--- title: "Importance of highway drainage" subject: "Transportation Engineering I" chapter: 4 key: "semester_6/transportation_engineering_i:drainage-importance" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/drainage-importance site: BCT Engineering --- # Importance of highway drainage Transportation Engineering I · Chapter 4: Highway Drainage - Asked in: 2080 Chaitra, 3 marks; 2078 Chaitra, 3 marks; 2070 Bhadra; 2064 Shrawan (old course) (asked 4 times) - Video: [Basic transportation engineering-Importance of highway drainage](https://www.youtube.com/watch?v=K1mzUQoHcSY) — Bhavik Joshi - 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-importance - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-4 ## Question Discuss the necessity/importance of highway drainage. ## Short answer Water is the greatest enemy of roads; proper drainage maintains pavement strength by keeping subgrade at design moisture, prevents stripping and pumping, ensures safety by avoiding hydroplaning, stabilizes slopes against erosion and landslides, prevents flooding, reduces frost heave, and keeps roads open during monsoon. ## Answer Water is the greatest enemy of a road. Proper drainage is necessary for the following reasons: 1. **Pavement strength.** Water entering the pavement and subgrade reduces the soil strength (CBR falls sharply when soaked), causing rutting, potholes and failure. Drainage keeps the subgrade in the design moisture state. 2. **Pavement life.** Water in bituminous layers causes stripping of binder, and in cement concrete causes pumping and erosion of the sub-base. Good drainage lengthens the life and lowers the maintenance cost. 3. **Safety.** Water on the carriageway leads to skidding and hydroplaning, and reduces visibility. 4. **Stability of embankments and slopes.** Saturated slopes slide, and runoff causes gully erosion, scour and landslides. This is especially serious on hill roads in Nepal during the monsoon. 5. **Prevents flooding and washouts** of the road and avoids damage to adjoining land and property. 6. **Frost heave and swelling damage** are reduced, since less water is available in the soil. 7. **Continuous service.** The road remains open in the rainy season, which is vital for the economy of the area. Studies show that a poorly drained pavement can fail in a fraction of its design life, so drainage is a part of road design from the beginning. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What happens to the soil strength when water enters the subgrade? A. CBR rises B. CBR falls sharply, causing rutting and potholes C. No change D. Only the surface is affected Answer: B. Soaked subgrade loses strength, causing rutting, potholes and failure. (Key point: Strength) 2. How does water damage bituminous and concrete pavements? A. It improves binder adhesion B. It only causes colour change C. Stripping in concrete; pumping in bituminous D. Stripping of binder in bituminous; pumping and erosion of sub-base in concrete Answer: D. Bituminous layers lose binder (stripping) and concrete pavements suffer pumping. (Key point: Pavement life) 3. Why is surface drainage a safety need? A. It increases friction B. It lowers vehicle speed safely C. It only affects pedestrians D. Water causes skidding and hydroplaning and reduces visibility Answer: D. Water on the carriageway leads to skidding, hydroplaning and poor visibility. 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