Flume Observations
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Flume Observations
February 24, 2025
Pictured on the left is our group having a great time experimenting with the flume sandbox.
Photo credit: Joe Wheaton
Flume Controls
1- Water Discharge (Q)
A small pump moves water from a bucket at the base of the table up through a tube to the top of the table where it is discharged.
Discharge (Q) can be adjusted by turning a knob that increases or decreases the flow.
The rocks at the outflow dissipate the energy of the discharge flowing into the flume.
2- Slope (S)
Slope can be controlled by adjusting the position of the flume, either raising or lowering the table to adjust the slope.
3- Sediment Discharge (Qs)
Sediment is filtered out at the drain (base level) of the flume and can be redistributed back into the flume. Sediment can be removed or added to increase sediment discharge.
Increase or decrease of sediment within the flume can also affect slope.
4- Profile:
The longitudinal profile reflects the elevation of a river from the headwaters to base level. As we adjust the flume we can reflect various sections of the river based on their flow capacity and their ability to transport or deposit sediment.
The flume essentially represents one small reach of a river, at various positions within the longitudinal profile.
5- Base level:
Base level is the lowest elevation that a river can erode to. In the case of the flume, the base level is controlled by a drain that can be adjusted to either increase or decrease base level.
Fluvial Geomorphic Processes
1- Bed Erosion:
Erosion of the river bed occurs when there is enough discharge and therefore force, to entrain the sediment on the bed of the river and cause bed erosion.
2- Bank Erosion:
Bank erosion usually occurs around bends in a river channel. These areas are referred to as cut banks. The river erodes the banks at these bends because the velocity of the water is the highest at the outside edge of the bed. The inside of the cut bank is referred to as the point bar because velocity is the lowest and this can result in deposition of fine sediment.
This video provides examples of both Bed and Bank Erosion.
Video credit: Christina Waddle
3- Deposition
Deposition generally occurs at lower flows where the river does not have enough velocity and force to entrain larger size grains, resulting in their deposition.
This photo reflects several depositional zones where coarse grain sediments are being deposited due to the rivers inability to transport this material.
4- Sediment Transport
This video shows both grain size sorting and sediment entrainment. At this low flow, only finer sediment (white and black grains) is being entrained and moved the river.
Video Credit: Bryan Blau
Fluvial Geomorphic Mechanisms
1- Grain Size Sorting:
Largest to Smallest
Grain size sorting can be observed in the flume reflecting entrainment and the role of different discharges. Larger grains require a higher discharge to entrain where as smaller, finer grains can be entrained at lower discharges.
Right: Here grain size sorting can be observed with well sorted large grains reflected on the banks of the channel and smaller fine grains being entrained within the channel.
All of the sediment within the flume was already wet when we started our experiments which is an important factor affecting cohesion. This would not be the case for natural systems.
2- Meandering
Meandering rivers generally occur at lower slopes and vary in sinuosity. It is very challenging to create meandering river patterns in the flume. We were able to create small meandering patterns only occasionally and most did not last for very long. This example reflects very low sinuosity and is not a single threaded channel.
3- Braiding
A braided river reflects a network of channels without vegetation that branch and rejoin extensively. Braiding is a common pattern that can be observed in the flume due to the lack of vegetation or other structure that would inhibit braiding.
4- Avulsion
Avulsion is the process of flow diverting out of its original course and into a new path.
This video provides an example of avulsion at 0:34-0:50.
Video credit: Christina Waddle
5- Chute Dissection
Chute dissection generally occurs at high flows when a river flows over banks or meander paths and results in headward erosion. This photo reflects chute dissection occurring at several locations likely due to a high flow event.
6- Structural Forcing
Structural forcing occurs when the river is inhibited and forced to flow to flow around or through something. This can be caused by geology, material within the channel or in this case, anthropogenic forces. We were able to simulate structural forcing by using this culvert. Our river was forced to flow through this small culvert, this causes erosion at the outflow of the culvert and also caused hyporheic flow around the structure.
7- Single-Threaded Meandering Channel?
We were unable to create a single-threaded meandering channel, likely due to the presence of actual vegetation that would cause the cohesion necessary.
It was discussed that the only flume experiment that has resulted in the succesful creation of a single-threaded meandering channel was due to growing alfalfa in the flume. The roots and vegetation likely caused the cohesion needed to created a single sinuous channel.
Events
1- Small Flood
Here we simulated a small flood with a recessional limb. The small flood resulted in significant sediment entrainment and transport but did not significantly alter the channel and was strong enough to move "natural" structures (i.e. vegetation and large wood) within the channel.
Video Credit: Christina Waddle
2- Big Flood
Here we simlulated a large flood by increasing our discharge substantially. This caused bank erosion, bed erosion and one large homogenous channel to form.
We were also able to see the formation of a delta near the base level of our stream.
Video Credit: Quinn Olpin
3- Channel Realignment (Grading)
This is the process of altering a channels flow path or gradient, it often results in a more homogenous channel. This is well reflected in our large flood where our channel became one large, relatively straight and uniform channel opposed to any meandering or braiding patterns.
4- From what you did, what seems to be the roll/impact of small foods vs. big floods?
Small Floods: Within the flume result in sediment entrainment and transport as well as cut bank erosion but don't alter the channel too substantially.
Large Floods: We observed that large floods often cause substantial erosion of the channel and its banks and well as homogenization. The channel becomes less complex as large flood events move significant amounts of water and sediment.
5- In your experimentation, did you observe overbank flows, bankfull flows and/or baseflow flows?
Overbank Flows:
Overbank flows occurred during flood events when discharge was increased significantly causing the the river to flow out over its banks. This often caused avulsion and even chute dissection.
Bankfull Flows:
Bankfull flows occurred at discharges that did not alter the channel significantly but maintained enough water to fill the channel. This can be seen in the small flood event, where sediment is entrained and transported but the water does not overtop the banks of the rivers.
Baseflow Flows:
Baseflow flows exist at low discharges and do not result in sigificant erosion or deposition. This was simulated with small discharges.
6- What role did hyporheic flow play in what you observed?
Hyporheic flow is the movement of water between a stream's surface and subsurface. Hyporheic flow was strongly reflected when we simulated structural forcing. Water started to seep out and flow out of the banks of the downstream side of our chute. It is important to note that all of the grains within the flume were already saturated when we started working in the flume, which would likely affect how much hyporheic flow is occurring.
7- What roll did recession limb flows seem to play in what you observed?
The recessional limb of a hydrograph reflects how quickly or slowly the flow a flood decreases. A sharp recessional limb will affect a river differently than a more gradual recessional limb. We were able to simulate a more gradual recessional limb in our small flood event.
A gradual recessional limb results in sediment deposition within the channel and can result in a more stable channel. A steeper, more aggressive recessional limb can cause more unstable channels and can ultimately result in more erosion.
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