--- title: "Requirements of highway drainage" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:requirements-highway-drainage" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/requirements-highway-drainage site: BCT Engineering --- # Requirements of highway drainage Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2079 Ashwin, 3 marks; 2078 Poush, 4 marks; 2077 Chaitra, 3 marks; 2072 Magh; 2071 Bhadra; 2064 Poush (old course), 3 marks (asked 6 times) - Video: [Highway Drainage System.Ideal Requirements and Types Detailed Explanation of Civil Engineering.](https://www.youtube.com/watch?v=euMqlCLZIbA) — AboutCivil Engineer's - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/requirements-highway-drainage - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question What are the requirements of a good highway drainage system? ## Short answer A good system quickly removes surface water, disposes of it safely, maintains non-silting and non-scouring velocities, intercepts outside water, keeps water table below subgrade, protects slopes, has adequate capacity, and is economical and easy to maintain. ## Answer A good highway drainage system should satisfy these requirements: 1. **Quick removal of surface water.** Rainwater should drain off the carriageway immediately through camber and cross slope, with no ponding on the surface. 2. **Safe disposal.** Water collected from the road and nearby catchment must be taken through side drains and cross-drainage works to a natural outfall (stream or gully) without damaging the road, adjacent property or farmland. 3. **Proper flow velocity.** Velocity in drains should be high enough to prevent silting (non-silting velocity, about 0.6 to 0.75 m/s) and low enough to prevent scouring (the non-scouring limit depends on the soil and lining). 4. **Interception of outside water.** Catch drains should stop water from the upslope area from reaching the road. 5. **Control of sub-surface water.** The water table should be kept well below the subgrade (about 0.6 to 1.0 m), and seepage and capillary rise should be controlled. 6. **Protection of embankments and slopes** from erosion and from saturation, using lining, turfing and energy-dissipating structures. 7. **Adequate capacity.** Drains and cross-drainage structures must carry the design discharge for a suitable return period (longer for culverts and bridges than for side drains), with enough freeboard. 8. **Economy and ease of maintenance.** Drains should be simple, easy to clean and made of locally available material, without causing hazard to traffic (safe slopes, no deep open drains near the carriageway). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which requirement relates to camber and cross slope? A. Economy B. Adequate capacity C. Quick removal of surface water without ponding D. Interception of outside water Answer: C. Camber and cross slope ensure water leaves the carriageway immediately. (Key point: Surface water removal) 2. What velocity range should drain flow satisfy? A. Below the non-silting velocity B. Above the non-silting velocity but below the non-scouring velocity C. Above the non-scouring velocity D. Zero velocity Answer: B. Velocity should be high enough to prevent silting (0.6-0.75 m/s) and low enough to prevent scouring. (Key point: Flow velocity) 3. What is the purpose of catch drains? A. Reduce camber B. Lower the water table C. Carry water across the road D. Stop upslope water reaching the road Answer: D. Catch drains intercept water from the upslope area before it reaches the road. 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