Key Terms
- Hydraulic Radius - the ratio of the cross sectional area of the channel and the length of its wetted perimeter. The hydraulic radius indicates a channel's flow efficiency. The greater the hydraulic radius, the of the channel the more volume it can carry.
- Wetted Perimeter- The portion of the perimeter of a channel that is in contact with the water.
- Cross-sectional Area - Total length of the bed and the bank sides in contact with water in the channel.
- Attrition- The rounding of particles of sediment carried in water by repeated collision with each other and the shore.
- Abrasion/corrasion- The erosion of the shoreline of the channel by sediment carried by the channel.
- River processes are divided into three main categories: Erosion, transportation and deposition. All three are dependent on the amount of energy the river possesses which is a function of the mass of water, height above sea level and gradient of the channel.
- The greater the velocity, the greater the turbulence that is generated and the more capacity for sediment load. It is influenced by channel shape, channel slope, roughness of the bed.
- The above diagram demonstrates the hydraulic radius of a channel. This radius is the best indicator of the shape of the channel.
The Bradshaw Model
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Erosion
There are four main processes of erosion.
The load of the river is an important factor of the stream's effectiveness to erode. There are three main mechanisms for a river to acquire load for erosion.
- Hydraulic action - the movement of sediment by frictional drag of moving water. On the outside bend of meanders it can remove materials from banks leading to undercutting and collapse of the bank.
- Abrasion/corrasion - the scouring of the bed and banks by sedimentary material carried by the river. This process of erosion can cause potholes.
- Attrition - reduction in the size of particles in a channel through collision with each other, the bed and the river bank. This means it is more common to find smaller, rounded particles in the lower course of a river and larger, more angular ones in the upper course.
- Corrosion - occurs where rocks are dissolved into the water. most common when limestone and chalk are exposed to the channel.
The load of the river is an important factor of the stream's effectiveness to erode. There are three main mechanisms for a river to acquire load for erosion.
- Vertical erosion - deepens the channel. Most likely to occur in the upper course when a channel has a bedload comprising of coarse and hard particles. Will often form potholes and deep, narrow gorges.
- Lateral erosion - increases a river's width. A large sediment load is required for this process in the middle and lower courses of the river.
- Headward erosion - increases the length of a river. Most commonly associated with waterfall formation and active near the source of the river.
Transportation
Sedminent that breaks free because of erosion or that is washed into the river from the valley sides can be carried along the river in three main ways:
Competent Velocity - The lowest velocity needed to move particles of a specific size.The bigger the particle, the greater the velocity needed to move
Hjulstrom curve - Illustrates the relationship between velocity and competence. It shows the velocity at which sediment will normally be eroded, transported and deposited.
- Bedload - larger materials such as boulders and cobbles that are too heavy to be carried by the stream may roll or slide along the bed which is known as traction. Pebbles may be bounced along the bed and this is known as saltation.
- Suspended Load - most of the sediment in the river is transported in this way, is what gives a river a muddy colour.
- Dissolved Load - weak acids in the water can erode and dissolve carbonate rocks.
Competent Velocity - The lowest velocity needed to move particles of a specific size.The bigger the particle, the greater the velocity needed to move
Hjulstrom curve - Illustrates the relationship between velocity and competence. It shows the velocity at which sediment will normally be eroded, transported and deposited.
- very fine particles may require higher velocities to be eroded due to the cohesive nature of fine clays and silts. Also, the velocity required to keep fine particles in suspension is less than the velocity required to erode them. They can stay in suspension while the water is almost still.
Deposition
Depositional processes occur hen the rivers' carrying capacity is reduced, and this can be for three reasons:
Deposition can occur at any point in the river's course and is not confined to the lower course.
- Water Velocity: can be caused by variations in geology, gradient or break in slope.
- Geology: This can change the chemical composition of the water and cause vegetation growth, which in turn reduces the velocity of the water and encourages deposition
- Evaporation: will reduce flow and encourage deposition
- Additional debris and water volume: where tributaries meet the main channel, water volume can decrease velocity and lead to deposition.
Deposition can occur at any point in the river's course and is not confined to the lower course.