Quantifying mangrove effectiveness for tsunami reduction under scenario variations in Denpasar, Bali

Abstract

Tsunamis are among the most devastating coastal hazards, particularly in Indonesia, which sits along active subduction zones. Denpasar, Bali, faces significant tsunami risk from potential megathrust earthquakes. Mangrove forests act as natural shields, dissipating incoming wave energy. Yet their effectiveness under varying tsunami intensities and vegetation conditions remains insufficiently quantified, especially for Denpasar. This study evaluates mangrove effectiveness in tsunami mitigation across variations in earthquake magnitude, vegetation density, and belt width. Numerical simulations were run using the TUNAM1-N2 model. Mangrove density was represented through adjustments to Manning's roughness coefficient and drag coefficient, while multiple belt widths were tested to assess their role in reducing tsunami impact. Results show mangroves substantially reduce inundation extent, with mitigation performance improving as vegetation density and belt width increase. Under extreme tsunami conditions, however, reduction efficiency declines once wave energy exceeds the vegetation's dissipative capacity. These findings underscore the importance of optimizing mangrove density and belt width as key design parameters for nature-based tsunami risk reduction in vulnerable coastal areas.