EXPERIMENT 2

Hypothesis

Carving an initial curve at the head of the table will help to begin the first meander. As the thalweg crosses to the opposite side of the channel, additional meanders will develop downstream.

Set up (Figs. 1a, b)

Bed fill:	Fine-grained (0.70 mesh) quartz sand
Bed thickness:	Not recorded
Bed gradient:	0⁰
Base level:	5 - 6 cm below bed surface (estimated)
Discharge rate:	45 mL/s (estimated)
Sediment feed rate:	0
Shape of initial channel:	~70⁰ bend beginning 17 cm from discharge entry point
Depth of initial channel:	2 cm (estimated)
Width of initial channel:	2.5 cm (estimated)
Discharge stage:	Bankfull
Adjustments from Experiment 1:	· Sand bed thickness was increased by ~3 cm to account for hinge thickness associated with the basin subsidence mechanism and allow for deeper downcutting. · The sand bed was saturated prior to beginning the experiment. · We began the experiment with a pre-carved channel extending from a curve at 17 cm from the discharge entry point to about half the length of the table (Fig. 1a). · The flexible rubber hose used to control base level was bent to the desired height and suspended from the side of the table by a stiff wire.
Procedure:	· Discharge was started and allowed to flow uninhibited for the duration of the experiment.

   

Figure 1a: Table set up, showing initial channel.

Figure 1b: Close up of carved channel bend.

Observations

1) Time: ~10:00. After following the carved channel, the discharge incised a straight channel to the basin margin.

2) ~10:02. Upon reaching the basin margin, headward erosion began to deepen the channel (Fig. 2).

3) ~10:08. The levee was breached along the outside of the carved bend, leading to splay development and laminar flow across the sand-bed surface (Fig. 3).

4) 10:11. Continued headward erosion from basin margin.

5) 11:08. Termination of experiment.

Figure 2: Headward erosion.

Figure 3: Crevasse and splay developed by overbank flow.

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

1) Discharge was concentrated in the channel, allowing erosion at its termination due to higher energy than when the flow exited the channel.  Possibly reversed headward erosion as sediment back filled into the channel.  This relationship needs to be further analyzed.

2) Steepening of gradient where the discharge dropped into the basin increased the rate of erosion.

3) Discharge was too high for the channel cross-sectional dimensions and overtopped the bank, cutting a crevasse through the levee.  Decreased flow velocity caused sediment to be deposited, forming the splay.  Now non-channelized flow moved as a sheet across the bed surface.

4) See Interpretation 2.

Technical Issues

About 3 minutes into the experiment, a leak was found along one of the table sides, and the experiment had to be temporarily halted to make repairs.  The above observations were made following the repair.  The base-level control continued to be unstable and change position when the table was  bumped.