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.