Prevent erosion by keeping soil covered and water slow: plant grass on every bare slope, especially cut banks and fill slopes, armour the places where water runs fast with rock 15-30 cm across, and build terraces on slopes steeper than 15 percent so runoff soaks in before it can cut. Grass is the first and cheapest defence, because roots hold the topsoil and cover breaks the force of raindrops.
Part of Flood Control and Drainage
Erosion prevention is the set of measures that stop rain and running water from carrying soil away from slopes, ditch banks, berms and stream edges.
Water Takes Soil In Steps, And Each Step Has A Cure
Raindrops hit bare soil and break it loose. Water collects into thin sheets, then into small channels, then into gullies that undercut banks and drop trees. Each stage has a remedy: cover stops the raindrop damage, slowing and spreading flow stops the channels forming, and armour holds the places where water is strongest.
What You Need
- Grass seed or cuttings — see Vegetation Cover.
- Rock — 15-30 cm across for armour. See Riprap.
- Labour to build terraces — see Terracing.
- Mulch — straw or similar to hold seed in place. See Mulching.
Method: Work In This Order
Step 1: Cover bare ground first. Plant grass on any bare slope, especially cut banks and fill slopes. Roots hold topsoil, ground cover breaks raindrop impact and tree roots stabilise slopes.
Step 2: Find the fast water. Use your flow map: ditch confluences, culvert outlets, stream banks and steep hillsides concentrate flow. See Water Flow Mapping.
Step 3: Armour the fast spots. Place rock of 15-30 cm on those places so the rock absorbs the energy and stays put.
Step 4: Slow the long slopes. On slopes steeper than 15 percent, step the ground into terraces so runoff is caught and soaks in. Edge each terrace with a small berm or stone wall.
Step 5: Check after storms. A new rill is an early gully. Fix it while it is small with seed, stone or a re-graded surface.
Common Mistakes
- Waiting to seed — bare earth erodes within months. Seed straight after earthworks.
- Armouring the wrong place — rock where the water is slow is wasted, and the real scour point goes unprotected.
- Ignoring small rills — they grow with every storm.
- Relying on rock alone — rock without a vegetated surface around it leaves bare soil to wash out beside it.
- Planting once and walking away — new grass needs a season to establish. Protect it from grazing and traffic.
Safety
Steep slopes and gullies are unstable
Eroded gullies and undercut banks collapse under a person’s weight, and working on a steep wet slope risks a slide. Work uphill of a bank edge, keep clear of the toe, wear grip footwear, and never work alone at a stream bank. About 15 cm of fast-flowing water can knock an adult off their feet.
What’s Next
- Learn next: Vegetation Cover — the first and cheapest defence
- Learn next: Riprap — for the places grass cannot hold
- Combine with: Erosion Control — the farming side of the same problem