2 Erosion and sedimentation processes
2.3 WATER COURSES
2.3.1 Sediment Movement
2.3.1.1 Sediment Characteristics
Sediments can be divided into cohesive and non-cohesive categories. When resting on the bed or banks of a channel cohesive sediments are held together by electrostatic/chemical bonds which resist erosive forces. Once in motion, however, they lose this bond to a certain extent, and may become non-cohesive in terms of further transport. The size of cohesive sediments is generally less than about 0.06 millimetres but there is some latitude in this figure. Non- cohesive sediments consist of larger particles, the movements of which are determined by the physical properties of the individual particles and the imposed hydro-dynamic forces. In some
circumstances river beds will consist of a non-homogeneous mixture of both types of sediment.
Particle size. Particle size has a direct bearing on the transportability of the sediment.
Sizes range from upwards of 2 metres equivalent diameter (very large boulders which roll along the bed of mountain torrents) to less than 0.0005 millimetres (very fine clay which remains almost indefinitely in suspension, assuming no flocoulation).
Naturally occurring sediments are irregular in shape and hence the definition of "size"
in terms of a single dimension of length is a simplification which can occasionally be mislead- ing. The following dimensions are commonly adopted:-
Sieve diameter - this is the size of sieve opening through which the particle will sand to coarse gravel). For finer particles sedimentation diameter is determined by settling analysis methods. Nominal diameter and triaxial dimensions are normally used for sizes greater than about 75 millimetres.
Naturally occurring sediments normally contain a range of particle sizes which can be displayed as a cumulative frequency curve. Figure 2.6 shows the size distribution of a range of sediments and indicates clearly that coarse river sediments tend to exhibit a larger geo- metrical standard deviation in sizes than fine river sediments.
Particle shape. Particle shape is one factor which influences the rate of sediment tran-
j
0.195 Miss. larbert La.
80 -. 0.275 Niobrara 0.59
70 - 0.90 Mountain Creek 29.72
60 _ 47.50
k7.09
I KY- ,‘.’
,j , .:.:
.(/..;; _’ ’ t/ *;.’
,!/ ;;‘:’ //
50 40 30 20 10
wo.03 0.05 0.1 0.5 1.0 5.0 10.0
DIMENSIONLESS GRAIN SIZE COMPOSlTlON (Di ID&
Fig. 2.6 Grain size distributions. From Sediment transport: an appraisal of methods (HRS INT 119), by W.R. White, H. Milli and A.D. Crabbe, 1978. (Crown copyright.
Reproduced by permission Controller HMSO, courtesy Hydraulics Research Station, Wallingford, England).
IO’
D SPHERE DIAMETER
IO3
KINEMATIC VlSCOSlTY
WD
u y SPECIFIC WEIGHT OF FLUID
IO2 g ACCELERATION OF GRAVITY
Id FOLYSTYRENE .03-.05
Fig.
40
2.7 Settling velocities. From Mechanics of sediment transport, by M.S. yalin (publi- shed by Pergamon Press, 1977).
_~---_- -..
2.3.1.2 Fall Velocity
Particle fall velocity is frequently used in predictive techniques for assessing sediment move- ment. It is a fundamental property of the sediment/water system which depends on the size, shape and density of the particle and the density and viscosity of water, Yalin (1972) has presented a non-dimensional plot, see figure 2.7 which provides the generalised relationship for spherical particles. Particle shape modifies this relationship and the deviations are most pronounced for large particles with high fall velocities, see Graf (19711, Romanovsky (1972), ASCE (1975).
2.3.1.3 Threshold of movement
As the flow of water over a bed of loose granular material is increased a point is reached when a few grains are dislodged by the flow and move a small distance in the direction of the current.
This condition, although it is difficult to define in an exact manner, is known as the threshold of movement. It has been described by many authors e.g. Shields (19361, Grass (1970), White,S J
(1970), Neil1 (1967), White, W R (1972), and Issledovanie (1976), in terms of the physical properties of the sediment and the imposed shear stresses. A convenient non-dimensional pre- sentation of the functional relationships is given in figure 2.8.
-
\ -
-\
-\
-
-
-
-
4 6
: f t . . . 1
8 1 2 4 6810 2 4 68102 2 4 6 8 lo3
09r
Fig. 2.8 Threshold of sediment movement. From Sediment transport in channels; a general function (HRS INT 104), by W.R. White, 1972. (Crown copyright. Reproduced by permission Controller HMSO, courtesy Hydraulics Research Station, Wallingford, England).
2.3.1.4 Bedload steps with comparatively long intermediate rest periods.
3. The average step made by a bedload particle is largely independent of the flow condition, the transport rate and the composition of the bed.
4. Different rates of transport are achieved by the individual particles moving more or less often.
Engineers, agronomists, environmentalists and others are concerned with the rate of tran- sport of the bedload and predictive methods have been developed within the last 100 years or so fine material entering the reach under consideration and the availability of erodible material within the reach. This is the fundamental difference between the fine material load and the rest of the suspended load.
42
_
SUSPENDED LOAD
DIMENSIONLESS GRAIN SIZE, D,
Fig. 2.9 Criteria for suspension of sediment. From Sediment transport; an appraisal of methods (HRS INT 119), by W.R. White, H. Milli and A.D. Crabbe, 1978 (Crown copy- right. Reproduced by permission Controller HMSO, courtesy Hydraulics Research Station, Wallingford, England).
A Typical ripple pattern, FCC1 E Plane bed, F< 1 and d( 0.4 mm
Weak boil -zzr
F Standing waves, F)l B Dunes with ripples superposed, F(( 1
,804
G Antidunes, F)l
LJ Washed-out dunes or transition, F(1 H Antidunes. F)l
Fig. 2.10 Bed features. From The Effect of bed roughness on depth-discharge relations in alluvial channels, by D-B. Simons, and E.V. Richardson, in U.S. Geological Survey Water-Supply Paper 1498-E, 1962.
6.0
antldunes
I
-0RMS OF BED ROUGHNESS
0 TransItlo”
RAPID FLOW A Plane l Standma waves
0 0.2 0.4 0.6 08 1.0
DEPTH, IN FEET
Fig. 2.11 Bed roughness. From Flume studies using medium sand (0.45 mm), by D.B. Simons, E.V. Richardson and M.L. Albertson, in U.S. Geological Survey Water-Supply Paper 1498-A, 1961.
44
2.3.1.7 Bed Features
River channels exhibit a variety of bed features (or forms) as follows:-
Plane bed. This is the condition where the bed, when measured over a wide area, is sensibly a plane surface. This does not mean that the bed is hydraulically smooth, however, the sediment forming the bed exhibits a grain roughness associated with individual particles.
Plane beds, see figure 2.10 can occur when there is no movement of sediment and under certain metrical, see figure 2.10. They are more two-dimensional inthatthey exhibit continuous crests across the stream. Movement is downstream and results from sediment toppling over the crest of move steadily upstream although individual sediment particles move rapidly downstream. The sand and water waves increase in heightuntil they break. This appears to cause a sudden reduc- tion in height. There is thus a continuous cyclical variation of wave heights.
References by Yalin (1972), Graf (1971), Raudkivi (1967), Znamenskaja (1976), Kennedy (1969), and Simons and Richardson (1960 and 1961) provide descriptive and analytical treatments of the topic of bed features. Figure 2.11 is taken from Simons and Richardson (1960) and sug- gests that the type of bed feature can be related to the local Froude number, Fr. This, how- ever, is probably an over-simplification of a complex hydro-dynamic problem.
2.3.1.8 Armouring of the Bed
Channel characteristics are discussed extensively by Ackers and Charlton (1970), and the follo- wing two sections are extracted largely from this reference.
2.3.2.1 The Geometry of Stable Channels
Natural channels rarely remain straight for distances greater than about ten channel widths (Leopold and Wolman 1960) and thus it would appear that a straight alignment is unusual under and bank materials of natural streams, irregular patterns frequently occur. Schumm (1963) clas- sified channels into five groups: tortuous, irregular, regular, transitional and straight.
Ackers and Charlton (1970) considered three groups: (a) straight or slightly irregular channels, (b) regular sinuous or meandering channels and (c) braided channels (i.e. multithread systems divided by islands or shallows).
2.3.2.2 Factors Affecting Channel Geometry
Although meandering channels have been studied in both field and laboratory for many years and many hypotheses put forward, there exists today no completely satisfactory explanation for their development. Local disturbances due to variations in bank material or obstructions, excessive energy of the stream, variations in stage, the existence of secondary currents, the development of convection currents and the occurrence of a transverse seiche have all been suggested as possible causes of a meander pattern (Fujihoshi 1950, Werner 1952, White C M 1939).
In spite of the various hypotheses, however, the true cause is still obscure. There exists no generally accepted means of predicting, under given conditions of fluid and sediment flux, whether meanders will occur, although there is considerable empirical information which is of value in predicting aspects of geometrical pattern which might result. There remain con- siderable differences of opinion about the types of streams which meander. Werner (1952) for example, states that meandering streams are noted for being shallow and that meandering does not occur if the water depth exceeds a certain value. Be also states that a certain bedload is necessary to initiate meandering and that superfluous silt content may prevent meandering.
Schumm (1963), on the other hand, believes that relatively wide shallow channels tend to be straight whereas relatively narrow deep channels tend to meander, but he also concluded that silt content affects the geometry, (Schumm 1960 and 1967).
Leopold and Wolman (1960) agree with Werner (1952) suggesting that in channels which are deep in relation to their width the velocity is nearly constant with depth, hence, helical cir- culation becomes negligible and in curved channels potential flow approximates. This results in an inverse variation of downstream velocity with the radius of the stream lines; the thread meandering channels offer larger resistance to flow than otherwise comparable straight channels.
This is supported by Blench and Qureshi (1964) who have stated that meandering channels of low forward by Werner (19521, who also suggested that superfluous sediment would prevent meandering, and by Inglis (1947) in his definitions of primary and secondary meandering. On the other hand, Schumm (19601, on the basis that sinuosity is not conducive to bedload transport, has stated that a decrease in sediment supply causes a channel to become narrower and steeper and increase its tendency to meander.
Ackers and Charlton (1970) carried out exhaustive laboratory tests on small artificialchan- nels in fine cohesionless sand and their main conclusions were as follows:- irregularities, and the regular sinuous channels.
(3) Excluding certain results which are suspect because of the growth of algae, the me-
46
ander length under steady conditions at flows between 0.25 and 2.0 cusecs in 0.15 millimetres sand correlates well with discharge, the best-fit equation being:
k=38.0~"~~~ (ft Set units).
(4) The width of a meandered channel is at least twice that of a straight channel in the same sediment at the same discharge: channels with prominent shoals that nevertheless remain straight are less than twice as wide.
(5) A multiple correlation of meander length with discharge and sediment concentration (i.e. sediment flux as proportion of fluid flux) does not prove any appreciable dependence on assumption is probably true for laboratory channels with constant flows and constant supplies of sediment. It can also be regarded as true for long reaches of natural channels where sig- nificant changes only occur over very long periods. These channels are not in equilibrium in terms of geological time spans but they can be regarded as such in terms of the human life span or the expected life of engineering works.
There are many situations, however, where conditions are not in equilibrium and a dispa- rity between sediment supply and sediment transporting capacity causes either erosion or de- position. cantly effects the economic viability of proposed schemes and methods of evaluating sedimenta- tion are discussed in more detail later.
causes a reduction in flow velocities which, in turn, results in deposition of some or all of the fine sediments in suspension.
Deposition on the flood plain can be detrimental to the environment if road and railways are rendered unusable but it is usually beneficial to agricultural land because of the enhanced cover of nutritious soil.
(3) Estuaries. In estuaries sediment movement is influenced additionally by the oscil- latory movement of the tides and the change from a fresh to a salt water environment. Deposi- tionoffine alluvial sedimentsis acommon feature of estuaries and this causes a reduction in cross-sectional area of the waterway. The engineering consequences of this may take the form of increased flood levels upstream of the estuary or difficulties in navigation due to the decrease in water depths. Sedimentation in estuaries is a difficult analytical problem which is discussed later.
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