Bridge site factors; river bank protection structures

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.