Soil erosion causes and preventive measures

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.

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