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S. A. SCHUMM
H. R. KHAN
Department of Geology and Engineering Research Center, Colorado State University,
Fort Collins, Colorado 80521
Experimental Study of Channel Patterns
ABSTRACT
A series of experiments was performed in a
large flume to determine the effect of slope and
sediment load on channel patterns. Sediment
loads and slopes were closely related, and as
slope and sediment loads increased, threshold
values of these variables were encountered, at
which channel patterns altered significantly. At
a very low slope and sediment load, the
channels remained straight, but at a discharge
of 0.15 cfs, a meandering-thalweg channel
formed at slopes greater than 0.002. With increased slope and sediment loads, thalweg
sinuosity increased to a maximum of 1.25. At
slopes greater than 0.016, a braided channel
formed. The model channels responded to increased sediment loads by maintaining steeper
gradients and by major channel pattern
changes, but at very gentle slopes and at steep
slopes, the channel could not be forced to
develop a meandering thalweg.
These experiments suggest that landforms
may not always respond progressively to altered conditions. Rather, dramatic morphologic changes can occur abruptly when critical
erosional and (or) depositional threshold values
are exceeded.
The meandering-thalweg channel was not a
meandering channel. A truly meandering
channel with a sinuosity of 1.3 formed when a
suspended-sediment load (3 percent concentrations of kaolinite) was introduced into the
flow. The clay stabilized the alternate bars, and
scour and deepening of the thalweg resulted.
This in turn lowered the water level at constant discharge, and the alternate bars emerged
to form point bars. A meandering-thalweg
channel was thus converted to a meandering
channel by the type of sediment load change
that has accompanied climatic and hydrologic
changes of the recent geologic past.
INTRODUCTION
A review of geomorphic and engineering
literature reveals that diverse patterns of rivers
have elicited equally diverse hypotheses to explain their origin. These range from purely
hydraulic (Callander, 1969; Shen and Komura,
1968; Tanner, 1960) and stochastic (Scheidegger, 1970, p. 231-234; Langbein and Leopold,
1966; Thakur and Scheidegger, 1968) to
geologic (Schumm, 1963). Efforts to resolve
these differences through quantification of
meander characteristics have produced empirical relations between channel and meander
dimensions and discharge and sediment characteristics (Jefferson, 1902; Leopold and
Wolman, 1960; Dury, 1964; Schumm, 1968),
but a generally acceptable explanation has not
been forthcoming.
Experiments in the hydraulic laboratories of
the world have concentrated on the phenomenon of meandering, but, in fact, it has not
been possible to induce a laboratory channel to
develop a truly meandering course. The excellent and oft-cited research by Friedkin, for
example, produced a channel with a meandering thalweg, but the channel itself did not
meander. This assertion is confirmed by his
statement (Friedkin, 1945, p. 8) that "Practically all photographs were taken at extremely
low flow so as to define the pattern of the
stream"; that is, in order to photograph the
meander pattern in his experiments, the water
level was lowered until the alternate bars
emerged, and the water was confined to the
low-water channel or thalweg (Figs. 3 and 4).
Friedkin's cross sections show clearly that at
bank-full stage the alternate bars were submerged. Therefore, the sinuosity of his channel
was very low, and it was a thalweg sinuosity
rather than a channel sinuosity that developed.
It appears then that model studies of river
meanders have not duplicated the field condition of a truly sinuous bank-full channel (for
example, see the more recent results of Wolman
and Brush, 1961; Ackers, 1964; Ackers and
Charlton, 1971).
The one factor lacking in these experimental
investigations appears to be consideration of
the changes of quantity and type of sediment
load that has occurred in natural rivers as a
Geological Society of America Bulletin, v. 83, p. 1755-1770, 22 figs., June 1972
1755
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1756
SCHUMM AND KHAN
result of climatic changes during the recent
geologic past. The change from predominantly
bed load to suspended load apparently caused
the transition from straight to highly sinuous
channels on the alluvial plain of the Murrumbidgee River of eastern Australia, and it is
suggested that similar changes have probably
caused the meandering of modern rivers of the
great plains of western United States and of
the Mississippi River (Schumm, 1968).
A series of experiments was designed to test
this theory in the hydraulic laboratory. In
short, the work of Friedkin (1945), Tiffany
and Nelson (1939), Quraishy (1944), Ackers
(1964), and others would be duplicated and
then an attempt to induce a meanderingthalweg channel to adopt a sinuous course by
changing the type of sediment load introduced
into the channel would be made. The model
river would be converted by appropriate manipulations of sediment load from a bed-load
channel to a suspended-load channel, and the
pattern should then change from straight with
alternate bars (meandering thalweg) to meandering.
In addition to Friedkin's (1945) basic study,
the generalizations by Leopold and Wolman
(1957) and Lane (1957) concerning the influence of water discharge and slope on river
patterns are significant. These workers found
that if data for mean annual discharge or mean
annual flood are plotted against gradient,
braided rivers plot above the position of
meandering rivers. In fact, for a given discharge
there appears to be a threshold slope above
which a braided channel pattern develops.
Further, it has been suggested by Lane (1957,
p. 38) and Ackers (1964), and confirmed by
recent experiments performed by Ackers and
Charlton (1971), that at very low slopes another threshold exists between straight and
meandering-thalweg channels.
Thus, if one wishes to model a range of river
patterns, gradient is a variable that must be
changed. Although slope can be changed at
will in the laboratory, in the field it is established in relation to discharge and sedimentload characteristics, and it is, therefore, a
dependent variable.
The primary objective of these experiments
was to develop a truly meandering channel.
Moreover, if a series of channel types from
straight through meandering to braided were
to form in response to changes of slope, then
the entire range of channel and thalweg
patterns could be investigated experimentally,
and the existence of threshold slopes could be
either confirmed or rejected.
EXPERIMENTAL PROCEDURE
Equipment
All of the experiments were performed in a
concrete recirculating flume that is 100 ft long,
24 ft wide, and about 3 ft deep. This facility is
located at the Engineering Research Center,
Colorado State University (Fig. 1). The flume
has double walls along its length. The inner
wall of each pair is permeable, and therefore the
water level or water table in sedimentary
material within the flume can be regulated by
controlling the water level between the double
walls.
A steel bridge spans the width of the flume
and is mounted on rails on the inner walls. The
bridge is moved by an electric motor drive, and
it is equipped with a point gage for measuring
elevations of the sand surface. Scales were installed along the length of the flume and on the
Figure 1. Straight channel. Water introduced into
an initial bend at a slope of about 0.001; channel
width 1.2 ft.
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EXPERIMENTAL STUDY OF CHANNEL PATTERNS
1757
bridge; these were used to establish a co-ordinate system, and channel morphology was
measured in relation to it.
Both water and sediment passed through the
model channel and entered a tail box at the
end of the flume. A 2^-in. axial flow pump was
used for recirculating the water. Discharge was
measured with a manometer, connected to a
2-in. Venturi meter. A smaller pump was also
installed to pump water into the side channels
and tail box from the sump located under the
floor of the laboratory.
Procedure
The flume was filled to a depth of 2 ft with
sand having a median grain size of 0.7 mm.
Unlike most materials used in experiments of
this type, the sand was poorly sorted. The
sorting index (S) was 2.22, where
4. ^50 I
-r~ + ^r
«50
016 /I '
Before each test, the sand surface in the
center of the flume was smoothed, and channel
was then excavated along the center line. The
initial channel was about 1 ft wide and about
3 in. deep. In each test the bed of the channel
was carefully graded to the desired slope, and
then sand was removed adjacent to the channel
in order to form banks of uniform height
(Figs. 1 and 2). It is this slope (S) along the
axis of the flume that is referred to throughout
the paper.
Before each experiment the tail box was
filled with water, and the ground-water table
in the sand was raised by pumping water into
the side channels. Water was introduced into
the channel by starting the recirculating
pumps, and discharge was gradually increased
to the desired amount. A discharge range of
from 0.10 to 0.30 cfs was possible; however, a
constant discharge of 0.15 cfs was used for the
tests described here. The water entered the
upper end of the flume through entrance
baffles, and after flowing through the model
channel it entered the tail box and was recirculated.
Sand was introduced at the entrance of the
channel to compensate for the sediment trapped
in the tail box. The rate at which sand was fed
at the entrance was determined by trial and
error. The proper rate was that which prevented scour and, therefore, maintained the
initial longitudinal slope of the channel. When
less than the required rate was introduced, the
S=
dm
Figure 2. Straight channel. Slope about 0.002;
channel width 2.2 ft. Upper part of channel has sinuous
thalweg but lower part remains straight.
channel deepened and the slope became
flatter. When more than the required rate was
introduced, the channel aggraded and the slope
became steeper. Water was allowed to flow
through the initial channel until major channel
adjustments were complete and the channel
became relatively stable. That is, no further
significant adjustments of channel shape,
dimensions, or pattern occurred. This situation
developed after as long as 24 hrs at very gentle
slopes but after only 2 or 3 hrs at steep slopes.
During each experiment, elevation of both
the bed of the channel and water surface was
measured by using the point gage. At the end
of each experiment a series of cross sections and
the longitudinal profile were measured, and a
map of the channel was prepared. This made
possible the calculation of channel and thalweg
sinuosity, hydraulic radius, width-depth ratio,
and so forth (see Tables 1-5). Sinuosity, as
measured, is the ratio of channel or thalweg
length to the straight line distance down the
axis of the flume. It is also the ratio of valley
slope to channel gradient. In addition, the rate
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1759
EXPERIMENTAL STUDY OF CHANNEL PATTERNS
TABLE 4.
ENTRANCE AT AN ANGLE
Slope, ft/ ft
0.015
0.016
0.018
Mean channel width, ft
Mean maximum depth, ft
Width-depth ratio
Mean area , ft 2
Mean velocity, fps
Bed-load discharge, gm/min
Bed-load concentration, ppm
Chezy C
5.1
5.30
5.47
0.060
85.00
0.0883
1.70
114.00
0.018
1.58
1.43
0.054
104.00
0.0625
2.40
1,003.00
3,900.00
134.00
0.021
1.82
Discharge 0.15 cfs.
etry of the experimental channels, the initiating bend was used to develop patterns of
uniform meander wavelength and meander
amplitude (Fig. 5). This procedure has been
utilized by most investigators (Friedkin, 1945;
Tiffany and Nelson, 1939).
In the third series of experiments, five tests
were made with water containing suspended
sediment (kaolinite), which was added to the
water in the tail box. In all tests of this type,
a concentration of 3 percent (30,000 ppm) by
dry weight was maintained. Some clay was
deposited in the channel during each experiment. The water was sampled frequently, and
the concentration was measured by using a
hydrometer. The hydrometer was calibrated
by using water containing known concentrations of suspended materials. When necessary,
additional kaolinite was added to the flow to
maintain the desired suspended-sediment concentration.
Kaolinite was selected as the material to be
used as suspended load because, although it is
readily transported in suspension, when deposited it is not as cohesive as other types of
TABLE 5.
Slope, ft/ft
Mean channel width, ft
Mean maximum depth, ft
Width-depth ratio
Mean area, ft 2
Mean velocity, fps
Meander wave length, ft
Meander width, ft
Bed-load discharge, gm/min
Suspended sediment concentration, ppm
Sinuosity
Chezy C
Shear, lb/ ft 2
926.00
3,605.00
120.00
0.020
825.00
3,110.00
1.22
Braided.
5.63
0.057
96.00
0.0704
2.14
1.93
738.00
2,880.00
103.00
0.017
Shear, lb/ft 2
Froude number
River channel pattern:
0.057
93.00
0.0777
0.020
clay. Once deposited it could be eroded, and
this was desirable.
EXPERIMENTAL RESULTS
Using the experimental procedures outlined
above, an attempt was made to reproduce the
results obtained by earlier investigators; that is,
we first attempted to obtain a model river
channel with a meandering thalweg. The results of each of the three sets of experiments
will be discussed separately.
Experiments with Straight-Entrance
Conditions
When water entered the initial channel, bank
erosion occurred as it widened to accommodate
the flow. The channel also became shallower
(Table 1), but it remained straight (see also
Wolman and Brush, 1961). No tendency to
meander was noted until the slope was steepened to 0.004. At slopes above 0.004, a meandering thalweg formed, but as indicated above,
the meandering-thalweg pattern was not uniform under straight-entrance conditions. Meander amplitude increased downstream, and the
SUSPENDED SEDIMENT TESTS
0.0026
0.0064
0.0075
1.40
1.84
2.00
2.20
0.15
0.145
0.156
0.162
7.6
12.7
12.8
0.0085
13.6
0.155
0.118
0.11
0.97
1.27
1.37
1.60
18.40
20.70
20.95
21.00
4.70
26.00
30,000.00
1.075
98.00
0.0159
River channel pattern; Meandering, Discharge 0.15 cfs.
7.75
58.00
30,000.00
1.24
98.00
0.0220
7.79
76.00
30,000.00
1.260
97.00
0.0229
0.094
7.50
85.00
30,000.00
1.260
87.00
0.0212
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1760
SCHUMM AND KHAN
pattern became better-defined in a downstream
direction. The decision was then made to introduce water at an angle to produce a repetitive meander pattern. Nevertheless, these
initial experiments with straight-entrance conditions produced some significant results. They
demonstrated that there is indeed a threshold
slope above which a channel has a tendency to
meander (Ackers and Charlton, 1971). In this
case a meandering-thalweg channel developed.
With straight-entrance conditions, the water
entered the channel directly and was not subject to unusual disturbance; yet at slopes
greater than 0.004, the thalweg meandered.
This leads one to suspect that a perturbation or
disturbance of the flow may not be an essential
cause of meandering.
It was established during these tests that as
the slope of the surface on which the model
channel was cut (valley slope) became steeper,
the greater was the quantity of sand required
in order to maintain a stable channel. Without
an increase in the rate at which sand was fed
into the channel, scour occurred at the head of
the flume, thereby reducing slope. The slope
and the quantity of bed load required to maintain a stable channel at that slope are closely
related (Table 1). This point will be considered
in detail later.
Experiments with an Initiating Bend
The experiments were begun anew at low
slopes with water introduced at an angle to the
axis of the flume. This, of course, forced the
development of a bend at the head of the
flume with all its effects on the hydraulics of
flow. Surprisingly, even under these conditions
a channel with a meandering thalweg did not
occupy the entire length of the flume until the
slope was increased from 0.001 to 0.0026.
A straight channel has a relatively deep and
narrow cross section, and its banks are straight
(Fig. 1, Table 2). Cross sections are almost
uniform throughout the length of the channel,
but two or three bends did iorm in the upper
part of the flume at slopes of about 0.002 (Fig.
2). The initial bend, of course, caused bank
erosion, and alternate bars were formed due to
the migration and deposition of bed materials.
However, the size of the bends decreased
sharply in the downstream direction, and the
channel became straight in the lower part of
the flume. Such a channel was considered to be
a straight channel, although a slight increase in
thalweg sinuosity did occur (Fig. 2).
The results of these experiments demonstrate
that a threshold slope exists below which a
meandering thalweg will not form even though
the water has moved through one bend. It has
been assumed that this type of disturbance,
especially the development of secondary flow
in bends, should produce a meandering channel.
At slopes of 0.0026 and greater, a meandering-thalweg channel formed. The channel itself
Figure 3. Meandering-thalweg channel formed at
slope of about 0.0008; channel width 4.8 ft. Note that
alternate bars are submerged.
Figure 4. Meandering-thalweg channel of Figure 3
at low water. Discharge was reduced to show meandering thalweg in essentially straight channel.
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1762
SCHUMM AND KHAN
Figure 7. Braided channel developed at slope of
about 0.016; channel width 5.6 ft. Two alternate bars
were formed in the upper part of the channel as a result
of the initial bend.
Figure 8. Details of braided channel. Part of a
longitudinal sand bar projects above water surface.
reached that permitted development of a meandering-thalweg channel. Thalweg sinuosity
increased to a maximum of 1.25 with increased
slope, and then the pattern became braided.
This is, of course, only a part of the story, and
in order to explain the reaction of the channel
to changed laboratory conditions, other factors
/
must be considered. For example, it was noted
1
earlier that in order to maintain a stable chan1
A'
nel (nonscouring-nonaggrading channel), the
1
\
1
rate of sand feed was increased as slope in1
J
creased. On Figure 10 the relation between
J
/
slope of the model channel and bed-load dis\
V
charge is displayed. As slope was increased, the
}
/
rate at which sediment moved through the
/
/
channel also increased. Further, the changes in
/
1
rate of increase of sediment load coincide with
/
/
the changes of channel pattern. From the
/
o
V_
/
geologic point of view, an increase of sediment
load causes an increase in slope, and accom\
panying these changes of slope are major
\
changes in channel pattern, channel crossA'
sectional geometry, and the hydraulics of flow
17
in the channels.
0
! Foot
i
_
Several matters require clarification. The
Scale
first is the fact that the meandering thalweg
Figure 9. Map and cross sections of braided
formed at slopes greater than 0.0047 with a channel. Dashed lines indicate positions of multiple
straight entrance but at a slope of 0.0026 with thalwegs.
'
l\
\
\
\\
\,
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1763
EXPERIMENTAL STUDY OF CHANNEL PATTERNS
2.0
1.0
.001 .002
.01
.005
Slope
.02
_Meandering —Tholweg _
Channels
£6
ft/ft
Braided
Channels
Figure 10. Relation between rate at which sand was
fed into the channels and slope of sand surface at
beginning of each experiment.
an initiating bend. Ippen and Drinker (1962)
performed a series of tests to study the distribution of boundary shear stresses in curved
trapezoidal channels. They showed that the
maximum shear stress in a bend exceeded in
intensity the mean shear for uniform motion in
a straight reach by over 100 percent. In addition, as the water moves around a bend, it
develops a centrifugal force that causes a
superelevation of the water level on the outside
of the bend. Pressure from the excess weight of
water piled up in this way causes a screwlike
type of secondary circulation that is termed
helicoidal flow. Thus, the existence of a bend
0
_ 0.2
£ O.I
O.
S
o
6
_ 4
/
0
•—- Meandeni
Cho
.01
Slope ft /ft
.02
Figure 11. Changes of channel dimensions and
width-depth ratio with slope.
0.002
0.004 0.006
Slope f t / f t
0.01
0.02
Figure 12. Relation between slope and velocity.
will induce additional bend formation at a
lower gradient than that required by a straight
channel.
In spite of the above and although water was
introduced into the flume through an initiating
bend that could have produced secondary flow,
a sinuous thalweg did not develop at very low
slopes. Perhaps at these low slopes the velocity
of secondary flow currents was not great
enough to cause lateral shifting of sediment
and the formation of alternate bars; however, as
slope increased with sediment discharge, a
threshold was reached above which alternate
bars and a meandering thalweg developed. At
the steepest slopes braided channels formed as
another threshold was exceeded. This change
occurred at a Froude number of about 1.0
(Tables 3 and 4). Under such flow conditions,
alternate bars and the sinuous thalweg are
destroyed.
The changes of sediment discharge with slope
and the changes of thalweg pattern as sediment
loads and slope increased were accompanied, of
course, by significant changes of channel
geometry and hydraulics of flow. At first,
channel depth decreases rapidly with slope
(Fig. 11), but it decreases slowly for meandering-thalweg channels and becomes almost constant for braided channels. Width on the other
hand increases rapidly to a slope of about
0.004, and then it increases less rapidly and at
what appears to be a uniform rate as slope increases. Changes in the rate at which channel
shape (width-depth ratio) adjusts appear to
occur at the previously noted critical slope
values (Fig. 11).
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SCHUMM AND KHAN
1764
As these morphologic changes occurred, both
velocity of flow and shear or tractive force increased, but at increasing rates for meanderingthalweg and braided channels (Figs. 12 and
13). Thus, as slope increases with sediment load
at constant discharge, both velocity and
tractive force increase significantly, thereby
providing an explanation for the different rates
at which sediment load increases with slope
(Fig. 10). The increase in velocity and shear is,
of course, closely realted to slope, but it is
also affected by the slow rate at which depth
decreases with slope (Fig. 11). In addition, a
change of bed configuration from ripples to
plane bed occurs when the channel braids. This
lessens channel roughness (Chezy C increases,
Tables 1-4). All of these factors act to increase
the rate at which velocity and shear increase as
channel patterns change from straight to
meandering thalweg to braided with increasing
slope and sediment load.
Suspended-Sediment Experiments
The previously described experiments have
provided new data on channel-pattern development in relation to sediment load and
slope, but, nevertheless, a truly meandering
channel did not develop. A meanderingthalweg channel formed, but although its
sinuosity reached a maximum value of 1.25,
channel sinuosity remained about 1 (Fig. 6).
Conclusions reached, through studies of rivers
of different patterns in the United States and
by a comparison of the modern Murrumbidgee
River with its associated paleochannels in
Australia, indicate that before a channel becomes sinuous a change in the type of sediment
load transported through the channel is necessary; that is, straight rivers tend to transport
bed load, whereas the most sinuous channels
transport predominantly suspended load
(Schumm, 1968). The Murrumbidgee River
now transports primarily silts and clay as suspended load at a slope of 0.7 ft per mi. Earlier
the channel was functioning with a slope of 1.5
ft per mi; however, as discharge increased and
bed load was reduced in response to climate
change, this gradient was not required. The
river reduced its gradient to about 0.7 ft per mi
by developing a sinuous course. In this series of
experiments the above geologic changes were
reproduced by introducing suspended sediment
into the meandering-thalweg channel while
sand load was reduced.
Initially a meandering-thalweg channel was
formed with a constant water discharge of 0.15
cfs. This flow contained negligible suspended
sediment, but as before, the bed load was fed
at the entrance to compensate for the coarse
sediment trapped in the tail box. After the
development of a meandering thalweg, kaolinite was mixed with the recirculating water in
the tail box until a concentration of 3 percent
(30,000 ppm) was obtained. Bed load was fed
into the channel, but at a markedly reduced
rate (Table 5).
The major change in type and quantity of
sediment load at constant water discharge,
especially the reduction in bed load, would be
expected to cause scour in the channel and a
cr
m
I
3
O
Q.
I
.006
Meandering —Thalweg
Channels
.004
.001
.002
.004
.006.008 .01
Slope
ft /ft
Figure 13. Relation between slope and shear.
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EXPERIMENTAL STUDY OF CHANNEL PATTERNS
gradient reduction. In fact, the scour that occurred was confined to the thalweg and occurred throughout its length, thus a significant
change of gradient by scour at the head of the
flume did not occur. A factor of major importance was the stabilization of the alternate
bars by deposition of the suspended load. Flow
over the alternate bars was shallow (Fig. 3),
and deposition of suspended sediment on the
bars further decreased flow depths, but of more
importance was the fact that the veneer of fine
sediment prevented erosion of the bars. The
banks of the channel were also partly stabilized
in this manner, and the thalweg was subjected
to scour. Lowering of the thalweg caused a
decrease of water level, and the submerged
alternate bars emerged to form point bars and a
sinuous channel. The channel became narrower,
deeper, and sinuous as a result of the introduction of suspended load and a decrease of bed
load in a meandering-thalweg channel. Figures
14 and 15 show the pattern and cross sections of
the channels before and after adjustment to
changed sediment loads. For comparison,
j
\
1765
•-
Figure 15. Maps showing channel before (A) and
after (B) introduction of suspended-sediment load.
Cross sections show changes of channel dimensions and
shape. Slope was 0.0064.
//
Figure 3 shows a characteristic meanderingthalweg channel, and Figures 16 and 17 show
the meandering channel that developed following introduction of suspended-sediment load.
Four experiments of the type described
above were performed at different slopes
(Table 5). In each case the meandering-thalweg
channel adjusted to the changes of sediment
load by a decrease of width and an increase of
depth, which resulted in a decrease of crosssection area, a decrease of width-depth ratio
0
I Fool
A
Cross Section
B
Scale
(Fig. 18), and an increase of sinuosity (Fig.
19). As all of these changes occurred at a
t
i
constant discharge, the decrease in size of the
channel cross section necessitated an increase in
0 3 6 Feel
flow velocity, and this was detected (Fig. 20).
i
Scale
\
In spite of increased sinuosity, which means
\
greater distance of flow and a decreased
gradient, the average velocity in pools increased about 10 to 15 percent after the suspended sediment was added to the flow. Several
/
factors act to increase velocity under these
Figure 14. Maps showing channel before (A) and circumstances. For example, suspended sediafter (B) introduction of suspended-sediment load. ment reduces turbulence, thereby increasing
Cross sections show changes of channel dimensions and velocity (Vanoni, 1946). In addition, deposishape. Slope was 0.002.
tion of the fine sediment on the bed and banks
i
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1766
SCHUMM AND KHAN
Figure 16. Meandering channel and point bars
formed after introduction of suspended load at slope of
0.0085. Channel width about 2.2 ft.
Figure 17. Recently emerged point bars with coating of fine sediment. Entire flow of water is confined to
deepened thalweg which is now a new sinuous channel.
reduces channel roughness, and the narrowing
and deepening of the channel also contributed
to flow efficiency and increased velocity.
A narrow, deep channel should develop with
a high concentration of clay in the flow, for
clay acts as a binding agent for the coarser
particles and enables banks to withstand a
higher shear stress. It has been recognized that
the greater the quantity of silt and clay in the
perimeter of a channel the narrower and deeper
it will be (Schumm, 1960). The higher velocity
of flow did cause some erosion of the concave
banks, and as a result, the sinuosity of the
channel increased slightly after the increase of
suspended-sediment concentration (Fig. 19).
Eventually, during a long run, both the bars
and the channel itself became coated with a
veneer of clay. This inhibited any further increase of sinuosity, and the sand load moved
directly through the narrow, deep channel to
the tail box.
With completion of this series of experiments, the principal objective of the research
was achieved. By duplicating, as far as possible,
in the hydraulic laboratory the effects of
changing climate on river systems during the
recent geologic past, a truly meandering channel was produced for the first time in the
hydraulic laboratory. Further, the conversion
of alternate bars to point bars was observed, and
it is suggested that this manner of flood-plain
formation can also occur in nature, during
changes in the hydrologic regimen of a stream.
SUMMARY AND CONCLUSIONS
The experiments demonstrate that threshold
values of slope and (or) sediment load exist
above which river patterns are significantly
altered. Threshold values of slope at a given
discharge have been recognized by Lane
(1957), Leopold and Wolman (1957), and
Ackers and Charlton (1971), and their relations
are shown on Figure 21 with our data. It is
very difficult to attempt to extrapolate from
the model study to natural river systems, but
the Lane and Ackers and Charlton curves fit
our data reasonably well. It should be recalled
that slope as used here is slope of the initial
channel or the sand surface on which the model
channel developed. It is analogous to valley
slope rather than to channel gradient.
Although during experimentation, slope was
changed and the sediment load was adjusted to
compensate, one may consider the results from
the view that if, as in nature, the sediment load
had been altered, slope in time would have
adjusted to produce the same experimental
results.
When total sediment-load data are available
for a range of river types, it probably will be
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1767
EXPERIMENTAL STUDY OF CHANNEL PATTERNS
2.0
20
1.0
,0.6
No Suspended
Sediment
3% Suspended Sed. Cone.
A No Suspended Sediment
•
3% Suspended Sed. Cone.
0.002
0.002
0.004
0.006
0.01
Slope ft /ft
Figure 18 Effect of suspended-sediment load on
width-depth ratio of channel.
possible to prepare a figure showing the
threshold values of sediment load at which
channel patterns change. For example, Figure
22 shows such a relation for our experiments.
This figure contains the identical data used to
plot Figure 10, but sediment load is plotted on
the abscissa as the independent variable. From
the geomorphic point of view this figure is of
interest. For example, an increase of sediment
load at low slopes will cause a very significant
increase in channel slope (slope varies as about
Suspended-load
ied-lpdd Channel
Figure 19. Comparison of channel and thalweg
sinuosity for bed-load and suspended-load channels.
0.004
0.01
Slope f t / f t
Figure 20. Effect of suspended-sediment load on
velocity of flow.
the third power of load), whereas an increase of
load in the intermediate (meandering-thalweg)
range or in the high (braided) range will
produce a lesser change of slope. Depending on
the position of a channel on Figure 22 when an
increase of sediment load occurs, a change of
slope may occur alone, or it may be accompanied by a change of pattern. If near a threshold, a relatively slight increase of sediment
load could completely alter the channel pattern.
An example of a pattern change from
straight to meandering thalweg can be cited
from observations made by Mahmood and
Akhtar (1962, p. 50) in Pakistan, where a cutoff on the Sutlej River caused introduction of a
large sediment load into a stable irrigation
canal. Aggradation occurred in the canal, and
the canal assumed a steeper slope corresponding
to the silt change and clearly showed meandering tendencies in the form of bar formations
and bank erosion. These authors suggest that
the increase in slope increased secondary circulation and induced a meandering-thalweg
channel. The results of our experiments indicate that a threshold was crossed, and the
straight canal began to form a meandering
thalweg.
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SCHUMM AND KHAN
o = Braided
a = Meandering—Thalweg
Straight
.001
1.0
10
100
Discharge, c f s
Figure 21. Relation between slope and discharge
and threshold slopes at each discharge as defined by
Lane (1957), Leopold and Wolman (1957), and Ackers
and Charlton (1971).
It must be emphasized that the effect of size
of sediment load has not been investigated. It is
probable that if the sediment load had been
composed of finer particles the threshold values
would have occurred at lower values of load and
slope at the same discharge.
The data collected during these experiments
do not permit an explanation of the thresholds;
however, we suggest that the lower threshold
reflects the velocity at which secondary currents become effective in distributing sediment
in the channel. The upper threshold is most
likely related to a high Froude number. Flow
at a Froude number of about 1.0 will tend to
move directly through the channel. Secondary
flow should disappear or become ineffective
under this condition, and the flow paths will
straighten, thereby destroying alternate bars
and the sinuous thalweg.
The experiments show clearly that a meandering-thalweg channel need not develop true
meanders. Only when the bed load was decreased and suspended sediment added did the
channel meander. Meandering occurs when a
channel is flowing on a surface that is too steep
for the sediment load and water discharge
transported by the river. To understand this,
let us recdnsider Figure 6, and a cycle of
progressively increasing and then decreasing
sediment load. In our experiments, if a channel
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EXPERIMENTAL STUDY OF CHANNEL PATTERNS
300
500
Sediment Load,
1000
2000
gm/min
Figure 22. Relation between slope and sediment
load.
is transporting a high sediment load on a steep
slope and then the load is reduced, the channel
will shift to the left on the curve of Figure 6,
but the surface on which the river flows, the
valley-slope, will remain steep. As load decreases, a braided channel will revert to a
meandering-thalweg channel, and if sufficient
suspended load is moved through the channel
to permit stabilization of the alternate bars,
the thalweg will scour, point bars will emerge,
and a meandering channel will form.
Apparently a reduction of peak or average
discharge will accelerate this process, for a
significant reduction of water level will also
cause alternate bars to emerge and become
point bars. If discharge reduction occurs, however, without a significant reduction of sediment load, then gradient must increase, and
perhaps the alternate bars would be destroyed
and a braided channel formed.
Although the development of a meandering
river appears to depend on changes in sediment
load and discharge of the recent geologic past,
this explanation apparently does not apply to
the variations of channel pattern along one
river. The experiments performed during this
research program suggest another explanation
for some causes of pattern variability. We have
two examples of this when the rate at which
sand was fed into the initial channel was not
adequate to prevent scour. In both cases the
channel scoured at the head of the flume,
1769
thereby decreasing the gradient of the channel.
The rate of sand feed was increased to prevent
further scour, but the effects of the altered
slope predominated. During one experiment at
slopes close to the lower threshold, the upper
part of the channel remained straight but a
meandering-thalweg channel developed in the
lower part of the flume. For the same discharge
and sediment load, a slight alternation of slope
produced two different patterns. At higher
slopes the same problem arose, but in this case a
meandering-thalweg channel developed in the
upper part of the flume and a braided channel
on the downstream steeper part of the flume.
The results of these experimental failures
provide information about channel pattern
variations along one river. If, for example,
valley slope changed in a downstream direction,
the channel, in order to maintain a uniform
gradient with constant discharge and sediment
load, will alter its pattern. Changes of valley
slope, when they occur, are usually the result
of geologic causes: slight vertical uplift or
depression or changes in tributary contribution
to the main stream during the late Pleistocene
and Holocene. The Mississippi River reach
below the junction of the Arkansas River is a
prime example (Schumm, 1971).
Finally, the experimental results provide
evidence that natural systems may not always
respond progressively to altered conditions.
Rather, a progressive change may be interrupted by an abrupt and dramatic adjustment
as critical erosional and depositional threshold
values are exceeded. There are threshold values
for sediment movement and for the hydraulic
characteristics of fluids (Froude and Reynold's
numbers) and, therefore, landscape components
(streams and hill slopes) should be expected to
behave similarly.
ACKNOWLEDGMENTS
This work was performed with financial
assistance from the National Science Foundation (Grant GA-4450). The junior author performed the experiments and collected the data
as part of the requirements for a Ph.D. degree
in civil engineering. The authors thank D. B.
Simons and E. V. Richardson of Colorado
State University for their critical comments on
the manuscript.
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SCHUMM AND KHAN
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MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 3,
1971
REVISED MANUSCRIPT RECEIVED JANUARY 27, 1972
PRINTED IN U.S.A.