1. Flume Controls
1.1 Water Discharge (Q)
Discharge in the channel has a significant role in its morphological evolution. Erosional strength, sediment transport capacity etc. are dependent on the discharge.
Flume Analogy:
In the flume experiment also, we could clearly observe the effect of various discharges on the evolution of the channel bed. The discharges at the mouth of the flume were tuned using a controller. The channel responded differently with different discharges. The erosional effect of the bigger discharge was higher than the lower discharges and notable changes in the channel geometry was seen in a quick succession of time.
The discharge was controlled by using an electronic controller device at one end of the flume which was connected to a set up that continuously pumped water to the head end of the flume.
1.1 Water Discharge (Q)
Discharge in the channel has a significant role in its morphological evolution. Erosional strength, sediment transport capacity etc. are dependent on the discharge.
Flume Analogy:
In the flume experiment also, we could clearly observe the effect of various discharges on the evolution of the channel bed. The discharges at the mouth of the flume were tuned using a controller. The channel responded differently with different discharges. The erosional effect of the bigger discharge was higher than the lower discharges and notable changes in the channel geometry was seen in a quick succession of time.
The discharge was controlled by using an electronic controller device at one end of the flume which was connected to a set up that continuously pumped water to the head end of the flume.
1.2 Slope (S)
The channel slope dictates the energy of the flow or the stream power and its capacity to transport the sediments down the river. Other conditions being the same, with increasing slope the sediment transport capacity of the channel increases.
Flume Analogy:
In the flume experiment, the slope was adjustable by changing the base inclination of the flume set up. The flume legs at one end could be adjusted to manipulate the slope.
1.3 Sediment Discharge (Qs)
Sediment discharge is the total amount of sediment transported by the water across a certain section. A sediment hungry river tends to depict erosion of the channel whereas the one with excess sediment will exhibit deposition.
Flume Analogy:
The discharge entering the flume was free from sediment, so the sediment discharge entering the flume was zero. However, the sediment discharge in the flume itself could be altered by adding or removing the sediment particles manually at any desired location.
Sediment discharge is the total amount of sediment transported by the water across a certain section. A sediment hungry river tends to depict erosion of the channel whereas the one with excess sediment will exhibit deposition.
Flume Analogy:
The discharge entering the flume was free from sediment, so the sediment discharge entering the flume was zero. However, the sediment discharge in the flume itself could be altered by adding or removing the sediment particles manually at any desired location.
1.4 Profile
The profile of the river influences the stream power and its strength to carry sediments. For a steep profile, the velocity is high and so is the transport capacity. So, even bigger sediments are transported where the profile is steep. In mild profile larger sediments are deposited and smaller sediments are carried by the flow.
Flume Analogy:
In the flume experiment the profile could be easily manipulated by the redistribution of the sand particles to the desired depth. Any kind of profile could be created as desired.
The profile of the river influences the stream power and its strength to carry sediments. For a steep profile, the velocity is high and so is the transport capacity. So, even bigger sediments are transported where the profile is steep. In mild profile larger sediments are deposited and smaller sediments are carried by the flow.
Flume Analogy:
In the flume experiment the profile could be easily manipulated by the redistribution of the sand particles to the desired depth. Any kind of profile could be created as desired.
1.5 Base Level
A base level is the lowest level to which the water can flow. If a freshly formed river is allowed to evolve freely for sufficiently long time, it would adjust itself to that base level.
Flume Analogy:
In the flume experiment, we observed that the river had different slopes depending on the base level. When the base level was higher (controlled by the outlet at the end of the flume), the river had a milder slope. As soon as the base level was lowered, the channel started to adjust itself to that level characterized by the erosion starting at the downstream part that progressively shifted towards the upstream.
In the flume experiment the base level could be controlled by the vertical movement of outlet tube located at the end. The tube could be moved up and down to respectively raise and lower the base level.
A base level is the lowest level to which the water can flow. If a freshly formed river is allowed to evolve freely for sufficiently long time, it would adjust itself to that base level.
Flume Analogy:
In the flume experiment, we observed that the river had different slopes depending on the base level. When the base level was higher (controlled by the outlet at the end of the flume), the river had a milder slope. As soon as the base level was lowered, the channel started to adjust itself to that level characterized by the erosion starting at the downstream part that progressively shifted towards the upstream.
In the flume experiment the base level could be controlled by the vertical movement of outlet tube located at the end. The tube could be moved up and down to respectively raise and lower the base level.
2. Fluvial Geomorphic Process
2.1 Bed Erosion
Bed erosion is a natural process occurring in the channel bed in which the bed particles are entrained into the flow and its carried away. Bed erosion and deposition are continuously happening. Bed erosion occurs when the net force acting on the sand particle has an upward component that entrains it into the flow and gets carried with it.
Flume Analogy:
In the flume experiment we observed bed erosion in almost all the flow scenarios. It was more prominent in case of the higher discharges than the lower discharges. If we refer to the video , we can clearly see the bed erosion occurring intensely near the flume outlet (when the base level was lowered). The channel bed got incised to adjust itself to the bed level.
2.1 Bed Erosion
Bed erosion is a natural process occurring in the channel bed in which the bed particles are entrained into the flow and its carried away. Bed erosion and deposition are continuously happening. Bed erosion occurs when the net force acting on the sand particle has an upward component that entrains it into the flow and gets carried with it.
Flume Analogy:
In the flume experiment we observed bed erosion in almost all the flow scenarios. It was more prominent in case of the higher discharges than the lower discharges. If we refer to the video , we can clearly see the bed erosion occurring intensely near the flume outlet (when the base level was lowered). The channel bed got incised to adjust itself to the bed level.
Bed Erosion-Click here for illustration video
2.2 Bank Erosion
Unlike, bed erosion in which particles from the channel bed are entrained into the flow ; the bank erosion shows a distinct pattern of the entrainment of the sediment particles from the bank of the river into the flow channel. Apart from particle entrainment, bank erosion predominantly occurs due to the toe cutting action of the flow and subsequent mass failure. Bank erosion changes the channel morphology in many ways. It causes the widening of rivers, change in river course etc. This was clearly observed in the flume experiment.
Flume Analogy:
In the flume experiment bank erosion was abundantly seen along the channel. In the following video, bank erosion has been clearly depicted and a dummy house has been installed to realize it better. At a certain moment the bank erosion reaches the house and topples it into the channel.
Unlike, bed erosion in which particles from the channel bed are entrained into the flow ; the bank erosion shows a distinct pattern of the entrainment of the sediment particles from the bank of the river into the flow channel. Apart from particle entrainment, bank erosion predominantly occurs due to the toe cutting action of the flow and subsequent mass failure. Bank erosion changes the channel morphology in many ways. It causes the widening of rivers, change in river course etc. This was clearly observed in the flume experiment.
Flume Analogy:
In the flume experiment bank erosion was abundantly seen along the channel. In the following video, bank erosion has been clearly depicted and a dummy house has been installed to realize it better. At a certain moment the bank erosion reaches the house and topples it into the channel.
Bank Erosion-Click here for illustration video
2.3 Deposition
Deposition of the sediment occurs when the channel doesn't have enough power to transport the sediments. It can be seen when the slope becomes milder or there is increase in base level. When the flow reaches a more wider section then also deposition can be seen. Morphological units such as bars are also a result of deposition. Points bars for instance are seen at the inner bends where the velocity is less.
Flume Analogy:
In the flume experiment deposition phenomenon was seen at numerous occasions. One example of it can be seen in this video where the deposition occurs near the mouth of the channel where the slope gets milder and the flow slows down making it unable to further carry all the sediments.
Deposition of the sediment occurs when the channel doesn't have enough power to transport the sediments. It can be seen when the slope becomes milder or there is increase in base level. When the flow reaches a more wider section then also deposition can be seen. Morphological units such as bars are also a result of deposition. Points bars for instance are seen at the inner bends where the velocity is less.
Flume Analogy:
In the flume experiment deposition phenomenon was seen at numerous occasions. One example of it can be seen in this video where the deposition occurs near the mouth of the channel where the slope gets milder and the flow slows down making it unable to further carry all the sediments.
Deposition-Click here for illustration video
2.4 Sediment Transport
Sediment transport is the process by which different particles are carried by the flowing water along its course of flow. The mechanism by which the particles are transported depends on the size of the particles transported. Sediment transport occurs in the form of bed load, suspended load and wash load.
Flume Analogy:
Sediment transport was clearly visible in almost all the flow conditions in the flume experiment. Although, it was difficult to differentiate between the suspended load and bed load while viewing from the top, it wont be unfair to assert that the sediment was transported mostly in the form of bed load.
Sediment transport is the process by which different particles are carried by the flowing water along its course of flow. The mechanism by which the particles are transported depends on the size of the particles transported. Sediment transport occurs in the form of bed load, suspended load and wash load.
Flume Analogy:
Sediment transport was clearly visible in almost all the flow conditions in the flume experiment. Although, it was difficult to differentiate between the suspended load and bed load while viewing from the top, it wont be unfair to assert that the sediment was transported mostly in the form of bed load.
3. Fluvial Geomorphic Mechanisms
3.1 Grain Size Sorting
Grain size sorting is the distribution of different sizes of sediment particles in a sediment deposit. A well sorted sediment deposit consists maximum particles of same size. Usually, grain size sorting improves with increasing flow path which can be evidently seen in the river mouths.
Flume Analogy:
In the flume experiment four different grain sizes were used (Largest to Smallest : Yellow > White > Black > Red). However, a reasonable grain size sorting was not observed. In theory, largest particle settles first and the smallest particle settles at last. In the flume experiment, the finest particle (red) got interlocked between the coarser ones and couldn't get entrained into the flow to the extent it was supposed to. Hence, abundant coarser particles could be seen even at the mouth of the flume and finer particles at the beginning of the flume hidden beneath the coarse grains. The grain size sorting was poor in the flume experiment in almost all the locations (when a sample over a depth was considered). While looking from the surface, some well sorted zones were also seen.
3.1 Grain Size Sorting
Grain size sorting is the distribution of different sizes of sediment particles in a sediment deposit. A well sorted sediment deposit consists maximum particles of same size. Usually, grain size sorting improves with increasing flow path which can be evidently seen in the river mouths.
Flume Analogy:
In the flume experiment four different grain sizes were used (Largest to Smallest : Yellow > White > Black > Red). However, a reasonable grain size sorting was not observed. In theory, largest particle settles first and the smallest particle settles at last. In the flume experiment, the finest particle (red) got interlocked between the coarser ones and couldn't get entrained into the flow to the extent it was supposed to. Hence, abundant coarser particles could be seen even at the mouth of the flume and finer particles at the beginning of the flume hidden beneath the coarse grains. The grain size sorting was poor in the flume experiment in almost all the locations (when a sample over a depth was considered). While looking from the surface, some well sorted zones were also seen.
3.2 Meandering
Meandering occurs when the channel develops a curvy, sinuous path with a series of loops and bends along its way. Meandering occurs at mild slopes resulting from a regular pattern of erosion at the outer bends and deposition at the inner bends.
Flume Analogy:
In the flume experiment a number of curvy channels with bends were observed but not a single perfect meander was seen. The channel couldn't retain the meander pattern for a long time and didn't keep on evolving.
Meandering occurs when the channel develops a curvy, sinuous path with a series of loops and bends along its way. Meandering occurs at mild slopes resulting from a regular pattern of erosion at the outer bends and deposition at the inner bends.
Flume Analogy:
In the flume experiment a number of curvy channels with bends were observed but not a single perfect meander was seen. The channel couldn't retain the meander pattern for a long time and didn't keep on evolving.
Meandering-Click here for illustration video
3.3 Braiding
When a river channel turns into multiple smaller channel then its called braiding. When the river flow is slow due to channel widening or due to a milder reach, there is a sediment build up forcing the river to change direction along with generation of multiple smaller channels. However, during high discharges the braided channel can convert to a single channel.
Flume Analogy:
In the flume experiment braiding was observed scantly. It was seen near the end the flume during smaller flows rather than larger flows. The flow would occasionally break through the deposited sediment to form a few braided channels.
When a river channel turns into multiple smaller channel then its called braiding. When the river flow is slow due to channel widening or due to a milder reach, there is a sediment build up forcing the river to change direction along with generation of multiple smaller channels. However, during high discharges the braided channel can convert to a single channel.
Flume Analogy:
In the flume experiment braiding was observed scantly. It was seen near the end the flume during smaller flows rather than larger flows. The flow would occasionally break through the deposited sediment to form a few braided channels.
Braiding-Click here for illustration video
3.4 Avulsion
Avulsion is a process in which water diverts out of the original course into a new permanent path abandoning the former course. It occurs during large flood events when the flow is highly erodible and a new main channel is formed in the adjacent floodplain.
Flume Analogy:
Avulsion was seen in the flume during high flows when the flow would find a new path after cutting through the peripheral deposit.
Avulsion is a process in which water diverts out of the original course into a new permanent path abandoning the former course. It occurs during large flood events when the flow is highly erodible and a new main channel is formed in the adjacent floodplain.
Flume Analogy:
Avulsion was seen in the flume during high flows when the flow would find a new path after cutting through the peripheral deposit.
3.5 Chute Dissection
When the flow spills over a steep gradient along a sand deposit, erosion starts to occur with a rapid headwards extension leading to the formation of a chute channel. This process is called chute dissection. Chute dissection can be seen taking place over the bars. It is also one of the mechanisms for the formation of braided channels.
Flume Analogy:
Chute dissection was seen on numerous occasions, mostly at the point bars. One example has been depicted in the video in which the chute dissection leads to the formation of a new channel through a point bar. It starts first with an initial spill of water over the sand deposit of the bar leading to the downstream erosion which sustains itself and transfers upstream until it joins the main channel, and hence a new channel is formed.
When the flow spills over a steep gradient along a sand deposit, erosion starts to occur with a rapid headwards extension leading to the formation of a chute channel. This process is called chute dissection. Chute dissection can be seen taking place over the bars. It is also one of the mechanisms for the formation of braided channels.
Flume Analogy:
Chute dissection was seen on numerous occasions, mostly at the point bars. One example has been depicted in the video in which the chute dissection leads to the formation of a new channel through a point bar. It starts first with an initial spill of water over the sand deposit of the bar leading to the downstream erosion which sustains itself and transfers upstream until it joins the main channel, and hence a new channel is formed.
3.6 Structural Forcing
Structural forcing refers to various anthropogenic interventions to the natural flow dynamics. Such interventions alter the natural flow processes and have an impact on the morphological evolution. Construction of hydraulic structures such as bridges and culverts, embankments and dykes, sand extraction, diversion of water etc. are few examples of structural forcing.
Flume Analogy:
In the flume experiment also, some structural forcing such as a culvert and vegetation were used. It was seen that the lateral movement of the channel at the location of the culvert was restricted. Similarly, the vegetation inhibited the erosion of the banks.
Structural forcing refers to various anthropogenic interventions to the natural flow dynamics. Such interventions alter the natural flow processes and have an impact on the morphological evolution. Construction of hydraulic structures such as bridges and culverts, embankments and dykes, sand extraction, diversion of water etc. are few examples of structural forcing.
Flume Analogy:
In the flume experiment also, some structural forcing such as a culvert and vegetation were used. It was seen that the lateral movement of the channel at the location of the culvert was restricted. Similarly, the vegetation inhibited the erosion of the banks.
3.7 Meandering Question
During the flume experiment, we tried to create a meandering channel. Different discharges were tried. With larger discharger there was no sign of any meander. However, with lower discharges a more curvy channel was obtained with more bends. But there was no uniformity in the pattern. It is safe to conclude that during the flume experiment a classic single thread meandering channel was not obtained.
The reason behind the inability to create a single thread perfect meander can be attributed to the lack of cohesion. In real life natural processes such property is contributed by the presence of vegetation. However, no vegetation effect was used in the flume experiment because of which a perfect meander was not achieved. The roots of the vegetation increases the relative strength of the banks in comparison to the non cohesive sand bed, which in fact is a necessary condition for river meandering (Braudrick CA, Dietrich WE, Leverich GT, Sklar LS. 2009).
During the flume experiment, we tried to create a meandering channel. Different discharges were tried. With larger discharger there was no sign of any meander. However, with lower discharges a more curvy channel was obtained with more bends. But there was no uniformity in the pattern. It is safe to conclude that during the flume experiment a classic single thread meandering channel was not obtained.
The reason behind the inability to create a single thread perfect meander can be attributed to the lack of cohesion. In real life natural processes such property is contributed by the presence of vegetation. However, no vegetation effect was used in the flume experiment because of which a perfect meander was not achieved. The roots of the vegetation increases the relative strength of the banks in comparison to the non cohesive sand bed, which in fact is a necessary condition for river meandering (Braudrick CA, Dietrich WE, Leverich GT, Sklar LS. 2009).
Click here for illustration video
We can see in the video that the meander is not a perfect meander. Although the overall configuration is meandering, a single thread meander was difficult to generate.
We can see in the video that the meander is not a perfect meander. Although the overall configuration is meandering, a single thread meander was difficult to generate.
4. Events
4.1 Small Flood
In the flume experiment, small flood was created using the discharge controller which was allowed to pass from the head of the flume. The response was slow for the small flood(compared to large flood). Both the bed erosion and bank erosion were observed. Mid channel bars and point bars were observed. There was continuous lateral movement of the channel. Later it developed a more sustained main channel.
4.1 Small Flood
In the flume experiment, small flood was created using the discharge controller which was allowed to pass from the head of the flume. The response was slow for the small flood(compared to large flood). Both the bed erosion and bank erosion were observed. Mid channel bars and point bars were observed. There was continuous lateral movement of the channel. Later it developed a more sustained main channel.
Small Flood-Click here for illustration video
4.2 Big Flood
In the flume experiment, large flood was created using the discharge controller which was allowed to pass from the head of the flume. Unlike the small flood, response was quick for the large flood, there was significant amount of erosion in the channel. Both the bed erosion and bank erosion were prominent and the sediment transport rate was very excessive which could be realized from the sediment collected on the filter below the flume outlet. Avulsion was also seen during the large flood event. The flow maintained a main channel after a certain time. Bars were formed here and there.
In the flume experiment, large flood was created using the discharge controller which was allowed to pass from the head of the flume. Unlike the small flood, response was quick for the large flood, there was significant amount of erosion in the channel. Both the bed erosion and bank erosion were prominent and the sediment transport rate was very excessive which could be realized from the sediment collected on the filter below the flume outlet. Avulsion was also seen during the large flood event. The flow maintained a main channel after a certain time. Bars were formed here and there.
4.3 Channel Realignment (Grading)
Channel realignment was done by creating a straight channel. It caused the water to speed up and caused intense erosion of the bed along with high bank erosion. The channel quickly took a new shape and form.
Channel realignment was done by creating a straight channel. It caused the water to speed up and caused intense erosion of the bed along with high bank erosion. The channel quickly took a new shape and form.
4.4 From what you did, what seems to be the role/impact of small flood vs big flood ?
The small and big floods have distinctively different impacts in the morphology of the channel as well as the processes leading to the morphological changes. Small floods have less erosional capacity than the bigger floods and hence takes more time to induce changes. It was also seen that the larger floods can lead to the formation of entirely new permanent channels. However, it cannot be entirely denied that small floods also develop new channel which was also seen in the flume experiment with small flood.
The small and big floods have distinctively different impacts in the morphology of the channel as well as the processes leading to the morphological changes. Small floods have less erosional capacity than the bigger floods and hence takes more time to induce changes. It was also seen that the larger floods can lead to the formation of entirely new permanent channels. However, it cannot be entirely denied that small floods also develop new channel which was also seen in the flume experiment with small flood.
4.5 In your experimentation, did you observe overbank flows, bankfull flows and/or base flows?
The experimentation was done in a flume with an impermeable base over which a channel bed was created using a mix of different sized cohesion-less granulated particles (simulating sand). It was seen that the channel bed was highly mobile and didn't show much resistant to the flow. The particles were highly erodible. Whenever, the discharge was increased, erosion would take off soon both in the bed and the banks. Due to this, the channel adjusted quickly to accommodate the flow and there were very less chances of overbank conditions. The bankfull flows at higher discharges were also fleeting due to similar reasons.
A distinct and separate base flow was not fed into the flume. Controllers were directly used to alter the flow. Hence, no base flow was observed in the flume experimentation.
The experimentation was done in a flume with an impermeable base over which a channel bed was created using a mix of different sized cohesion-less granulated particles (simulating sand). It was seen that the channel bed was highly mobile and didn't show much resistant to the flow. The particles were highly erodible. Whenever, the discharge was increased, erosion would take off soon both in the bed and the banks. Due to this, the channel adjusted quickly to accommodate the flow and there were very less chances of overbank conditions. The bankfull flows at higher discharges were also fleeting due to similar reasons.
A distinct and separate base flow was not fed into the flume. Controllers were directly used to alter the flow. Hence, no base flow was observed in the flume experimentation.
4.6 What role did hyporheic flow play in what you observed?
The hyporheic flow played a significant role in the current flume observations. Due to the unconsolidated granulated particles used in the experiment a major portion of the flow was occurring through the sand particles itself. In many instances the hyporheic flow was resulting in the piping effect causing an erosion event that propagated upstream. Sometimes, it accelerated the bank erosion at points where the flow rejoined the main channel.
The hyporheic flow played a significant role in the current flume observations. Due to the unconsolidated granulated particles used in the experiment a major portion of the flow was occurring through the sand particles itself. In many instances the hyporheic flow was resulting in the piping effect causing an erosion event that propagated upstream. Sometimes, it accelerated the bank erosion at points where the flow rejoined the main channel.
4.7 What role did recession limb flows seem to play in what you observed?
During the recession limb the sediment transport gradually decreased and the channels were more stable. This is mainly due to the gradual decrease in the flow rate. Deposition was more prominent than erosion. Bank erosion decreased considerably. At the mouth of the channel, finer particles were deposited.
The recession limb exhibited a distinct shift of the course of the flow towards a single channel.
During the recession limb the sediment transport gradually decreased and the channels were more stable. This is mainly due to the gradual decrease in the flow rate. Deposition was more prominent than erosion. Bank erosion decreased considerably. At the mouth of the channel, finer particles were deposited.
The recession limb exhibited a distinct shift of the course of the flow towards a single channel.