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

Table 2-1
Acceptable Equivalent
Roughnes4 Values

Design Problem

k,ft

Dischlllge Capacity

0,007

Maximum VEHocity

0.002

Proximif to Critical
Depth

Tranqu. Flow

0,002

Rapid Flow

0,007

Nots:
1. To pr9Vent Und8Sira~8 undulating waves, ratios of flow depth
to crilieal depltl between 0.9 ana 1,1 should be avoided where
economically feasible.

1959). These values will normally be much larger than
the spherical diameters of the bed materials to account for
boundary irregularities and sand waves. When friction
coefficients can be determined from experienced flow
information. k values should then be computed using the
relations described in Equation 2-6. The k values so
determined apply to the surfaces wetted by the
experienced flows. Additional wetted surfaces at higher
stages should be assigned assumed k values and an
effective roughness coefficient computed by the method
oudined in Appendix C if the incre:lSed wetted surf:lCCS
are estimated to be appreciably smoother or rougher.
Values of k for nall1r.li channels may also be estimated
from Figures 8 and 9 of Chow (1959) if experimental d:Ita
are not available.

d, Flow ciassijicarion, There are several different
types of flow classification. Those treated in this para-
graph assume that the channel has a uniform
cross-sectional rigid boundary. The concepts of ttanquil
and rapid flows are discussed in (1) below. The applica-
bility of the newer concepts of steady rapid flow and pul-
sating rapid flow to design problems are tre:lted in (2)
below. All of these concepts are considered from the
viewpoint of uniform flow where the water,surface slope
and energy grade line are parallel to the bottom slope.
F10w classification of nonuniform flow in channels of
unifonn solid boundaries or prismatic channels is dis-
cussed in (3) below. The design approaches to flow in
nonprismatic channels are treated in other portions of this
manual.

(1) Tranquil and rapid flows.

EM 1110-2-1601
1 Jul 91

(a) The dislinction between ttanquil flow and rapid
flow involves critical depth. The concept of specific
energy H" can be used to derme critical depth. Specific
energy is dermed by

He = d + 11 v2
7g

(2-8)

where

d = depth

11 = energy cortection factor

V2{2g = velocity he:ld

Plate 6 shows a specific energy graph for a discharge q
of 100 cubic feet per second (cfs) (two-dimensional
flows). E:lch unit discharge has its own critical depth:

( 2 )'13
d = SL
. 9

(2-9)

The development of this equation is given by pp 8-8 and
1l-9 of Brater and ICing (1976). It may be noted that the
critical depth occurs when the specific energy is at a
minimum. Flow at a depth less than critical (d < de) will
have velocities greater than critical (V > V.,), and the
flow is described as rapid. Conversely, when d > <1.
and V < Ve . the flow is ttanquiL

(b) It may be noted in Plate 6 that in the proximity
of critical depth. a relatively large change of depth may
occur with a very small variation of specific energy.
F10w in this region is unstable and excessive wave action
or undulations of the water surface may occur.
Experiments by the US Army Engineer District (USAED).
Los Angeles (1949). on a rectangular channel estab1islled
criteria to avoid such instability, as follows:

Tranquil flow: d > 1.1<1. or F < 0.86
Rapid flow: d < 0.9<1. or F > 1.13

where F is the flow Froude number, The Los Angeles
District model indicated prototype waves of appreciable
height oc:ur in the unstable range. However. there may

2-3