Gypsum's role in the Friuli Venezia Giulia sinkholes

A sinkhole, also known as a sink, shake hole, swallow hole, swallet, doline, is a natural depression or hole in the surface topography caused by the removal of soil or bedrock, often both, by water. Sinkholes may vary in size from less than a meter to several hundred meters both in diameter and depth. These morphologies may be formed gradually or suddenly, and are found worldwide. In the mountain areas of the Friuli Venezia Giulia Region in the northern part of Italy (Figure 1), several are the instability phenomena caused by the dissolution of gypsum deposits. The evolution of these phenomena is usually progressive but slow, sometimes can be catastrophic. Among the phenomena that evolve in a slow and progressive way, it is possible to include: A) erosions and dissolutions at the foot of the slope with a consequent trigger of instability conditions B) the dissolution of gypsy clasts within alluvial deposits or loose moraine can cause the depletion of geotechnical characteristics C) the dissolution of gypsum or limestone bedrocks covered by loose deposits is able to create vertical subsidence (solution dolines). Figure 1: In red the studied area. Among the second type of phenomena, the ones that evolve in a sudden way, it is possible to recognize the genesis of collapse dolines (collapse doline) and the subsequent formation of alluvial dolines (suffosion dolines) or of subsidence in the bedrock (subsidence doline). Sinkholes can also form from human activities, not only from natural flush processes: sinkholes may occur in urban areas due to water main breaks or sewer collapses when old pipes give way. They can also occur from the over pumping and extraction of groundwater and subsurface fluids. Anyway, independently from their genesis, these phenomena can be extremely damaging to infrastructures and dangerous for the populations. With this contribution we propose to assess the state of evolution of some sinkholes of great geomorphological interest, some of them are already described in literature and related to the presence of gypsum bedrock (Marinelli, 1897 and 1916; Gortani, 1904 and 1965; Cucchi & Piano , 2002 and 2003). The interest area is a stretch of several kilometers of the Tagliamento Valley, between the villages of Ampezzo and Enemonzo, where there are frequent points of absorption associated with collapse dolines, or better alluvional doline or subsidence doline large and deep collapsed in the cemented fluvial-glacial deposits and in the recent alluvial deposits. The aim of the study is to check the stage of development of the sinkholes in the Alta Val Tagliamento and to realize a detailed distribution map of these phenomena.