EXPERIMENT 4

It is with great pleasure that we announce significant progress and a major advance in our objective of generating a sustainable, migrating meandering channel.

Hypothesis

Adding sediment from a point source to a channel carved into a level bed of unconsolidated sand will result in deposition of sediment along point bars, balancing erosion to cutbanks and instigating development of a meandering channel pattern (Figs. 1a, b).

Fig. 1a: For meandering to occur, the channel must maintain a constant width.  This is accomplished by a balance between cutbank erosion and point bar deposition.

Figure 1b: Sediment eroded from a cutbank is deposited on the first point bar downstream.  To avoid scouring at the head of the table, sediment must be introduced with the discharge, simulating what would have been derived from upstream cutbank erosion.

        

Set up

Bed fill:	Fine-grained (0.70 mesh) quartz sand
Bed thickness:	5 - 6 cm
Bed gradient:	0⁰
Base level:	11 cm below top of table side
Discharge rate:	45 mL/s (estimated)
Sediment feed rate:	3 mL/min (75% feed potential feed rate)
Shape of initial channel:	Curve (dimensions not recorded)
Depth of initial channel:	1 cm (estimated)
Width of initial channel:	2 cm
Discharge stage:	Bankfull
Adjustments from Experiment 3:	· Sand identical to that in the table bed was fed to the discharge prior to entering the table. · The initial curve was randomly generated.
Procedure:	· Discharge and sediment feed were started and allowed to flow uninhibited for the duration of the experiment.

 

         
Observations

Experiment commenced 11 Oct 10:30.

1) Unanticipated rise in base level.

2) The carved channel widened, producing a braided pattern with multiple shallow channels.

3) Base level deliberately lowered to original elevation.

4) The initial carved bend at the head of the system showed signs of cutbank erosion with a small point bar developing immediately downstream (Figs. 2a and b).

5) A delta formed where the stream entered the basin (Fig. 3).

6) The channel avulsed near the discharge point, flowed in a straight line to the south table wall, then followed the wall to the basin while producing an alluvial fan at the head of the table (Fig. 4).

7) The avulsed channel incised into the alluvial fan.  Sediment had ceased to be fed onto the table because of a malfunction with the sediment delivery system.

Experiment terminated 11 Oct @ 19:38.

 Figure 2a: Cutbank erosion and point bar deposition.

Figure 2b: Cutbank erosion and point bar deposition.

Figure 3: Delta forming at channel mouth.         

           

Figure 4: Channel avulsion and straightening and formation of an alluvial fan.           

             

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

1) Discharge rose due to a faulty base-level control.

2) The channel widened as a result of cutbank erosion without coeval point bar deposition.  A high base level prevented downcutting, focusing energy and erosion on channel margins.

3) Base level was lowered to provide space for downcutting, with the intent of favoring a single, relatively deep channel, rather than multiple, relatively shallow channels.

4) Once sediment was fed into the system, deposition began to occur in the form of point bars, partially balancing erosion along cutbanks and allowing the river to maintain a constant width.

5) In previous experiments, we found that if no sediment entered the system, once a graded profile was established, no sediment exited the system (Nothing in, nothing out.), representing a state of equilibrium.  In this experiment, once sediment was fed onto the table, a partial balance was established between cutbank erosion and point bar deposition.  As a result, sediment was being transported along the entire length of the channel, depositing and prograding as a delta at the channel mouth.

6) The amount of sediment entering the system proved to be more than could be transported by the available discharge; therefore, the channel became blocked, avulsed, and evolved to an alluvial fan due to in-channel deposition, as the system increased its gradient to establish a steeper graded profile that could handle the introduced load and move to a new equilibrium.

7) Water was wicking from the discharge hose into the overlying sediment feed, clogging the feed mechanism and stopping sediment delivery.  Because of the lack of sediment, the discharge incised into the alluvial fan, forming a flatter graded profile.

             
          

Technical Issues

Mixing of sediment and water within the same mechanism proved incompatible, as water would wick up into the sand, clogging the sediment feed and shutting off the sediment supply.  Continued issues with base-level fluctuation.