River Discharge & Channel Characteristics (DP IB Geography)
Revision Note
River Discharge & Stream Flow
River discharge is the volume of water passing a given point over a set time
As rivers move downstream the characteristic features change
Bradshaw model
River Channel Characteristics
Component | Definition |
|---|---|
Discharge | The volume of water passing a specific point in the river per unit of time increases downstream due to tributary contributions |
Occupied channel width | The width of the river channel typically increases downstream as more water from tributaries is added |
Channel depth | The depth of the river channel increases downstream as more water accumulates |
Average velocity | The speed at which water flows within the river generally increases downstream with a greater volume of water and steeper gradients |
Load quantity | Load quantity increases as the material is made smaller through erosion |
Load particle size | Load particle size becomes smaller as the material is made smaller through erosion |
Channel bed roughness | Channel bed roughness decreases as the river’s energy decreases allowing for accumulation of finer sediments leading to a smoother channel downstream |
Slope angle | The slope angle decreases as a river moves downstream |
Hydraulic radius | A cross-sectional area of the flow divided by the wetted perimeter |
How to measure discharge in a river
Worked Example
Calculating Discharge
Step One-Depth
Calculate the mean depth
All units of measurement should be the same
The mean depth should be calculated in meters not centimetres
Depth measurements for Site One
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | Mean |
|---|---|---|---|---|---|---|---|---|---|
Depth in mm | 0.05 | 0.12 | 0.17 | 0.23 | 0.30 | 0.35 | 0.28 | 0.18 | 0.21 |
To calculate the mean depth add the 8 measurements together and divide by 8
This gives a measurement of mean depth = 0.21m
Step Two-Cross-sectional area
Cross-sectional area (m2) = width (m) x mean depth (m)
If the width is 4mx mean depth 0.21m the cross-sectional area = 0.84m2
Step Three - Velocity
Time Measurements for Site One
Time Measurement | Left | Center | Right |
|---|---|---|---|
1st | 35 | 28 | 37 |
2nd | 42 | 30 | 39 |
3rd | 36 | 27 | 45 |
Mean | 37.7 | 28.3 | 40.3 |
To work out the mean time taken for the float to travel 10 metres for site one the following calculations need to be completed:
37.7+28.3+40.3-106.3
106.3 is then divided by 3 (number of positions) to give a mean time for site one of 35.43 seconds
Divide this by 10 to get the velocity in m/s
35.43/10=3.543 seconds
The surface velocity for site one is 3.543 m/s
Step Four-Discharge
Discharge = Cross-sectional Area 0.84m2x Velocity 3.543 m/s
Discharge = 2.98 m3/s (cumecs)
Factors affecting stream flow
Hydraulics is the study of water flow in channels
Water flow is determined by gravity and frictional resistance with the channel bed and banks
Channel volume and shape affect the stream's energy
When water flow is turbulent there will be eddying patterns
Turbulence supports the lifting and suspension of fine particles
Turbulent flow conditions include complex channel shapes, high velocities and cavitation
Laminar flow is characterized by smooth and layered movements and is common in groundwater and glaciers but not in rivers
Laminar flow occurs in shallow, smooth, straight channels with low velocities
Rivers sediments remain undisturbed on the bed under laminar flow conditions
When water velocity is low turbulence is reduced
When water levels rise the mean velocity and the hydraulic radius enable the stream to appear to be more turbulent
Velocity
Friction causes uneven velocity distribution in a stream
The water closest to the bed and banks moves slowly
Water in the centre of the channel travels the fastest
Maximum velocity occurs mid-stream, about one-third down
Channel shape influences the velocity
Channel shape
Stream efficiency is measured using hydraulic radius (cross-sectional area divided by wetted perimeter)
Higher ratios indicate greater efficiency and less frictional loss
Channel shape is influenced by both channel material and river forces
Solid rock leads to slow changes and alluvium allows rapid changes
Silt and clay create steep, deep, narrow valleys, while sand and gravel promote wide, shallow channels
Channel roughness
Channel roughness introduces friction, reducing water velocity
Friction arises from bed irregularities, boulders, trees, vegetation and water-bed and bank contact
Manning's n is a formula describing the relationship between channel roughness and velocity
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