--- title: "Energy dissipating structures" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:energy-dissipating" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/energy-dissipating site: BCT Engineering --- # Energy dissipating structures Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2081 Chaitra; 2068 Bhadra (old course), 8 marks; 2062 Jestha (old course), 8 marks (asked 3 times) - Video: [Hydraulics Topic 5.4 Energy Dissipator Structure](https://www.youtube.com/watch?v=1ngN-4SlCf0) — MOOC UTHM Johor - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/energy-dissipating - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question Explain the different types of erosion control and energy dissipating measures/structures used in the highway drainage system and their function. ## Short answer Erosion control includes drain lining, check dams, turfing/bio-engineering, rip-rap/gabion aprons, mitre drains. Energy dissipators: drop structures/stepped drains, stilling basins with baffle blocks, splash/rip-rap aprons, baffle blocks/dentated sills, rough channels/rapids, flip buckets. They reduce velocity to non-scouring limits, prevent undermining, gully formation, and safely release water. ## Answer 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. ```text 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). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are energy dissipating structures needed at drain outlets? A. To store water B. To raise the water table C. To increase velocity D. High-velocity flow scours and undermines structures Answer: D. High velocity energy causes scour, undermining and slope erosion. (Key point: Need for dissipation) 2. How do check dams reduce erosion? A. Reduce effective gradient, trap silt and slow flow B. Raise discharge C. Remove water completely D. Increase the slope Answer: A. Small walls across steep drains lower the effective gradient and slow flow. (Key point: Check dams) 3. What is the role of a stilling basin? A. Support the culvert deck B. Collect rain on carriageway C. Prevent capillary rise D. Lined depression where hydraulic jump forms to dissipate energy Answer: D. Baffle blocks and end sill help form the hydraulic jump in the stilling pool. (Key point: Stilling basin) 4. Which is a bio-engineering erosion control measure? A. Pipe culvert B. Turfing with grass, vetiver and shrubs C. Gully inlet D. Perforated pipe drain Answer: B. Vegetation binds the soil on slopes; jute nets and geotextiles protect meanwhile. 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