--- title: "Necessity of catch drain, causeway, EDS" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:drainage-elements-necessity" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/drainage-elements-necessity site: BCT Engineering --- # Necessity of catch drain, causeway, EDS Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2073 Magh, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/drainage-elements-necessity - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question Clarify the necessity of the following elements of highway drainage: i) Catch drain ii) Causeway iii) Energy dissipating structure iv) Sub-surface drainage ## Short answer A catch drain intercepts hillside runoff to prevent slope erosion and saturation. A causeway provides an economical road crossing on wide shallow streams where floods are brief, allowing overtopping. Energy dissipaters reduce flow velocity to prevent scour. Sub-surface drainage lowers the water table to protect subgrade strength and pavement life. ## Answer #### i) Catch drain A catch (intercepting) drain is a channel dug above the cut slope on the hill side. Its necessity: - It intercepts runoff from the upslope catchment and carries it to a stream or a culvert before it reaches the road. - It prevents erosion and gullying of the cut slope. - It keeps the side drain from overflowing, and the slope and subgrade from becoming saturated, which reduces the chance of landslides. #### ii) Causeway A causeway is a low-level crossing built on the stream bed. Necessity: - Where wide, shallow streams carry flow only during short monsoon floods, a bridge would be uneconomical. - It is cheap, quick to build, and low in maintenance on low-traffic roads. - It lets the flood flow over the road without damaging the embankment, which is protected by aprons and cut-off walls. #### iii) Energy dissipating structure (EDS) Needed where water flows with high velocity down steep slopes or leaves a culvert or drain on a steep outfall. - It reduces the velocity and energy so that the flow does not scour the bed or undermine foundations. - It prevents gully erosion and slides on the downstream slope. - Examples: drop structure, stilling basin, rip-rap apron, check dams, baffle blocks. #### iv) Sub-surface drainage Needed where the water table is high, seepage emerges in cuttings or springs are present, or the subgrade soil is fine-grained. - It lowers the water table and intercepts seepage, so the subgrade stays in the design moisture state (soaked CBR is not lost). - It prevents pavement failure, pumping, frost heave and slope failures. - It extends the life of the road and reduces maintenance cost. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a catch drain provided above a cut slope? A. To store water for use in construction B. To intercept upslope runoff before it reaches the road and so protect the slope from erosion and saturation C. To carry traffic loads D. To lower the water table inside the pavement Answer: B. It intercepts runoff from the upslope catchment, preventing gullying, overloading of the side drain and saturation that causes landslides. (Key point: Catch drain) 2. Why is a causeway preferred to a bridge on a wide, shallow stream with short monsoon floods? A. Bridges cannot be built on shallow streams B. Floods never reach a causeway C. A causeway is cheap, quick to build and low in maintenance, and lets floods flow over the road D. A causeway keeps traffic running during all floods Answer: C. A bridge would be uneconomical there; a causeway lets flood flow over the road, with aprons and cut-off walls protecting the embankment. (Key point: Causeway necessity) 3. What is the main purpose of an energy dissipating structure at a steep outfall? A. Reduce velocity and energy so the flow does not scour the bed or undermine foundations B. Increase the velocity of flow C. Raise the water table D. Increase discharge capacity of the culvert Answer: A. EDS such as stilling basins, aprons and baffle blocks reduce velocity and energy, preventing scour and gully erosion. (Key point: Energy dissipating structure) 4. Why is sub-surface drainage needed where the water table is high or seepage emerges? A. To increase the moisture of the subgrade B. To make the pavement thinner only C. To collect rainwater from the carriageway D. To lower the water table and intercept seepage so the subgrade keeps its design strength and the pavement does not fail Answer: D. It keeps the subgrade in the design moisture state, preventing pumping, frost heave and pavement or slope failure. 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