Understanding the Hjulström Curve: River Erosion & Sediment Transport
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| Hjulström Curve |
What is the Hjulström Curve?
Created by Swedish geographer Filip Hjulström in 1935, the Hjulström Curve is a fundamental graph used by hydrologists and geologists to determine whether a river will erode, transport, or deposit sediment.
The graph plots the relationship between the velocity of water (how fast it’s moving) and the size of the sediment particles (from tiny clay to large boulders). It provides a visual answer to a simple question: "At this speed, what happens to this rock?"
Back to TopHow the Curve Works
The graph plots two primary lines against a logarithmic scale of particle size (x-axis) and water velocity (y-axis):
- Critical Erosion Velocity Curve (Upper Line): The minimum velocity needed for a river to pick up (entrain) a particle of a specific size.
- Zone of Erosion: This is the upper part of the graph. Here, the water velocity is high enough to lift particles off the riverbed. Larger particles, like gravel and cobbles, require significantly more energy (higher velocity) to be dislodged.
- Settling/Fall Velocity Curve (Lower Line): The maximum velocity at which a river can be flowing before a particle of that size is deposited on the bed.
- Zone of Deposition: When the river slows down and loses energy, it can no longer carry its load. The largest particles settle first, while the smallest remain in motion until the water is almost still.
- Zone of Transportation: The area between these two lines where the water has enough energy to keep particles moving but not necessarily enough to pick new ones up. Once a particle is picked up, it requires less energy to keep it moving than it did to lift it initially. In this "middle ground," sediment is carried along by the current through traction, saltation, or suspension.
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The Clay Anomaly: Why Small Particles Are Hard to Move
One of the most interesting features of the Hjulström Curve is the "dip" for sand-sized particles. You might assume that because clay is smaller than sand, it is easier to erode. The opposite is true.
Clay particles are cohesive. Due to their flat shape and chemical bonds, they "stick" together. This means a river actually needs a higher velocity to erode clay than it does to erode fine sand. However, once clay is finally eroded, it can be transported at very low velocities because it is so light.
Back to TopWhy it Matters for Geologists and Engineers
The Hjulström Curve isn't just academic; it has real-world applications in civil engineering and environmental science.
- Flood Management: Predicting where a river might wash away its banks during a storm.
- Dredging: Understanding where silt will build up in harbors and shipping lanes.
- Paleogeography: Helping geologists look at ancient rock layers and determine how fast the water was moving millions of years ago based on the size of the stones left behind.
Check for Understanding
1. Which sediment type requires the highest velocity to erode despite being very small?
2. What happens to sediment when a river’s velocity drops below the settling velocity line?
3. True or False: It takes more energy to keep a particle moving than to start it moving.