Lecture 5 - River Bank Erosion

Question 1

What drives bank erosion in forms of energy?

Answer 1

Potential, kinetic and thermal energy.

Question 2

What are the 3 energy states in stream energy?

Answer 2

It highlights the 3 energy states (potential, kinetic and mechanical through thermal)

Question 3

River bank composition is distinguished by what?

Answer 3

Cohesive and non-cohesive sdiments

Question 4

What is in a cohesive river bank

Answer 4

• solid rock
• generally silt and clay-sized material
• resistance varies on the bonds between particles

Question 5

What is a non-cohesive river bank?

Answer 5

comprised of a more sandy/gravel-sized material

Question 6

What are the 2 bank composition types?

Answer 6

Fine and coarse

Question 7

Why are roots and vegetation so important to bank composition?

Answer 7

with 16-18% root volume to 5 cm depth, offered 20,000 times more protection than a bank without vegetation (Smith, 1978)

Question 8

What are the pre-weakening bank erosion processes?

Answer 8

• freeze-thaw
• slaking and positive pore water pressure (air being forced into small holes by water pressure, weakening the material)
• soil piping (natural water pipes below the surface, weakening the surrounding river bank)

Question 9

What is shear stress in bank erosion processes?

Answer 9

When the river bank material has exceeded the critical shear stress of entrainment (entrainment is to carry sediment)

Question 10

What is Mass Failure?

Answer 10

Erosion by the effect of gravity on bank material

Depends on bank structure, material properties and the bank geometry

Question 11

What is Thorne and Tovey’s (1981) cantilever system? (Bank erosion)

Answer 11

A model that shows 3 cantilever failures;

• shear - falls straight down
• beam - falls out bank diagonally
• Tensile falls vertically but leaves an overhang which is only supported at the surface

Question 12

What are the 2 main ways to measure and monitor erosion?

Answer 12

Indirectly - reconstruction of historical channel patterns

Directly - Actual measurement of contemporary erosion

Question 13

How to measure erosion indirectly?

Answer 13

• Using data from OS maps
• Ariel photographs and satellite imagery

Question 14

How to measure erosion directly?

Answer 14

• Using steel rods stuck in the river bank with measurements

-Using a self-reading erosion pin (more expensive than steel rods)