1.3 Hydrology and fluvial geomorphology: river channel processes and landforms Flashcards
Describe and explain abrasion/corrasion
Abrasion/corrasion takes place when coarse and angular rock fragments are dragged and bounced along the channel bed and sides, scraping and removing fragments of rock from the channel floor and sides.
Some of the bedload may become trapped in depressions in the bed of the channel and spun round by the current. This often results in the process of eversion which involves the formation of potholes which may enlarge and merge and contribute to the deepening/lowering of the channel bed.
Describe and explain solution/corrosion
Solution (corrosion) takes place when the acids in river water put certain minerals in solution. It is particularly effective in calcareous rocks such as limestone and chalk.
Describe and explain cavitation
Cavitation occurs during turbulent flow when the water is full of air bubbles – such as in the plunge pools below waterfalls. These air bubbles implode and create small cavities in the rock of the channel bed and sides.
Describe and explain hydraulic action
Hydraulic action occurs where the weight and force of the river flow penetrates cracks, joints, and bedding planes in the channel bed and sides, leading to the erosion of bed rock and the undercutting of the sides of the channel.
What are the factors affecting the rates of erosion
Factors affecting rates of erosion:
Load: the heavier and sharper the load the greater the potential for erosion
Velocity: the greater the velocity the greater the potential for erosion
Gradient: increased gradient increases the rate of erosion
Geology: soft, unconsolidated rocks such as sand and gravel are easily eroded
pH: rates of solution are increased when the water is more acidic
Human impact: deforestation, dams, and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion
Describe traction
Traction is the rolling/sliding/pushing of larger, heavier sediment, such as pebbles, along the river bed by the river flow.
Describe saltation
Saltation is the bouncing along the river bed of small particles light enough to be picked up/entrained for only a short distance.
Describe suspension
Suspension is the transport of particles light enough to be carried/suspended above the river bed by the flow of the river.
Describe solution
Solution is the transport of dissolved sediments in solution that are the products of solution weathering.
Define deposition
Deposition is the geological process in which sediments, soil, and rocks are added to a landform of landmass (check this definition)
What are the causes of deposition
There are a number of causes of deposition, such as:
- A shallowing of gradient, which decreases velocity and energy
- A decrease in the volume of water in the channel
- An increase in the friction between water and channel
Define sedimentation
Sedimentation is generally described as the opposite process of erosion that is the terminal end were the sediment transport (check this definition)
Describe and understand where and when erosion, transportation and deposition operate (knowledge of the hjulstrom curve)
The transport of sediment in a river channel depends on:
- The speed/velocity of river flow
- The load particle size
It is sometimes expressed as a measure of a river or stream’s competence.
The Hjulstrom curve represents this in the form of a diagram which displays the river channel flow velocities required for the sediment particles of different sizes to be picked up (entrained) and transported. This will take place whenever the river velocity is great enough to erode or entrain sediment. This could occur in periods of high discharge or where river channels long gradient is steep. It is usually combined with a deposition curve so that it is possible to see the speeds at which material will be deposited. It allows an estimate of the balance between the three river functions of erosion, transportation, and deposition.
The different velocities illustrate the speeds required to erode, transport, and deposit particles of a given size. Larger particles and fine sized particles are, surprisingly, both entrained at high velocities (surprisingly because it would seem logical that the very small clay particles would be the first particles to be picked up).
The reason for this is that the tiny clay particles actually “stick” together as a result of weak electrical bonding. At low velocities sediment settles and is deposited. For coarse particles this is just below the erosion velocity. Clay particles settle at very low velocities.
Define river velocity
River velocity is the mean speed of the river flow in metres per second measured at a point in a river by a current meter or over a set distance along the channel (often 10 metres) and time
What are the factors that control the velocity of a river channel
The gradient of the channel bed, ie the gradient of the long profile
The volume of water in the channel
The shape of the river channel
The channel roughness, is a measure of how rough or smooth the channel bed and sides are, the amount of friction acting on the water to slow it down and how fast the river channel allows water to flow through its. The Manning flow equation (sometimes called Manning’s “n”) is a method by which channel roughness can be measured using fieldwork data.
Define river discharge
River discharge is the volume of water passing a point in a given unit of time, which is calculated by the equation Q = A x V, where:
Q = Discharge, in cubic metres per second (Cumecs)
A = Channel cross sectional area, in square metres
V = Velocity, in metres per second
Describe laminar flow
Laminar flow is found in a smooth, shallow, straight channel with a slow velocity. The water flows in layers/sheets sliding over one another parallel to the river bed. It is usually found in the lower section of a river’s course
Describe turbulent flow
Turbulent flow is found in rough, shallow channels which have complex and rapid changes in shape, along with an increased velocity. This produces a series of spinning eddies which mix with one another. They often trap air bubbles inside them producing a “white water” effect. It is in this type of channel that cavitation takes place, as all the air bubbles are constantly imploding.
Describe helicoidal flow
Helicoidal flow occurs where the river flow takes on a corkscrew shape and is found in water that travels around the bend of a meander. The flow, while still moving downstream, also moves across the top of the water surface of a meander from the inside bank towards the outside bank where it then drops/descends towards the bed of the channel and returns towards the inside bank as a reverse flow
Explain what the thalweg is
The thalweg is the name given to the line of both maximum depth and maximum speed/velocity found along a river channel. It tends to swing across a river channel and is found on the outside of meaner bends, where it causes the outside the bank to be undercut and collapse, forming a river cliff.
Need to distinguish between a description of the flow and the causes of it***
Draw and describe a straight channel
These are rare in nature and will not last for long – rarely for longer than 10 times their channel width. For example, they may follow the route of a fault line. They are found when rivers have:
- Low energy
- Small discharges
- Gentle gradients
Draw and describe a meandering channel
Meandering channels are a much more common form; about 80% of channels are meandering.
Meanders are characterised by a winding shape in their plan view and have an asymmetrical cross profile. Meanders are produced when the line of fastest and deepest flow – the thalweg – starts to follow a winding/sinuous path through alternating sections of deep and shallow water called pools and riffles. Meanders form when there is: moderate gradient, moderate flow and the channel sides and bed are formed in alluvium.
When the thalweg runs past the outside of the river bank, the more powerful river current erodes and undercuts the outer bank, which will eventually collapse and the process will be continually repeated, increasing the size of the meander.
It also produces a secondary movement/flow called helical (or helicoidal) flow which is a spiral shape flow which piles up/raises/elevates the water on the outside of the meander and produces a return/counter current which moves across to the inside of the meander. In a meander cross profile/section the features include point bars/slip off slopes on the inside bend where deposition occurs as the current slackens.
On the outside bends undercutting and collapse of the river channel banks takes place by the faster current (following the line of deepest flow – the thalweg) which will produce river cliffs/bluffs). Lateral/sideways erosion of the river channel is then taking place. This lateral movement is called meander migration and it is responsible for widening the floodplain.
Draw and describe a braided channel
Braided channels consist of two or more channels in an interlocking pattern. They are divided by low bars and islands made up of sands and gravels which have been deposited by the river.
Braided river channels are found in rivers that have extremes of river discharge, which are sometimes called “unstable flow regimes”. In periods of high discharge these rivers pick up and transport very large amounts of sediment (ie they have large bed loads and incoherent, easily eroded banks). Sand and gravel bars and islands are formed in periods of low discharge. Two conditions often cause this reduction in river discharge:
1) A reduction in the gradient of the stream
2) The reduction of discharge after precipitation or the spring melting of snow and ice.
Vegetation may quickly colonise these island sin the braided channels which will help trap more sediment and increase the height of the eyot. This vegetation may become quite permanent and well established with trees and only then the eyot will only be covered briefly by water during very high discharge.
Braided channels occur where there are:
- Abundant amounts of bedload
- Easily eroded banks
- High and variable discharges
- Steep long gradients/channel slopes
Such conditions are most often found in tropical climates, deserts, and in arctic tundra climates.
