Encyclopedia of Environmental Science and Engineering, Volume I and II

1068 SEDIMENT TRANSPORT AND EROSION

Shen,^15 Karaki,^15 Graf,^16 Brown,^1 Simons, and Senturk and

the ASCE Sedimentation Engineering Manual.

In some instances^15 turbulence flumes (a concrete lined

reach with baffles to create severe turbulence) have been con-

structed across a stream channel in order to suspend the bed

load and thus to sample it by suspended load techniques.

THE MECHANICS OF SEDIMENT TRANSPORT

IN A STREAM

General

The nature of sediment transport in a stream depends on the

shear intensity of the flow and the type of bed material. The

diagram in Figure 5 shows the sequence of bed forms (waves)

associated with increasing levels of shear on a fine granular

bed material.^3 This figure also shows, schematically, the typi-

cal changes in the Darcy friction factor and the sediment con-

centration with increasing flow velocity. The primary mode

of transport of particles, in the case of ripples,^17 is discrete

steps along the bed; however with increasing shear more of

the bed material becomes suspended until the particle motion

is nearly continuous for anti-dunes.

Dunes and ripples are triangular in shape with relative

flat upstream slopes and sleep downstream slopes. The

water surface waves are out of phase with the dune forma-

tion while ripple formations appear to be independent of the

free surface.

Dune wave lengths,  d , are related to the depth of flow

and in general,

l d  3 feet (2)

whereas ripple wave lengths  r are shorter,

2"  lr  18" (3)

Dune heights, H d , are related to the depth of flow, with the

limiting height approaching the average flow depth. The

ratio of dune length to height is given by^17

815 

l

Hd

. (4)

The maximum ripple height is about 0.1 feet.

Both ripples and dunes progress downstream. The tran-

sition from dunes to anti-dunes occurs at a Froude number

close to 1.0.

Anti-dunes, as indicated by Figure 5, are in phase with

the surface wave. The may be stationary or move upstream.

The maximum height of an anti-dune is approximately equal

to the flow depth at the trough of the surface wave.

Simons, 17,18 on the basis of experimental data, developed

the relationship shown in Figure 6 between stream power and

bedform for varying fall diameters. Simons and Richardson^19

also studied the variation of Chézy’s C with bed form. Their

results are summarized in Table 2.

0 123456

V feet/sec.

Friction

Factor

f

0.02

0.04

0.06

0.08

0.1

1

10

100

1,000

10,000

100,000

Concentration of

Bed Material

C ppm.

Flat

Bed

Ripples Dunes Transition Antidunes

C

f

FIGURE 5 The behavior of a mobile stream bed (adapted from Leopold, Wolman, and Miller).

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