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
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
Measuring river discharge
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
