Flume Observations: Fluvial Geomorphic Processes

Project Overview

Visit a flume (date: February 24, 2025)
Make observations of sediment transport processes on a small scale
Simulate large and small flood events to observe their impact on channel morphology

Instructions for this assignment can be found here.

Flume Controls

A flume is a tool used to simulate the longitudinal profile of a channel on a small scale. As discharge, sediment supply, and base level change in a flume, changes in the longitudinal profile of the simulated channel can be observed.

On this flume, discharge (Q) was controlled by a knob that could increase or decrease the amount of water flowing into the channel from a tube at the most upstream part of the flume.

Slope was controlled by the angle at which the flume table was set, and could be changed by increasing or decreasing the table angle.

Sediment discharge (Qs) was obsvered by using a net positioned underneath the drain of the flume which caught any sediment grains that were transported downstream by flow.

Base level was controlled by a moveable drain outlet, which simulated an increase in base level when moved up and a decrease in base level when moved down.

Diagram of the flume setup used for the following observations.

Fluvial Geomorphic Processes

Bed Erosion

Bed erosion occurs when flows have enough energy to entrain sediment along the channel bottom, leading to erosion (aka lowering) of the channel bed.

Our group observed bed erosion in this channel, which was indicated by the large amount of visible sediment transport and the steep height of the banks compared to other side channels.

Bank erosion

Bank erosion occurs when banks of a river channel are undercut by erosion, forming cut banks. The outer edge of the bank is cut inward because the velocity of the flow is highest at that location.

Our group observed that under high enough flows, bank erosion led to the formation of new side channels.

Deposition

Deposition occurs when flows do not have enough energy to entrain certain grains of sediment, resulting in larger grain sizes being deposited on the banks.

Our group observed deposition here where the deposition of larger sediment grains (yellow) was forming a new bar.

Sediment Transport

Sediment transport occurs when flows have enough energy to overcome forces holding sediment on the channel bed.

Sediment transport was observable when colored "grains" were clearly entrained and visibly transported downstream.

Fluvial Geomorphic Mechanisms

Grain Size Sorting

Photograph courtesy of Quinn Olpin.

Flows differ in their ability to perform work on a channel. When a flow does not have enough energy to move large sediment grains, those grains are deposited while grain sizes small enough to be transported by available energy remain in the channel.

This process can result in clear sediment sorting by grain size on the inside and outside of the channel. In the flume, yellow grains were the largest, whereas black and white grains were smaller. Under a lower flow, this resulted in nearly all yellow grains depositing on the banks of the channel while white and black grains continued to be transported downstream.

Meandering

A meandering river is a river with low slope and a sinuous morphology. The sinuous pattern of channel meanders is created by erosion of banks balanced with deposition past eroded banks that forms point bars.

Our group was unable to produce a single-threaded meandering stream, but we were able to produce meandering within multi-threaded channels. This photograph illustrates how meandering was observed in main channels with other threads branching off.

Original photograph taken by Joe Wheaton.

Question: 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?

Our group was unable to create a single-threaded meandering stream with the controls at our disposal. We discussed that meandering channel patterns are nearly impossible to create in a flume environment because it is difficult to artifically reproduce the cohesion of natural soils. Successful models of single-threaded meandering have only been achieved in experiments where alfalfa was present.

Braiding

Original photograph taken by Quinn Olpin.

Channel braiding is a term that describes when a channel is split into many small channels, or threads, that form an interconnected netweork due to high rates of sediment transport.

We observed channel braiding multiple times in the flume. Braiding is a common channel pattern to reproduce in a flume because sediment supply is easily increased and obstructions such as fake plants or other structural elements can be removed to reduce confinement or structural forcing.

Avulsion

Avulsion occurs when a channel abandons its original course and begins flowing in a new channel.

Our group observed avulsions multiple times in the flume, especially when discharge was increased leading to increased bank erosion.

Original video filmed by Christina Waddle.

Chute Dissection

Chute dissection occurs when discharge is high and erosion causes headcutting off the side of an established channel.

Structural Forcing

Structural forcing occurs when flow is forced into a certain path by a structural element such as a culvert.

Our group simulated structural forcing by placing a bridge/culvert into the main channel on the flume. We attempted flooding by increasing discharge and sediment supply, but it took a lot of effort to get the channel flowing powerfully enough to start flowing around the structural forcing element (in addition to flowing through it).

Events

Small Flood

Small flood events were induced by slightly increasing discharge to the channel (ie, low to medium flows or medium to high flows).

Impact: Small flood events usually induced channel changes on a small scale. This included smaller-scale bank erosion, increased rates of sediment transport, and increased hyporheic flows.

Big Flood

Big floods were induced by strongly increasing discharge to the channel (ie, low to high flows).

Impact: Big flood events usually induced channel changes on a large scale, such as channel realignment, due to high rates of erosion and sediment transport.

Channel Realignment

Channel realignment occurs when a large flow causes a channel's flow pattern to shift from multiple channels to a large, homogeneous channel.

Channel realignment occurred when our group simulated a long flood event with considerable duration. The flood (video above) realigned the channel so that smaller channels were abandoned and a large, straight channel was created along the side of the flume.

Overbank, Bankfull, and Base Flows

Question: In your experimentation, did you observe overbank flows, bankfull flows and/or baseflow flows?

Overbank flows were often obsvered during large flood events, when enough discharge was present to overflow existing banks. Overbank flows often led to the formation of new channel features via bank erosion.

Bankfull flows occurred when discharge was relatively high, but not so high that banks were overtopped. Sediment transport, deposition, and erosion occurred during bankfull flows but did not change the channel morphology nearly as dramatically as overbank flows.

Baseflows were observed when the discharge was lower and remained constant for long periods of time without being increased. Baseflows often led to a shift from majority erosion to majority deposition since baseflow discharges had less energy to move larger grains of sediment.

Hyporheic Flows

Question: What role did hyporheic flow play in what you observed?

Hyporheic played a much larger role in channel creation and pooling than I expected. In attempting to create a single-threaded meander, our group was surprised to see most water traveled through sediment to create backwater pools or channels off to the side that were not connected to the main channel. Hyporheic flows caused much greater floodplain diversity by creating these features that were not connected to the main channel, but were coming from the same water source.

Recession Limb Flows

Question: What role did recession limb flows seem to play in what you observed?

"Recession limb flows" refer to the period at the end of a flood when discharge is lowering from an increased level to more normal flows. As time passes from the end of the flood, the amount of energy the flow has to transport sediment is decreased. Our group observed that flows on the recession limb of a flood usually resulted in higher rates of sediment deposition and abandonment of higher-elevation, less-eroded channels. This was due to the decrease in energy that occurred after the flood ended.

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