What this lesson teaches
I can explain how mass movement and water erosion move material down slopes, and what controls them.
Syllabus 9173, Topic 2.2, 1. Geomorphic Processes. What influences the movement of materials on slopes?
Make a guess
A hillside is cleared of plants just before the rainy season. What happens first when heavy rain hits the bare soil?
- Raindrops knock soil particles loose.
- Nothing, because erosion needs a river.
- The whole slope slides at once.
- The soil is protected by its own weight.
Show the answer
Raindrops knock soil particles loose.
Splash erosion starts when drops hit bare soil. On a slope, more particles land downhill, and loosened soil is then carried away by surface water.
Raindrops knock soil loose, water flowing over the surface carries it away, and concentrated flow cuts small channels called rills.
Water erosion on slopes has three forms. Splash erosion starts when raindrops hit bare soil and knock particles loose. On a slope, more of them land downhill. Impacts can also break up soil clumps and seal the surface, so less water soaks in.
Rainwash is soil carried away by a thin sheet of water flowing over the slope. Rillwash happens where water gathers and cuts small channels, called rills. Rills deepen and join as runoff continues.
Rain intensity, drop size, slope length and steepness all affect erosion. So do soil texture and how wet the soil already is. Erosion depends on the supply of loose soil as well as the water's power to carry it.
Plants, leaf litter and mulch shield soil from raindrops and slow water. Roots hold soil together. Clearing slopes, leaving soil bare or packing it down with machines increases erosion. Contour planting, cover crops and terraces help, if maintained. But badly maintained terrace drains can concentrate water and cause damage.
Water erosion is different from mass movement. In erosion, flowing water carries grains away. In a slide, gravity moves a whole mass. Erosion can remove support at the base of a slope and trigger a later slide, so the two are connected.
- Splash erosion
- Raindrops knock soil loose.
- Rainwash
- A thin sheet of flowing water carries soil away.
- Rillwash
- Concentrated flow cuts small channels.
Step by step
Loosen
Raindrops knock particles loose.
Carry
Surface water moves them downslope.
Concentrate
Water gathers and cuts rills.
Worked example: A bare plantation track
A packed track absorbs less rain than nearby covered soil. Water gathers in wheel ruts and carries soil into a stream. The cause is compaction, less infiltration and rillwash, not just heavy rain.
Watch out for this
Planting trees stops slope erosion at once.
Young trees leave much bare ground. Ground cover and upkeep matter.
Check your understanding
Which change most directly reduces splash erosion?
- Covering bare soil with mulch.
- Removing leaf litter.
- Channelling the runoff into one unprotected ditch.
Show the answer
Covering bare soil with mulch.
Yes. Mulch shields soil from raindrops.