Sediment transport and sediment properties
7.1 BASIC CONCEPTS .1 Definitions
Sediment transport is the general term used for the transport of material (e.g. silt, sand, gravel and boulders) in rivers and streams.
The transported material is called the sediment load. Distinction is made between the bed load and the suspended load. The bed load characterizes grains rolling along the bed while sus- pended load refers to grains maintained in suspension by turbulence. The distinction is, how- ever, sometimes arbitrary when both loads are of the same material.
Note
The word 'sediment' refers commonly to fine materials that settles to the bottom. Technically, how- ever, the term sediment transport includes the transport of both fine and large materials (e.g. clay, silt, gravel and boulders).
7.1.2 Bed formation
In most practical situations, the sediments behave as a non-cohesive material (e.g. sand and gravel) and the fluid flow can distort the bed into various shapes. The bed form results from the drag force exerted by the bed on the fluid flow as well as the sediment motion induced by the flow onto the sediment grains. This interactive process is complex.
In a simple approach, the predominant parameters which affect the bed form are the bed slope, the flow depth and velocity, the sediment size and particle fall velocity. At low velocities, the bed does not move. With increasing flow velocities, the inception of bed movement is reached and the sediment bed begins to move. The basic bed forms which may be encountered are the ripples (usually of heights less than 0.1 m), dunes, flat bed, standing waves and antidunes. At high flow velocities (e.g. mountain streams and torrents), chutes and step-pools may form. They consist of succession of chutes and free-falling nappes (i.e. supercritical flow) connected by pools where the flow is usually subcritical.
The typical bed forms are summarized in Fig. 7.1 and Table 7.1. In Table 7.1, colunrn 3 indi- cates the migration direction of the bed forms. Ripples and dunes move in the downstream direction. Antidunes and step-pools are observed with supercritical flows and they migrate in the upstream flow direction. Field observations are illustrated in Fig. 7.2.
Flat bed (no sediment motion)
Ripples (Fr<^)
Bed form motion
Dunes ( F r < 1 )
White waters (hydraulic jump)
V
Bed form motion
(a)
Standing waves (Fr = ^)
Antidunes ( F r > 1 )
Flow direction
•
Erosion Deposition
Dune migration (in the downstream direction) Flow direction
•
Erosion Deposition
(b)
Bed form migration
Antidune migration (in the upstream direction)
Fig. 7.1 Bed forms in movable boundary hydraulics: (a) typical bed forms and (b) bed form motion.
Table 7.1 Basic bed forms in alluvial channels (classification by increasing flow velocities) Bed form
(1)
Flow (2)
Bed form motion Comments (3) (4) Flat bed
Ripples Dunes Flat bed Standing waves Antidunes Chute-pools
No Flow (or Fr«\) NO
Fr « 1 D/S Fr < 1 D/S Fr < 1 NO Fr=l NO Fr > 1 U/S Fr > 1 U/S Step-pools Fr > 1
No sediment motion
Three-dimensional forms; observed also with air flows (e.g. sand ripples in a beach caused by wind) Three-dimensional forms; sand dunes can also be caused by wind
Observed also with wind flow
Critical flow conditions; bed standing waves in phase with free-surface standing waves
Supercritical flow with tumbling flow and hydraulic jump upstream of antidune crests
Very active antidunes
Cascade of steps and pools; steps are often caused by rock bed
References: Henderson (1966) and Graf (1971).
Notes: D/S = in downstream flow direction; Fr = Froude number; U/S = in upstream flow direction.
( b ) ^
Fig. 7.2 Field observa- tions of bed forms:
(a) Great Sand Dunes National Monument, Alamosa and Saguache Counties CO in April 1966(courtesyof Dr Lou Maher) - dune bed forms carved by Aeolian action, (b) Sand dunes in Cape River after the flood, 270km North of Clermont QLD, Australia (8 July 1983) (courtesy of Mrs J. Hacker).
Fig. 7.2 (c) Standing waves at Takatoyo beach (Japan) in June 1999 - flow from bottom rigiit to top left, (d) Gravel antidune at Brigalow Bend, Burdekin River, Australia in August 1995 (courtesy of Dr C. Fielding). Bed forms left after a peak flow of 8000 m^/s-flow from the left to the right, (e) Gravel antidune at Brigalow Bend, Burdekin River, Australia in August 1995 (courtesy of Dr C. Fielding). Bed forms left after a peak flow of 8000 m^/s - looking down- stream, with an observa- tion trench digged in the foreground.
7.2 Physical properties of sediments 155
Notes
1. Both ripples and dunes are observed with wind-blown sand and in open channel flow (Fig. 7.2a and b).
2. Note that, in alluvial rivers, dunes form with subcritical flow conditions only. Antidunes are associ- ated with supercritical flow while standing waves are characteristics of near-critical flow conditions.
3. The transition between dune and standing-wave bed forms occurs with a flat bed. The flat bed is an unstable bed pattern, often observed at Froude numbers slighlty below unity: e.g. Fr = 0.77 (Kennedy, 1963), 0.83 (Laursen, 1958) and 0.57-1.05 (Mien and Rastan, 1998).
Discussion
Ripples are associated with the presence of a laminar boundary layer. Their size is independent of the flow depth d. Usually their characteristic dimensions (length / and height h) satisfy / « lOOOc? and h < 100 J.
Dunes are associated with a turbulent boundary layer. In rivers, their size is about proportional to the flow depth (see also Table 12.3). In open channels, dunes take place in subcritical flow.
With standing-wave and antidune bed forms, the fi*ee-surface profile is in phase with the bed form profile. In natural streams, antidunes and standing waves are seldom seen because the bed forms are not often preserved during the receding stages of a flood. Kennedy (1963) investigated standing- wave bed forms in laboratory while Alexander and Fielding (1997) presented superb photographs of gravel antidunes.
7.2 PHYSICAL PROPERTIES OF SEDIMENTS