--- title: "Bridge site factors; river bank protection structures" subject: "Transportation Engineering II" chapter: 5 key: "semester_7/transportation_engineering_ii:bridge-site-factors-river-bank-protection" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/bridge-site-factors-river-bank-protection site: BCT Engineering --- # Bridge site factors; river bank protection structures Transportation Engineering II · Chapter 5: Introduction to Bridge and Tunnel Engineering - Asked in: 2074 Asoj, 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/bridge-site-factors-river-bank-protection - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-5 ## Question What are the factors affecting the choice of location of bridge site? Discuss the river bank protection structures. ## Short answer Bridge site choice is governed by stream stability, flood and waterway data, foundation conditions below scour depth, alignment, clearances, economy, geology, seismicity, and construction access. Bank protection structures include revetments, gabion walls, spurs, guide bunds, retaining walls, dykes, and launching aprons. ## Answer #### Factors affecting the choice of bridge site A bridge site is chosen so that the bridge is safe and economical, and serves both the road alignment and the river with the least disturbance. Factors to consider (IRC:5 / IRC:SP:13 and Nepal DoR bridge practice): 1. **Stream/river behaviour:** a straight, stable reach with a well-defined, non-meandering channel; avoid bends, confluences, and braided and aggrading reaches. 2. **Waterway and flood data:** a narrow, stable crossing with high banks; adequate waterway for the design flood (usually a 100-year return period for major bridges as per IRC:5 and DoR, 50-year for small bridges and culverts as per IRC:SP:13); reliable HFL, discharge and velocity records. 3. **Foundation conditions:** firm strata or rock at a reasonable depth; the foundation must be taken below the scour depth (Lacey's regime theory, IRC:78) at an affordable cost; avoid soft, liquefiable or landslide-prone soils. 4. **Approaches and alignment:** the bridge should be square (normal) to the river flow; approach roads should be straight, of limited height and not on unstable slopes. For major bridges the road alignment is adjusted to suit the best bridge site, while for minor bridges and culverts the bridge follows the road alignment. 5. **Navigation and clearance:** vertical and horizontal clearance for boats, floating logs or boulders; vertical clearance between HFL and the soffit of 150 mm to 1.5 m depending on the discharge (IRC:5), with more in Nepal's boulder-carrying rivers. 6. **Economy:** total cost of bridge, approaches and river training works; nearness of construction materials and labour, and access for equipment. 7. **Geology and seismicity:** avoid active faults and steep gorges with rockfall; follow the seismic provisions of NBC 105 and IRC:6. 8. **Existing structures and utilities:** distance from other bridges, dams and weirs (backwater/draw-down effects), settlements, and land acquisition. 9. **Environmental and social aspects:** minimum afflux, no damage to property upstream, aesthetics, and environmental clearance requirements. 10. **Construction convenience:** space for temporary works and diversion, and a low-flow period for working. #### River bank protection structures 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). Main methods: 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 (pile, timber) and impermeable (stone, boulder); attracting, repelling or deflecting, depending on their angle. 4. **Guide bunds:** earthen embankments with a stone-pitched river face, curved at the upstream end, on both sides of the bridge waterway to guide the flow centrally through it (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 to 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. Which river reach is preferred for a bridge site? A. A meandering reach with bends B. A confluence of two streams C. A straight, stable reach with a well-defined channel D. A braided, aggrading reach Answer: C. A straight stable reach gives predictable flow; bends, confluences and braided or aggrading reaches should be avoided. (Key point: Stream behaviour) 2. How should the bridge be placed relative to the river flow? A. Square (normal) to the flow B. Heavily skewed to the flow C. Parallel to the flow D. Angle does not matter Answer: A. A bridge normal to the flow needs the shortest length and the least obstruction and scour problems. (Key point: Alignment to flow) 3. Which foundation condition is desirable at a bridge site? A. Soft liquefiable soil at shallow depth B. Firm strata or rock at reasonable depth with affordable scour depth C. Landslide-prone slopes D. Very deep soft clay Answer: B. Firm strata at reasonable depth and manageable scour depth (Lacey's regime theory) keep the foundation safe and economical. (Key point: Foundation and scour) 4. What do spurs (groynes) do? A. Project from the bank to deflect flow away and cause silting between them B. Run parallel to the flow to confine the river C. Only support the bridge deck D. Carry the approach road Answer: A. Spurs project into the stream, deflect flow from the bank and train the channel. (Key point: Spurs/groynes) ## Related in Transportation Engineering II - [Bridge components sketch; river bank protection](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-sketch-components-river-bank-protection) — Transportation Engineering II, chapter 5 - [Tunnel lighting factors; river bank protection methods](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-lighting-factors-river-bank-methods) — Transportation Engineering II, chapter 5 - [River bank protection; tunnelling in hard soil](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Ariver-bank-protection-tunnelling-hard-soil) — Transportation Engineering II, chapter 5 - [Bridge site factors and tunnel drainage](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-tunnel-drainage) — Transportation Engineering II, chapter 5 - [Bridge site factors and components with sketches](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-components-sketches) — Transportation Engineering II, chapter 5 - [Bridge site factors; importance of tunnel ventilation](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-tunnel-ventilation-importance) — Transportation Engineering II, chapter 5 ## Related in other subjects - [Factors affecting choice of BIL](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Abil-choice-factors) — Transmission and Distribution System Design, chapter 3 - [Bheri river suspension bridge collapse](https://www.bctengineer.com/question.md?key=semester_8%2Fengineering_professional_practice%3Abheri-suspension-bridge-collapse) — Engineering Professional Practice, chapter 7 - [Building near river bank; stop construction?](https://www.bctengineer.com/question.md?key=semester_8%2Fengineering_professional_practice%3Abuilding-on-river-bank-flood) — Engineering Professional Practice, chapter 7 - [Dry and wet river intakes; unstable bank](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Awet-dry-intake-unstable-bank) — Water Supply Engineering, chapter 5