1. Flume Controls

How do you control the following in the flume?

1.1 Q (Water Discharge)

The water discharge is controlled on the stream table by turning a manual knob (A) on the pump to increase or decrease the flow. Water is then discharged onto the top of the table, which is displayed by (B). The images below displays how the water discharged in controlled.

1.2 S (Slope)

The slope of the table is controlled by how the table itself is supported. Here, the table is supported with one end higher than the other end, creating a table slope where the headwaters are higher than the outlet. Other methods to change the slope within the table is to pile sediment to create a slope (local changes in topography) as shown in the video below or to increase or decrease the base level elevation outlet.

1.3 Qs (Sediment Discharge)

Sediment discharge can be seen by the amount of sediment that exists the table system through the outlet at the bottom of the table as shown in the video below. It is controlled by lowering or raising this drain. Sediment discharge can also be controlled by adjusting the water discharge or adjusting the grain size ratios in the flume.

1.4 Profile

The channel longitudinal profile can be manipulating by changing the topography by digging, creating a new channel, or building "mountains" in the flume. The image below shows that we used the toy bulldozer to manipulate the channel longitudinal profile.

1.5 Base Level

The base level of the flume is the level or elevation of the outlet. The channel will adjust its profile to the elevation of the base level. In the table, the base level is controlled by the level of the outlet, which can be moved up or down. This is shown in the video below.

2. Flume Geomorphic Processes

Simulate and record your observations of:

2.1 Bed Erosion

Bed erosion is the process where the channel bed is degraded or lowered due to erosion. Bed erosion will occur in sediment source areas, where sediment transport critical shear stress thresholds have been exceeded. Bed erosion can be seen in the video below as the channel bed is slowly being degraded and lowered due to erosion.

2.2 Bank Erosion

Bank erosion is the process where sediment is being eroded/removed from the bank by the flow of the channel and the channel becomes wider. Bank erosion will also occur in sediment source areas and areas where the discharge is too large for the channel. Bank erosion can be seen in the video below as sediment is being eroded/removed from the bank by the flow of the channel.

2.3 Deposition

Deposition occurs when the critical shear stress to transport sediment is no longer exceeded. The sediment is no longer able to be transported through the channel as it is too heavy for movement and is instead deposited and results in aggradation. This commonly happens when the slope decreases, inside bend bars, inside mid-channel bars, or a riffle which is a channel standing bar. Deposition is demonstrated in the video below.

2.4 Sediment Transport

Sediment transport occurs when sediment is being moved by water. For transport to happen, the critical shear stress must be exceeded. The transport of sediment will result in measurable sediment discharge as sediment leaves the stream table. The video below demonstrates sediment being moved by water and therefore, being transported.

3. Fluvial Geomorphic Mechanisms

Simulate and record your observations of the specific manifestation of fluvial geomorphic processes as these mechanisms of adjustment:

3.1 Grain Size Sorting

Grain size sorting is indicated by the grouping of the individual particles by size. In this flume experiment, there were 4 grain sizes indicated by 4 different colors of red, black, white, and yellow. Red was the smallest, then black, then white, and yellow was the largest grain size. As shown in the image below, sediment grain sizes are visibly sorted. The black and yellow grains are seen to be transported (A), with the yellow grains grouping together and being deposited (B). The white grains are seen to be surrounding the yellow grains indicating their grain size sorting (C).

**Note there was very little red sediment seen in this flume experiment so it is not visible in the image below**

3.2 Meandering

A meandering channel is a single-threaded sinuous channel that moves back and forth across the floodplain as the outside bend of the channel is eroded and deposition occurs on the inner bend. The video below demonstrates a meandering channel. Although in order to demonstrate this, we had to force the meandering channel to occur by digging a sinuous channel before turning on the flow.

3.3 Braiding

Braiding channels are those that come together and separate again splitting the discharge of a single river between multiple channels. There are multiple shifting bars that separate the channels. The image below displays a braiding channel.

3.4 Avulsion

Avulsion is the sudden shift in water flow from one channel to another. Avulsion will occur after an interference or degradation and aggradation processes have altered the channel and the flow will change or jump channels to a more efficient way downhill. The video below demonstrates avulsion.

3.5 Chute Dissection

Chute dissection is when a new surface channel is formed that serves as a cutoff connecting either two other channels or a downstream segment of the same channel. The finger in the video below displays where the chute dissection will form.

3.6 Structural Forcing

Structural forcing occurs when a structure (boulders, vegetation, woody debris, beaver dams, etc.) is placed within a channel and the channel needs to adjust either through bank erosion, degradation, aggradation, riffle formation, new channel formation, etc. The video below demonstrates structural forcing by vegetation. It is hard to see in this video but sediment is being deposited behind the vegetation which is how the channel is adjusting to this structure.

3.7 While meandering can be observed, were you able to produce a classic single-thread, meandering channel in any of your experiments? If so, explain how it occurred, if not, do you think it is possible with the controls you had at the disposal in this flume?

A few times a single-threaded meandering channel existed naturally, though not for very long. When the discharge was small was the only time that conditions were right to create a true meandering channel. During one of our flume experiments, we forced a meandering channel to occur by digging a meandering, sinuous channel before turning on the flow. In real life, cohesive sediments like clay and vegetation will be present and hold together better than what was simulated in the flume.

4. Events

Simulate and record your observations of three specific events:

4.1 Small Flood

A flood is defined as when the discharge is great enough to initiate overbank flow. A small flood was simulated in the flume experiment by manually increasing the discharge into the table to 51 mL/s. The small flood resulted in an altered channel and increased sediment transport than what was seen at lower flows. Sediment discharge out of the table also increased as the result of the small flood. As shown in the video below, the tree was being impacted by the newly transported sediment and was being flooded with water. We thought that the floodplain of the houses was going to be filled with water, but the small flood was not large enough for that to happen, which is also demonstrated in the video.

4.2 Big Flood

A flood is defined as when the discharge is great enough to initiate overbank flow. A big flood was simulated in the flume experiment by manually increasing the discharge into the table to 98 mL/s. Channel alteration and significant erosion occurred on an even larger scale than seen during the small flood. Through avulsion, new channels were created to meet the size of the flood before making one very wide channel. As shown in the video below, the channel widened even more compared to the small flood and the floodplain where the houses were sitting started to be degraded more, where eventually they would be taken out by the flood.

4.3 Channel Realignment (Grading)

Channel realignment (grading) is channel modification by humans who build a new flow path for a water course. The humans are realigning the channel. In the video below, we are modifying the channel to straighten the channel, therefore realigning it.

Analysis

4.4 From what you did, what seems to be the role/impact of small floods vs. big floods?

The impact of a flood on a system results in channel alteration and increased sediment transport. During the small flood, channels were altered, erosion increased, and the sediment discharge out of the table increased. The big flood, however, altered more of the channel, completely overwhelmed the existing channels and made the channel wider. Through avulsion, new channels were created to meet the size of the flood before making one very wide channel. Erosion and sediment discharge was also seen on an even larger scale during the large flood compared to the small flood.

4.5 In your experimentation, did you observe overbank flows, bankfull flows, and/or baseflow flows?

All three flow types were observed during the experiments we created using the flume table being that we manually changing the flow throughout. Bankfull flows were seen when the experiment started and the channels were first being formed. The bankfull flow can also be described as the channel forming flow. When discharge was manually decreased following channel formation, this simulated the baseflow conditions, where the flows were less than that required to alter the existing channels. Overbank flows were seen when the discharge was increased to more than that of the initial bankfull channel forming flow producing a flood. This discharge was greater than what was used to form the channels so overbank flow and floodplain development occurred.

4.6 What role did hyporheic flow play in what you observed?

Hyporheic flow, also called interstitial flow, is the seeping flow of water through the sand, gravel, sediment, and other permeable soils under and beside the open streambed. The sediment in the channel was non-cohesive therefore water often flowed in the subsurface of the channel. This allowed for new channels to form through headcutting up to the source of the existing channel. This happened multiple times during the course of the flume experiment.

4.7 What role did recession limb flows seem to play in what you observed?

Recession limb flows are when the discharge decreases and the river's level falls, therefore incising the channel. As discharge was manually decreased, the sediment transport capacity decreased resulting in deposition and aggrading of the channels. With decreased flow, the number of flowing channels decreased as water searched for the path of least resistance to move downhill. The rate of channel avulsion and chute dissection also decreased. In receding flows, it is also possible to see bank failure from erosion that had occurred during the higher flow. The figure below demonstrates the recession limb of the hydrograph as most of the overland flow has now been discharged and it mainly throughflow that is making up the river water.