Assignment 5:

Flume Observations

Flume Observations: Fluvial Geomorphic Processes

8 Feb. 2021

Introduction

This page contains observations made while examining the EM-River EM2 Stream Table (hereafter "flume") as part of an effort to better understand fluvial geomorphic processes (Fig.1). Observations concerning (1) flume controls, (2) fluvial geomorphic processes, (3) fluvial geomorphic mechanisms, and (4) specific events are included herein.

Fig. 1. The flume used for observations in the following sections pictured here with various meandering channels, woody debris, and inlet/outlet controls at either end.

1. Flume Controls

The following factors may be controlled through manual adjustment of the flume or sediment: water discharge Q, slope S, profile, base level, and sediment discharge Qs. See Figs. 2-6.

Water Discharge, Q

Fig. 2. Discharge Q (cfs) is controlled by the dial at the base of the flume. Flow ranged between ~5 cfs and 50 cfs during observations.

Slope, S

Fig. 3. Slope is controlled by both the speed of the water (discharge Q) and the spread of the sediment. A plastic jug is used to scrape, pile, and grade an appropriate slope.

Profile

Fig. 4. The profile of the flow is largely determined by the slope and total discharge Q. The depth and width of the river change along its profile, widening with slower flow (towards the bottom of the flume) and cutting deeper channels as flow increases speed (near the top, in this picture).

Base Level

Fig. 5. The base levels of the river are at the head and outlet of the flume. The head is controlled through discharge Q and large rocks placed to diffuse initial flow. The outlet control is a plug that may be set at a high or low elevation to cause a flood and delta structure or intense sediment transport, respectively, through the flume.

Sediment Discharge, Qs

Fig. 6. Sediment discharge is the rate at which sediment is transported. Sediment transport is when grains are picked up in the flow and taken downstream. Sediment transport is controlled by grain size and total discharge Q. Faster speeds of water transport more sediment.

2. Fluvial Geomorphic Processes

The following fluvial geomorphic processes were observed in the flume: bed erosion, bank erosion, deposition, and sediment transport. These are illustrated in figures 6-9.

Bed Erosion

Fig. 7. Bed erosion occurs when flow begins to separate grains by size, cutting deeper into the landscape. This picture shows a channel where red grains indicate an area where the channel cut deeply, or eroded the bed.

Bank Erosion

Fig. 8. Bank erosion can occur with the rapid drawdown of water or channel incision; when a high base level is suddenly dropped, banks become unsaturated and crumble into the channel.

Deposition

Fig. 9. Deposition occurs when sediment transported through the system is left on inner banks or at a delta where the river meets a larger body of water, shown in this picture.

Sediment Transport

The process of sediment being transported through a river system is explained in Figure 6, on sediment discharge.

3. Fluvial Geomorphic Mechanisms

The following fluvial geomorphic mechanisms were observed in the flume: grain size sorting, meandering, braiding, avulsion, chute dissection, structural forcing. These are illustrated in figures 10-15.

Grain Size Sorting

Fig. 10. Grain size sorting in the channel; the red grains are the finest, and white and yellow grains are the largest. The channel naturally sorts grains as flow picks up sediment along its course.

Meandering

Fig. 11. Channel meander occurs when a channel does not take a direct course downstream, but rather creates a sinuous pattern across the landscape, as shown in this figure.

Braiding

Fig. 12. Channel braiding occurs when the channel is split with bars of sediment throughout. Braiding is common especially at low flows and mild slopes.

Avulsion

Fig. 13. An avulsion is when the river abandons its original course and creates a new path or joins another channel. This figure shows a channel with a fork, indicating an avulsion either occurred recently or that the right-most channel will eventually join the channel on the left of the fork, causing an avulsion.

Chute Dissection

Fig. 14. Chute dissection refers to a channel splitting or dissecting as it creates an avulsion or braided pattern.

Structural Forcing

Fig. 15. Anthropogenic influences on the channel, including the culvert shown here, can result in changes to flow patterns by forcing flow in a certain direction. This may be referred to as structural forcing.

While meandering of the channel was observed, we were not able to produce a classic single-thread, meandering channel in this experiment. I don't think creating this type of channel would have been possible with the controls we had at this flume because from my understanding, a single-thread meandering channel requires cohesion in the surrounding soil, for example, caused by plants.

4. Events

The following events were observed in the flume: a small flood, a big flood, and channel realignment/grading. These events were observed through other fluvial geomorphological processes; see the video on deposition (Fig. 9) for an example of a "big" flood. Fig. 15 on structural forcing shows a small flood downstream of a culvert. Channel realignment is shown in Fig. 14, which shows chute dissection leading to natural channel realignment. Another channel realignment, due to anthropogenic influences, is shown in Fig. 15, which shows a culvert which re-aligned the flow.

From these observations, there were significant differences between a small flood and a big flood. The small flood we observed simply made a difference along the channel, creating some meander and braiding as the water level decreased. The big flood, shown in Fig. 9 transported an enormous amount of sediment, and left large deltas in its wake as the water rapidly drew-down.

In this experiment, we mainly observed baseflow flows in the meandering/braided channels. When experimenting with flume controls, other types of flow became evident. When we re-graded the flume, new channels were created at bankfull flow, which began to cut the channel until it became the baseflow. Inducing flow to stop created flooding and overbank flows upstream of the flood.

Hyporheic flow was really neat to observe in the flume; the best example is seen in Fig. 14 on chute dissection. In this video, hyporheic flow is the main cause of chute dissection. The flow through the substrate beneath the surface-level channels also broke through near the culvert in Fig. 15 or through small pileups we created to dam flow. Hyporheic flow was instrumental in connecting channels and providing a way for the channel to create avulsions as it meandered through the flume.

Recession limb flows happen when total discharge is decreased, mimicking the tail-end of a hydrograph. As we experimented at the flume, recession limb flows appeared to cause meander in the channel. As the flow velocity decreased, the channel began to develop more of a braided pattern.

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