Learning objectives To understand why the Hjulstrom curve is a logarithmic graph To interpret the Hjulstrom curve

Hjulstrom Curve The main factor that controls whether a river erodes, transports or deposits its load is its VELOCITY.. The Hjulstrom curve studied this relationship Based on research from the 1930s Velocity Particle size

Can you think of any other scales in Geography that are logarithmic? A logarithmic scale? Logarithmic scales are used when there is a WIDE range of data to record on one graph Both the x-axis and y-axis are logarithmic. There is a ten-fold increase between each of the equally spaced points on the axis. Can you think of any other scales in Geography that are logarithmic? Look at your graph….

Critical erosion velocity Critical deposition velocity The graph shows two line; Critical erosion velocity Critical deposition velocity What would you expect the graph to look like? Velocity Particle size

CRICITCAL EROSION VELOCITY This shows us the velocity needed to pick up or erode particles of different sizes. GENERAL TREND = the larger the particle the higher the velocity needed to erode it The smallest particles of clay are microscopic , flat platelets that stick together easily. They are very cohesive. Due to their nature they need lots of energy to pick them up, even though they are a small particle. This creates a curve on the graph – hence the name Hjulstrom Curve!! HOWEVER THERE IS AN EXCEPTION TO THE TREND – CAN ANYONE SPOT IT!

CRICITAL DEPOSITION VELOCITY This shows us the velocity at which a river deposits its material. GENERAL TREND = As the particle size increases the velocity at which it drops it increases

There is a gap between the two lines. Transportation What does the gap between the two lines tell us? Notice how the gap decreases as the particle size increases… There is a gap between the two lines. What does this show us? It show us that when there is a small particle size a slight decrease in velocity does not mean that deposition will occur. However as particle size increases to gravel size even a small decrease in velocity will cause deposition.

60 second challenge Can you draw the Hjulstrom Curve in 2 minutes? Every member of the class must participate!!

Critique Problems with Hjulström: Velocity WHERE? Bed? Banks? Mean? (varies enormously within channel, so hard to apply the graph to real river) Is SIZE (calibre) of the load the important factor? What about different densities? SHAPE of load is important too (why?) SHEAR STRESS is key, not VELOCITY (a function of water depth and gradient) TURBULENT FLOW

Kinaesthetic Hjulström! INSTRUCTIONS Stand up when you are eroded. Move around when you are transported. Sit down when you are deposited.

Velocity = 1.0 cm/s

Velocity = 8.0 cm/s

Velocity = 2.0 cm/s

Velocity = 10.0 cm/s

Velocity = 0.5 cm/s

Velocity = 100 cm/s

Velocity = 9 cm/s

Velocity = 1000 cm/s

Velocity = 80 cm/s

Velocity = 8000 cm/s

Skills exam practice

Key words game…