EXPERIMENT 10

Hypothesis

Thoroughly mixing sand and kaolinite as the alluvial bed material will result in more cohesion than sand alone and less than sand covered by a thick layer of kaolinite and bentonite, potentially allowing adjustments to discharge and sediment load that will find a balance between cutbank erosion and point bar deposition.  

Set Up (Fig. 1)

Bed fill:	Fine-grained (0.70 mesh) quartz sand mixed with kaolinite.  Sand:clay = 3:1.
Bed thickness:	8 cm
Bed gradient:	~1⁰
Base level:	Even with channel mouth (11.5 cm)
Discharge rate:	Not recorded (Rates from previous experiments are not reliable.)
Sediment feed rate:	~0.67 mL/min
Shape of initial channel:	Same as Experiment 9
Depth of initial channel:	~1 cm
Width of initial channel:	~3.5 cm
Discharge stage:	Bankfull
Adjustments from Experiment 9:	· 3:1 ratio of sand and kaolinite mixed to make bed fill.
Procedure:	· Discharge and sediment feed were started and allowed to flow uninhibited for the duration of the experiment.

Figure 1: Initial setup with sand/clay mixture for bed fill and a bend near the head of the channel to facilitate initial meander development.

Observations

Experiment initiated 18 November 12:23.

1) 13:32: Sediment had filled the channel near the feed.  Discharge had broken through the head of the first bend, causing flooding and splay development. (Fig. 2).  The sediment feed rate was lowered to ~0.5 mL/min.

Figure 2: Sediment filled channel, flooding, and splay development.

20 November

2) 14:00: The sediment feed was stopped to encourage channel incision.

3) 11:25: The discharge was increased to bankfull and base level was lowered by 0.5 cm to encourage channel incision.

4) 11:45: The channel was artificially deepened to allow a higher discharge and greater sediment transport.

21 November

5) 07:50: The initial bend shows erosion with minor lateral migration.  A submerged point bar has developed on the inside of the bend (Fig. 3). Cutbank erosion and point bar deposition appear to be roughly equal.  The discharge level had dropped to about 1 cm below bankfull; therefore, discharge volume was increased to bring it back to bankfull.

Figure 3: Cutbank erosion and development of a point bar on the initial meander.  At bankfull stage, the point bar is submerged.

6) 12:10: Further erosion and migration of the cutbank.  Deposition on the point bar appears to be keeping pace.

7) 16:40: Two additional meanders have developed downstream from the initial bend (Fig. 4).

Figure 4: Enlargement of the first meander and development of two others.

8) 18:05: Discharge is maintained at bankfull and continues to flow over the tops of point bars. We discussed possibly varying discharge stage to make point bars alternate between being submerged and exposed but decided, for now, to maintain the flow at bankful.  Base level was lowered by about 0.5 cm to encourage channel incision.

22 November

9) 08:37: Little change from the last observation.

10) 10:38: Dropped base level another ~0.5 cm to encourage thalweg incision.

11) 11:47: Turned the sediment feed off to encourage thalweg incision.

12) 13:00: A chute channel had incised across the point bar at the initial bend (Fig. 5).

Figure 5: A small, straight chute channel incised across the initial point bar.

13) 14:00 Re-oriented the angle of the discharge hose to direct flow away from the point bar and enhance flow within the thalweg, rather than across the bar top.

14) 15:00 The thalweg has returned to the main channel and no longer cuts across the point bar.

15) 16:00: Re-started the sediment feed, using a mixture of 3 parts sand to 1 part kaolinite, matching the composition of the bed fill.  Before this, pure sand was fed onto the table.  The sediment feed rate is ~0.9 mL/min.

23 November

16) 17:20: Meanders have become better defined along their margins, but the channel is very shallow near head of the table at the discharge/sediment feed entry point (Fig. 6).

Figure 6: Each of the three meanders has migrated laterally and downstream.  Downstream migration is particularly evident in the middle meander.

17) Time not recorded: The first and second meanders have become increasingly well defined and continue to migrate.  Meander three has straightened and transformed to a deltaic distributary and is widening the channel mouth (Figs. 7a, b).  The sediment feed rate reduced to ~0.6 mL/min.

Figure 7a: Continued downstream and lateral migration of the first and second meanders. The third meander has transformed to a delta distributary.

Figure 7b: Continued migration of meanders.  Notice emergent point bars.

26 November

18) 10:15: Base level had dropped significantly, apparently because someone not related to the experiment tampered with the base level control. A new, relatively straight channel was incised through headward erosion from the basin toward the source (Figs. 8a, b). Base level was raised in an unsuccessful attempt to re-establish the meander pattern.  The experiment was terminated.

Figure 8a: The experiment was terminated prematurely due to unauthorized drop in base level, resulting in incision of a straight channel.

Figure 8b: Close up of Figure 8a.

Interpretations (Each interpretation is tied by number to an above observation.)

1) The sediment feed rate was too high to be transported by the discharge, leading to in-channel deposition, which blocked the channel, causing flooding and development of a splay.

2 - 4) Stopping the sediment feed, increasing discharge, lowering base-level, and, finally, artificially deepening the channel were done to create a through-flowing thalweg.

5) The adjustments recorded in Observations 2 through 4 seem to have been successful, and an apparent balance between erosion and  deposition was achieved, leading to cutbank migration and development of a small point bar associated with the initially-carved channel.

6 - 9) Enhancement of the relationship of Observation 5, with development of additional meanders that are migrating both laterally and downstream.  The current conditions seem to be achieving our objective.

10 - 11) The desired meandering channel is forming; however, we would like to produce a deeper, better-defined thalweg.  To try and achieve this, we dropped base level and ceased feeding sediment.

12 - 15) The adjustments recorded in Observations 10 through 11 led to headward erosion across the first point bar and development of a chute channel.  To correct this, we changed the orientation of the discharge hose to be parallel with the meander bend, rather than pointed toward the point bar.  This adjustment successfully caused the flow to reoccupy the channel thalweg and abandon the chute across the point bar surface.  Additionally, the sediment feed was restarted to re-establish deposition on the point bar.  In this case, rather than run pure quartz sand, as before, a three part sand to one part clay mixture, the same as that used in the alluvial bed, was fed to the table with the thought that the clay would diminish point bar erosion and incision.

16 - 17) The current balance between sediment character, sediment feed rate, discharge, and composition of bank/fill material appears to be close to creating the desired balance between cutbank erosion and point bar deposition.  A moderately-meandering pattern consisting of three bends resulted; however, because of deltaic deposition and consequent basinward extension of the channel, the third meander has transformed to a straighter distributary channel.

Technical Issue

Someone not in the class and not associated with the project lowered base level, resulting in steepening of the gradient, headward erosion, and development of a straight channel that cut from the basin across all three point bars to the head of the table.  Because of proximity to the close of the semester, it was decided to terminate the project, rather than try to re-establish the meandering pattern.  Had the experiment continued, we desired to try to develop a deeper, better defined thalweg and a tighter, more complex meandering pattern.

EXPERIMENT 9

Hypothesis

Adding a thick layer of clay (kaolinite and bentonite) across the entire alluvial surface will increase bank cohesion, slowing rates of cutbank erosion and allowing point bar deposition to keep pace.

Set Up

Bed fill:	Fine-grained (0.70 mesh) quartz sand overlain by thin layer of kaolinite, a layer of bentonite (~1 cm), and another thin layer of kaolinite
Bed thickness:	5 - 6 cm
Bed gradient:	0⁰
Base level:	Even with channel mouth
Discharge rate:	45 mL/s (estimated)
Sediment feed rate:	0 mL/s
Shape of initial channel:	Single semi-circular 45⁰ bend with a 10 cm radius, followed by a straight channel
Depth of initial channel:	~2 cm
Width of initial channel:	4 cm (estimated)
Discharge stage:	~1 cm below bankfull
Adjustments from Experiment 8:	· An ~0.5 cm thick bed of inter laminated Kaolinite and bentonite covering the entire alluvial bed surface. · Base-level stability resolved by inserting 3/4" black water-line tubing into 1" similar tubing with a plastic handle running through the top.
Procedure:	· Discharge and sediment feed were started and allowed to flow uninhibited for the duration of the experiment.

Observations

Experiment initiated 1 Nov

1) 11:15: The bed surface had swollen, developing large mounds and cracks.  The initial channel was carved.  (Fig. 1).

2) 12:15: The channel had narrowed and portions of its banks had collapsed, restricting flow (Fig. 2).

Figure 1.  Mounds and cracks developed on alluvial bed surface.

Figure 2.  Channel constriction due to swelling and collapsing of clay.

2 November

3) 7:20: Sand had accumulated within the channel upstream from collapse structures.  The stream avulsed and flowed along the head of the table to a sidewall, where it then followed the sidewall to the basin (Fig. 3a, b). The channel was cleared of collapse structures and sand, and the avulsion crevasse was plugged, allowing reoccupation of the carved channel. Base level was raised ~0.5 cm.

Figure 3a.  Initial channel filled with sand near the table head and point of avulsion.

Figure 3b.  Initial and avulsed channel paths.

5 November

4) 14:00: A meander had developed just downstream from the initial, artificially carved bend (Fig. 4).  Banks continued to collapse and restrict stream flow (Fig. 5).  Collapsed blocks were removed artificially with each inspection of the table.  This would cause a drop in discharge level upstream of the prior obstruction, increasing flow velocity and allowing transport of sand the entire length of the table.

6 November

5) 14:00: A weakly meandering thalweg had developed (Fig. 6).

Experiment terminated 8 Nov

6) Little additional change, other than continuing slow collapse along banks.

Figure 4: Development of a cutback due to removal of sand and undercutting of the clay layer.

Figure 5: Collapsed bentonite blocks.

Figure 6: Over a period of several days a weak meandering pattern developed.

Interpretations (Each interpretation is tied by number to an above observation.)

1) Permeation of "ground" water into the alluvial bed led to expansion of the bentonite layer, developing the mounded topography.

2) Swelling of the bentonite layer caused channel narrowing.  Steepness of the channel banks resulted in mass wasting.

3) Clay blocks that collapsed into the channel restricted the flow, causing the upper channel to fill with sand.  Because the stream could no longer follow the initial channel, it flowed over its bank and moved along the table walls to the basin.  The initial channel was reactivated artificially by removing blockages and plugging the avulsion point.

4) Keeping the channel clear of bentonite blocks allowed the discharge to flow more rapidly.  As the flow crossed the channel from the carved bend, it removed sand, undercutting the clay layer and leading to collapse and formation of a cutbank.

5) The process described under Interpretation 4 continued, along with deposition of sand on point bars, producing a weak meandering pattern.

6) Had we allowed the experiment to continue, a more pronounced meandering pattern probably would have developed, though, as the clay became stripped from the surface, ultimately a braided channel similar to previous results is likely.

Technical Issues

No new technical problems.  New base-level control appears to function well, as do temporary "fixes" to the discharge and sediment feed mechanisms.

EXPERIMENT 8

Hypothesis

Adding a thin layer of kaolinite to the sand-bed surface will increase bank cohesion, decreasing rates of cutbank erosion and allowing point bar deposition to keep pace.

Set Up (Figs. 1a, b. c)

Bed fill:	Fine-grained (0.70 mesh) quartz sand with thin layer of kaolinite on surface in a narrow strip within and along the channel
Bed thickness:	5 - 6 cm
Bed gradient:	0⁰
Base level:	Even with channel mouth
Discharge rate:	45 mL/s (estimated)
Sediment feed rate:	~0.75 mL/min
Shape of initial channel:	Single semi-circular bend with a 10 cm radius, followed by a straight channel
Depth of initial channel:	~1 cm
Width of initial channel:	4 cm (estimated)
Discharge stage:	Bankfull
Adjustments from Experiment 7:	· Kaolinite added to surface within and along channel.
Procedure:	· Discharge and sediment feed were started and allowed to flow uninhibited for the duration of the experiment.

Figure 1a: Initial channel with a thin layer of kaolinite in a strip along either side.

Fig. 1b: Sifter used to apply kaolinite to bed surface.

Fig. 1c: Initial channel bend.

Observations

1) Kaolinite within channel quickly removed and transported to the basin (Fig. 2).

2) Development of a point bar and opposing cutbank (Fig. 3).

3) Widening of channel and transition into a well-developed braided channel pattern (Fig. 4).

Figure 2: Transportation of eroded kaolinite to the basin.

Figure 3: Development of a point bar and cut bank erosion on opposite side of channel.

Figure 4: Final braided channel pattern.

Interpretations (Each interpretation is tied by number to an above observation.)

1) Kaolinite as an unconsolidated powder is easily removed by flow within the channel.

2) Clay added to the floodplain surface became saturated, providing some increase in cohesion, allowing initial development of a weak meandering pattern and cutbank erosion temporarily slowed.

3) The thin layer of kaolinite did not sufficiently slow erosion to prevent significant widening of the channel.  Once widening exceeded the width of the  narrow band of kaolinite along the channel, the rate of erosion increased to rates of previous experience, transforming the system to a braided pattern.




Technical Issues

Continued problems with wicking of water into sediment feed.

Continued instability of base-leve control.

EXPERIMENT 7

We were getting pretty tired of developing braided streams and alluvial fans, but we have to admit, we're getting really good at it!

You'd think by now we'd be so accustomed to watching the stream table that it would seem boring, but we’re still fascinated.


Hypothesis

Continued modification of the load:discharge ratio will lead to a balance between cutbank erosion and point bar deposition.

Set Up (Figs. 1a, b)

Bed fill:	Fine-grained (0.70 mesh) quartz sand
Bed thickness:	5 - 6 cm
Bed gradient:	0⁰
Base level:	Even with channel mouth
Discharge rate:	45 mL/s (estimated)
Sediment feed rate:	~0.75 mL/min
Shape of initial channel:	Single semi-circular bend with a 10 cm radius, followed by a straight channel; a few minutes into the experiment, the channel radius was reduced.
Depth of initial channel:	~1 cm
Width of initial channel:	4 cm (estimated)
Discharge stage:	Bankfull
Adjustments from Experiment 5:	· The sediment feed rate was decreased. · A narrow metal chute replaced the plastic soda bottle for mixing of sediment and water at the head of the stream.
Procedure:	· Discharge and sediment feed were started and allowed to flow uninhibited for the duration of the experiment.

Figure 1a: Initial set up.  Bend A was carved with a 10 cm radius. Bend B had a radius of ~3 cm.

Figure 1b: Modified set up.  Bend B of Fig. 1a was determined to be too sharp.  The channel was re-carved with 10-cm radii for both bends.

Observations

1) Within 5 min, rapid widening of channel occurred along the bend.  Experiment paused to reshape initial channel bend.

2) Within 5 min of restarting, rapid filling of channel occurred along the bend, accompanied by overbank flooding and laminar flow across the floodplain (Fig. 2).  An artificial levee was constructed ~5 cm from the cutbank (Fig. 3) of the initial bend and reduction of sediment feed to ~0.6 mL/min.

3) Within 15 min of levee construction, the channel was still filling rapidly along the bend, causing flooding and erosion of the artificial levee.  Discharge rate was increased to ~50 mL/s.

4) Erosion outpacing deposition with widening of the channel and development of a meandering thalweg.

5) The experiment paused to artificially deepen the thalweg (Fig. 4).

6) Almost immediately upon restarting the discharge and sediment feed, the channel began to widen, partially filling the channel with sediment (Fig. 5).

7) At some point, the sediment feed became clogged and stopped delivering sediment.

8) Development of a braided channel pattern across the entire width of the table (Fig. 6).

Figure 2: Within minutes the channel filled with sediment and flooded.

Figure 3: An artificial levee was built around the bend to prevent flooding.

Figure 4: Although the channel filled with sediment, a thalweg developed and flowed along a sinuous path. We shut off the sediment feed and discharge to incise a deeper channel along the thalweg's path.

Figure 5: Within minutes of restarting the load and discharge, the channel widened and filled with sediment.

Figure 6: The experiment terminated with a well-developed braided channel pattern.

Interpretations (Each interpretation is tied by number to an above observation.)

1) Rapid widening of channel due to an overly sharp curve at Bend B.

2) The bend was filling with sediment due to a load:discharge ratio that was still too high.  This led to blockage of the channel and flooding.  The levee was constructed to help constrain the flow, and the sediment feed rate was decreased to prevent further in-channel deposition.

3) In-channel deposition continued.  Discharge was increased to try and balance the excessive load.

4) The increased discharge proved to be an over correction, and erosion now outpaced deposition, widening channels.  However, a meandering thalweg did develop within the confines of the channel margins.

5) It was thought that artificially deepening the thalweg would encourage point bar deposition along with the already occurring cutbank erosion.

6) With the lower sediment feed and higher discharge, the load:discharge ratio is now too low, leading to further erosion without deposition.

7) Sediment is depositing faster than it can be removed and, therefore, clumping as it becomes damp.  Eventually, it piled to the level of the delivery device, wicking moisture into the augers, leading to clogging and stopping of the sediment supply.

8) Non-cohesive bank material led to erosion rates that favored widening of the system and production of multiple shallow channels, rather than a single deeper channel.

Overall interpretation: As we go back and forth on modifying load and discharge, we alternatively form either an alluvial fan or a braided channel pattern, without production of a meandering pattern between.  Following this experiment, we think that part of the problem is the lack of cohesion in the quartz sand that makes up the bed-fill.  This is consistent with the findings of earlier studies (e.g. Schumm and Khan, 1972; Commission et al., 1945).

Dr. Little reminded us that although we didn't develop a meandering stream, we are still succeeding, because we are learning and witnessing the principles of fluvial geomorphology. For example, our stream had reached complete equilibrium. Because sediment had stopped feeding (due to getting clogged again), there was no sediment entering the system and, therefore, there was no sediment leaving the system. EQUILIBRIUM!



Technical Issues

Continued problems with wicking of water into sediment feed.

Continued instability of base-leve control.

References

Commission et al., 1945, A laboratory study of the meandering of alluvial rivers

Schumm and Kahn, 1972, Experimental Study of Channel Patterns