Flood Hazard Assessment using Rainfall Data (1995–2025) with a Hydrograph Approach and an Eco-Geomorphic Overview in Padang City, Indonesia

Abstract Padang City experiences recurrent flooding because intense rainfall, rapid runoff from eastern catchments, low-lying floodplains, and coastal–estuarine backwater effects interact across a developed watershed continuum. Existing assessments often examine rainfall, runoff response, spatial hazard, and ecological functions separately, limiting integrated flood-risk reduction and land-use planning. This study aims to 1) analyze areal rainfall frequency for 1995–2025 using the Annual Maximum Series of 24-hour Rainfall (R24/AMS), the Log-Pearson Type III (LP-III) distribution, and the Kolmogorov–Smirnov (K–S) goodness-of-fit test; 2) generate a preliminary regional-scale runoff hydrograph using the Nakayasu Synthetic Unit Hydrograph (SUH) to characterize event magnitude and response timing under design rainfall; and 3) map flood-hazard zones using GIS-based weighted overlay scoring that integrates hydrological, geomorphological, and ecological parameters. Rainfall records indicate interannual variability and repeated ≥150 mm/24 h events, including 447 mm on 07/03/2024 and 261 mm on 25/11/2025. Design 24-hour rainfall increases from 160.87 to 334.14 mm across 2–50-year return periods, producing 4-hour intensities of 22.13–45.97 mm/h. The preliminary Nakayasu scenario yields peak discharges of approximately 135–250 m³/s with short times to peak and should be interpreted as screening evidence rather than basin-specific engineering estimates. The flood-hazard map classifies 537.97 km² (77.41%) as Safe, 96.74 km² (13.92%) as Low, 36.76 km² (5.29%) as Moderate, and 23.49 km² (3.38%) as High. Moderate- and high-hazard areas cluster in fluvial–marine lowlands such as floodplains, backswamps, coastal alluvium, river corridors, and estuaries. These findings provide a basis for Ecosystem-based Disaster Risk Reduction (Eco-DRR) through retention-space protection, river setbacks, wetland restoration, upland recharge conservation, and erosion–sediment control.