Booklet 5 - River channel processes Flashcards
define fluvial erosion
Break up and removal of rock by water.
Erosion - process in which rock is broken up and transported by natural forces such as wind, water or glaciers.
Define load
particles of sediment and dissolved matter carried along by the river
4 main processes which erode the river channel
- Abrasion
- sediment in the river is thrown or scraped along the banks and beed of the river
- river load acts as sandpaper, removing pieces of rock as it rubs against the bed and banks. Further encouraged/ erodes faster is the bed/bank rock is permeable and has cracks. - Hydraulic action
- the force of the water hits against the river channel and this repeated action weakens the riverbank and cause it to breakup. The water flow will then remove the articles that have been broken away from the river bank. - Cavitation
- air bubbles trapped in water get compressed into small spaces like cracks in river banks. Bubbles will eventually explode, creating small shockwaves that weakens the rocks, to a point where it will fall apart. - Solution
- only affects certain types of rocks
- water is slightly acidic and will react with rocks which contain calcium carbonate. Calcium carbonate alters chemically to form calcium bicarbonate which will dissolve to form bicarbonate. - Attrition
- pieces of rick in the river lad knock together, breaking chunks of rock off one another and gradually making the load particle smaller and rounder.
Define fluvial transportation
material the river erodes is called the dement, when the river moves the sediment along its course, it is called the river’s load. The river water will move this load downstream through the process of transportation.
Factors influencing erosion
- Load size and amount
- large, heavier and sharper load particle = greater potential for abrasion and attrition - Velocity of river
- faster speed of water = greater potential for cavitation, hydraulic action and abrasion - Gradient of slope
- steeper slopes = increases speed of water flow = increases rate of abrasion, hydraulic action, cavitation and attrition - Geology
- softer rock = weaker = faster erosion
- more air spaces and joints = increased cavitation
- certain rock types = more easily eroded by solution (if they contain calcium carbonate) - Human activity
- deforestation = increased river discharge + building bridges = creates areas of faster flowing water = increases erosion processes - pH of water
- more acidic = increased chance of solution
define suspended load
describes the smallest particles (silts and clays) held up by the river
Define bed (tracted) load
heaviest load that is dragged along the riverbed.
4 ways of transportation
- Traction
- transports largest particles such as boulders. particles are rolled along the bed of the river = eroding the bed by abrasion and particles by attrition - Saltation
- medium sized particles such as cobbles and gravel are transported. load is bounced along the bed of the river, particles are too heavy to be carried by the water by suspension. - Suspension
- fine particles like sand, silt and clay are transported. carried in the water - Solution
- particles are dissolved into the water, only found in rocks (limestone and chalk) have been eroded by he process of corrosion / solution
define capacity and competence
Capacity - maximum volume of load that a river can transport at a specific point.
Competence - largest sized particle , measured by diameter, that a river can transport at a specific point
Both are affected by river’s energy
4 factors influencing a river’s energy and velocity
- Discharge
- higher the discharge = higher the velocity
- energy of river will be at its highest at times of high rainfall and rapid snow melt - Gradient of channel bed
- steeper gradient = increased velocity = more energy - Hydraulic radius
- measure of channel efficiency, higher HR = greater channel efficiency = greater velocity - Roughness of channel
- describes the quantity of large and angular bedload in which increase wetted perimeter
- channel in upper course is often shallower and cluttered with large angular bedload = higher friction = low channel efficiency
- velocity increases downstream
How to measure hydraulic radius
measured by dividing the cross sectional area by its wetted perimeter (length of bed and bank in contact with water)
define Fluvial deposition
if the river no longer has the energy to transport the load, it will deposit it on the channel bed
4 reasons that cause the river to lose energy
- Decrease in precipitation
- decrease discharge = decrease in velocity - River meets the sea or lake
- flows down a body of stationary water = velocity slows down - Inside of a meander bend
- river flows slower on the inside of the meander, deposits load - On the flood plain
- during a flood event, river overflows onto floodplain, water is shallow on floodplain thus velocity slows down due to friction
Sequential deposition
Boulders, Cobbles, Pebbles, Sand, Silt, Clay (course –> fine load)
Define sediment yield
amount of sediment per unit area removed from a drainage basin by flowing water during a specified period of time. measured in tonnes per km^2
Define denudation
wearing down of the land surface by natural processes including erosion by the wind, glaciers, and flow of water. measured in mm per year
Example of spatial variation in sediment yield: Indonesian vs Japanese rivers
Indonesian and Japanese rivers both have high sediment yields
Indonesian:
1. intense tropical rainfall, experiences annual heavy rainfall = increased of river discharge = higher velocity to erode river rocks + rain can loosen soil = creating runoff and increased sediment transport as it carries sediment into river channels.
2. high soil erodibility , tropical soils = readily detached and transported into rivers
Japanese:
1. frequent earthquakes + landslides = add large amounts of sediment into river systems. during rainfall landslides deposit loose soil and rock that are easily eroded and transported by rivers
2. steep and short rivers = fast flowing water increases erosion power of river = carry large amounts of sediment downstream quickly through suspension until it reaches the river mouth.
types of flow
laminar flow
turbulent flow
helicoidal flow
define laminar flow
water is organised in sheets or laminae parallel to the channel bed. Water particles stay within their layers and move along the stream at that level. Layers of water moving at different paces.
A smooth, straight river channel with low velocity is required. It is rare, would most likely be found in groundwater flow and maybe in lower course of the river, near the mouth.
define turbulent flow
occurs where there are higher velocities and complex channel morphology such as a meandering channel with alternating pools and riffles.
Chaotic flow that involves eddying and vertical movement. Turbulence causes marked variations in pressure within the water. As turbulent water swirls (eddies) against the bed or bank of the river, air is trapped in pored, cracks and crevices and put momentarily under great pressure. As the eddy swirls away, pressure is released: air expands suddenly creating a small explosion that weakens the bed/bank material.
Turbulent flows erodes vertically by abrasion to create potholes. Hollows in the bedrock contain pebbles which are swirled around by the eddies, which grinds the beds via abrasion. Additionally, attrition would occurs as eddies forces the material in the pothole to knock against each other.
define helicoidal flow
a ‘corkscrewing’ motion responsible for the formation of meanders. occurs in channels with alternating riffles and pools and where the channel is meandering.
define eddying
erratic horizontal and vertical spirals
Explain the process of helicoidal flow
water flows in a corkscrew motion. thalweg flows from the outside bank of the meander, to the outside bank of the next meander. water current corkscrews/spirals by moving downwards along the outer bank and upwards along the inner. As a result it will transport sediment from the outer bank to the inner bank of the meander.
