Estimation of debris flow using Curve Number analysis for The Mount Agung Volcanic event, Bali, Indonesia

After the 2017 eruption, rainfall-induced debris flow was generated within several rivers in where upstream area at Mount Agung. On 27 November 2017, a debris flow occurred in the Yeh Sah River after a rainfall intensity of 32 mm/day was monitored at the Rendang Observation Station. Estimation of rainfall-runoff-induced debris flow at this event is difficult and uncertain because this event occurred at 01.00 – 06.00 local time. This study focused on analyzing curve number value to estimate debris flow based on the 27 November 2017 event at Yeh Sah River Basin. Daily rainfall on 27 November 2017 was distributed to 6 hours using the PSA coefficient and then applied to generate runoff potential using the modified NRCS-CN for the hydrologic loss method and the SCS Unit Hydrograph for the transform method, applied in HEC-HMS 4.10. Hydrological model results are verified with the debris flow modeling using 2-D non-Newtonian features in HEC-RAS 6.3.1 with geometry based on topography 2016 and 2020. The results of the hydraulic model are validated with debris plain boundaries obtained from Google Earth imagery in 2018 and field observations in 2022. The result showed that the estimation of the debris flow event on 27 November 2017 using curve number values which consider five days of rainfall depth and land use/land cover slope of Yeh Sah River Basin was more comparable to the real condition.