EXPERIMENT 6

Incomplete Entry - needs to be finished.


Hypothesis

Decreasing the ratio of load to discharge will create a balance between erosion and deposition that will favor development of a meandering channel pattern, rather than construction of an alluvial fan.

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:
Shape of initial channel:	Single bend followed by a straight channel (dimensions not recorded)
Depth of initial channel:	1 cm (estimated)
Width of initial channel:	3 cm (estimated)
Discharge stage:	Bankfull
Adjustments from Experiment 5:	· The sediment feed rate was decreased. · An attempt was made to better determine actual discharge rates (Table 1).
Procedure:	· Discharge and sediment feed were started and allowed to flow uninhibited for the duration of the experiment.

Table 1: To determine discharge rates, we filled a beaker from the discharge hose, taking measurements in ten second intervals and relate those to partial turns of the control knob. Turns of 1/4 and 3/4 of a rotation showed consistent results; however, 1/2 rotation turns showed a higher flow rate when turned down from a higher rates than when turned up from lower rates. For consistency, when we want to have the water at a 1/2 turn flow rate, we will turn the water off and make the half turn to keep the flow a constant rate of 45mL/sec.

                       


Observations

1) 

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

1) 


Technical Issues

EXPERIMENT 5

This experiment is unchanged from Experiment 4.  We are doing a re-run with changes made to improve consistency in the sediment feed system.



Hypothesis

Because we have not changed the variables from Experiment 4, we anticipate similar results.  If the sediment feed functions properly, it is likely an alluvial fan will develop because of an over supply of sediment.  However, if the sediment feed functions properly, there should not be development of a subsequent incised channel.



Set Up

The parameters of Experiment 5 are identical to those of Experiment 4.  The one adjustment is a detachment of the discharge hose from the sediment feed housing.  Discharge is now fed by placing the hose directly into the plastic soda bottle located on the sand bed surface.  Sand is dropped into the bottle by the feed mechanism, where it mixes with the discharge (Fig. 1).

Figure 1: Discharge and sediment now mix within the plastic soda bottle, rather than within the previous housing.

Observations

1) An alluvial fan developed from the mouth of the soda bottle onto the table, and the original channel was abandoned (Fig. 2a, b).

2) Headward erosion developed from the basin toward the toe of the alluvial fan (2a, c).

Figure 2a: View of entire stream table with alluvial fan at the head and headward erosion at the toe.

Figure 2b: Close up of alluvial fan.

Figure 2c: Close up of headward erosion.

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

1) As hypothesized, an alluvial fan developed because of a load greater than could be transported by the available discharge.

2) Headward erosion proceeded from the basin toward the alluvial fan, because of the change in position of the entry point for discharge entering the basin.

Technical Issues

Though the wicking problem was greatly reduced, dampening of sediment caused it to accumulate on the rounded side of the plastic bottle above the level of discharge entry.  The sediment would pile until it collapsed into the bottle, causing a discontinuous, fluctuating sediment supply.  Occasionally, the pile would rise to the level of the sediment delivery mechanism, resulting in clogging.

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.

EXPERIMENT 3

Hypothesis

Carving an initial channel that has a single bend near its head and then runs in a straight line to the end of the table will encourage development of a meandering pattern.

Set up (Fig. 1)

Bed fill:	Fine-grained (0.70 mesh) quartz sand
Bed thickness:	Not recorded
Bed gradient:	0.5⁰
Base level:	5 - 6 cm below bed surface (estimated)
Discharge rate:	45 mL/s (estimated)
Sediment feed rate:	0
Shape of initial channel:	R/w = 4 cm
Depth of initial channel:	0.5 cm
Width of initial channel:	2 cm
Discharge stage:	Bankfull
Adjustments from Experiment 2:	· Based on a USGS study using a larger table, we scaled our channel gradient to be 0.5⁰. · Using a published formula relating curvature radius to channel width for natural streams, we adjusted the width, depth, and curvature of our initial bend. · We began the experiment with a pre-carved channel extending the full length of the table, following an initial bend (Fig. 1a).  This seems to be a characteristic of all successful experiments for larger tables. · A rock was suspended from the base-level hose to try and increase stability.
Procedure:	· Discharge was started and allowed to flow uninhibited for the duration of the experiment.

Figure 1: Initial carved channel with bend.

Observations

1) Upon beginning the experiment, the discharge began to scour the channel floor and deposit within the channel immediately downstream, creating a blockage to flow and causing overbank flooding with subsequent sheet flow across the floodplain (Fig. 2).

Figure 2: Flooding and subsequent sheet flow.

Interpretations

1) The discharge velocity was too high entering the channel, causing erosion and chute development. The additional sediment from from the chute then decreased flow velocity, causing deposition within the channel.  In-channel deposition decreased channel cross-sectional area, restricting the flow and leading to flooding and sheet flow across the sand-bed surface.

Technical Issues

The discharge rate needs to be adjusted to decrease the amount of initial erosion.  We continue to have problems with maintaining a stable base level.

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.