20 deg of curvature per 100 ft of channel.
up to a maximum incre:lSC of about 0.003. The small in-
crease in resistance due to curvature found by Scobey was
substantiated by the USBR field tests (Tilp and Scrivner
1964) for r/W > 4. Recent experiments have indicated
that the channel bend loss is also a function of Froude
number (Rouse 1965). According to experiments by
Hayat (Rouse 1965), the free surface waves produced by
now in a bend QI\ cause an increase in resistance.

2-6. Special Considerations

a. Freeboard.

(1) The freeboard of a channel is the venical dis-
tance measured from the design water surface to the top
of the channel wall or levee. Freeboard is provided to
ensure that the desired degree of protection will not be
reduced by unaccounted factors. These might include
erratic hydrologic phenomena; future development of
urban areas; unforeseen embankment settlemenl; the accu-
mulation of silt. Irash, and debris; aquatic or other growth
in the channels; and variation of resistance or other coeffi-
cients from those assumed in design.

(2) Local regions where water- surface elevations are
difficult to determine may require special consideration.
Some eXanlples are locations in or neat channel curves.
hydraulic jumps, bridge piers. aransitions and drop
structures. major jUllctions. and local storm inflow SlrOC-
tures. As these regions are subject to wave-action
uncertainties in water-surface computations and possible
overtopping of walls, especially for rapid flow. conserva-
tive freeboard allowances should be used. The baclcwater
effect at bridge piers may be especially critical if debris
accumulation is a problem.

(3) The amount of freeboard cannot be fIXed by a
single. widely applicable formula. It depends in large part
on the size and shape of channel. type of channel lining,
consequences of damage resulting from overtopping, and
velocity and depth of flow. The following approximate
freeboard allowances are generally considered to be satis-
factory: 2 ft in rectangular cross sections and 2.5 ft in

2.14

trapezoidal sections for concrete-lined channels: 2.5 ft for
riprap channels; and 3 ft for earth levees. The freeboard
for riprap and earth channels may be reduced somewhat
because of the reduced hazard when the top of the riprap
or earth channels is below natural ground levels. It is
usually economical to vary concrete wall heights by O.5-ft
increments to facilitate reuse of forms on rectangular
channels and trapezoidal sections constructed by channel
pavers.

(4) Freeboard allowances should be checked by
computations or model tests to determine the additional
discharge that could be confmed within the freeboard
allowance. If necessary. adjustments in freeboard should
be made along either or both banks to ensure that the
freeboard allowance provides the same degree of protec.
tion against overtopping along the channel.

b. Sediment transport. Flood conlrOl channels with
tranquil flow usually have protected banks but unprotected
inverts. In addition to reasons of economy, it is some-
times desirable to use the channel slre:lmbed to percolate
water into underground aquifers (USAED, Los Angeles,
1963). The design of a channel with unprotected inverts
and protected banlcs requires the determination of the
depth of the bank protection below lhe invert in regions
where bed scour may occur. Levee heights may depend
on the amount of sediment that may deposit in the chan-
nel. The design of such channels requires estimates of
sediment trnnspon to predict channel conditions under
given flow and sediment characteristics. The subject of
sediment trnnsport in alluvial channels and design of
canals has been ably presented by Leliavslcy (1955).
Fundamenlal information on bed-load equations and their
background with examples of use in channel design is
given in Rouse (1950) (see pp 769-857). An excellent
review with an extensive bibliography is available (Chien
1956). This review includes the generally accepted
Einstein approach to sediment transport. A comparative
lre:ltment of the many bed-load equations (Vanoni.
Brooks, and Kennedy 1961) with field data indicates that
no one formula is conclusively bener than any other and
that the acc=y of prediction is about :100 percent. A
recent paper by Colby (1964b) proposes a simple, direct
method of empirically correlating bed. load discharge
with mean channel velocity at various now depths and
median grain size diameters. This procedure is adopted
herein for rough estimates of bed-load movement in nood
cona-ol channels.

c. Design curves. Plate 27 gives curves of bed-load
discharge versus channel velocity for three depths of now
and four sediment sizes. The basic ranges of depths and