Post-flood structural rehabilitation of a reinforced concrete bridge: a multi-tiered intervention strategy in a dynamic alluvial river system in Nepal

The Danav Bridge in Rupandehi District of western Nepal experienced compound flood-induced failure during the September 2024 monsoon, an event with an estimated return period well in excess of 200 years and a peak upstream discharge of approximately 3200 m³/s at the Tinau at Butwal gauge, nearly double the previously recorded station maximum. The failure involved coupled hydraulic, geotechnical, and structural mechanisms: aggressive pier scour, thalweg migration, a 40 cm vertical pier settlement, and a differential deck misalignment of 22.9 cm above Pier P3. Existing rehabilitation frameworks typically address these phenomena in isolation; no integrated, field-validated framework documenting their compound interaction exists for Himalayan foreland alluvial bridge contexts. This case study presents a phased, multi-tier rehabilitation strategy developed through post-event field investigation, hydraulic back-calculation from DHM gauge data, and multi-temporal satellite imagery interpretation. Interventions comprised river channel realignment, gravel-boulder packing with M15 pressure cement grouting, M20 precast plum concrete scour aprons at five piers, bearing replacement with deck jacking, and a downstream stepped check dam, all executed without full traffic closure using locally available materials. Post-intervention campaign monitoring documented an 87% reduction in pier-nose scour depth (3.8 m to 0.5 m), 95% recovery of differential deck settlement (22.9 cm to 1.1 cm), and 46% improvement in spanwise flow velocity uniformity (CoV 0.41 to 0.22). The case provides a field-validated, resource-constrained rehabilitation framework applicable to analogous alluvial river bridge infrastructure across the Himalayan foreland and comparable developing-region contexts.