# HKDSE Geography/M2a/Fluvial Processes

HKDSE Geography
* Objectives
* Living with our physical environment
** M1: Opportunities and Risks
** M2a: Rivers
** M2b: Coasts
* Facing changes in the human environment
** M3: Industry
** M4: Sustainable City
development mutually exclusive?
* Confronting global challenges
** M5: Famines and Agriculture
** M6: Tropical Rainforests
** M7: Global Warming
* Elective modules
** E1: Dynamic Earth
** E2: Climate
** E3: Transport
** E4: Zhujiang Delta
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).

### Three Directions

| | Headward erosion | Vertical erosion/Downcutting | Lateral erosion |
| Diagram | | | |
| Primary location | Source | River bed of upper course | River bank of middle and lower courses |
| Result | The river is lengthened. | The river is deepened. | The river is widened. |
| Description | Loose rock debris is brought down by overland flow running downslope. The source is extended backward and the backwall retreats upslope, accelerated by mass movement. | The riverbed is eroded. | The two riverbanks are eroded when the river swings from side to side. |

### 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 vs. Abrasion
* 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.

### Four Processes

| Name | Load | Description | Weight |
| Solution | Dissolved load | Soluble minerals are dissolved and carried downstream | Lightest |
| Suspension | Solid load (suspended load) | Small particles are carried downstream in water without touching the bank | Light |
| Saltation | Solid load (suspended load) | Load is moved in a series of hops and bounces along the bed | Heavy |
| Traction | Solid load (bed load) | Load rolls and slides on the bed | Heaviest |

### Factors

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

## Deposition
### 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)