HKDSE Geography/M2a/River Channels and Flow
This chapter discusses two things:
- Channel geometry - what the channel looks like.
- River flow - how fast a river flows and how much energy it has.
These two topics are closely related and are thus put under a single roof.
- A channel is the path through which the river flows; it is only part of the river itself and does not include the water in the river.
- Velocity is the speed at which water flows. (m/s)
- Cross-sectional area is the area of a river's cross-section. (m2)
- Discharge is the volume of water passing a given point of a river in one second. (m3/s)
- Volume is the amount of water a river holds. (m3)
- Bankfull discharge is the maximum discharge a river can support without flooding.
- Capacity is the maximum volume of water a river can hold without flooding.
Volume is the amount of water in the river. Discharge is the amount of water passing through a certain point of a river in a second.
Using a protest march as an analogy, 'volume' would be the number of people in the march, and 'discharge' the number of people passing by a certain landmark in one minute.
If you study economics, 'volume' is like a stock concept and 'discharge' a flow concept.
Discharge is positively related to volume. Discharge is also positively related to velocity. The discharge is determined by factors that affect these.
Factors Affecting Water Volume and thus Discharge
These are factors that affect how much water there is, the 'software' of the river channel.
River discharge is positively related to overland flow, and therefore negatively related to infiltration.
- Slope gradient ↑ → Infiltration ↓, Surface run-off ↑ → Discharge ↑
- Vegetation cover ↑ → Interception ↑ → Infiltration ↑, Surface run-off ↑ → Discharge ↑
- Rock/soil permeability ↑ → Interception ↑ → Infiltration ↑, Surface run-off ↑ → Discharge ↓
Permeability is especially of note here. Here are just some factors that affect permeability:
|Increase permeability||Decrease permeability|
These relationships also matter:
- Drainage density ↑ → Discharge ↑
- Size of catchment area ↑ → Discharge ↑
- Stream order ↑ → Discharge ↑
The principle behind the above is that 'if there are more rivers, they can collect more.'
- Precipitation/Rainfall/Snowmelt ↑ → Discharge ↑ (take note of RH, temp., etc. relationships above)
- Evaporation ↑ → Discharge ↓ (take note of RH, temp., etc. relationships above)
Factors Affecting Velocity and thus Discharge (Channel-Related Factors)
These are factors that affect how fast the river flows, the 'hardware' of the river channel. It is related to the channel.
- Channel friction reduces velocity.
- The two sides of the river are river banks.
- The bottom of the river is the river bed.
- Wetted perimeter (WP) is the perimeter of the bank and bend in contact with water.
- Hydraulic radius (HR):
- Width-depth ratio is the ratio of the bank length to the bed length.
- A symmetrical river channel has the same bank shape on either side. If the shapes of the two banks are different, it's an asymmetric river channel.
- Load is the materials transported by the river (more below).
- Channel roughness is how rough the channel is.
The above parameters are related to discharge thus:
- WP↑ → Channel friction ↑ → Efficiency ↓ → Velocity ↓ → Discharge ↓
- HR↑ → Channel friction ↓ → Efficiency ↑ → Velocity ↑ → Discharge ↑ (this overrides the WP point!)
- Channel roughness ↑ → Channel friction ↑ → Efficiency ↓ → Velocity ↓ → Discharge ↓
- Asymmetric river channel → Channel friction ↑ → Efficiency ↓ → Velocity ↓ → Discharge ↓
- Amount of load ↑ → Internal friction ↑ → Efficiency ↓ → Velocity ↓ → Discharge ↓
Only one point to be made here:
- Discharge ↑ → Energy ↑
Thus all the factors above also affect energy.
Variation of Velocity and Energy Within a Channel