FLUVIAL PROCESSES
Flume Controls
Water Discharge
The discharge of river (Velocity *Area) controls the transport of sediment. As the sediment transport (bedload material) is directly proportional to the stream power and velocity.
The discharge in the flume can be easily controlled by using an electronic valve with a controller. During the experiment, increase in discharge increased the sediment transport and altered the morphological aspects of the stream by changing the alignment with bank and bed erosion.
Slope
Higher the slope higher is the velocity of the river (velocity is directly proportional to the square root of slope). This increase in velocity aids in entrainment and transport of the sediment from the bed and banks of river.
The slope of the flume could be controlled by adjusting the flume base slope and the initial slope can be controlled by adjusting the sediment depth.
Sediment Discharge
Sediment discharge is dependent upon the carrying capacity of the river (discharge, velocity) and also on the size and availability of the sediment in the river.
The availability of sediment in the flume can be controlled either by adding or removing the sediment in proximal area.
Profile
Steeper profile shows higher transport of sediment whereas a milder profile shows deposition pattern. As the profile gets milder, coarser sediments in the river are deposited whereas fines are carried still in suspension.
Base Level
The base level helps to control the slope of the river hence influences the stream power and the sediment transport in the river. The base level in the flume was controlled by the height of outlet valve.
Fluvial Geomorphic Processes
Bed Erosion
Sediment from the river bed gets entrained when the lifting component of velocity is higher. Bed erosion occurs when the sediment from river bed gets entrained and transported. Progressive erosion of bed causes channel deepening. Significant bed erosion was observed in the flume which can be seen in the video here.
Bank Erosion
The sediment at the banks are sometimes entrained similar as bed materials, but mostly toe cutting incites bank erosion. Similarly bank erosion occurs when the flood recedes rapidly causing the wet sediment to get heavier and hence topple in the river. Progressive erosion of bank will cause, channel widening and erosion in one bank causes shifting of main channel. Bank cutting can be observed in the flume experiment, and shifting of channel with progressive bank erosion was also observed in the experiment at the meandered reach of channel (see video).
Deposition
The transported sediment gets deposited when the stream power decreases, with the slope getting milder, channel getting wider or discharge being reduced (case of diversion). The bigger grains deposit first being the most heavy. Deposition was observed in the bars and near the outlet in the experiment (see video).
Sediment Transport
Sediment transport occurs in the water column, near the bed, by rolling or saltation and above in the water column in suspension. Hence the sediment load are categorized widely into bed load and suspended load. Bed load transport was observed in the experiment (see video https://www.youtube.com/watch?v=jpexS4-9IF0&ab_channel=hightideexp for reference)
Fluvial Geomorphic Mechanisms
Grain Size Sorting
When deposition occurs, the heavier and bigger grains get deposited first whereas the finer particles get carried away further where they deposit. So grain size sorting can be visible in the floodplains or bars or fans, when deposition occurs (see video). Different grains used in the flume experiment is shown in the figure in the right.
Braiding
When the channel width is sufficient and flexible and the channel is dynamic with erosion and deposition phenomena occurring in the channel, deposition occurs at certain areas giving rise to islands in the center or sides and making a number of active channel in the river cross-section known as braiding.
Braided channels were observed in the experiment (see video) mostly in the lower reach, where the channel was actively involved in sediment erosion and deposition.
Meandering
When the river/channel starts to follow a curved pattern or is sinuous with loops and bends, the channel is said to be meandering. Meandering causes the channel to erode outer bend while depositing the sediment in the inner bend. They are usually formed in a mild slope rivers.
Meandering process was observed in the flume, but single thread meandered channel was not observed. This might be due to the use of only non-cohesive soil in the flume. Similarly vegetation was not used in the channel banks, roots of vegetation act as a grip to the soil giving rise to a converged single thread channel.
Avulsion
Rapid cutting and erosion of the sediment was observed in the banks as well as a progressive upstream erosion was observed due to subsurface flow of water, due to the formation of channel which moved upstream.
Chute Dissection
A rapid erosion of sediment occurs when the water flows over a steep section with head cuts moving in the upstream section. In the flume numerous headcuts was visible in the bars, and braids.
Structural Forcing
Use of intervention to train the river can be termed as structural forcing. Digging materials off the bed, construction of dykes and embankments to adjust the flow path etc. influences the river flow and fluvial processes.
Use of culvert block in the flume forced the channel to pass through it, defining the alignment as well as slope of upstream section by creating a fixed bed in the section of culvert. (see video for reference https://www.youtube.com/watch?v=HOLFmDl4HM4&ab_channel=LittleRiverResearch%26Design )
Events
Flood events and its impact
Small flood events help to carry sediment and define the channel more prominently without altering the channel course.
Big flood event are powerful enough to change the channel course and establish a new channel course with high erosion and deposition.
Channel Realignment (Grading)
Channel realignment was visible in the flume experiment when sediments were deposited in the smaller channel (at the location of bifurcation). Similarly, realignment in the meandered section due to bank erosion and shifting was also observed.
FLOWS (overbank, bankfull, baseflow)
Figure in the left shows different flow conditions and the velocity distribution at different locations in the section. Transport capacity is highest in the deepest section where the velocity is maximum. So bed erosion takes place here, whereas deposition of heavier particles takes place in the banks where the velocity is lower, decreasing the transport capacity. On the other hand sediment availability is higher when the flow is higher, as water comes into contact with larger reach.
All three observations were done in the flume, by controlling the discharge with the help of the controller.
Hyporheic Flow
Due to the use of non-cohesive sediment the subsurface was highly porous giving rise to sub-surface flow. A notable amount of discharge passed through the sub-surface which was visible when head cutting started in the channel. (see video for reference)
Role of Recession Limb
The falling limb is said to carry less sediment ( less Qs for same Q), due to lower availability of sediment in the proximal areas. Sometimes, recession limbs carries fine sediment which were exposed due to sediment transport due to the peak discharge. Moreover, recession limb helps in sediment sorting along and across the channel.
The rapid decrease of discharge in recession limb is often a reason behing bankfall (bank erosion).
Water In Action 