Geographia Technica, Vol 21, Issue 1, 2026, pp. 105-126
GEOSPATIAL IDENTIFICATION OF SOIL EROSION HOTSPOTS EXACERBATING THE CATASTROPHIC FLASH FLOOD IN BALI, INDONESIA
Ni Made TRIGUNASIH 
, Moh SAIFULLOH , I Nyoman SUNARTA , Ida Bagus Putu BHAYUNAGIRI , Zulkarnain ZULKARNAIN
ABSTRACT: The catastrophic flash flood that struck the Biluk Poh Watershed in Bali on October 16, 2022, was a complex disaster. While triggered by extreme rainfall, its catastrophic scale was critically exacerbated by landscape-level factors, particularly massive sediment mobilization. This study investigates the spatial contribution of soil erosion to the flood's severity by quantifying its extent and intensity using the Universal Soil Loss Equation (USLE). The analysis integrates CHIRPS rainfall data, ALOS PALSAR DEM, vegetation indices from Sentinel-2, and field-based measurements of soil structure, texture, and organic matter. High-resolution erosion modelling (10 m) revealed soil loss ranging from 0 to 4664.78 t ha⁻¹ yr⁻¹, with a watershed average of 168.03 t ha⁻¹ yr⁻¹. The results show a landscape of stark contrasts: while 39.30% of the area experiences only Slight erosion (0–15 t ha⁻¹ yr⁻¹), a combined 55.97% suffers from Moderate (18.56%), Heavy (29.42%), and Very Heavy (7.99%) erosion. Erosion was most severe in the Penyaringan and Yeh Embang Kauh villages. These findings demonstrate that uncontrolled erosion from these specific hotspots, which are concentrated upstream and along steep river corridors, functioned as the key exacerbating factor. They supplied the massive sediment loads that blocked the main river channel, catastrophically worsening the flood event triggered by high rainfall. This study provides the first high-resolution erosion assessment for the Biluk Poh Watershed, offering critical evidence for targeted mitigation. The urgency of these findings is underscored by the catastrophic September 2025 Bali floods, which demonstrated that the sediment-driven mechanisms modeled in this study are a primary, escalating, and regional-scale threat.
Keywords: Erosion modeling; Bali flooding; Flash flood; Disaster risk reduction; Watershed management.
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