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