HKDSE Geography/M2a/Fluvial Processes

The three fluvial processes are erosion, transportation and deposition.


 * Erosion is the process in which materials are removed by an agent.
 * Transportation is the process in which eroded materials are carried away.
 * Deposition is the process in which materials are 'dumped'.

Erosion
There are three directions and four processes. Remember the names and what they mean (but don't memorise the definitions).

Four Processes
Four processes:
 * Hydraulic action: Water exerts pressure on the river bank and undercuts it. Cracks are widened.
 * Corrosion/Solution (Solvent action): Water dissolves soluble minerals from the bed or bank, e.g. calcium in limestone. It is a form of chemical erosion.
 * Abrasion (Corrasion): River load acts as a tool to wear away the bed or bank, e.g. large, angular boulders downcut the bed.
 * Attrition: Load knocks against each other, the bed or the bank. They break down into smaller pieces and get smaller and smaller.

Attrition is the wearing down of the load. Abrasion is the wearing down of the bed or bank.

When load knocks against the bed or bank, abrasion occurs on the bed or bank, and attrition occurs in the load.

Factors
These factors affect erosion:
 * Nature of load:
 * Load weight ↑ → Erosion ↑
 * Load angularity ↑ → Erosion ↑
 * Amount of load: Load amount ↑ → Erosion ↑
 * Discharge and velocity:
 * Discharge ↑/Velocity ↑ → Energy ↑ → Erosion ↑
 * Geology of rocks: Joints ↑ → Erosion ↑
 * Additional factors for corrosion:
 * Water acidity ↑ (pH value ↓) → Erosion ↑
 * Hot and humid climate → Erosion ↑
 * Rocks composed of soluble minerals → Erosion ↑

Transportation
The materials carried by transportation are load.

Factors

 * Energy ↑ → Velocity ↑ → More, heavier load
 * Fluvial Erosion ↑ → Load ↑
 * Soil Erosion ↑ → Load ↑
 * Deposition ↑ → Load ↓

One Process
One process occurs in deposition: Sorting. The heaviest load is deposited first. Cobbles are deposited, then pebbles, then sand, then silt, and finally clay. Thus deposited materials are deposited in layers.

Factors

 * Input of water ↓ / Output of water ↑
 * Drought, dry season, high evaporation rate → Amount of water (discharge) ↓ → river energy ↓ → Deposition ↓


 * Velocity ↓
 * Inland flow of seawater → Velocity ↓ → Deposition ↑
 * Load ↑ → Internal friction ↑ → Velocity ↓ → Deposition ↑
 * Examples: Siltation, landslide
 * Channel friction ↑ → Deposition ↑
 * Examples: entering a channel with long wetted perimeter
 * Gradient ↓ → Velocity ↓ → Deposition ↑
 * e.g. waterfall, entering a flood plain
 * Water enters a large body of water
 * shallow lake, shallow sea
 * At inner banks (will be discussed later)