A preliminary study on the possible effect of deforestation in debris flows deposits

The note focuses on a preliminary study of the effects of deforestation on the activation of debris flow. The area of interest is located in Nottoria, at the south of Norcia (Perugia, Italy). In 2012, after an intense thunderstorm, a debris flow occurred involving the village. The mobilized debris is mainly made of blocks of limestone channelling into the main road and then pouring into the valley lateral to the village. The debris reached in some point 1m of thickness while, in some areas, deep erosion due to water runout was significant. The debris consists of calcareous pebbles of varying sizes diffused in a marly-clayey matrix. Another debris flow was recorded in 2015. The main causes have been attributed to intense rainfalls involving valleys characterized by the outcrop of limestones intensely fractured. This information has been confirmed by the inspection of the Regional Geological Map (1:10000). The figure below shows the accumulation area, the propagation channel, and the source areas of the debris flow. Highly fractured side cliffs in which vegetation has grown were observed. Debris flow polygon has been imported from IdroGEO database (ISPRA).

A recent survey has revealed the presence of tall beeches and oaks. Furthermore, evidence of deforestation is clear in the area of possible triggering of debris. It is interesting to note that different deforestation parcels were generated between 2005 and 2011, while the debris flows occurred in 2012 and 2015. Moreover, from a recent survey, there appear to be potential source zones upstream of the phenomenon mapped in the official databases (IdroGeo, ISPRA).

A preliminary study has been undertaken to highlight the effects of deforestation occurring near the trigger zone. The research aims at investigating the role of vegetation in shallow slope stability or, from another point of view, the effect of deforestation in slope instability.  In-situ investigation has been planned, consisting of a geo-electric survey. Geophysical surveying techniques has been proved to be useful for predicting matric suction values through the relationships among soil porosity, saturation degree and electrical resistivity.

 

A laboratory investigation on physical properties of debris will be carried out in the geotechnical laboratories of ISPRA and University of Perugia. The need to set up specific in situ instrumentation to detect roots geometry, typology and depth is recognized, with the goal of quantifying the hydro-mechanical effects of roots on the material shear strength through in situ tests (e.g. direct shear apparatus, corkscrew, prospecting based on sound signal propagation).

There is no proof of debris re-activation after the seismic event of 30 October 2016 M_w 6.5 Norcia earthquake, 4 years later the occurrence of the first debris flow and 1 year later the second one. This point requires further investigation. The beneficial (or not) contribution of vegetation along the debris flow channel needs to be carefully modelled in order to design safe countermeasures for the mitigation of hydrogeological and seismic risk.

 

References

IdroGEO, https://idrogeo.isprambiente.it/ (website accessed on 09/01/2024).

Umbria Region, Geological Map (Carta geologica dell’Umbria - Dataset - Open Data Umbria regione.umbria.it).

Geological Survey of Umbria Region (https://www.regione.umbria.it/-/cartografia-geologica)