Flume Observations
Since I was unable to observe the flume in person I wanted to thank everyone who posted videos. I greatly appreciate it!
1. Flume Controls
1.1 Q (water discharge)
Discharge, calculated, Q=Ax*v, usually measured in cubic feet per second. Generally, discharge describes the volume of water passing through a location in the stream per unit of time. The average cross-sectional area is measured by multiplying the average depth and the average width. This measurement can be used to determine base flow, bankflow, and high-water events for a given stream or river. In the flume discharge is controlled by the volume of water that is being pumped into the flume itself in addition to the channel dimensions (Figure 1).
Figure 1. Photograph of water input into flume. Credit: Triton Abeyta
1.2 S (Slope)
Slope, is the change in elevation (high to low) over change in distance. The slope is an important measurement because it dictates the rate of runoff and the stream potential energy (which in turn effect the impelling and resisting forces of a system). Flume experiment: In the flume the slope is determined by the angle of the table itself. As the slope is increased (table lifted on the side parallel to flow) the velocity of the water increases. This results in an increase in entrainment and transport of sediment upstream and an increase in deposition downstream. Entrainment and transport are subject to grain size and embeddedness. In the flume, it appears that the grain size is a relatively uniform sand-dominated system (Figure 2). This results in higher sediment transport because the sediment is relatively light and not subject to cohesion.
Figure 2. Video showing the effect of slope increase on sediment and flow. Credit: Little River Research and Design
1.3 Qs (Sediment Discharge)
Sediment discharge is defined as the total volume of sediment moved by the stream over a defined amount of time or total mass of sediment moved over a defined amount of time. In the field this is measured by taking water samples, drying, and weighing them or constructing traps (wholes) that catch sediment. In the flume this can be measured by depositional areas and the relative elevation changes at different locations. At the outlet of the flume there is a sieve that catches the sediment that moves through the system. This could be used to measure net transport out of the system (Figure 3).
1.4 Profile
The profile is a cross sectional representation of the bedform and flow within a channel. This can be expressed as a longitudinal profile where the channel is examined from upstream to downstream (assessing changes in topography over changes in elevation). Lateral profiles are used to look at the shape of the channel at a particular location (from river right to river left). In the flume the profile can be assess either longitudinally or laterally as well. Figure 3 shows a view of the channel looking upstream, which you could use to obtain either profile.
1.5 Base Level
Base level refers to the lowest elevation a river can erode its channel to. When analyzing rivers in a larger context, this would refer to sea level. On a local scale this measure can vary depending on the erosion capacity within the channel in addition to sediment supply and calibre. In the flume the baselevel is located around the drain (Figure 3). In Figure 11 after a large flood event base level changes elevation due to bed load transport that aggrades the channel.
Figure 3. Example of sediment transport . Video credit: Megh Raj Kc
2. Fluvial Geomorphic Processes
2.1 Bed Erosion
Bed Erosion is the erosion of the bed material itself; this occurs within the channel. This results when shear stress is higher than the critical threshold and materials are entrained and transported elsewhere. This is subject to the material grain size and the velocity and depth of the water. In the flume you can visually see the movement of sediment downstream resulting in a deeper channel (figure 5).
2.2 Bank Erosion
Bank erosion occurs when particles detach from the bank and become entrained (Fryiers and Brierly, 2013). Depending on the antecedant soil conditions and composition bank erosion can be influenced by hydraulics or mechanisms. In Figure 5, as flow increases particles are detaching from the bank and moving downstream causing the channel to widen. The bed is also eroding and incising the channel.
2.3 Deposition
Deposition occurs when the velocity decreases to a point at which particles fall out of the water. This often occurs in result a barrier (large wood, debris, dams) in addition to geomorphological/ hydraulic features. When a particle meets the shear zone and is pushed into an eddy, this may result in the particle decreasing in speed and settling out. In other cases, small bars along meander bends form. The velocity on the outside of the bend in the thalweg is the greatest and decreases as it moves outward, resulting in decreased velocity, depth, and shear stress to induce motion (Figure 5).
2.4 Sediment Transport
Sediment is transported prior to entrainment of the grains from lift, drag, and turbulent forces (entrainment threshold). Entrainment thresholds are greater for larger particles and lower for smaller particles, the exception being cohesive materials that need higher shear stresses to entrain particles. Transportation of the grain occurs when the occurs materials are moved downstream. This process can occur at different scales; dissolved, suspended, or bedload transportation. In the flume, sediment transport can be noted visually as particles move downstream (Figure 5). Figure 6 shows sediment transport from an underwater perspective.
Figure 5. Video of flume experiment that shows bed erosion, bank erosion, deposition and sediment transport. Credit: Joseph Wheaton
Figure 6. Underwater video of sediment transport Credit: ____
3. Fluvial Geomorphic Mechanisms
3.1 Grain Size Sorting
Grain Size sorting is the process of selective deposition of grain materials due to varied flow velocities within a channel. The velocity of the flow dictates which grains can be transported and the fall velocity determines when a particle will be deposited. Along a bend in a river, the thalweg has the highest velocity, and the velocity decreases along the inside of the bend this results in deposition of materials (point bars). The Hjulstrom diagram gives a general idea of when these processes occur for differing grain sizes and velocities, but sediment cohesion and embeddedness must also be considered. In the flume, grain size sorting is very apparent due to the different colors of sediment that are associated with size. In FIG_ the inside curve of the bend has characteristically smaller grained materials (red/black/white) that are deposited until a point bar formed. In the channel, the remaining grains are larger (yellow/black) (Figure 7).
3.2 Meandering
Meandering of a river occurs as the flow pattern creates a wave like pattern. The shear stress exerted on the outer bank erodes it and deposits the sediment on the opposite bank creating point bars. Meandering increases sinuosity of the channel. In Figure 3 (above) for a brief period the channel is a single meandering stream at low flows. When flow increased, the channel reconnected to smaller channels. The video below shows how a single channel meanders at a smaller scale and his helpful for visualizing and understanding this process (Figure 8).
Figure 8. Single channel meander formation. Credit: Little River Research and Design
Figure 7. Picture of a channel with grains sorted. Photo credit :
3.3 Braiding
Braiding in a channel describes a system with 3 or more channels. Channels are split by bars or islands. This occurs in more alluvial systems that are not confined by valleys. Braiding also occurs as a result from obstructions causing erosion/deposition and differing preferential flow patterns. In the flume, braided channels resulted from depositional features (bars/islands) the diverted flow and created or maintained channels (Figure 9) .
3.5 Chute Dissection
Chute dissection occurs on formerly wet bars or islands. Overbank flows on bars create channels that are reoccupied and further eroded at higher flows that create chute channels (Fryirs and Brierley, 2013). In the flume, the chute channels occurred because of seepage (Wheaton) (Figure 10). In the flume experiment, chute channels occured in the bar below the vegetation (Figure 11).
Figure 9. Flume experiment braided channel. Credit: Megh Raj Kc
Figure 10. Video of chute disection in flume. Joesph Wheaton describing the process.
3.4 Avulsion
Avulsion is a shift in the preferential flow of a river. The conditions and mechanisms under which this occur vary. In the flume experiment prior to the vegetation addition the braided channel the downstream depositional feature blocked flow and redirected it (Figure 11). Before the vegetation was reinforced and placed downstream the channel was braided and had several depositional formations. Prior to the introduction of the vegetation, a larger depositional feature was created downstream eventually aggrading and resulting in channel flow diverting to the opposite bank.
3.6 Structural Forcing
Structure forcing occurs when an object obstructs the channel flow resulting in depositional and erosion patterns around the object. This occurs from natural (large wood, vegetation, beaver dams) and anthropogenic factors (dams, culverts). The location of the obstruction within the channel dictates the depositional and erosional patterns. For example, a mid-channel debris jam will result in divergent flow around the jam which carves out channels, and deposition above and below the jam resulting in bars. Locally, the presence of structures within the channel act as barriers to trap sediment and increase geomorphic complexity. Within the flume classmates places small pieces of vegetation in the channel. From the video, it does not appear that a significant amount of deposition occurred as a result (Figure 11). This link is a great example of structural forcing from a log jam. https://www.youtube.com/watch?v=QCNx0QKbe-o&list=PLVa74th2F4P9p-mj-KIrlls8D5l6D1tb-&index=34
Figure 11. Video showing structure forcing, avulsion, and chute dissection. Credit: Joseph Wheaton
3.7 Observations of meandering
As observed in fig _ and fig_ a single channel meandering throughout the entire flume didn't occur. It did occur in the middle of the flume. I think the tendency to braid and aggrade in the lower reach is due to the lower slope and lack of valley confinement. Simply put, there is more room for the water to move onto the floodplain and dissipation of flow energy over a larger surface area. The middle reach was partly laterally confined on river right, but not on river left, allowing for floodplain connectivity and the associated flow dissipation. In the upper reach the main channel was laterally confined by the flume edge itself. Since I wasn't present for this experiemtn I'm unsure how packed the sediment was in this experience. If the sediment was densly packed it would take more energy to erode bank material.
4. Events
4.1 Small Flood
In fig a small flood event is observed. During the small flood sediment was mobilized and transferred downstream. Overbank flows at the top of the flume pushed large sediment (yellow) into the channel and downstream. The overbank flows erode the bank outer bank and deposits sediment in between the newly created channels. The main channel has fast flow that erodes the bed material causing the channel to widen. Small bends begin to form as depositional areas increase. The event subsides and small rills and gullys remain on the outside bank in addition to avulsion channels in the floodplain (Figure 11).
4.2 Large Flood
Large flood events have more potential energy to move sediment and rearange geomorphologic features within the channel. In the example stream discharge is high enough to transport all sediment calibres downstream (notice how far the yellow material moves before deposition). Thie high sediment influx creates bar and aggrades the channel. This is visible by the change in dominate color distribution, where the heavier sediment; yellow and white, becomes dominant through the reach. Due to the sediment movement the lower portion of the flume has has a wider floodplain area with more complex geomorphic units. During the flood the middle of the channels widen and deepen. The deposition of material from the hillslopes create depositional features. Depositional features with larger material exceed the entrainment threshold and induces meandering. (Figure 11).
4.3 Channel Realignment (Grading)
Channel realightment (Grading) in the flume was the result of sediment deposition from the large flood event. at the lower end of the flume. The process of incision of the channel and overbank flows eroded the channel and hillslope. The bed-material load and wash load was deposited in the lower reach of the flume which aggraded the main channel and smaller side channels.
Figure 12. Flume experiment with various flow regimes. Credit: Joseph Wheaton
4.4 Impact of flood magnitude
From my observations of my classmates" observations and youtube videos, it seems that small and large floods have different effects on river channels. Small floods have lower overall stream power and tend to create more small-scale depositional features within the channel, often resulting in channel braiding. Load is primarily transported as wash and suspended load in this instance. The floodplain connects to old channels and further erodes them, but the preferential flow path is not significantly altered. Larger floods appear to "level off" the stream transporting all sizes of sediment over a longer distance (Figure 13). In the floodplains, large floods fill in small channels and chutes and occasionally result in avulsion. Load is transported as bed material load in this instance. Overall, small floods rework geomorphic units on a small scale and large floods result in larger scale erosion and aggregation of the channel and floodplain. These observations are limited because the duration of the flood appears to be relatively short in both instance. Magnitude and duration of floods effect the geomorphic effectiveness of events.
4.5 Flow Observations
In these experiments, I believe overbank flows, bankfull flows and/or baseflow flows were observed (Figure 14). Overbank flows occurred during floods along the hillslope on river right of the flume in Figure 11. This resulted in aggravation of the lower floodplain areas with sediments of all calibre and grading. Bankfull flows were observed briefly before they either eroded the banks or became overbank flows during both the small and large floods. On river right at the top of the flume the hillslope eroded and also limited the ability for the channel to migrate, on river left the stream had a relatively low bank which required less flow to reach bankful stage. Following the small flood, the flow returns to baseflow, as return flow and throughflow decline the sediment within the channel settles and only minimal flow is present.
4.6 Hyporehic Flow
Hyporheic flow is the area in porous soil that surface water and groundwater interact. Hyporheic flow is influenced by large topographic features like boulders, cascades, aquatic vegetation, and large woody debris. Water from the channel enters hyporheic areas where there are steep drops and re-enters in lower slope reaches like pools or runs (Boano et. al., 2014). In Figure 11, the structural forcing induced by vegetation captured sediment above and below the structure. More topographic complexity drives flow to diverge and influences hyporheic flow (Boano et al., 2014). Hyporehic flow induced chute dissection in the channel through seepage (Figure 10). Hyporehic flow likely was a factor in bank erosion as well, easing the shear stress required to disloge particles.
4.7 Recession Limb Flow
The hydrograph for the flood events in the flume would be right skewed, having a large peak and a gentle receding limb. The receding limb is slow and gentle because water is inflitrating into soils. If the soil becomes saturated, saturation excess overland flow occurs resulting in overbank erosion and downstream deposition. The receding flow occurs as soils slowly filter out excess water. This results in a gradual decrease in discharge to base flow levels. This process is influenced by the magnitude and duration of the rainfall event, antecedent soil conditions, and soil composition. The large flood in Figure 11 was preceded by a small flood, thus the soil was closer to the saturation threshold, producing more overbank flow and chute dissection. The shape of the flume "catchment" was relatively elongated, and it takes less time for water to reach the trunk stream than in more circular basins. The narrow headwaters with steep banks allowed for water to flow at high velocities over a smaller area and dissipate at lower energy in the lower reaches.
Figure 14. Figure from Fryiers and Brierly, 2013
Figure 13. Post Large flood flume profile. Credit: Megh Raj Kc