Energy dissipating structures

When water leaves a drain, culvert or steep channel, it has high velocity and energy. This energy scours the bed, undermines structures and erodes slopes. Erosion control and energy dissipating structures reduce the velocity, spread the flow, and protect the soil.

A. Erosion control measures (reduce velocity and protect the surface)

1. Lining of drains: stone pitching, brick/concrete lining, or grouted masonry in steep or erodible drains.
2. Check dams: small stone, gabion or concrete walls across a steep drain. They reduce the effective gradient, trap silt and slow the flow.
3. Turfing / bio-engineering: grass, vetiver, shrubs, brush layering and fascines on slopes bind the soil. Jute nets and geotextiles protect the slope while plants grow.
4. Rip-rap and gabion aprons: loose stones or wire-crate mattresses laid at inlets, outlets and stream banks.
5. Mitre / turnout drains and cascades which lead water away from the road at short intervals and divide the flow.

B. Energy dissipating structures

1. Drop structure / stepped drain (cascade): the drain is built as a series of steps. The water falls over each step and loses energy in the pool below.
2. Stilling basin (plunge pool): a lined depression at the foot of a drop or culvert outlet where the jump forms; baffle blocks and an end sill help the hydraulic jump.
3. Splash apron and rip-rap apron downstream of culvert outlets. Loose stones spread the flow and resist scour.
4. Baffle blocks or dentated sills at the end of an apron break up the jet.
5. Rapids (rough channel) on a steep slope: a channel with rough bed (boulders, cross ribs) which keeps the velocity within safe limits.
6. Flip bucket / trajectory bucket for high drops.

  Drain ----\
             \  drop
              |_____  stilling pool
                    \___baffle__/ -> safe outflow
                     rip-rap apron

Functions

• Reduce the velocity to a non-scouring limit.
• Prevent undermining of culvert outlets, bridge abutments and embankment toes.
• Prevent gully formation and landslides on hill roads.
• Safely release the water into a natural stream.

The type is chosen by discharge, drop height, soil, and available material (stone is often used in Nepal for gabions and masonry check dams).