Debris Flow Phenomena: A Short Overview?

Summary: On the day of 29 August 2003 in the extreme northern part of Italy, the Friuli-Venezia Giulia region was invested by a violent and intense rainfall that caused several instability phenomena. Along the sides of the Val Canale valley were mobilized over a thousand of landslides, most of whom were of first generation. The rainfall started at midnight, firstly affecting the areas belonging to the upper sector of the mountains around Cucco, Malborghetto and Ugovizza, then it gradually moved downwards with increasing intensity (Calligaris et al., 2010). A total value of 293 mm rainfall was recorded by the Pontebba rain gauge from 02h00 to 18h00. Around 18h30, some impressive waves plunged with unexpected violence, power and noise, an avalanche of water, big rocks and trunks crashed against courtyards, houses and went inside windows (Russo, 2003). This event caused the death of two people, 300 lost their homes, 260 buildings were damaged and substantial damages occurred to infrastructures that remained out of action for several days. The event was so extreme and particular that the return time has a considerable variation in its value depending on the period: between 1 and 24 h, the time of delivery is between 50 and 100 years; for 12 h it is between 200 and 500 years, while, for a period from 3 to 6 h, return period varies between 500 and 1000 years (Borga et al., 2007; Calligaris et al., 2010). This area has been chosen as test site due to the important amount of data availability. Many studies have been realized to characterize the debris flow occurred. The Geological Survey of FVG Region had the necessity to better understand the occurred phenomena in order to prevent future disasters and to proceed to a better and more coscientious territorial planning. In this context, many approaches were used. Back analysis simulation thought commercial and innovative software permitted to delimit the flooded areas. The debris flow is often considered to be a mixture of viscous slurry, consisting of finer grain sizes and water, and coarse particles (Scotto di Santolo, 2008). The volume and the composition of the mixture are the main factors that contribute to determine the hazards associated with such phenomena, since they govern the mobility and impact energy of the debris (Iverson, 1997; Jakob, 2005). During the last years, several simulation models and approaches have been implemented (Cesco Bolla, 2008; Pirulli, 2005; Rickenmann, 1999) and created to reconstruct a debris-flow phenomena, but a believable scenario can be obtained only by resorting to real parameters that are suitable to characterise the involved material (Sosio et al., 2006). Thus, it is necessary to calibrate those computational codes through back-analysis simulations and laboratory analysis (Tecca et al., 2006). The input variable needed by the codes are so aleatory that is a strongly felt need to isolate each one of them and to go deep inside their meaning. Rheological parameters of viscous debris flows are one of these variables; they are influenced by a great amount of factors and are therefore extremely difficult to estimate. Viscosity and yield stress are the one that define the debris flow behaviour. For these reasons laboratory rheometer analysis have been realized in order to better characterize them, to apply the obtained results at the available codes and to proceed to new outlines of the invested areas

Evaporite sinkholes in the Friuli Venezia Giulia Region (NE Italy)

4noreservedmixedStefano, Devoto; Chiara, Calligaris; Luca, Zini; Franco, CucchiDevoto, Stefano; Calligaris, Chiara; Zini, Luca; Cucchi, Franc

Evaporite Dissolution Rate through an on-site Experiment into Piezometric Tubes Applied to the Real Case-Study of Quinis (NE Italy)

The present paper deals with a field experiments on evaporite rock samples and groundwater investigations in the Quinis test site, a hamlet of the Enemonzo municipality in NE Italy, were sinkholes occurred in the past and are still occurring causing severe damage to the existing infrastructures. The area is characterised by a Carnian evaporitic bedrock made of gypsum and anhydrite mantled by alluvial and colluvial deposits. In order to evaluate the loss of weight and volume of the subcropping evaporites as responsible for sinkholes, a field-experiment was carried out. Inside seven piezometers, at dierent depths, evaporitic rock samples were exposed to the naturally occurring variable climatic conditions such as degree of humidity, dierent air flow and hydrodynamic. The rock samples were installed at the beginning of April 2017 in the dry sections of piezometric tubes, in the vadose zone and in the phreatic zone. Data related to water level fluctuations were recorded by using data-logger devices and highlight significant changes in the water table. After 13 months of data recording (May 2018), rock samples were removed, reweighted and the volume loss measured. In addition, water from piezometer-experiment, representative of the groundwater circulation, were collected at dierent depths. The obtained results indicate that rock sample reduction is dependent on the hydrological regime and water chemistry and not on the number of days during which the samples remained submersed. In particular, the water geochemistry highlights the possible role in gypsum/anhydrite dissolution due to NaCl water admixing in a complex scenario. In additional, the geochemical data highlight the occurrence of some potentially toxic elements (As, Fe, Mn) at concentrations of concern in some water. This approach represents a novel contribution in the study of karst hazard in evaporites adding a tile to the knowledge of the fast evolutionary processes which cause sinkhole formation

Geophysical and geomorphological integrated investigations to identify mantled sinkholes

In a mantled evaporite karst system, sinkholes are not always easily identifiable. These phenomena are not deeply studied in the Italian peninsula, because they do not cause so frequent and severe damages as for example in other countries like the United States and Spain. However, they are quite frequent in specific areas where gypsum is exposed or mantled, especially in some alpine valleys where evaporites acted as a tectonic lubricant during the main alpine orogenetic compression phases. This is the case of some portions of the north-eastern corner of Italy where, along the high Tagliamento valley, there are several villages built in areas where the alluvial deposits of the Tagliamento River created thick terraces over a buried evaporitic bedrock. In the past, within this area, several sinkhole events were recorded, few of them occurred below or close to the built houses and the farmers usually refilled the voids in order to continue to exploit the land, while ignoring the actual cause of those sudden events. This is the case of Quinis village, a hamlet of Enemonzo, where the farming activities masked for a period the precursor signs of the sinkhole occurrences. The outline of the sinkhole events appear to be particularly difficult in an urbanized environment where the geomorphological surveys do not guarantee a recognition of their characteristic shapes, while trenching activities are not always possible and are of high impact. For these reasons, geophysical techniques are a valid approach in the study of these phenomena. In an area under cultivation, just outside from the inhabited zone, an integrated geophysical investigations (2D, 3D electrical resistivity tomography, and gravimetric measurements) have been performed in order to image the subsurface below a smoothed surface depression which represents a possible hint of a mantled sinkhole. In the 3D acquisition we used a \u201chorseshoe\u201d electrodic geometry, with 72 electrodes having a constant spacing equal to 2m and located all around the resistivity anomaly previously recognized along a 2D resistivity profile crossing the surface depression. A gravimetric profile with 10m-spaced data was at first realised along the 2D resistivity profile. Later 74 additional measurements were collected within a 5m grid centred in correspondence of the depressed area. The geophysical investigations permit to locate an approximately 15 m wide zone vertically extending for tens of meters. The evaporitic bedrock in the sinkhole area lies at a depth exceeding 100 m and is masked in the electrical data by an aquifer reaching a depth of about 45 m below the surface. The experience realised in the area permitted to identify the best approach to investigate these type of buried phenomena, highlighting that the 3D electrical tomography is a useful tool to image the lateral extension, the depth and the shape of the sinkhole

Evaporite Dissolution Rate through an on-site Experiment into Piezometric Tubes Applied to the Real Case-Study of Quinis (NE Italy)

The present paper deals with a field experiments on evaporite rock samples and groundwater investigations in the Quinis test site, a hamlet of the Enemonzo municipality in NE Italy, were sinkholes occurred in the past and are still occurring causing severe damage to the existing infrastructures. The area is characterised by a Carnian evaporitic bedrock made of gypsum and anhydrite mantled by alluvial and colluvial deposits. In order to evaluate the loss of weight and volume of the subcropping evaporites as responsible for sinkholes, a field-experiment was carried out. Inside seven piezometers, at different depths, evaporitic rock samples were exposed to the naturally occurring variable climatic conditions such as degree of humidity, different air flow and hydrodynamic. The rock samples were installed at the beginning of April 2017 in the dry sections of piezometric tubes, in the vadose zone and in the phreatic zone. Data related to water level fluctuations were recorded by using data-logger devices and highlight significant changes in the water table. After 13 months of data recording (May 2018), rock samples were removed, reweighted and the volume loss measured. In addition, water from piezometer-experiment, representative of the groundwater circulation, were collected at different depths. The obtained results indicate that rock sample reduction is dependent on the hydrological regime and water chemistry and not on the number of days during which the samples remained submersed. In particular, the water geochemistry highlights the possible role in gypsum/anhydrite dissolution due to NaCl water admixing in a complex scenario. In additional, the geochemical data highlight the occurrence of some potentially toxic elements (As, Fe, Mn) at concentrations of concern in some water. This approach represents a novel contribution in the study of karst hazard in evaporites adding a tile to the knowledge of the fast evolutionary processes which cause sinkhole formation

Effect of s strong rainstorm on the hydrodynamics of the Puerto Princesa underground river (Palawan, Philippines)

Fascinating and fragile environments as are the underground estuaries, need to be studied, understood and protected for present and future generations. Even if wide and abundant bibliography related to tides and their behaviour with respect to the external estuaries is available, none dealed with the estuary caves and the related hydrogeology. This paper aims to partially fill this gap presenting a preliminary study done at the Puerto Princesa Underground River (PPUR), in the Palawan Island (Philippines). The data was collected during the last expedition (November 2016) organised by La Venta, in which some of the authors took part. During the survey, the cave has been instrumented with in continuous diver data-logger devices (CTD) recording temperature (T), electrical conductivity (EC) and water level fluctuations. Longitudinal and vertical water hydrogeological profiles of the cave were realised with the aim of understanding the dynamics of the waters during different hydrogeological regimes in combination with salt water intrusion. In addition, a bathymetric profile was done to better identify the point where to realise the vertical logs. A significant rainstorm occurred during the expedition, lasting for approximately 12 hours resulting in 80 mm of rain, and its effects gave the researchers the opportunity to analyse the hydrodynamics of the flowing waters during a flood. In normal flow conditions, the longitudinal logs highlighted the presence of freshwater inlets and the vertical logs testified to a clear stratification of the waters (freshwaters at the surface and brackish waters at the bottom). During floods, the EC and T data showed evidence of a fast substitution of the resident waters.Key words: anchialine caves, sea water intrusion, hydrodynamics, karst springs, PPUR, Philippines.Učinek močnega naliva na hidrodinamiko podzemske reke Puerto princese (Palawan, Filipini)Očarljivo in občutljivo okolje, kot so podzemni estuariji, je treba preučevati, razumeti in zaščititi za sedanje in prihodnje generacije. Četudi imamo na voljo široko in bogato bibliografijo o plimovanju in delovanju plimovanja v zunanjih estuarijih, nobeno delo ne obravnava estuarskih jam in s tem povezane hidrogeologije. Namen tega članka je delno zapolniti to vrzel in predstaviti predhodno študijo na podzemni reki Puerto Princesa (PPUR) na otoku Palawan (Filipini). Podatki so bili zbrani med zadnjo ekspedicijo (novembra 2016), ki jo je organizirala La Venta, v kateri so sodelovali tudi nekateri avtorji. V raziskavi je bila jama opremljena z zveznimi merilci (CTD) temperature (T), električne prevodnosti (EC) in nihanja nivojev vode. Vzdolžni in navpični hidrogeološki vodni profili jame so bili izmerjeni, da bi razumeli dinamiko voda med različnimi hidrogeološkimi nivoji in med vdori morske vode. Poleg tega je bil opravljen tudi batimetrični profil, da bi lažje določili, kje izvesti navpične meritve. Med ekspedicijo se je zgodil močan naliv, ki je trajal približno 12 ur, padlo je 80 mm dežja, njegov učinek pa je raziskovalcem omogočil analizo hidrodinamike tekočih voda med poplavo. V običajnih vodostajih so vzdolžne meritve pokazale na dotoke sveže vode, vertikalne meritve pa so pokazale  jasno stratifikacijo lastnosti vode (sveža voda na površini in brakična voda na dnu). Med poplavami so podatki EC in T potrdili hitro zamenjavo vode.Ključne besede: anhialine jame, vdor morske vode, hidrodinamika, kraški izviri, PPUR, Filipini.