Flume Lab

Module 6: This week in class we had an opportunity to explore fluvial geomorphic mechanisms and processes by way of a sediment-transport flume

Flume Controls

The flume is a highly controlled situation where the distinct forces in a fluvial system are able to be accounted for

Water discharge ( Qw ) is a direct factor of how water is being added to the system, i.e flow rate, which is directly controlled by the settings at the base of the table. Within the flume discharge won't occur till saturated conditions occur can be accelerated if flow settings are increased.
Slope (S) has two major factors of influence; the slope of the table which is predetermined by the legs of the table and the topography of the sediment within the flume.

Sediment discharge (QS) is primarily controlled the base control(plug at bottom of table) and the critical shear stress. Critical shear stress is influenced by the fluid friction required to move sediment. So, Qs could also be attributed to characteristics of sediment and flow rate.
Profile can be explained as the patterns of sediment( i.e. topography), which as highlighted in Qs is influenced by flow rate and base level controls
Base level is able to be controlled by raising and lowering the drain at the bottom of the table

2. Fluvial Geomorphic processes

Processes described and simulated in videos below

2.1 Bed Erosion- critical shear stress point is reached, sediment is being entrained with little deposition at the site allowing for the channel to incise

2.2 Bank Erosion- flow is directed to the outside of the channel and meets the outside of a bend in a river, this leads to the excess energy eroding material

2.3 Deposition- occurs where the velocity of flow is low enough to allow sediment to drop out of entrainment, heavier sediment is dropped first. The highest deposition rates occur on the inside of river bends and in flood plains

2.4 Sediment transport- When critical shear stress is met, thus eroding sediment, and flow is maintained at a rate that allows sediment to slide, roll, and saltate along the channel boundary or if the sediment is light enough to suspend in flow.

3. Fluvial Geomorphic Mechanisms

o- grain size sorting

x-poor grain size sorting

3.1 Grain size sorting is most easily observed along the longitudinal profile of a river in gravel rivers. Near the headwaters larger sediment is able to be entrained but will not travel very far as slope will continue to decrease as you move towards the floodplain, thus the closer to the headwater you are sediment of a larger calibre will be able to be observed. As opposed to floodplains where high deposition rates occur but it is often a factor of small sediment calibre that is more easily supported by a lower flow rate

3.2 Meandering is observed when the fastest velocity on the outside of a bend meets the outside of an opposing bank leading to erosion causing sinuosity in the planform profile . The critical shear stress point is met on the outside bend but not on the inside of the bend, allowing for deposition. In nature this process continues to erodes and leads to geomorphic units like oxbow lakes.

3.3 Braiding can be observed in rivers that have multiple channels where a diffluence is the separation of a channel and a confluence is where two channels join again. Observed in sand/silt dominated rivers where bars are formed by heavier sediment

3.4 Avulsion is when a new channel forms that does not rejoin the previous main channel. Either creating a new channel for the river or creating a new river that diverges from the previous

3.5 Chute Dissection is any lateral or longitudinal cuts, often leading to features like braided river or oxbow makes. This differs from avulsion in that it does not lead away from the main channel.

3.6 An example of Structural Forcing as artificial riparian vegetation. Structural forcing can be any kind of obstacle that obstructs and diverts flow around.

3.7 Recreating meandering

In the example above of meandering we were unable to achieve this under natural conditions but rather had to force the conditions by creating excessive mounds of sediment that forced flow around them. When attempting to allow the flow to forms its own sinuosity the channel seemed to not be able to create more than one bend before straightening out or braiding. I contribute the difficulty of this to the limit of resisting forces and lack of diversity in grain size/sediment calibre. I'm curious if we had created a dense riparian area is we could have seen more success, but ultimately without some outside influence and carefully placed sediment I do not think meandering can occur "naturally" on its own.

4. Events

4.1 Small Flood

Small Flood event at 51 mL/s based on previous channel structure where we had been running flow around 20 mL/s. At the increase of flow rate geomorphic work was occurring much more rapidly.

4.2 Big Flood

Flood of a large magnitude which we had increased flow rate to 98 mL/s based off previous flood conditions. Geomorphic work was occurring more quickly than previously with head cutting and erosion leading to an avulsion.

4.3 Channel Realignment

In fig 4.1 I drew on the image where the old channel was and where the new channel was able to straighten out. In this scenario we formed a channel but kept flow at a rate of 20 mL/s which allowed for a lot of deposition near the "headwaters". The channel sought an easier path and cut through a bank to realign. The green shows the old path where slope had dramatically changed from the sediment, and red shows the new channel path.

fig 4.1

Analysis

Small vs. Big flood

The greatest differences I observed between the small and big flood events where the rate at which work was being done. We did not adjust base level but with the influx of water in the system, the larger flood event seemed to be preforming work more rapidly through channel cutting upstream. While the small flood was under less saturated conditions and the flood plane was able to maintain its structure.

Overbank, bankfull, and baseflow

When it was intentional baseflow was observed but not much experimenting was done with the pump turned off. Bankfull flow was observed during the big flood and half way through the small event near the mid channel. Overbank flow did not occur till flood plane conditions had reached flooding conditions, primarily due to the massive amount of sediment we had placed at the head of the table.

Hyporehic Flow

Hyporehic flow can be described as through flow that is able to surface and supplement baseflow. I think this was present under highly saturated conditions and and was more obvious with headcutting on the downslope of a mound.

Recession Limb Flow

We did not adjust for a recession limb much but after the larger flood event there was a moment where flow was turned down, less sediment was transported in this time and got deposited. No significant observations were made during the recession limb that was roughly 1/4 of the flood event.

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