--- title: "Tunnel lighting factors; river bank protection methods" subject: "Transportation Engineering II" chapter: 5 key: "semester_7/transportation_engineering_ii:tunnel-lighting-factors-river-bank-methods" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/tunnel-lighting-factors-river-bank-methods site: BCT Engineering --- # Tunnel lighting factors; river bank protection methods Transportation Engineering II · Chapter 5: Introduction to Bridge and Tunnel Engineering - Asked in: 2070 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/tunnel-lighting-factors-river-bank-methods - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-5 ## Question What are the factors to be considered in tunnel lighting? What are the different methods of river bank protection work? ## Short answer Tunnel lighting factors include traffic speed and stopping sight distance, portal luminance, tunnel length, traffic volume, eye adaptation, day/night circuits, wall reflectance, luminaire glare and flicker, emergency power, and geometry. River bank protection methods include revetments, gabion walls/mattresses, spurs/groynes, guide bunds, retaining walls, longitudinal dykes, launching aprons, bio-engineering, and check dams. ## Answer #### Factors in tunnel lighting The factors to consider in tunnel lighting are: 1. **Speed of traffic and stopping sight distance**, which govern the length and luminance level of the threshold zone. 2. **Outside (portal) luminance** at the approach, which depends on the sky, the surroundings and the orientation of the portal; the threshold luminance is a fraction of it. 3. **Length of the tunnel:** short tunnels (below about 50–100 m) may need only daylight; long ones need full zone lighting. 4. **Traffic volume and type** (one-way or two-way, heavy vehicles, pedestrians). 5. **Eye adaptation time:** the transition zone must reduce the luminance gradually. 6. **Day and night requirements**, with separate day and night circuits. 7. **Wall and road surface reflectance**, cleanliness and tunnel finish (light-coloured walls help). 8. **Luminaire type, mounting position, glare and uniformity** of luminance, and avoiding flicker frequencies in the range of about 2.5–15 Hz (worst at about 4–11 Hz). 9. **Power supply and emergency lighting**, maintenance and cost, and energy saving through dimming. 10. **Safety and geometry:** curves, sight distance and tunnel shape. #### Methods of river bank protection River bank protection prevents erosion and scour of the banks near bridge approaches and guides the flow to pass smoothly through the waterway (IRC:89 "Guidelines for Design and Construction of River Training and Control Works for Road Bridges"; Nepal DoR river training and bio-engineering guidelines). The main methods are: 1. **Revetments (pitching):** stone pitching (dry or grouted), concrete blocks or gabion mattresses laid on a filter layer over a graded bank slope, with a toe wall or launching apron to prevent undermining. Used for banks with moderate flow. 2. **Gabion/crate walls and mattresses:** wire-mesh boxes filled with boulders; flexible, free-draining, cheap and widely used in Nepal. Gabion mattress aprons adjust to scour. 3. **Spurs/groynes:** short structures projecting from the bank into the stream that deflect the flow away from the bank, cause silting between them and train the channel. Types: permeable (piles, timber) and impermeable (stone, boulders); by their angle to the flow they are attracting, repelling or deflecting. 4. **Guide bunds:** earthen embankments with a stone-pitched river face and curved heads at the ends, built on both sides of the bridge approach to guide the flow centrally through the waterway (IRC:89). 5. **Retaining walls/embankment walls:** masonry, RCC or gabion walls along the bank where space is limited. 6. **Longitudinal dykes/marginal bunds:** embankments parallel to the flow that confine the river. 7. **Launching apron and toe protection:** loose stones at the toe that launch down the slope when scour occurs, protecting the foundation of the protection work. 8. **Bio-engineering:** vegetation (grass, bamboo, shrubs, trees) planted on the bank, often combined with gabions; cheap and environment-friendly for smaller flows (DoR Roadside Bio-engineering Guideline). 9. **Check dams/sills and permeable fences:** reduce velocity and trap sediment in steep streams. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The threshold zone length and level in tunnel lighting are governed mainly by: A. Colour of walls only B. Speed of traffic and stopping sight distance C. Number of luminaires only D. Width of footpath Answer: B. Vehicles must be able to stop safely, so speed and stopping sight distance set the threshold zone. (Key point: Tunnel lighting) 2. Why is a transition zone needed in tunnel lighting? A. To reduce luminance gradually so the eye can adapt from daylight B. To increase glare C. To save the cost of lamps by turning them off D. To ventilate the tunnel Answer: A. Luminance must fall gradually from the bright outside to the interior level to avoid a 'black hole' effect. (Key point: Eye adaptation) 3. Which tunnel may need only daylight and no full zone lighting? A. Very long tunnel B. Short tunnel below about 50-100 m C. Two-way heavy traffic tunnel D. Curved tunnel Answer: B. Short tunnels can be seen through, so daylight may be enough. (Key point: Short tunnels) 4. The threshold luminance is: A. A fraction of the outside (portal) luminance B. Equal to the interior reflectance C. Independent of surroundings D. Always zero Answer: A. Outside luminance depends on sky, surroundings and portal orientation; the threshold level is a fraction of it. 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