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1.1 & 1.5 Discharge Q and Base Level:
Discharge is the volume of water that flows through a river system. Discharge is one of the primary variables that impacts sediment transport and the morphology of a river channel. In the flume discharge is controlled by the blue knob that changes how much water is pumped through the flume.
Base level is the lowest elevation that a river system flows into. Base level controls can be local like cutler reservoir for the logan river or ultimately for most river systems the ocean.
1.2 Slope Control
The slope of a river channel heavily dictates the amount of energy a river system has. Slope generally is greater at the headwaters of the system and decreases as distance downstream increases. In the flume the slope of the system was dictated by its base. To increase slope for the flume you simply raise the upstream side.
1.3 Qs sediment discharge
The volume or mass of sediment that a system transports is the sediment discharge. Sediment discharge is dependent on numerous factors such as, discharge, slope, and grain size. Sediment discharge in the flume was the amount of sediment that fell into the net and was largely dictated by discharge and base level.
1.4 Profile
The longitudinal profile of a river depicts the change of elevation from the headwaters of a river to its mouth. Longitudinal profile is graphed with elevation on the y-axis and distance from the headwaters on the x-axis. The longitudinal profile of a stream offers insite on how the slope of the river bed changes throughout the system. In the flume adjusting the base level as well as the slope of the system controled the profile.
2.1 Bed Erosion
Bed erosion occurs when the river has sufficient energy to entrain sediment from the bed of the river and carry the sediment downstream. In the flume bed erosion was apparent when we increased discharge or simulated a flood event.
2.2 Bank Erosion
Bank erosion was simulated in the flume by creating an artificial channel with steep banks. Due to the lack of cohesive properties of the flume sediment the steepened banks of the channel collapsed. Additionally when discharge was increased the banks were further eroded widening the channel.
2.3 Deposition
Deposition is when sediment transported by a river is deposited due to a decrease in energy. Although deposition occurs throughout a river system, as the slope of a river channel decreases the amount of deposition increases due to loss of energy. In the flume deposition was greatest at the tail waters of the systems or the "delta". Deposition additionally could be seen from the creation of point bars in the flume.
2.4 Sediment Transport
Sediment transport is the transportation of sediment throughout a river system. The amount of sediment that a river can transport is dependent on the slope of a river, discharge, and the sediment size. How sediment is transported throught a system falls into two catagories either bed load dominated or suspended load dominated. Due to the size of the sediment in the flume it was primarily bed load dominated.
3.1 Grain Size Sorting
Grain size sorting is the process of selective entrainment of sediment due to the amount of stream power present. Grain size in the headwaters of system is generally larger while the grain size at the tail end is finer. This is caused by the ability of a stream to move finer sediment larger distances and carry it downstream. In the flume sediment size from largest to smallest is yellow, white, black, and red. In the video you can see that the yellow sediment was not moved while the white and black sediment was. This was due insufficient stream power to entrain the yellow sediment.
3.2 Meandering
Meandering occurs when a river system in latterly unconfined. This leads to the river creating a sinuous, bending path through a valley. In the flume we attempted to create a single meandering channel by structural forcing the banks with fake trees. But, lack of soil cohesiveness caused the river cut through the banks.
3.3 Braiding
Braiding occurs in systems that are laterally unconfined and bed load dominated. These channels are separated by bed material rather than floodplain like an anastomosing channel. Braiding is more apparent at base flows or low flows. Braiding was apparent in the flume due to the lack of cohesive properties and the lack of lateral confinement.
3.4 Avulsion
Avulsion is the rapid abandonment of a channel due to the creation of a new preferential flow path. Avulsion can be observed following a large flood event. Avulsion in the flume could be seen through the abandonment of a channel after a large flood.
3.5 Chute Dissection
Chute dissection occurs when a point bar or mid-channel bar is dissected leading to a new channel running through the bar. This occured in the flume when flows were lowered and the new preferential path was through a point bar that was previously formed.
3.6 Structural Forcing
Structural forcing is the process of armoring or enforcing a river channel to alter its flow path. In the flume we added fake trees to the banks of the channel to try and force meandering.
3.7 While meandering can be observed, were you able to produce a classic single-thread, meandering channel in any of your experiments? If so, explain how it occurred, if not, do you think it is possible with the controls you had at your disposal in this flume?
We tried to create a single-thread meandering channel by trying to increase the cohesiveness of the soil by burying fake trees in the banks. This structural forcing worked for a short time with very low flows however the channel quickly realigned and became braided again. To achieve a single thread meandering channel we would need riparian vegetation to increase how cohesive the sediment is.
4.1 Small Flood
We simulated a small flood by increasing discharge. A small flood lead to an increase in erosion and a second channel forming at the headwaters. Before the simulated flood there was a large degree of braiding in the system. During the flood some of these braided channels turned into one. We also observed the channel widening and deepening as well as avulsion during the small flood.
4.2 Big Flood
To simulate a big flood we rapidly increased discharge. This lead to a substantial increase of sediment transportation and an increase in bank and bed erosion. Additionally, where previously the channel was braided after the big flood there was only one over windened channel.
4.3 Channel Realignment (Grading)
Channel realignment was simulated by single thread straight channel. As we increased discharge the channel widened and regraded.
4.4 From what you did, what seems to be the roll/impact of small flood vs. big flood?
Small floods lacked the pure power of a large flood. During small floods there was an increase in erosion and sediment transport. Small floods lacked the power to completely straighten the channel and changed the profile of the river slower than a large flood. Large floods lead to a completely different flow path and mobilized much more sediment.
4.5 In your experimentation, did you observe overbank flows, bankfull flows and/or baseflow flows?
In the flume I didn't observe overbank flows but mainly observed base or bankfull flows. When flows increased to enough to achieve overbank flows the channel would usually just erode the banks creating new flow paths. We decreased the discharge to observe baseflow. During baseflow erosion decreased substantially.
4.6 What role did hyporehic flow play in what you observed?
Due to the sand like properties of the flume sediment hyporhic flows played a large part in channel formation. In numous cases we saw subsurface flow slowly eroding under the bank ultimantly leading to a new channel froming. The movement of water through the sediment hylighted the lack of cohisive properties of the sediment.
4.7 What roll did recession limb flows seem to play in what you observed?
In the flume when discharge was decreased simulating a recession limb the river channel stabalized. Additionally, there was a significant decrease in sediment discharge and transport. For our group we were able to lower the discharge enough that only red and white sediment was able to be entrained.
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