Erosion Flashcards
What is abrasion (corrosion)?
-wearing away of the bed and bank by the load carried by a river
-mechanical impact produced by the debris eroding the rivers bed and banks
-increases as velocity increases
What is attrition?
-wearing away of the load carried by a river creating smaller, rounder particles
What is hydraulic action?
-force of air and water on the sides of rivers and in cracks
-direct force of flowing water and cavitation, as fluid accelerate, pressure drops and may cause air bubbles to form
What is corrosion (solution)?
-removal of chemical ions, especially calcium
-key factors controlling rate of corrosion are bedrock, solute concentration of the stream water, discharge and velocity
What factors effect the rate of erosion?
-load: the heavier and sharper load the greater the potential for erosion
-velocity: the greater the velocity the greater the potential for erosion
-gradient: increases gradient increases the rate of erosion
-geology: soft rocks like sand and gravel are easily eroded
-pH: rates of solution are increases 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 rate of erosion
How is load transported downstream?
-smallest particles (silts and clays) are carried in suspension as the suspended load
-larger particles (sands, gravels, small stones) transported in a series of hops as the salted load
-pebbles are shunted along the bed as the tracked load
-in areas of calcareous rock, material is carried in solution as the dissolved load
What does capacity and competence of a stream refer to?
capacity - largest amount of debris that a stream can carry
competence - diameter of the largest particle that can be carried
What is the critical erosion velocity?
lowest velocity at which grains of a given size can be moved, the relationship between these variables is shown by means of a Hjülstrom curve
What are the main features on Hjülstrom curves?
-smallest and largest particles require high require high velocities to lift them, clay resists entrainment due to cohesion and gravel due to weight
-higher velocities are required for entrainment than for transport
-when velocity falls below a certain level, those particles are deposited
What is laminar flow?
-smooth, straight channel with a low velocity is required
-allows water to flow in sheets parallel to the channel bed
What is turbulent flow?
-occurs where there are higher velocities and complex channel morphology such as meandering channel with alternating pools and riffles
-turbulence causes marked variations in pressure within the water, as turbulent water swirls (eddies) against the bed or bank air is trapped in pores, cracks and crevices and momentarily under great pressure
What is helicoidal flow?
-associated with the presence of alternating pools and riffles in the channel bed, and where the river is carrying large amounts of material
-thalweg travels from outside bank to outside bank in meanders, transports sediment from the outer bank to the inner bank where it is deposited as a sand bar
