Channel: The confines of the river, encompassing the bed and two banks.
Wetted Perimeter: The total length of the bed and the banks in contact with the river.
Cross-sectional area: The width of the river multiplied by the depth of the river. Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.
Velocity: This is the speed that the water in a river is travelling at. The unit of measurement is normally metres a second (m/s). River velocity can be measured using a flowmeter (pictured right), or more commonly by timing a floating object over a set distance (pictured left). Velocity is then calculated by dividing the time (seconds) by the distance (metres).
Discharge: This is the amount of water in a river at a given point. Discharge is normally measured in cumecs (cubic metres a second). It is calculated by multiplying the cross-sectional area by the velocity.
As you move from the source to the mouth, both the discharge and velocity of a river increases.
Just because a river has a big cross-sectional area, it does not necessarily mean it is efficient. A rivers efficiency is gauged by calculating it hydraulic radius. Hydraulic radius is calculated by dividing a rivers cross-sectional area by its wetted perimeter. If a river has a lower hydraulic radius it means more water is in contact with the bed, banks and surface, making it less efficient. If it has a high hydraulic radius, it means less water is contact with bed, banks and surface, making it more efficient. A river is most efficient just before it reaches bankfull discharge. If it is below bankfull discharge it means a greater proportion of its flow is in contact with the bed, banks and the surface. If it is above bankfull discharge then it means the river is in flood and therefore in contact with the floodplain increasing friction.
Bankfull discharge: The maximum discharge that a river can hold before it bursts its banks.
Hydraulic radius is a fairly simplified way at looking at a river's efficiency, a more detailed way is Manning's coefficient. Manning's coefficient uses hydraulic radius in its formula, as well as looking at bed roughness and gradient.
A rough bed can increase the amount of friction. A steep river gradient can increase the amount of gravitational pull.
When a river has less energy it tends to have a much more laminar flow. If a river has surplus energy it tends to have a more turbulent flow and is able to erode and transport more.
Long profile: The long profile looks at how a rivers' gradient changes from the source to the mouth. A rivers' gradient is much steeper near the source and a lot more gentle near the mouth. Later in the topic we will look at how a rivers' features change as you move from the source to the mouth. We will also look at changes in erosion, transportation and deposition (Floodplain management).
Two simple models which look at changes in velocity, cross-section, bed roughness, etc. are the Bradshaw model and the Schumm model (see models below).
Base level: This is the lowest level that a river can erode its bed to. It is basically sea level. A river can not erode its bed below sea level because rivers are unable to travel upwards.
Contrary to popular belief, average (mean) river velocity actually increases as you move from the source towards the mouth. Despite the river gradient and therefore gravity being much greater nearer the source, the poor efficiency of the river channel means it is less efficient. 95% of a river's energy is used overcoming friction. Therefore in a river's upper course (near the source) where a river's cross-sectional area is smaller and its hydraulic radius lower there is more friction and the river is slower.
BRADSHAW MODEL
SCHUMM MODEL
Load: Material transported by a river e.g. stones, sand, boulders.
Discharge
Bank: The sides of the river channel.
Channel: The confines of the river, encompassing the bed and two banks.
Wetted Perimeter: The total length of the bed and the banks in contact with the river.
Cross-sectional area: The width of the river multiplied by the depth of the river. Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.
Discharge: This is the amount of water in a river at a given point. Discharge is normally measured in cumecs (cubic metres a second). It is calculated by multiplying the cross-sectional area by the velocity.
As you move from the source to the mouth, both the discharge and velocity of a river increases.
Bankfull discharge: The maximum discharge that a river can hold before it bursts its banks.
A rough bed can increase the amount of friction. A steep river gradient can increase the amount of gravitational pull.
When a river has less energy it tends to have a much more laminar flow. If a river has surplus energy it tends to have a more turbulent flow and is able to erode and transport more.
Two simple models which look at changes in velocity, cross-section, bed roughness, etc. are the Bradshaw model and the Schumm model (see models below).
Base level: This is the lowest level that a river can erode its bed to. It is basically sea level. A river can not erode its bed below sea level because rivers are unable to travel upwards.