Integrating Flood Inundation Modeling into Urban Spatial Planning: A Case Study of the Bareng River, Malang City

Abstract

Rapid urbanization in Malang City, East Java, has increased flood exposure along the Bareng River, where dense riparian settlements restrict channel conveyance. This study integrates hydrological–hydraulic modeling and spatial exposure assessment to support flood-hazard-based spatial planning. Design rainfall (2–100-year return periods) was estimated using the Log Pearson Type III distribution, and design flood discharge was derived using the Gama 1 Synthetic Unit Hydrograph. A coupled one-dimensional–two-dimensional HEC-RAS model, developed from a UAV-derived Digital Surface Model, simulated the 2-year (Q2) design flood. Results show that the channel generally conveys the Q2 discharge, but a constricted middle reach produces localized overbank flow, resulting in 2.47 ha of inundation with depths of 0.03–0.91 m. Validation against 10 years of flood records from the Malang City Disaster Management Agency (BPBD) yielded F_Bates = 0.88 and F_Di Baldassarre = 0.77. F_Bates was adopted as the primary planning indicator because it reduces underestimation of flood-prone areas, while uncertainty buffers are recommended to account for limited rainfall records and boundary-condition uncertainty. The modeled inundation affects 213 buildings, predominantly residential (85%), providing a basis for proposed flood hazard zoning to inform future revisions of the Regional Spatial Plan (RTRW) and Detailed Spatial Plan (RDTR).