Impact of a volcanic eruption on the sediment connectivity of a Chilean river basin: the Calbuco study case

Volcanic eruptions are natural disturbances capable of introducing large quantities of sediment
into river systems as to upset the transport regime for several years. Such a disturbance can have
a strong impact on the water and sediment flows and consequently on the transport capacity.
Moreover, changes in morphological settings and land cover lead to an alteration of the sediment
connectivity within the catchment. This study aims to investigate the changes of sediment
connectivity in a catchment affected by an explosive volcanic eruption using the Index of
Connectivity (IC) with a multi-temporal approach. Potential variations were analyzed at the
catchment scale over a period of 6 years, before and after the eruption. The study area, located in
southern Chile, is the Blanco Este River basin (39,6 km²), affected by the eruption of the Calbuco
volcano (April 2015, total volume of sediment expelled of about 0,28 km³) which profoundly
changed its vegetation cover, geomorphology and hydrology. IC analyses were based on lowresolution
and freely available data (i.e., GDEM, Landsat 8 satellite images). Through supervised
image classification and field data survey, a Manning's n coefficient for overland flow is derived as
weighting factor (W) due to its suitability to represent the impedance to sediment flows in
catchments characterized by land cover variations. Following the eruption, bare soil cover on the
basin doubled (from 5% to 10% of total basin area). Consequently, the multi-temporal analysis
results in an overall increase of IC with the median value ranges from -3,58 to -3,26 in pre-eruptive
(2015) and first post-eruptive scenario (2016), respectively. The connectivity maps show that the
higher IC values (i.e. range from -1,23 to 1,66) are persistently located in three areas: at the base of
the volcanic dome, on the steepest slopes near the main channel and in a sub-basin on the right
side of the catchment. Moreover, the Difference of IC (DoIC) among different scenarios highlighted
the major variations. Such changes are found along the volcano slopes, in a flat area located in the
upper part of the basin and along the lower valley of the Rio Blanco Este. The study proposes a
useful methodology to evaluate the sediment connectivity, and its evolutionary trends, in
environments affected volcanic eruptions starting from low-resolution data and field survey. These
results may help to better define types, location and typologies of interventions to improve the
river management approaches, considering the ongoing cascading processes. This research is
funded by the Fondecyt 1200079 project.