Jens Ammon
Fluvial Geomorphic Processes - Laboratory Flume Demo
Flume Controls
Slope (S)
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The slope of the slope of a system is a large factor in the total energy of a system and how sediment is transferred. Generally, slope angle decreases moving from headwaters to base level.
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The slope of the flume is controlled by raising one side of the sandbox.
Profile​
The longitudinal profile is the elevation change along the river length. Adjustments to base level impact the entire channel profile.
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Sediment discharge for the flume system is the sediment that falls through the base level drain.
Discharge (Q)
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Water discharge is the volume of water that flows through, and acts on, a fluvial system. Discharge is a rate, with units of volume/time. Discharge is often labeled as the flow exiting a system.
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In the flume, discharge is controlled with the water recirculation pump, recycling the drained water back to the top of the flume.
Sediment Discharge (Qs)
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Sediment discharge is the amount of sediment moved by the river. This sediment flux varies depending on if it is an erosional system or depositional system.
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Sediment discharge for the flume system is the sediment that falls through the base level drain.
Base Level​
The base level of a system is the lowest elevation that all water flows to. For most systems, the ultimate base level is the ocean, but it can be any body of water depending on the scale of the system.
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In the flume, base level was controlled by raising or lowering the plastic drain chute.
Fluvial Geomorphic Processes
Bed Erosion
Bed erosion is the entrainment of sediment off of the river bed. The river must have ample energy to initiate erosion and sediment movement.
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Bed erosion can be seen all along the flume channel. Note some grains fall out of suspension and others are lifted into suspension.
Bank Erosion
Bank erosion occurs as the channel moves laterally across the landscape. Bank erosion is common on the outside curve of bends.
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Bank material is falling into the flume channel as it is undercut by the water flow.
Deposition
Deposition occurs when the stream no longer has enough energy to keep the sediment suspended. Deposition often occurs when the stream profile flattens or the channel widens, causing flow to slow.
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Deposition can be seen in the flume as delta deposition, where the channel meets the base level pool.
Sediment Transport
Sediment transport is the material moved by a river. Larger sediment is generally transported as bedload, and finer sediment generally comprises suspended load.
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The black and red grains in the flume are finer and the yellow and white are larger. Different patterns develop depending on the stream power.
Fluvial Geomorphic Mechanisms
Grain Size Sorting
Larger grains require more energy to both entrain and keep suspended. Therefore areas of higher stream power will have coarser bed material, and areas of slower flow will have finer bed material.
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Flume sediment size order from smallest to largest is red, black ,white, yellow. Note that the yellow and white grains were deposited first to make the mid-channel bar.
Meandering
A meandering river is a very sinuous, laterally unconfined river. They are typically found in flatter areas of low elevation change. Erosion occurs on the outer edge of each bend, with a point bar being deposited on the inside bend.
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While a single channel meandering river is not attainable with the non-cohesive flume sediment, small sections of meandering channels do arise.
Braiding
A braided river consists of multiple interwoven sinuous channels in a laterally unconfined system. The mid-channel bars between channels are temporary and unvegetated. Braided rivers typically have high sediment transport rates.
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Note the multiple active channels in the flume and the varying sizes of mid-channel bar.
Avulsion
An avulsion is when the active channel switches to another path, either a previously abandoned channel or a newly formed channel. This switch often occurs due to structural forcing and vegetation restricting flow in the main channel.
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In this flume example, debris is causing deposition, which then restricts flow and causes the channel to avulse.
Chute Dissection
Chute dissection occurs when flow erodes into sub-preferential flow paths, such as when flow levels drop and the river begins carving into previously fully submerged deposits.
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In the flume new chutes are being cut due to seepage.
Structural Forcing
Structural forcing occurs when debris, vegetation or built structures cause a change in channel flow path. Such structures can cause deposition that restricts flow in the main channel, leading to an avulsion.
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In this flume example, the rock is splitting flow into two channels.
Meandering Channels in the Flume?
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A sustained single meandering channel is not possible in the experimental flume environment. In order to maintain a single channel, sediment must have cohesive strength, either through clay or vegetation. The sediment used in the flume experiment is too similar to incohesive sand to support a single meandering channel.
Events
Small Flood
A small flood event was simulated by increasing the discharge of the system (turn up the pump) from a medium rate to a high rate. This event caused the main upstream channel to widen and deepen, and reworked the downstream braided system into one dominant channel. The upstream section remained erosion dominated, with deposition occurring near base level.
Large Flood
A large flood event was simulated by increasing discharge from a low rate to a high rate. This event cause total reworking of the entire channel, with the complete removal of the initial mid-channel features and reworking of the floodplain areas. Sediment discharge was much higher for this event.
Channel Realignment (Grading)
A channel realignment event was simulated by digging out a new channel, causing divergent flow.
Big vs Small Flood
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The large flood event caused much more sediment movement, both in the channel and in the floodplain, than the small flood event. As seen in the previous videos, the small flood event consolidated the multiple braided channels into a single large channel. The large flood event also consolidated flow into a single channel, but there was much more lateral movement of the channel through the floodplain with the larger amount of floodwater and energy.
Overbank Flow / Bankfull Flow / Baseflow
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- Overbank flow can be seen in the structural forcing, flood, and avulsion videos. In all of these scenarios, deposition caused the water flow to breach the channel banks and form a new channel.
- Bankfull flow can be seen in the sediment transport and braided river videos. In these videos, there is rapid sediment and bedload transport and moderate lateral channel movement.
- Baseflow can be seen in the meandering and deposition videos. In these cases, the low amount of water flow leads to a deposition dominated system.
Hyporheic Flow
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Due to the coarse sediment size relative to the channel size, the hyporheic zone was fairly large for the flume channels. Hyporheic flow can be seen as seepage in the flume (chute dissection video) and it likely helped entrain bed material in the channel. Hyporheic flow/seepage was a large contributor to the formation of new channels and avulsions, as seen by the multiple small channels trying to start in the avulsion video.
Recession Limb Flow
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Recession limb flow, or the return from flood conditions to baseflow conditions, can be seen as the large single-channel flood system changes to a sinuous and braided system. With decreasing discharge, sediment transport rates gradually decline, mid-channel bars and features are deposited, and floodplain areas are again abandoned.