Hydro: Fluvial Processes

Question 1

What are the 3 types of fluvial processes?

Answer 1

Erosion, transportation and deposition

Question 2

Name the types of erosion

Answer 2

Abrasion/corrosion, hydraulic action, attrition, solution

Question 3

Where does abrasion occur?

Answer 3

in upstream regions and in rock-cut steams

Question 4

How does abrasion/ corrasion occur?

Answer 4

Occurs when coarse and angular fragments of hard rocks are transported as part of bedload, rubbing and wearing away exposed rock outcrops

Question 5

What is the outcome of abrasion/corrasion?

Answer 5

Downcutting that deepens channels/vertical erosion

Question 6

What is the extreme form of abrasion/corrasion?

Answer 6

Pothole drilling - occurs in fast flowing rivers with strong eddy motions, creating a shallow depression. When fragments of load/pebbles are trapped in these hollows, turbulent eddies will by localised erosion, swirl them around to drill potholes into the bed rock

Question 7

Where does hydraulic action take place?

Answer 7

In the middle and lower courses of river and in alluvial channels

Question 8

How does hydraulic action take place?

Answer 8

The sheer force of flowing water is sufficient to dislodge particles or fragments of unconsolidated material, leading to bank collapse at concave banks of meanders where river velocity is the highest.

Question 9

What is the outcome of hydraulic action?

Answer 9

Lateral erosion as banks undergo erosion

Question 10

What is an extreme form of hydraulic action?

Answer 10

Cavitation. It occurs when bubbles burst against the bank and the resultant shock waves hit and slowly weaken river banks, leading to lateral erosion.

Question 11

Describe attrition

Answer 11

A form of erosion that attacks the load rather than the channel. The collision of the fragments causes the particles to become rounded and decrease in size/calibre downstream

Question 12

How does solution take place?

Answer 12

Occurs in rocks that can be dissolved by water (eg. Limestone due to concentration of carbonic acid)

Question 13

What are the 3 components of river erosion?

Answer 13

Vertical downcutting, lateral erosion, headward erosion

Question 14

How does vertical downcutting occur?

Answer 14

Coarse bedload, with high velocity of flow, abrades and potholes the channel floor. When eventually neighbouring potholes are joined together, a rock-walled gorge is formed due to lowering of the river bed.

Question 15

How does lateral erosion occur?

Answer 15

It occurs when a river meanders as erosion will be concentrated at and below the water surface due to higher velocity. Results in undercutting, collapse and retreat of the concave banks with time.

Question 16

Name 2 instances when headward erosion can take place.

Answer 16

1. At the head of the river
   - Erosion from the underground as erosion will extend the the valley headward.
2. At points where the river profile is is locally steep. Associated with waterfalls where vigorous vertical erosion is concentrated in the plunge pool at the base of the waterfall

Question 17

Name the types of load that undergo transportation

Answer 17

Bedload, suspended load, dissolved load

Question 18

How is bedload transported?

Answer 18

Traction: rolling and sliding along stream bed, occurs near source of stream where valley side gradients are steep and can deliver coarse debris to stream channels.

Saltation: smaller grains bounce along the river bed under the hydraulic force of moving water. They are lifted bodily from the stream bed by turbulence and will land downstream a short distance away.

Question 19

How are suspended load transported?

Answer 19

Small and light particles (such as silt and clay are held up by turbulence in water and form suspended load.
They usually form the greatest proportion of total load and amount increases towards river mouth.
Increase in turbulence and velocity -> increase qty and size of particles that can remain in suspension

Question 20

What comprises dissolved load?

Answer 20

Soluble materials in the form of chemical ions that are removed in solution (eg. Limestone)

Question 21

What factors affect dissolved load?

Answer 21

Dissolved load is affected by the characteristics of the upstream drainage basin (geology ie. carbonate and vegetation cover ie. humid acid). Most dissolved load is derived from the rocks through which groundwater passes before it enters the river basin as baseflow.

Question 22

What load does velocity affect? How does it affect them?

Answer 22

Bedload and suspended load, NOT dissolved load. Total amount of load and max particle size being moved increases.
Proportion of bed and suspended load fluctuates with velocity.

Question 23

Draw/ Imagine the hjulstrom curve in your head

Answer 23

y-axis
- 0.0625, 0.125, 0.25, 1, 2, 4, to 2048

x-axis particle size
- clay silt sand pebbles cobbles boulders

curve
TED, CEV, CSV, 0.5mm

Question 24

Describe the hjulstrom curve.

Part 1 - top curve

Answer 24

Top curve shows that particles 0.5mm in diameter have the lowest CEV, increasing with both increasing and decreasing particle sizes. Boulders have higher CEV due to greater weight. Silt and clay have higher CEV due to higher cohesiveness between particles and their small diameter is smaller than the thickness of the laminar au later where the flow is hydrodynamically smooth and protected from entrainment.

Question 25

Define CEV

Answer 25

The lowest velocity of flow at which particles of a certain size, resting loosely on the channel floor, are set in motion.

Question 26

Describe the hjulstrom curve.

Part 2 - bottom curve

Answer 26

The bigger the particle size, the higher the river velocity needed to transport the particle. This is shown by the bottom curve as big particles have a higher settling velocity (CSV).

Question 27

Define CSV

Answer 27

Velocity at which particles of a given size become too heavy to be transported and fall out of suspension to be deposited.

Question 28

Describe the hjulstrom curve.

Part 3

Answer 28

The velocity required to maintain particles in suspension (transport) is less than the velocity needed to pick them up (erode). Velocity required to maintain very fine clays is close to nil even tho their CEV is high. Thus, material eroded upstream can still be kept in suspension even if there is a fall in velocity downstream. Boundary between transportation and deposition is narrow for course particles. Smaller drop in V -> deposition

Question 29

Compare river capacity and river competence.

Answer 29

River capacity: river’s ability to transport a particular volume of sediment. Varies with 3rd power of river’s velocity. Eg. Double in steam velocity -> volume of load it can carry will increase by 2^3 times

River competence: diameter of the largest sediment that can be moved as load. Varies with 6th power of river’s velocity. Eg. Double in river velocity -> can transport particles 2^6 times heavier

Question 30

Describe downstream changes in sediment load.

Answer 30

Amount of sediment load increases downstream due to contribution from tributaries, erosion of channels and feeding of weathered material from valley sides
Individual sediment particles tend to become rounded and smaller downstream due to attrition and gentle valley slopes can only deliver finer material.

Question 31

How does deposition occur?

Answer 31

When river’s no longer have the competence or capacity to carry all its load, starting with the largest particles

Question 32

What are the conditions for deposition?

Answer 32

1) sudden input of load causing river to be overloaded
2) loss of energy when river broadens out (increase friction) and when discharge volume is reduced from low precipitation

Question 33

What features are associated with deposition?

Answer 33

Alluvial fans, pointbars (lateral accretion) and flood plains (vertical and lateral accretion)

Question 34

How are alluvial fans formed?

Answer 34

1) steep sided valleys, steep gradients, heavily loaded with sediment
2) sudden drop in gradient, sudden decrease in river’s velocity and energy
3) deposition forms an alluvial fan, a cone shaped mass of alluvium with an apex where more coarse sediment are found.

Question 35

How are pointbars formed? (Lateral accretion)

Answer 35

In a meander, undercutting tends to occur along the concave banks where eroded material slumps into the river. Some sediment is transported to the opposite convex bank to form point bars. Concave retreats convex advances due to accumulation of more alluvium. A floor plain is created when the almost level point bar deposited undergoes lateral accretion to cover the plain with alluvium.

Question 36

How are flood plains formed? (Vertical accretion)

Answer 36

Flood plains may be formed when river overflows its banks during floods so that new alluvium is laid down on top of old ones. Silt in suspension settles out due to decrease in velocity. Deposition of course particles is greatest slog the margins of the river channel, forming natural levees.