Designing for Resilience: An Integrated Engineering Framework for Irrigation Infrastructure in Climate Vulnerable Himalayan Basins

Abstract

Designing resilient irrigation infrastructure under hydrological uncertainty is a critical challenge in climate-vulnerable, sediment-laden Himalayan basins. This study develops an integrated engineering framework through comprehensive assessment of Nepal's Bagmati River at Pandhera Dobhan, combining forty-one years of discharge data (1979–2019), historical and contemporary sediment records (1990–2024), and seasonal water quality monitoring. The Log-Normal distribution provided the best fit for flood frequency analysis (χ² goodness-of-fit, p < 0.05), yielding a 100-year design flood of 7,638 m³/s (95% CI: 6,110–9,166 m³/s). Applying a recommended climate safety margin of 15–20% to address non-stationarity raises the practical design value to 8,800–9,200 m³/s. Mann-Kendall analysis revealed no significant trend in annual peak flows (τ = 0.08, p = 0.32) but identified increasing pre-monsoon flows (τ = 0.22, p = 0.03; Sen's slope: +15.3 m³/s/yr). The flow duration curve establishes Q₉₅=11 m³/s as the environmental flow requirement. The river exhibits extreme seasonality, with suspended sediment concentrations ranging from 40.5 ppm (±12 SD) in the dry season to monsoon peaks of 2,214 ppm (±458 SD) in a supply-limited transport regime (R² < 0.3). Post-2015 Gorkha earthquake monsoon sediment concentrations increased by 48.7% relative to the 1990–1997 baseline (p = 0.008). From particle size analysis, D50 = 0.4 mm was determined for the settleable sediments. The phosphate values measured were 3.5 mg/L (±0.7 SD), which is very much higher than the 0.1 mg/L eutrophication limit value. The electrical conductivity stayed lower than 200 μS/cm, meeting FAO. These interconnected constraints are synthesized into a quantitative "Resilience Trilemma" framework, from which specific design parameters are derived: a settling basin of 3,347–6,300 m² surface area (overflow rate: 0.77 cm/s), Manning's n of 0.0225 for unlined canals, freeboard of 0.8 m for flows exceeding 10 m³/s, and a design duty of 0.88 lps/ha. Validated against operational data from the Bagmati Irrigation Project, this framework provides a transferable, climate-smart model for irrigation infrastructure design in rapidly changing mountain basins worldwide.