---
title: "Soil erosion causes and preventive measures"
subject: "Transportation Engineering I"
chapter: 4
key: "semester_6/transportation_engineering_i:erosion-causes-prevention"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/erosion-causes-prevention
site: BCT Engineering
---

# Soil erosion causes and preventive measures

Transportation Engineering I · Chapter 4: Highway Drainage

- Asked in: 2067 Mangsir (old course), 8 marks
- Video: [What Is Soil Erosion & Conservation? | SOIL CONSERVATION | Dr Binocs Show | Peekaboo Kidz](https://www.youtube.com/watch?v=uo_ntewAemw) — Peekaboo Kidz
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/erosion-causes-prevention
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4

## Question

What are the causes of soil erosion? Describe preventive measures of soil erosion and energy dissipation.

## Short answer

Erosion is caused by high velocity runoff, concentrated flow, vegetation removal, heavy rain, and erodible soils. Prevention includes proper drainage, lined drains, slope protection with vegetation and pitching, and retaining structures. Energy is dissipated with check dams, drop structures, stilling basins, and rip-rap at outlets.

## Answer

**Soil erosion** is the detachment and transport of soil particles by running water, wind or gravity. Along roads it appears as rills, gullies, scour at culvert outlets and washing of embankment and cut slopes.

#### Causes
1. **High velocity of runoff** due to steep slopes and long drain lengths, exceeding the non-scouring velocity of the soil.
2. **Concentrated flow** from culvert outlets, drains and mitre drains discharging onto loose soil.
3. **Removal of vegetation** and exposure of bare soil during construction, with steep cut and fill slopes.
4. **Heavy rainfall** (monsoon in Nepal) with raindrop impact and surface wash.
5. **Erodible soil** such as silt, fine sand and dispersive soil, with poor compaction.
6. **Poor or missing drainage**, saturated slopes and seepage.
7. **Stream scour** near bridges and bank-side roads.

#### Preventive measures
- **Proper surface drainage**: catch drains, side drains with controlled velocity, mitre drains at frequent intervals, and well-spaced cross drains.
- **Lining of drains** (stone pitching, masonry or concrete) where velocity is high.
- **Slope protection**: turfing, vetiver and other bio-engineering (brush layering, fascines, live check dams), jute net and geotextile, and stone pitching or shotcrete on steep slopes. Flatter slopes and benches.
- **Retaining and breast walls, gabions** at toes.
- **River training**: spurs, guide banks and bank protection near bridges.
- **Minimise the exposed area** during construction and restore vegetation quickly.

#### Energy dissipation
- Check dams, drop structures (stepped drains) and rapids for steep drains.
- Stilling basins, splash aprons and rip-rap at culvert outlets.
- Baffle blocks and dentated sills which break up the jet.
- Release the water at stable natural outlets.

```text
 catch drain
   ___\                 turfing / bio-engineering
      \__ slope ________\____ side drain
          stone pitching     check dam
            outlet -> apron + rip-rap
```

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does high runoff velocity cause soil erosion on a road slope?
   A. It exceeds the non-scouring velocity of the soil, detaching and carrying particles
   B. It compacts the soil
   C. It reduces the rainfall
   D. It binds the soil with silt
   Answer: A. Steep slopes and long drains give velocity above the non-scouring limit, so soil particles are detached and transported. (Key point: Velocity cause)

2. Which situation is an example of concentrated flow causing erosion?
   A. Rain falling evenly on a grassed slope
   B. A culvert or drain outlet discharging onto loose soil
   C. Wind blowing over a paved surface
   D. A well-compacted lined drain
   Answer: B. Culvert outlets, drains and mitre drains discharging onto loose soil concentrate the flow and produce scour and gullies. (Key point: Concentrated flow)

3. Which is a slope-protection measure against erosion?
   A. Removing vegetation to expose soil
   B. Steepening the cut and fill slopes
   C. Turfing, vetiver and other bio-engineering, jute net and stone pitching
   D. Leaving drains unlined at high velocity
   Answer: C. Vegetation, bio-engineering, geotextile and pitching protect the surface; flatter slopes and benches also help. (Key point: Slope protection)

4. What measures dissipate energy at culvert outlets and steep drains?
   A. Straight steep unlined channels
   B. Releasing the water onto loose fill
   C. Removing all check dams
   D. Stilling basins, splash aprons, rip-rap, baffle blocks, check dams and drop structures
   Answer: D. These break up the jet and reduce velocity so the water leaves at a stable outlet without scour. (Key point: Energy dissipation at outlets)

## Related in Transportation Engineering I

- [Energy dissipating structures](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aenergy-dissipating) — Transportation Engineering I, chapter 4, asked 3×
- [Causes of moisture variation in subgrade](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Amoisture-causes) — Transportation Engineering I, chapter 4, asked 13×

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- [Mitigation measures of erosion](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aerosion-mitigation) — Engineering Geology II, chapter 5
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