Flood Susceptibility and Sediment Transport Analysis of Stromboli Island after the 3 July 2019 Paroxysmal Explosion

Omar S Areu-Rangel,Matteo Del Soldato,Nicola Casagli,Federico Di Traglia,Rosanna Bonasia

doi:10.3390/su12083268

Abstract

On 3 July 2019, Stromboli volcanic island experienced a paroxysmal explosion that triggered wildfires on vegetated areas in the south, southwestern, and eastern part of the island. This study analyzes the runoff and the transport of sediment originating from rainfall, to verify whether the vegetation loss due to wildfire changed the hydrogeological structure of the affected area and the flooding hazard. A preliminary hydrological study was conducted to analyze the superficial runoff due to rainfall. According to local planning, the hydrogeological study and flood risk assessment were carried out for the return periods corresponding to 50, 100, and 300 years. The flooding levels were calculated using the hydrodynamic module of the IBER software. The IBER sediment transport module was applied in a non-stationary regime for erosion and sedimentation analysis. The results showed that the fire caused an increase of the water discharge rates between 0.06 and 0.16 m2/s, for the 50 year return period, in the Ginostra inhabited area. The great differences of the flood levels between pre- and post-eruptive scenarios, for the highest return periods, were recognized. The analysis of sediment transport showed that rains could exert an erosion and re-sedimentation effect that would transport from 0.1 m to more than 1 m of re-mobilized material in the Ginostra area, which could cause inconvenience in the inhabited area of the island.

Highlights

Mass-wasting may cause a long-lasting hazard factor in a volcanic setting due to the persistence of non-equilibrium slope conditions [1,2,3]
IBER performs the flood hazard assessment considering the impact that the flood can have on people, buildings, cultivated fields, etc., when a certain hydraulic parameter exceeds pre-established critical conditions
For the hydrological context of Stromboli island originated after the wildfire, the flood level was not the proper parameter to be used for the assessment of the hydraulic hazard

Summary

Introduction

Mass-wasting may cause a long-lasting hazard factor in a volcanic setting due to the persistence of non-equilibrium slope conditions [1,2,3]. Explosive eruptions can severely disrupt the environment around volcanoes by depositing large volumes of erodible fragmental material or inducing wildfires on vegetated volcano slopes, altering the boundary conditions of fluvial systems, increasing the erosion rate and the drainage mass flux (water and sediment) in the affected basin (i.e., [4,5,6]), and altering river water quality [7]. Lava flows typically have a minor effect at the basin scale, dislocating rivers’ paths and producing temporary damming, changing the hydrology and sediment transport [11]. The effect of wildfires on sediment transport, streamwater chemistry, landscape disturbances, and debris flows has been studied in various geological and geomorphological contexts [12,13,14,15], currently, there is no documentation of basin disturbance due to wildfires in volcanic zones, except for an analysis of the hydrological structure and sediment transport in volcanic river systems after an eruption [16]. Areas affected by wildfires are prone to accelerated erosion, due to the increased surface runoff [17,18,19] and the development of soil water repellency [20]

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