Module 6: Flume Observations

February 19, 2021

FLUME CONTROLS

Q (Water DIScharge)

Q (Water discharge) is the volume of water moving through a channel per unit time. A common unit for discharge in the US is cubic feet per second, or CFS. This was controlled in the flume by adjusting the pump that recirculated the water.

S (SLOPE)

Slope is a measure of steepness. For the flume, slope was controlled by the supports for the table. If we wanted to, we could have changed the slope by adding another item under one side of the table.

Qs (Sediment Discharge)

Sediment discharge is the amount of sediment (measured by weight or volume) that is transported per unit time (ex. pounds/year). In the flume, we controlled this indirectly by controlling the water discharge, table slope, base level, and grain size.

Profile

The profile is the longitudinal shape of the channel. This was controlled by creating sediment "dams", placing structures, etc.

When the channel cut down through the graded sediment, a knickpoint would have been present on the profile. On the other hand, base level rise created a "flat" section along the profile of uniform water surface elevation.

BASE LEVEL

Base level is the lower limit for an erosion process in a given fluvial system. For example, a dam would represent a base level control on a river. Base level was controlled in the flume by a flow outlet. The height of this flow outlet could be adjusted to simulate base level change. This video shows base level being lowered.

FLUVIAL & GEOMORPHIC PROCESSES

BED EROSION

Bed erosion is the process of bed material excavation by a channel. I observed this process by watching grains saltate, roll, and slide along the bed of the channel. Over time, the bed showed evidence of incision.

BANK EROSION

Bank erosion is the process of bank material excavation by a stream. This process is shown in the video as bank material collapses into the channel.

DEPOSITION

Deposition is the process of sediments being transported and deposited by a channel. Over time, these deposits can form geomorphic units, such as bars. I observed a deltaic deposit forming as the channel made contact with the reservoir (base level).

SEDIMENT TRANSPORT

Sediment transport is the movement of organic and inorganic particles by a channel. In this video, the sediment is shown being transported by the channel both as bed load and suspended load.

FLUVIAL GEOMORPHIC MECHANISMS

Grain Size Sorting

Grain size sorting describes the distribution of sediments and can be useful for understanding sediment transport and deposition conditions. As grain size increases, so does the amount of energy required to move it.

Because the flume grains are color-coded, grain sorting was very easy to identify. In this photo, the fine grains (red) have been deposited on the inside of a bend due to a change in energy as the channel bends.

Meandering

Meandering occurs when a channel has faster moving water at a bend, causing the bank to erode (cut bank). The channel has lower energy on the inside of the bend, causing deposition to occur and create a point bar. A channel is considered to be meandering where these forms occur.

In this photo, the channel meanders and creates a point bar. The fine (red) sediment has been deposited on the bar.

Braiding

Braiding occurs where there is a high sediment load, allowing for easy bar formation. The resulting network of channels, separated by many small and often temporary bars, takes on a braided shape. These adjust according to slope, sediment load, and discharge.

In the flume, I noticed a lot of braiding occur as we changed things like discharge and base level.

Avulsion

Avulsion occurs when one channel is rapidly abandoned in favor of another.

I didn't get an effective video for this, but I noticed it happen during a large flood event and also when we changed the base level. The river was adjusting to changing discharge by making a new path for the main flow.

In this video, woody debris forces the channel to widen and leads to chute dissection (yellow arrow).

Structural Forcing & Chute Dissection

Structural forcing occurs when structure (woody debris, boulders, etc) force changes in hydraulics which force changes to geomorphic processes.

Chute dissection occurs when a channel chooses a path that dissects a bar. Preferential flowpaths exist within the network, so when discharge decreases, water concentrates into these flowpaths.

The Classic Meandering Channel

Although we observed meandering, we were unable to produce a classic single-thread, meandering channel in the flume. It wasn't possible for us to achieve this because the grain sizes that we had to work with (sand) lacked sufficient cohesive qualities. Without the bank cohesion (provided in natural systems by clay and vegetation), the meander bends shift before they can achieve the classic meandering channel.

EVENTS

Small FLOOD

During small flood events, the changes seemed to happen slower but were more noticeable. At lower flow, there wasn't as much suspended sediment, but channels seemed to carve deeper.

BIG FLOOD

The large flood was impressive. There was a lot of "overbank" flow that moved a significant amount of sediment, but obscured the braided nature of the channels that were present. Once flow was lowered, the many braided channels became very apparent. Structural forcing didn't play as large of a role in the large flood event because the flows weren't concentrated.

Channel realignment

To simulate this in the flume, we created a straight channel with a fairly homogenous bed elevation and observed the changes that took place as processes like bank and bed erosion reshaped the channel.

Impact of small floods vs big floods

The large floods moved significantly more sediment and sometimes changed the location of the main channel, so they were able to do a lot of geomorphic work. They obscured the braided pattern of the network and usually featured one main channel with the rest of the flow spread out fairly evenly. The small floods didn't carry as much sediment, but did a lot more in terms of creating new headcuts and creating a braided network of channels.

Overbank flows, Bankfull Flows, Baseflow

We observed all three flow types. At minimum discharge, we observed the flume's "baseflow" conditions. When we turned the flow up, we saw bankfull flow very briefly before the flow exceeded the bank height (overbank flow). The sand wasn't cohesive, so bankfull and overbank flow were difficult to observe-- often, the sand would collapse.

The role of Hyporehic flow

Hyporheic flow is the flow of water through sediment and soil outside of the channel. Because the sediment in the flume was all sand, which has a lot of pore space, water was able to easily flow between the grains. This created very saturated conditions and allowed headcuts, avulsions, and chute dissections to take place frequently.

Role of Recession Limb Flows

Recession limb flows occur when discharge is decreased and the water level drops. These slower flows formed the braided network by cutting down through "big flood" deposits to create a network of channels. They didn't move as much sediment, but the channels they formed were more pronounced-- deeper and more narrow.

Page updated

Google Sites

Report abuse