Optimizing flood risk management through nature-based solutions (NBS): a case study in the Attanagalu Oya Basin, Sri Lanka

Abstract

Floods are among the most frequent and destructive natural disasters in Sri Lanka, causing widespread damage to life, property, and infrastructure. This study evaluates the effectiveness of Nature-Based Solutions (NBS) in mitigating flood risks in the Attanagalu Oya Basin, Sri Lanka. Hydrological and hydraulic simulations were conducted using the HEC-HMS and HEC-RAS models. The HEC-HMS model was calibrated and validated with high reliability (NSE = 0.779 for calibration and 0.775 for validation). Due to the unavailability of observed flood depth data, the HEC-RAS model was validated using the flood extent shapefile from the 2010 flood event, through visual comparison with simulated inundation maps. Results showed that NBS including rainwater harvesting systems, leaky dams, and permeable pavements, reduced peak discharge by 21%, 14%, 13%, and 10% for 5-, 10-, 25-, and 50-year return periods, respectively. Flood inundation area under a 50-year event decreased by 32.5% (from 40 km² to 27 km²). These interventions enhanced infiltration, reduced surface runoff, and delayed peak flows. The study identifies NBS as an effective and sustainable approach that supports traditional flood control, future flood risk management, and urban planning in Sri Lanka’s flood-prone regions.