1.2 - River Discharge
The concept of river discharge and its variation downstream
River discharge is a key measure in understanding how rivers function. It represents the volume of water flowing past a specific point in a river over a set period of time, typically measured in cubic metres per second (cumecs).
Understanding river discharge
- Definition and measurement - Discharge is calculated as the volume of water passing a point per second, expressed in cumecs.
- Variation along the river - Discharge generally increases as a river progresses from its source to its mouth due to the accumulation of water from tributaries and surface runoff.
The Bradshaw model of channel variables
The Bradshaw model provides a framework for understanding how various characteristics of a river channel change from upstream to downstream. It highlights predictable patterns in river behaviour and morphology as the river progresses along its course.
Key changes in channel variables from upstream to downstream
| Variable | Upstream Characteristics | Downstream Characteristics |
|---|---|---|
| Discharge | Low volume of water | High volume due to tributary input |
| Occupied channel width | Narrow channel | Wider channel |
| Water depth | Shallow depth | Deeper due to increased volume |
| Water velocity | Slower due to friction | Faster with less friction |
| Load quantity | Smaller amount of material | Larger amount due to erosion |
| Load particle size | Larger, angular particles | Smaller, rounded particles |
| Channel bed roughness | Rough with boulders and irregularities | Smoother due to erosion and deposition |
| Slope angle (gradient) | Steeper incline | Gentler slope |
The main types of river erosion and factors affecting them
Erosion is a fundamental process in shaping river landscapes, involving the wearing away of the river bed and banks. Different types of erosion occur due to the interaction between water, sediment, and the river's surroundings, and several factors influence the rate and extent of this process.
Types of river erosion
- Abrasion (corrasion) - The wearing away of the river bed and banks by the load carried by the river.
- Attrition - The wearing away of the load carried by the river, creating smaller, rounder particles.
- Hydraulic action - The force of water and air on the sides of rivers and in cracks.
- Solution (corrosion) - The removal of chemical ions, especially calcium, which causes rocks to dissolve.
Factors influencing the rate of erosion
- Load characteristics - The heavier and sharper the load, the greater the potential for erosion.
- Velocity and discharge levels - The greater the velocity and discharge, the greater the potential for erosion.
- Gradient of the river - Increased gradient increases the rate of erosion.
- Geological composition - Soft, unconsolidated rocks, such as sand and gravel, are easily eroded.
- Water pH - Rates of solution are increased when the water is more acidic.
- Human activities - Deforestation, dams, and bridges interfere with the natural flow of a river and frequently end up increasing the rate of erosion.
The processes of river transportation
Transportation in rivers refers to the movement of sediment and other materials from one location to another. The method of transportation depends on the size of the material, the energy of the river, and the flow conditions.
Main types of river transportation
- Suspension - Small particles are held up by turbulent flow in the river.
- Saltation - Heavier particles are bounced or bumped along the bed of the river.
- Solution - The chemical load is dissolved in the water.
- Traction - The heaviest material is dragged or rolled along the bed of the river.
- Floatation - Leaves and twigs are carried on the surface of the river.
The theory of river channel load and the Hjulström curve
The theory of river channel load explores how rivers manage the sediment they carry, focusing on concepts like capacity and competence. The Hjulström curve is a graphical tool that illustrates the relationship between water velocity and sediment movement, helping to predict when erosion, transportation, or deposition will occur.
Key concepts of river channel load
- Capacity - The largest amount of debris that a stream can carry.
- Competence - The diameter of the largest particle that can be carried.
- Critical erosion velocity - The lowest velocity at which grains of a given size can be moved.
Understanding the Hjulström curve
The Hjulström curve plots water velocity against particle size to show the conditions under which sediment is eroded, transported, or deposited.
Main features of the Hjulström curve
- Velocity requirements for different particle sizes - The smallest and largest particles require high velocities to lift them.
- Entrainment vs. transport - Higher velocities are required for entrainment than for transport.
- Settling velocity - When velocity falls below a certain level (settling or fall velocity), particles are deposited.