Embedded Files
1- Flume Controls
The water discharge can be directly controlled by the pump power/recirculation rate. The discharge can indirectly be increased by increasing the slope or sinuosity. Raising the height of the drain at the end determines the relief of the landscape which decreases the slope. The water discharge can be viewed in the videos below as the volume of water running across the cross-section of the flume at a given time.
Slope can be controlled in the channel through raising and lowering the top and bottom of the flume. Slope is decreased by increasing the drain height and the sinuosity.
Sediment discharge increases as more sediment is fed into the system and decreases as more sediment is deposited. The sediment transport across the flume was at it's greatest when the slope was increased and there was greater discharge. To increase sediment discharge irrespective of these 2 features, more sediment can be placed at the top of the flume. The sediment discharge can be viewed in the videos below as the volume of sediment moving across the cross-section of the flume at a given time.
The profile refers to the altitude as a function of distance from the top to the bottom of the flume. The profile can be adjusted by setting the amount of sand in each area of the flume or by placing structural features to incur deposition and erosion at certain points along the profile.
The baseline is lowest altitude the flume drops to, or the end of the profile. The baseline can be set by adjusting the drain height at the bottom of the flume. The water discharge can be viewed in the videos below as the volume of water running across the cross-section of the flume at a given time. The baseline can be seen in the videos as the relative elevation the water drains at.
2- Fluvial Geomorphic Processes
Bed erosion is entrainment of sediment from the bottom of a channel. It is a source of sediment. Mobilization of sand particles from the bottom of the channel through bed erosion can clearly be seen in the below video.
Bank erosion is entrainment of sediment from the sides of a channel and is a source of sediment. Sediment being eroded from the banks can be seen at the bottom of this video. This is occurring through shear stress put on the banks from discharge and from incision in the channel causing bank destabilization and failure.
Deposition is the accumulation of sediment and occurs downstream of where sediment is being transported. This can be seen in the video at 0:11 as the sediment goes from moving to not moving in areas that serve a sink (slope is not steep enough to continue the momentum).
Sediment Transport is the movement of sediment downstream. Sand can be seen moving from the top of the video to the bottom as discharge keeps the sediment moving.
3- Fluvial Geomorphic Mechanisms
Grain Size Sorting occurred in the channel as heavier grain sizes dropped out sooner. In the flume Orange was the heaviest/largest, followed by white and then black was the lightest/smallest. Red sand wasn't obvious. Sorting by size was evident at 0:11 as grains along the depositing front are covered up. Heavier grains (orange) can be seen depositing first, with white and black being deposited subsequently on top of the new channel bottom because they are lighter.
Meandering is the phenomenon of increased sinuosity in the channel as it creates it's own gravel bars and finds areas of least resistance in the floodplain. The heavier sand keeps going straight and is deposited as a bar, the lighter sand and flow go around the deposition causing helicoidal flow structures and the curve. This leads to a channel that is not straight due to it's own adjustments. Meandering can be seen in the photo to the right.
Braiding was evident in areas with multiple stream paths and anabranches. This occurred in the video to the right (although this is almost more bordering on anastamosing instead of braided) in a relatively flat area in the floodplain where the flow split because of heavier sand and then rejoined again.
Avulsion is the abandonment of an old channel after the formation of a new channel. This usually occurs after the original channel has been blocked or if the flow destabilizes the bank enough to "short cut" and create a new flow path. This can be seen in this video as a new channel is formed at the bottom of the video, while the upper 2 channels dry up.
Chute Dissection occurs in this video at 0:11 on the right side of the screen where a new channel being formed can be seen. (The main channel is also an example.) Chute dissection is the formation of a new channel
Structural Forcing is impact of structures in the channel which changes the geomorphology. The woody debris causes the formation of a new channel below, while there is flooding and deposition above the dump truck and incision and sediment transport below the dump truck.
Can you produce a single thread channel?
We did not observe a single thread channel. I do not think it is possible with the controls we had with this flume. Banks need to stablize via more cohesive substrate with finer grains like clay. Vegetation would also help stabilize banks enough to reduce braiding. This was shown in Braudrick and Dietrich's flume using alfalfa sprouts. (Braudrick et al 2009)
4- Events
A Small Flood saw bankful flows and overbank flows. New channel formation occurred to account for increased discharge. We simulated the small flood by increasing the discharge.
A Big Flood caused the entire flume to readjust leading to an entirely new riverscape. Sediment transport dynamics were intense enough that you can see large differences in flow vectors in between the small flood and large flood videos. We simulated the large flood by maximizing discharge and turning the pump up as high as it would go.
Channel Realignment is the adjustment of the channel within the floodplain which can occur rapidly with large events or over time with long-term erosion and deposition patterns. Minor channel realignment can be seen in the small-event to large-event video to the right as the channel on the right of the flume deposits sediment at the inside gravel bar at the bend. The channel as well as the bend then adjust downstream. This was simulated the through the large flood but could also be done through structural forcing.
5- Questions
From what you did, what seems to be the roll/impact of small flood vs. big flood?
Small floods lead to overbank flows and bankful flows. Increased sediment discharge occurred from increased bank and bed erosion and some geomorphic change occurred. The large flood in addition to seeing the features associated with small floods really reshaped the entire riverscape and caused formation of new channels to accommodate the extra discharge. Mass sediment discharge occurred and the profile was reshaped. Both can be seen in the event videos above.
In your experimentation, did you observe overbank flows, bankful flows and/or baseflow flows?
I observed overbank flows during rapid increases in discharge, bankful flows as discharge moderately increased, and base flows during the recession limb. Overbank flow can be seen in the flood transitioned from small to large when I turned the pump dial way up and generally caused geomorphic change. Bankful flow occurred in the small and large flood videos after the channels had adapted and flow was moderately stable. Sediment discharge was still present with bankful flows. The baseflows can be seen in the recession limb video below and no geomorphic change or sediment discharge occurred.
What role did hyporheic flow play in what you observed?
Hyporheic flow or groundwater flow, served to dissipate some of the discharge from the main channel and start dissecting chutes. In areas where the bank destabilized enough from hyporheic flow new flow paths emerged and avulsion occurred. Hyporheic flow can be seen at the very bottom of the video to the right.
What role did recession limb flows seem to play in what you observed?
The recession limb was the lagging decrease of flow after discharge was decreased. Gravel bars are formed in areas as flow decreased, leaving structural geomorphic areas with the orange sand accumulated. These can be seen in the video to the right and will enforce structural integrity of the riverscape in future high flow events.
Citations-
Braudrick CA, Dietrich WE, Leverich GT, Sklar LS. 2009. Experimental evidence for the conditions necessary to sustain meandering in coarse-bedded rivers. Proceedings of the National Academy of Sciences of the United States of America 106 : 16936–16941. DOI: 10.1073/pnas.0909417106
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