Module 6

Flume Observations: Fluvial Geomorphic Processes

1. Flume Controls

1.1 Water discharge (Q)

In the flume, the discharge is controlled by the recirculation pump that flows through the system and then is continuously recycled through the system. The blue knob controls the rate of discharge throughout the system. Discharge is the volume of water that acts on and flows through a fluvial system.

1.2 Slope (S)

The slope is controlled by raising one end of the sand box. This system's slope decreases from the headwaters to the base area. Increasing the slope increases the energy of the system and affects sediment movement.

1.3 Sediment discharge (Qs)

To control sediment discharge, you can change water discharge by increasing or decreasing the volume that flows through the flume, changing the slope, changing the base level, or using structures/vegetation. Sediment discharge is the sediment moved through the flume into the base level drain.

1.4 Profile

The profile can be changed by raising or lowering the slope of the table, moving sediment around, or changing the base level. It is longitudinal and assesses the elevation changes along a river.

1.5 Base level

The base level of the system can be controlled by raising or lowering the plastic tube where the water drains out. The base level is the lowest elevation at which a water system drains out. Varys depends on the scale of the system.

2. Fluvial Geomorphic Processes

2.1 Bed Erosion

Bed erosion can be seen throughout the flume experiment, entrainment of the bed of sediment occurs when there is enough energy to move certain sizes of sediment. In this video we see a high flow creating a lot of suspension of smaller sediment and bedload transport of larger sediment which causes the channel to become more incised.

2.2 Bank Erosion

Bank erosion can also be seen along the flume channel as the side channels erode and the channel moves laterally. It commonly occurs on the outside curve of the channel bends. In this video you can see sediment being removed from the outside curve of the channel and then deposited on the inside curves.

2.3 Deposition

When the energy of the flume system decreases in certain areas, sediment deposition occurs. More sediment is deposited near the base level as the water begins to pool. In this video, once the flow decreases, we begin to see deposition of the finer sediment that was suspended.

2.4 Sediment Transport

Sediment transport can be seen by looking at the various colors of grains move throughout the experiment. Depending on the flow and turbulence, certain sediments will move. These factors, plus the grain size, will affect the differences and where this sediment will move within the stream. White and yellow grain sizes are larger and the black and red are finer. So larger sediments will move primarily as bedload and the finer sediment will be in the suspended load. In this video, we see how the larger sediment is deposited along the riverbanks and midchannel, and the sediment discharge of the finer sediment is more significant. Once the flow is lowered, the smaller sediment begins to settle in the channel bed.

3. Fluvial Geomorphic Mechanisms

3.1 Grain Size Sorting

The more energy a stream has, the more it will be composed of larger and coarser material, and if it has less energy, it will have smaller and finer bed material. In the experiment, we increased the flow, and then the larger grains were deposited into the mid-channel, point bars, and diagonal bars. The smaller grains were slowly being eroded away, suspended, and then went through the base-level drain.

3.2 Meandering

Meandering river is a type of river that bends and curves throughout a floodplain and is caused by erosion and deposition. Meandering rivers are laterally unconfined and very sinuous, typically in low gradient areas where the river can flow slowly and have more room to spread.

As you can see in the video, the river began turning into a meandering river as we lowered the base level, decreased slope, and slowed the flow. The outer banks have a higher flow velocity, which erodes, and then deposition occurs on the inner banks where the flow velocity is lower. We could not create a classic single thread meaning channel because the sediment in the flume experiment is non -cohesive.

3.3 Braiding

Braided rivers are signified by interwoven channels and separated by mid-channel bars or side bars. To create one in the experiment, we had the flow be variable and lowered the base level slightly to increase slope.

3.4 Avulsion

Avulsion is when an active channel switches to another channel that is either a new channel or a nearby but inactive channel.

To create an avulsion, we allowed a channel to form first. Once we had our channel, we increased the stream velocity. As you can see in the video, we had outer bank erosion occur, which then created a new channel redirecting our flow.

3.5 Chute Dissection

Chute dissection occurs when a river cuts a new, shorter path that bypasses the longer, existing flow path. This often happens in mainstream rivers when a high flow period causes neck erosion, creating a shorter, more direct flow route. To simulate this, we just increased the flow and lowered the base level to increase flow velocity, which caused erosion and created a chute.

3.6 Structural Forcing

Structural forcing means that built structures, debris, and vegetation interfere with the active channel causing a change to the flow path. To create this, we used fake vegetation, which, at the beginning of the video, caused the flow to be forced river left.

3.7 Meandering in the flume experiment

A single thread meandering channel was impossible to produce in the flume experiment due to the noncohesive sediment, and the flume experimental area constrains lateral movement. If the flow and slope were low, you could get some active channels to meander slightly, but the sediment eroded too quickly to create a classic single thread. If the sediment was adhesive, the experiment area was more extensive, and there was more time to allow the slow and steady flow to create the meandering channel, it could be possible.

4. Events

4.1 Small Flood

A small flood event was created by having the flow start at a medium rate, then turning up for a high rate for a short period, and then ensuring the slope was not too great by keeping the base flow in the middle. I used a video by Bryan Blau as our group forgot to record our small flood event.

4.2 Large Flood

Increasing the flow rate to a high flow rate for an extended period of time stimulated a large flood. This caused a significant amount of bed shear stress, leading to extensive bed and side channel erosion, which caused the channel to become wider, deeper, and channelized in some areas and lots of floodwater runoff leading to additional erosion. This event had high amounts of sediment discharge.

4.3 Channel Realignment

Channel realignment occurs when a river is reshaped or modified to improve channel flow efficiencies, reduce erosion, or adjust the channel's slope. It can occur naturally through processes such as chute dissection or avulsion. For our flume experiment, we graded out the sediment to create a difference right where the flow starts.

4.4 The role/impact of a small flood vs. a large flood

A small flood and a large flood caused an increase in erosion, leading to the active river channel becoming broader and more profound. The small flood served as an event that moved finer sediment deposits and didn't have overbank flow. The small flood events contribute to the stream evolution over time as they create new small erosions throughout the channel, which can lead to new channels in the future. Also, the small flood could move smaller debris, which the newly placed debris could alter stream flow.

A large flood event moves large and small amounts of sediment and debris. This movement can lead to new river features formed after the flood recedes and the large debris and sediment have moved. This event more than likely can lead to a more direct flow path. Also, A large flood can deconstruct the river structure through sediment pulses, leading to large amounts of sediment being distributed downstream. Lastly, large floods have high quantities of overbank flow which will flow into the flood plain. The overflow plays a role in floodplain formation, recharging groundwater and providing the flood plain with nutrients.

4.5 Overbank flows, Bankfull flows and/or Baseflow

Overbank flow was observed during the large flood event when the water went over the riverbank into the surrounding sediment.

Bankfull flow was seen during the small flood event as well as periods when we had smaller channels and would increase the flow, causing bank full flow to occur, like when we created braided rivers.

Baseflow was the most common flow throughout the flume experiment, as we often started our experiments with a low flow that did not fill the channel's banks.

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

Hyporheic flow was seen during chute dissection and avulsion. Hyporheic flow/seepage occurred as the bank remained, and water began to flow from the other side. Eventually, the bank eroded away and caused chute dissection, as seen in the chute dissection video above. The hyporheic flow mainly contributed to starting new channels, entrainment of bed sediment, the bed composition, and avulsions. It also contributed to the very fine red sediment being trapped beneath other larger-sized sediment.

4.7 The role of Recession Limb Flow

The role of recession limb flow can be seen after water flow goes from a high flow to a low flow rate. In the video, the flow is moderately high, which has created a braided river system. The river system has moved larger sediment into the mid-channel banks and the side channels bars. Once the flow is decreased, the finer sediment that was suspended during the high flow becomes deposited on the riverbed, which is the red sediment. We also see specific smaller channels become disconnected from the main flow as the sediment is deposited and the flow is returned to a baseflow cutting off the main channel from those areas (video provided by Joe Wheaton).

Throughout the flume experiment, this sediment deposition occurs after high flood events, creating a new geomorphological feature along the river channel as sediment transport is minimal.

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