An EM-River EM2 Stream Table was used to demonstrate fluvial geomorphic processes such as erosion, transport and deposition in real time. The flume demonstrations were conducted on February 9th, 2021.
Flume Controls
1.1
Discharge: the volumetric flow rate of water that is transported through a given cross-sectional area.
Discharge was controlled by adjusted the flow rate of the pump which introduced water at the "headwaters" of the flume.
1.2
Slope: is the measure of the steepness from the tallest to shortest points.
Slope was controlled by adjusting the height of the outlet base level, but it could also be controlled by adjusting the legs of the flume.
1.3
Sediment Discharge: the volumetric rate of sediment being transported through a given cross-sectional area.
Sediment discharge was controlled by either increasing discharge, which would increase erosion and the transported sediment or by lowering the base level outlet, which would cause channel incision.
1.4
Profile: the longitudinal shape of the channel.
The profile of the flume was controlled by placing blocking structures in the channel that would lead to aggradation and ultimately change the longitudinal shape of the river.
1.5
Base Level: the lower limit that flowing water can erode sediment.
The base level was controlled by adjusting the outlet height. An increase in outlet height (base level) would lead to deposition directly above the outlet and lowering the outlet point would cause channel incision above the outlet.
2. Fluvial Geomorphic Processes
2.1
Bed erosion: the bottom portion of the channel or bed being eroded.
In the beginning of this Video 1 the process of bed erosion can be seen by the yellow sediment being transported.
Video 1: Bed Erosion Demonstration.
2.2
Bank Erosion: Erosion of the outer bank sediment. This process is dominated on outside bends of channels.
In Video 2 the process of bank erosion can be seen by the sediment on the right side of the channel that is initially stable being eroded by both the water and the transported sediment.
Video 2: Bank Erosion Demonstration.
2.3
Deposition: the accumulation of sediment that was being transported..
In Video 3 the process of deposition can be seen by a substantial sediment load being transported in the channel then settling and becoming stationary.
Video 3: Deposition Demonstration.
2.4
Sediment Transport: The active movement of sediment from one location to another.
In Video 4 the process of sediment transport can be seen by the the black, yellow, red and white sediment moving.
Video 4: Sediment Transport Demonstration.
3. Fluvial Geomorphic Mechanisms
3.1
Grain Size Sorting: the distribution of sediment grains in a riverscape dependent upon the size of the individual grain.
This process can be described by the force necessary to move an individual grain. The larger the grain the more force that is necessary to break the threshold of motion. Therefore for a given flood larger grains will not move as far and are deposited first. This can be seen in Figure 1 by the banks being predominantly yellow grains which are the largest and the channels consisting of the smaller white and black grains since they require les force to break the threshold of motion.
Figure 1: Grain Size Sorting Demonstration.
3.2
Meandering: a series of regular sinuous curves in a single thread river. Meandering is commonly confined to single thread rivers.
Figure 2 depicts a slightly meandering channel since there is a series of two curves. It should be noted that there creating single thread meandering channels in flumes is extremely difficult since vegetation cohesion isn't present.
Figure 2: Meandering River Demonstration.
3.3
Braiding: A network of channels separated by islands which are generally temporary.
Figure 3 depicts a braided channel within the flume, which can be seen by the multiple channels contains islands between them.
Figure 3: Braided River Demonstration.
3.4
Avulsion: the rapid abandonment of a river channel and the creation of a new channel.
Video 5 depicts the process of avulsion which can be seen by the yellow sediment being eroded and a new channel being formed in its place.
Video 5: Avulsion Demonstration.
3.5
Chute Dissection: occurs when the inner bank bar is dissected by the formation of a new channel.
3.6
Structural Forcing: Using natural/non-natural structures to reinforce the channel to minimize erosion.
Video 6 depicts a clear structure reinforcing the bed and banks of the channel and minimizing erosion. Directly following the clear structure bed scour (a form of erosion) is seen to be occurring.
Video 6: Structural Forcing Demonstration.
3.7
During my Flume Observation a classic single-thread meandering channel was not produced, although it would have been possible if more structural elements were available to reinforce the banks. The key component that is necessary to produce a classic single-thread meandering channel is cohesion, whether that be from vegetation or structural forcing.
4.1
Small flood:
In Video 7 from 0:25-1:10 a small flood event occurs which causes avulsion through the delta preceding the outlet and results in a new main channel and remnant side channels that are no longer active. When normal flows return a meandering channel is formed that is bounded by the banks of the small flood event. It should be noted that during this demonstration the sidewall of the flume did have an effect on the resulting flow paths.
Video 7: Small Flood Event 0:25-1:10.
4.2
Big Flood:
In Video 8 from 1:10-1:50 a large flood event occurs which results in avulsion and significant amounts of erosion. It is evident that the large flood broke the threshold of motion for the larger yellow grains and resulted in grain size sorting which can be seen by the yellow sediment accumulating in one area. During this event the sidewall of the flume had a major effect on flow paths.
Video 8: Large Flood Event 1:15-1:50.
4.3
Channel Realignment: Creating a new flow path. This is commonly done by humans to create more accessible water ways for shipping barges (ie Mississippi River).
4.4
From the flume demonstrations the impact of small flood appears to be a replenishing of new sediment to the floodplain that in the real world would contain nutrients. This would have many ecological benefits and be essential to a healthy ecosystem. Large flood events seems to rework both channel geometry and geomorphic units. The dominant sediment size in a reach can drastically change from a large flood event and the confining boundaries post large flood event seem to be the remnant channel.
4.5
My group specifically tried to observe these by changing the flows rates from 15 mL/s to 25 mL/s to 40 mL/s. One thing we noticed was that this was most difficult immediately after a large flood event because the bankfull flows and overbank flows were extremely large floods. By allowing lower flow rates to create banks this exercise became much easier.
4.6
I observed hyporheic flow into remnant channels that were no longer connect to active channels but still had flowing water in them. I think this process would be especially important for wetland environments that may not be directly connected to the water source. Wetland environments host some of the most biodiverse habitats so I think this process is essential for increasing biodiversity.
4.7
Recession limb flows had a dominant role in setting the stage for fluvial geomorphic processes to occur and create new geomorphic units. This was done by depositing sediments throughout the river.
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