River route location considerations

A river route (valley route) is a highway alignment that follows a river valley instead of crossing the ridges. It is common in Nepal's hills because the river banks give a gentle, continuous gradient, although the road must cross many tributaries.

Basic considerations in locating it

1. Above flood level - the road formation should be kept above the Highest Flood Level (HFL) with adequate freeboard (about 0.6-1.0 m); avoid sections that are frequently flooded.
2. Bank stability - choose the stable bank (the inner/convex bank of a bend, or a rocky or terraced bank); avoid reaches where the river is cutting, shifting or undercutting the bank (the outer/concave bank of a bend). Provide bank protection (gabions, spurs, retaining walls).
3. Gentle ground and gradient - follow a gradient close to that of the river, avoiding unnecessary ups and downs. Good valley slopes make construction easy.
4. Geology and slope stability - avoid landslide, rock-fall and debris-flow areas, old slips and weak rock; cut as little as possible.
5. Crossing of tributaries - cross them at the upper, narrow, stable reaches at the best bridge site (rock abutments, a narrow channel) and at right angles to the flow; minimise the number of crossings.
6. Choice of bank - choose the side with fewer tributaries and fewer main-river crossings, while still serving the settlements.
7. River training - the road should not constrict the river; keep a safe distance from river meanders.
8. Material and economy - local aggregates from the riverbed, low earthwork, least cost over the life cycle, minimum land acquisition, and continued access to settlements.
9. Environment and flood plains - avoid prime agricultural land, protect the river ecology, and follow the NRS 2070 and IRC:SP:48 environmental requirements.