Multi-Scenario Modelling Framework for Afforestation Strategies to Mitigate Runoff and Flooding Across Storm Return Periods in Tropical Large River Basins: The Upper Citarum Case

Flooding is a recurring natural disaster in Indonesia, particularly in the Upper Citarum River Basin, causing significant damage to lives and property. This study examines the impact of afforestation on mitigating runoff, peak discharge, and flood risks under various storm conditions. Hydrological (HEC-HMS) and hydraulic (HEC-RAS) models were applied to simulate six afforestation scenarios: No Forest (NF), Medium Afforestation (MF), afforestation based on Bandung’s 2029 spatial plan (F-2029), Riverbank Afforestation (RF), spatial plan for 2029 (SP-2029), No Built-up (NB), and Land Cover 2021 (LC-2021) as the baseline. Six rainfall return periods (RP) (2, 5, 10, 20, 50, and 100 years) were generated using 32 years of maximum daily rainfall data from 11 rain stations. The results indicated that land cover changes significantly affected runoff, with urbanization increasing flood risks and afforestation improving water retention. Riverbank afforestation (RF) and reduced built-up areas (NB) were the most effective in reducing flood risks, while reduced forest cover (NF) and urban expansion (SP-2029) increased runoff and flood potential. Model validation with historical flood data revealed strong consistency in identifying flood-prone areas, particularly in Margahayu, Bojongsoang, Dayeuhkolot, and Baleendah. The prevailing flood pattern was observed in the 2- and 5-year RP scenarios, indicating frequent but less severe flood events. These findings underscore the importance of afforestation and land cover management for flood mitigation and offer valuable insights for policymakers to enhance resilience to flooding in the Upper Citarum Basin.