Flume Lab
Flume Controls
Q (Water Discharge)
S (Slope)
The slope of the flume lab is controlled by elevating one side of the table over the other. In the image to the right, the far end of the table is slightly higher than the other end.
Qs (Sediment Discharge)
Sediment discharge is the weight of sediment being transported downstream by a river. It is impacted by the distribution of grain sizes, roughness, and the water discharge rate.
Sediment discharge can be controlled in the flume lab by moving grains around, increasing/decreasing discharge, and or placing structural features in the flume.
Profile
The profile of the flume's channel is naturally defined by the channel as it aggrades or erodes grains from its banks. Aggradation can be increased by hand by raising the base level, erosion can be manually increased by lowering the base level (base level control mechanism is shown in the back of this photo and shown in action in the Base Level section).
Base Level
The base level of the channel can be controlled in the flume by raising or lowering the base level control mechanism.
Fluvial Geomorphic Processes
Bed Erosion
Bed erosion occurs when the discharge and/or slope of a channel gets high enough to entrain and transport sediment grains from the river's bed.
Bank Erosion
Bank erosion occurs when a flow undercuts a bank and eventually causes it to slump/fall into the active channel.
Deposition
Sediment deposition occurs when entrained grains lose momentum in a channel and are deposited onto the channel bed.
Sediment Transport
The material a river moves is known as sediment transport. In this flume, the grain size from largest to smallest is represented as yellow, white, black, and red. Sediment is transported from the upstream channel down through the base level drain. The largest grain sizes are typically transported as bedload, while finer grain sizes are transported as suspended load.
Fluvial Geomorphic Processes
Grain Size Sorting
As stream power increases, the size of the grains in the bed will be larger as smaller grains are entrained and transported downstream. Conversely, low stream power will have finer grain bed material.
Meandering
Meandering channels are formed in unconfined floodplains where a river has space to move. As the channel moves through areas of low slope, it begins to form bends and become more sinuous, or "meander". The channel erodes away at the the river/stream bank on its outer edge and deposits sediment on the inside.
(Video Credit: Emma Tombaugh)
Braiding
Braiding occurs in areas of high stream power where floodplains are laterally unconfined. A braiding river system has multiple channels which flow into and around mid channel bars. Braiding river systems are further characterized by high sinuosity and sediment transport.
Avulsion
An avulsion occurs when a channel within a river system is abandoned for a newer, more efficient path lower in the floodplain. The video to the right shows this process in action after discharge is increased in the flume. Video credit: Joe Wheaton.
Chute Dissection
Chute dissection is the process of a stream cutting through a mid channel or point bar in a river river system. It typically occurs as a river is adjusting to a lower base level.
Structural Forcing
Structural features are naturally added to rivers as wood accumulates or less erodible rocks fall into floodplains/channels. These features "force" a change in a river's hydraulics. Examples of structural features are circled in the image to the right within the flume.
Could a classic single thread, meandering channel be produced?
As stated before, the grain sizes in this lab were represented by yellow, white, black, and red grains; in order of grain size from large to small. Even though there were diffferences in grain sizes, there wasn't enough of a difference between them and their individual distributions to produce a classic meandering channel. Due to this, the floodplain in the flume had a lot of subsurface flow and the overall permeability was too high to create the variability necessary for a classic meandering channel.
Events
Small Flood
A small flood was created by increasing the discharge to a moderately high rate. This created a wide single channel upstream and a braided system further downstream at the base level.
Big Flood
A big flood event was induced by increasing the discharge to an extremely high rate. This completely reworked the main channel into a single wide channel down the middle of the flume. Video credit: Emma Tombaugh
Channel Realignment (Grading)
Channel grading is depicted in the video on the right; as the sediment is dredged to the edges of the floodplain, the water that has been held at the top of the flume flows down and realigns the channel to the middle of the floodplain.
What seems to the roll/impact of small floods vs. big floods?
The big flood was better at pushing through structural features and transporting sediment. In addition, the big flood was better at creating a straight channel through the flume while the small flood still had a lot of anastomosing channelization.
Were overbank flows, bankfull flows, and/or baseflow flows observed?
Overbank flows were definitely observed in our experiments (see the grading/realignment video for an example of overbank flows transporting massive amounts of sediment down the flume). Bankfull flows were also observed during both large and small flood events, though they were more frequent in the big flood event. Baseflows were observed after regular flow was kept constant for about a minute and the river had time to perform its work in the flume and establish its path.
What role did hyporheic flow play in observations?
Hyporheic flow is the underground flow of water under surface water along a stream's channel. As noted earlier on this page, there wasn't a lot of grain size variability in the flume so the overall floodplain in the flume was highly permeable. For this reason, a lot of stream power that could've been used to create complexity in the channel was lost to hyporheic groundwater.
Recession Limb Flow
A good example of a flood event recession limb was observed in our experiments at the end of our small flood. As the flood receded, deposition was allowed to occur more often, point bars were formed, and channel meandering is increased as the discharge decreases.
