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:
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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
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Bed thickness:
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5 - 6 cm
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Bed gradient:
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0⁰
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Base level:
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Even with channel mouth
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Discharge rate:
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45 mL/s (estimated)
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Sediment feed rate:
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0 mL/s
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Shape of initial channel:
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Single semi-circular 45⁰ bend with a 10 cm radius,
followed by a straight channel
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Depth of initial channel:
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~2 cm
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Width of initial channel:
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4 cm (estimated)
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Discharge stage:
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~1 cm below bankfull
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Adjustments from Experiment 8:
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· 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:
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· Discharge and sediment feed were started and
allowed to flow uninhibited for the duration of the experiment.
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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).
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Figure 1. Mounds and cracks developed on alluvial bed surface.
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Figure 2. Channel constriction due to swelling and collapsing of clay.
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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.
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Figure 3a. Initial channel filled with sand near the table head and point of avulsion.
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Figure 3b. Initial and avulsed channel paths.
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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
Experiment terminated 8 Nov
6) Little additional change, other than continuing slow collapse along banks.
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Figure 4: Development of a cutback due to removal of sand and undercutting of the clay layer.
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Figure 5: Collapsed bentonite blocks.
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Figure 6: Over a period of several days a weak meandering pattern developed.
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Interpretations
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.
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.
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.
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