River Channel processes and landforms Flashcards
Long profile
a graph of a river that marks the change in altitude from the
upper course to its lower course. river processes are related to the long profile because every river is
trying to achieve a smooth, concave long profile
* when this is not the case, the river will be working to smooth its long
profile out by erosion, transport or deposition to establish balance.
Dynamic equilibrium
the river channel shape is constantly adjusting to changes in inputs of
water and material, so that it can transfer the water and material
downstream
Abrasion
the wearing away of the bed and bank by the load carried by the river. Its effectiveness is dependent on the concentration,
hardness and energyof the impacting particles, as well as the resistance of
the bedrock. Abraision increases as velocity increases.
Attrition
when load particles collide against each other and wear one another off. Creating smaller and rounder particles.
Hydraulic Action
Force of air and water on the sides of the river and in cracks. Includes the direct force of flowing water and cavitation. As fluids accelerate pressure drops causing air bubbles to form.
Cavitation
air bubbles trapped in the water get compressed into small
cracks in the river’s banks; these bubbles eventually implode, creating a
small shockwave that weakens the rocks
—> the shockwaves are very weak, but over time, the rock will be
weakened to the point at which it falls apart
Solution
The removal of chemical ions esp calcium. Key factors controlling solution are velocity, solute concentration, discharge and bedrock. Maxiumum rates occur when fast flowing streams pass over soluble rocks (limestone).The water’s acidity will only affect certain types of rocks and
dissolve them
—> highly effective if the rock in the channel contains calcium
carbonate.
Types of Fluvial Erosion
Vertical erosion
Lateral Erosion
Headward Erosion
Lateral Erosion
River erodes the river banks and widens the river channel
Vertical erosion
River erodes downwards, deepening the river channel
Headward Erosion
River erodes in an upstream direction thus lenghtening the river valley.
Soil strucure affecting rates of erosion
differences in the coastal crust make some rocks more solid than others,
hence rendering them more difficult to erode. Soft unconsolidated rocks such as sand and gravel are easily eroded.
pH affecting rates of erosion
Increasingly acidic pH increases rates of solution
Human Activities affecting rates of erosion
removing trees, fertilising soils and overgrazing by livestock disrupts the
roots that stabilise sediment and soil, making easier to erode
Load affecting rates of erosion
the heavier and sharper the load the greater the erosion
Gradient affecting rates of erosion
as the slope becomes steeper, water gains velocity, heightening the
frictional force and enabling it to erode the banks and bed more rapidly.
Suspended load
smallest particles (silts and clays) carried
Saltated load
Larger particles (sands, gravels, very small stones) are transported in a series
of ‘hops’
Traction load
Pebbles are shunted or rolled along the bed as the bed
Solution
areas on calcareous rock material is carried as dissolved load.
Capacity of a stream
largest amount of debris that a stream can carry
Competence of a stream
diameter of the largest particle that can be carried.
Causes of deposition
a reduction in gradient, which decreases velocity and energy
hence resulting in the heavier load being
dropped
a decrease of volume of water in the channel the force of the river is reduced, hence
it becomes unable to carry the heavier load, which is eventually
deposited
an increase in the friction between water and channel the heavier sediment rubs against the river bed and
banks, causing it to eventually become stuck to and deposited on the
river bed.
Deposition
Mostly takes place in lower course. Sometimes takes place in middle course on inside bends of river meanders whilst outside bends experience erosion
Alluvium
the finest (lightest) material may continue to be carried as the river approaches the sea.
Sedimentation
the sediment deposited settles on the river bed, building up
in layers and reducing the river’s depth
Sediment yield
the amount of sediment reaching or passing a point of
interest in a given period of time.
Denudation
the geological process in which moving water erodes the river
bed, leading to a reduction in elevation and relief or landforms and
landscapes
Critical erosion velocity
the lowest velocity at which grains of a given size
can be moved
critical erosion curve
the minimum velocity required to lift a particle of a
certain size
critical deposition curve
the maximum velocity at which a river can be
flowing before a particle of a certain size is dropped
3 important features on Hjulstorm curve
smallest and largest particles require large velocities
Higher velocities are required for entrainment than for transport
When velocity falls below a certain level those particles with a particular
settling or fall velocity are deposited.
Factors controlling Velocity and discharge
gradient of the channel bed: steeper gradient = higher velocity
volume if water in the channel: controlled by precipitation levels
—> low levels of water = slower velocity
—> higher levels (banfull) = more rapid flow
channel roughness: rough channels can create friction, which slows down
flow
channel shape: bends/curves in the river can slow the slow, reducing the
velocity.
wetted perimeter
the entire length of the river bed, banks and sides in
contact with water. the greater the wetted perimeter, the greater the friction between
the water, bed and banks, and the slower flow of the river
River velocity upstream
large angular boulders
create a rough channel shape and therefore a large
amount of bed friction and reducing the velocity
Mannings Equation
a measure of how efficient the channel is at
transporting water and sediment
—> it is the ratio between the length of wetted perimeter and cross–
section of a river channel
laminar flow
when water travels smoothly in a regular path
* a smooth, straight channel with low velocity is required
* this occurs when water travels over the sediment in the riverbed without
disturbing i
Turbulent Flow
characterised by disorder in the movement of water and
irregular flowing patterns
* occurs when there are higher velocities and complex channel
morphology such as meanders, or when the riverbed is uneven. turbulence is associated with cavitation and hydraulic action
* turbulent water swirls called eddies will form an obstacle like a boulder
in a river
* the water within the eddy may reverse the direction of flow and make
it flow upstream
* vertical (downward) turbulence creates hollows in the channel bed
* pebbles and gravel can get trapped in these hollows, and get swirled
through eddying and eroding the riverbed through abrasion creates potholes in time
Eddies
turbulent water swirls that form an obstacle like a boulder
in a river
* the water within the eddy may reverse the direction of flow and make
it flow upstream
helicoidal flow
corkscrew movememt of water transporting eroded
material downstream from one meander bed to the next
Pools or riffles
will develop around water
twists and turns around obstructions
—> results in areas of slower and faster water movement.
Riffle-pool sequence
as a stream’s flow
structure alternates between areas of shallow to deeper water
Thalweg
line of maximum velocity and it travels from one outside meander bank to the next.
Formation of meanders
:1. Due to banks of sediment at the bottom of the river (which are deposited at times of low flow, that is, a low velocity and low discharge, meaning deposition increases), the river weaves around these alternating shallow and deeper sections (riffles and pools) on what was an initially straight channel A riffle is a a shallow section of a channel while a pool is a deep section. These riffles and pools develop at equal points along the river channel with each pool being about 5× the length of the channel.
In a pool, the channel is more efficient, while at a riffle, the channel is less efficient creating irregular flow. .2. This movement targets one bank of the section after the riffle, subsequently leading to erosion (carried out by hydraulic action and abrasion) – this becomes the outside bend of the meander.3. This leads to the formation of a river cliff on the outside bend. The material eroded further upstream is deposited on the opposite bank (inside bend) on the slip off slope. This is due to a corkscrew movement in the river (helicoidal flow) which results in water levels on the outside of a meander bend to be elevated, giving a faster velocity (the Thalweg line is located here). The deposition on the inside bend gives an asymmetrical cross section of the channel. .4. Erosion and deposition continues which leads to the meanders becoming more curved.
River cliff
The Helicodal flow (cock screw like flow of water that goes through rivers) causes surface water to flow towards the outside bank which causes lateral erosion through hydraulic action. Erosion of the outside bank also takes place through abrasion; as the velocity is greater here the river is able to carry larger sediment which increases the force of abrasion. The combined effect of this erosion causes the outside bank to be undercut and eventually causes it to collapse thereby forming a river cliff.
River Sinuosity
is a measurement of how much a river varies from a straight line. It’s a ratio between the channel length and displacement (straight line distance) between two points in the river’s course:
A sinuosity of 1 means that the channel is perfectly straight. A sinuosity greater than 1 means that the river meanders.
Increases as depth increases
slip off slope
The build-up of deposited sediment is known as a slip-off slope. Gently sloping bank found on the inside of a river bend.
Cross section of a Meander
would show that on the outside bend, the channel is very deep and concave. This is because the outside bend is where the river flows fastest and is most energetic, so lots of erosion by hydraulic action and corrasion takes place. River cliffs form on the outside bend as the river erodes laterally. The inside bend is shallower with a gentle slip-off slope made of sand or shingle that is brought across from the outside bend by the helicoidal flow of the river. The river flows much slower on the inside bend so some deposition takes place, contribution to the slip-off slope.
Straight channels
rare and typically have a central ridge of deposited material
Braided River channels EG- Tagliamento’s channel in Italy.
channel that is divided into smaller sub-channels by small, temporary islands called eyots. develop in rivers with a lot of sedimentary load, a steep gradient where the discharge of the river changes regularly and where there is easily erodable bank material.
Braided River channel formation
When the volume of load exceeds the river’s capacity or the discharge of the river drops, the river is forced to deposit its load in the channel thus creating a mid channel eyot. As the discharge decreases after a flood the bedload is deposited forming eyots that grow downstream.
Eyots
Bedload that is deposited after discharge of a river decreases. These grow downstream.
Islands
Eyots upstream that become stabilizd with vegetation. Helps to localise and narrow the channel in order to increase velocity so river can transport load.
Wavelengths of Meanders Influenced by
Channel Width
discharge and nature of bed and banks
Meander Wavelenghts
6-10 times channel width
5 times radius of curvature
wavelength increases in streams that carry excess load
Meander belts
peak to peak amplitude of meanders usually 14-20 times channel width
Factors causing meanders
Friction-causes turbulence making flow unstable producing bars.
Sand bars
sinuosity-
helicoidal flow
How meanders change over time
Migrate downstream and erode river cliffs
Migrate laterally and erode floodplain
Become exagerrated and become cut offs
Become intrenched or ingrown
Incised meanders
well developed on horizontally bedded rocks as river cuts through sediment and into underlying bedrock. Come in 2 types intrenched and ingrown
Ingrown Meanders- in the lower part of the Seine
are a result of lateral meander migration and are aysmmetrical in the cross section
Intrenched Meanders- EX GOOSENECKS OF SAN JUAN USA
Symmetrical and occur when vertical erosion is faster than the lateral migration of meanders, typically when sea levels drop.
Pools and Riffles
Formed by turbulences. Eddies cause the deposition of coarse materials (riffles) at high velocity points and fine sediment (pools) at low velocity points.
Waterfall Formation
Develop when there is a change in rock type from more resistant to less resistant (granite to limestone) river erodes less resistant rock faster resulting in a sudden change in gradient. As the river flows over the resistant rock, it falls onto the less resistant rock, eroding it and causing an undercut. When water flows over the waterfall it creates a plunge pool at its base and the splashback from the falling water undercuts the resistant rock. The unsupported rock is known as the cap rock which will collapse causing the waterfall to migrate upstream
Gorge Recession
Occurs when waterfalls migrate upstream, common when local rocks are not resistant to powerful river erosion. Similarly in arid areas (water for weathering is scarce) gorges are formed by long periods of river erosion
V shaped valleys
V-Shaped valleys are found in the upper course of the river and are a result of both erosion by the river and weathering. V-Shaped valleys are deep river valleys with steep sides that look like a letter V when a cross section of them is taken, hence the name. They’re found in the upper course because this is where the river has the greatest gravitational potential energy and so the greatest potential to erode vertically. It does so during periods of high discharge. When the river’s discharge is high, it is able to transport its large bedload by traction eroding the river’s bed and valley by corrasion, deepening it. Not much lateral erosion takes place so the channel and valley remains relatively narrow.
As the channel and valley deepens the sides of the valley are exposed and become susceptible to weathering. The valley’s sides also undergo mass movements resulting in large volumes of material falling into the river’s channel, adding to its erosive power and causing the valley sides to take up a V shape. The steepness of the valley sides and whether the valley actually looks like a V is dependent on the climate, vegetation and rock structure among things. In cold, wet climates, freeze thaw weathering is abundant and rainwater can act as a lubricant, aiding mass movements. Vegetation can impede mass movements because it will help bind the soil. If the valley is composed of hard rock the valley sides will be very steep because they won’t be weathered easily
Levees
During a flood the largest & heaviest load is deposited first and closest to the river bank, often on the very edge, forming raised mounds. The finer material is deposited further away from the banks causing the mounds to appear to taper off. Repeated floods cause the mounds to build up and form levees.
Levees aren’t permanent structures. Once the river’s discharge exceeds its bankfull discharge, the levees can be burst by the high pressure of the water.
Flooplain
large, flat expanses of land that form on either side of a river. The floodplain is the area that a river floods onto when it’s experiencing high discharge. When a river floods, its efficiency decreases rapidly because of an increase in friction, reducing the river’s velocity and forcing it to deposit its load. The load is deposited across the floodplain as alluvium. The alluvium is very fertile so floodplains are often used as farmland.
The width of a floodplain is determined by the sinuosity of the river and how much meander migration takes place. If there’s a lot of meander migration, the area that the river floods on will change and the floodplain will become wider.
Backswamps
It is where deposits of fine silts and clays settle after a flood. These deposits create a marsh-like landscape that is often poorly drained and usually lower than the rest of the floodplain.
Terraces
remenants of old floodplains which have been eroded.
River bluffs
line of relatively steep slopes at the end of a terrace
Oxbow lakes
strong erosion takes place on the outside bend of a meander while deposition takes place on the inisde bend. As a result, the neck of a meander narrows. During extremely high discharge (e.g., a flood), it’s more efficient for a river to flow accross the neck of a meander rather than around it. When discharge returns to normal levels, the river continues following this new course. The meander is left connected to the channel as a cutoff. Deposition eventually separates the cutoff from the main channel leaving behind an oxbow lake. With its main source of water disconnected, the lake eventually dries up leaving behind a meander scar.
Deltas
Deltas are depositional landforms found at the mouth of a river where the river meets a body of water with a lower velocity than the river (e.g. a lake or the sea). For a delta to develop, the body of water needs to be relatively quiet with a low tidal range so that deposited sediment isn’t washed away and has time to accumulate.
Delta formation
When a river meets a stationary body of water, its velocity falls causing any material being transported by the river to be deposited.
3 layers of Deltas
bottomset bed- built along the sea floor by turbidity currents. composed of fine material such as clay ( salt water causes particles to flocculate) his makes the clay particles sink due to their increased weight producing the bottomset bed. The bottomset bed stretches a fair distance from the mouth of the river as the fine sediments can be transported a reasonable distance from the river’s mouth.
foreset beds
on top of bottomset beds inclined layers of coarse material is deposited.The foreset bed makes up the majority of the delta and is dipped towards deep water in the direction that the river is flowing in.
Topset bed
Composed of fine material and are part of rivers floodplain and are horizontally bedded.
Types of Delta
Arcuate
Cuspate
Birds Foot
Lacustrine
Arcuate Deltas
Fan shaped. Found in areas of regular longshore drift or other currents. These factors keep the seaward edge of the delta smooth. (shaped like a triangle)
Cuspate Delta
are vaguely shaped like a V with curved sides. Cuspate deltas form when a river flows into a sea with waves that hit it head on, spreading the deposited sediment out.
Birds foot delta
They extend reasonably far into a body of water and form when the river’s current is stronger than the sea’s waves and deposition occurs along the edge of distributaries.
lacustrine deltas
When a river enters a lake and it deposits some or all of its load. As the delta builds up and out it may fill the lake basin
rapids
When a waterfall retreats the waterfall often
becomes smaller in size until it almost disappears
producing a steeper, more turbulent section of river
profile, often with a rough bed where outcrops of
resistant rock are found
