Encyclopedia of Environmental Science and Engineering, Volume I and II

1074 SEDIMENT TRANSPORT AND EROSION

(4) “base level” 
 the local level to which a stream tends to

cut its bed.

Lane contended that there is a natural tendency for a bal-

ance between the products in Eq. (38). For example, if one

of the factors, Q s , is decreased then, in order to balance the

equation, the stream slope might also tend to decrease, i.e.

degradation. On the other hand an increase in Q s could lead

to an increase in S, i.e. aggradation.

The Regime Approach

“The dimensions (width, depth, and slope) of a channel

to carry a given discharge, with a given silt load, are fixed by

nature, i.e. uniquely determined.” A channel whose dimen-

sions are so established is said to be in regime.

In the geological sense 3,10 a river system is never really

in equilibrium. According to W.M. Davis, who postulated a

geomorphological cycle, 3,10 the agents of uplift and gravity

(represented mainly by steam erosion) are always opposing

each other. However, from the engineering point of view, a

stream can be considered to be in “equilibrium” over a period

of a few decades if its average behavior or average dimen-

sions remain unchanged. There are always fluctuations, of

the channel geometry, about this average; thus the steam is

sometimes said to be in “dynamic equilibrium.”

Of course, a stream may be aggrading or degrading (on

the average in Engineering time) and thus it is not in equi-

librium. The regime theory could be used to predict the ulti-

mate dimensions of a stream that is not in regime.

Kennedy^40 and Lindley^41 collected data from canals in

India (Pakistan) and proposed an equation for the non-filtering,

non-scouring velocity, v,

v  C 1 y n , (38)

where C 1  0.84; n  0.64; and y  depth of flow.

Kennedy was followed by Lacey, Inglis, and Blench who

developed equations for channel slope and width.

Lacey 42,21 introduced the equations

vfR=117. (39)

0.93 mm.

0.47 mm.

0.19 mm.sand

0.27 mm.

.0001

.001

.01

.1

1

10

100

1000

.2 .4 .6 .8 1.0 2 4 610 20 40 60 80

T

T

FIGURE 11 Bed material transport function (after Bishop et al.).

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