Integrated flood risk modeling and nature-based solutions in the Petung Sub-Watershed, Pasuruan, East Java, Indonesia

Flood events in low-lying and estuarine areas pose persistent challenges to urban and environmental resilience, particularly in regions such as the Petung Sub-watershed, Pasuruan, East Java. Characterized by low elevation, flat topography, and tidal influence, this area is increasingly susceptible to inundation. This study aims to evaluate flood hazards and assess the effectiveness of structural mitigation strategies through integrated hydrological and hydraulic modeling. Rainfall data were analyzed using the Thiessen Polygon and Mononobe methods, with design discharge estimation performed via the Snyder Synthetic Unit Hydrograph within the HEC-HMS framework. Flood inundation modeling employed the HEC-RAS 6.2 two-dimensional unsteady flow model for 25- and 50- year return periods. These simulation results showed that downstream areas experience the most severe inundation, caused by backwater effects and channel degradation. River normalization alone was insufficient to prevent overflow, whereas the application of levees, particularly those designed using the Building with Nature (BwN) approach, demonstrated hydraulic performance by reducing flow velocity and water surface elevation. These findings the importance of integrating conventional engineering with nature-based solutions to enhance flood resilience in vulnerable watersheds.