Acoustic module of the Acquabona (Italy) debris flow monitoring system

International audienceMonitoring of debris flows aimed to the assessment of their physical parameters is very important both for theoretical and practical purposes. Peak discharge and total volume of debris flows are crucial for designing effective countermeasures in many populated mountain areas where losses of lives and property damage could be avoided. This study quantifies the relationship between flow depth, acoustic amplitude of debris flow induced ground vibrations and front velocity in the experimental catchment of Acquabona, Eastern Dolomites, Italy. The analysis of data brought about the results described in the following. Debris flow depth and amplitude of the flow-induced ground vibrations show a good positive correlation. Estimation of both mean front velocity and peak discharge can be simply obtained monitoring the ground vibrations, through geophones installed close to the flow channel; the total volume of debris flow can be so directly estimated from the integral of the ground vibrations using a regression line. The application of acoustic technique to debris flow monitoring seems to be of the outmost relevance in risk reduction policies and in the correct management of the territory. Moreover this estimation is possible in other catchments producing debris flows of similar characteristics by means of their acoustic characterisation through quick and simple field tests (Standard Penetration Tests and seismic refraction surveys)

Some considerations on the application of the FLO-2D model for debris flow hazard assessment

The two-dimensional flood routing model FLO-2D, with the capabilities of simulating non-Newtonian sediment flows, is becoming more widely used to route debris flows over alluvial fans of alpine torrents and to delineate hazard areas of inundation. Nevertheless the different applications described in literature are not comparable, because they base on different assumptions related to the numerous parameters governing the debris flow simulation. This paper reports the applications of the FLO-2D computer model and discusses the assumptions made for the replication of two well documented debris flow events at Fiames (Belluno) and Rio Dona (Trento) in the Eastern Dolomites. The simulations results are consistent with the field observations in terms of maximum flow depths and extent of the inundated areas in the two study sites. These two applications have enhanced the experience for the requirements of model input data in small alpine catchments, in particular the assessment of the main rheological parameters of flows, that are crucial in the design of debris flow countermeasures; they lead to propose a simplification in choosing the FLO-2D rheological parameters; and facilitate the results comparison and their interpretation among different FLO-2D simulations

Field observations of the June 30, 2001 debris flow at Acquabona (Dolomites, Italy)

On June 30, 2001, a debris flow occurred in the Acquabona Creek, a small catchment of the Eastern Dolomites, Italy. This debris flow originated shortly after an intense rainstorm, characterised by a peak intensity of 8.6 mm per 10 min; it transported a total volume of 30,000 m3, consisting of poorly sorted gravely sand with boulders up to 3 m in diameter. The sediment erosion yield rate reached as high as 20 m3/m. In order to verify the accuracy of the field measurements, the total volume of debris deposits was calculated using three different topographic measurement techniques: 3D laser scanning, terrestrial stereo-photogrammetry survey and total topographic station survey. Data collected so far show that no debris flow has occurred at Acquabona with a rainfall intensity lower than 4.6 mm per10 min. Channel cross section measurements indicate that debris flow velocity ranges from 2.0 to 7.2 m/s along the lower flow channel and peak discharge ranges between 22 and 300 m3/s. Field estimates of the rheological properties indicate a yield strength ranging from 2,088 to 5,313 Pa and Bingham viscosity between 70 and 337 Pa.s. It is not still possible to identify a rainfall intensity and amount threshold for debris flow triggering, but the data so far collected emphasise that debris flows do not occur with a rainfall intensity lower than 4.6 mm per 10 min

Lateral Spread: From Rock to Soil Spreading

The main conceptual and terminological issues related to lateral spreading are presented and accompanied by a brief outline of the state-of-the-art on the topic. Then the geomorphic features related to the two main types of spreading (rock spreading and soil spreading) are illustrated, with reference to the geological conditions in which they take place, as well as to their causes and evolution. The role of monitoring and modelling is then considered as useful tools to better understand the mechanical behavior of unstable slopes affected by such processes. Finally, some considerations on the hazard and planning implications are provided

Comparison of numerical models of two debris flows in the Cortina d'Ampezzo area, Dolomites, Italy

The accurate prediction of runout distances, velocities and the knowledge of flow rheology can reduce the casualties and property damage produced by debris flows, providing a means to delineate hazard areas, to estimate hazard intensities for input into risk studies and to provide parameters for the design of protective measures. The application of most of models that describe the propagation and deposition of debris flow requires detailed topography, rheological and hydrological data that are not always available for the debris-flow hazard delineation and estimation. In the Cortina d\u2019Ampezzo area, Eastern Dolomites, Italy, most of the slope instabilities are represented by debris flows; 325 debris-flow prone watersheds have been mapped in the geomorphological hazard map of this area. We compared the results of simulations of two well-documented debris flows in the Cortina d\u2019Ampezzo area, carried on with two different single-phase, non-Newtonian models, the one-dimensional DAN-W and the twodimensional FLO-2D, to test the possibility to simulate the dynamic behaviour of a debris flow with a model using a limited range of input parameters. FLO-2D model creates a more accurate representation of the hazard area in terms of flooded area, but the results in terms of runout distances and deposits thickness are similar to DAN-Wresults. Using DAN-W, the most appropriate rheology to describe the debrisflow behaviour is the Voellmy model. When detailed topographical, rheological and hydrological data are not available, DAN-W, which requires less detailed data, is a valuable tool to predict debris-flow hazard. Parameters obtained through back-analysis with both models can be applied to predict hazard in other areas characterized by similar geology, morphology and climate

Field observations of a debris flow event in the Dolomites

A debris flow event occurred in June 1997 in the Dolomites (Eastern Alps, Italy). The phenomenon was directly observed in the field and recorded by a video camera near its initiation area. The debris flow originated shortly after an intense rainstorm (25 mm in 30 min) whose runoff mobilised the loose coarse debris that filled the bottom of the channel in its upper part. The analysis of the steep headwater basin indicates a very short concentration time (9-14 min) that fits the quick hydrological response observed in the field. The debris flow mobilisation was not contemporaneous with the arrival of the peak water discharge in the initiation area probably due to the time required for the saturation of the highly conductive channel-bed material. Channel cross-section measurements taken along the flow channel indicate debris flow peak velocity and discharge ranging from 3.1 to 9.0 m/s and from 23 to 71 m3/s, respectively. Samples collected immediately after deposition were used to determine the water content and bulk density of the material. Channel scouring, fines enrichment and transported volume increase testify erosion and entrainment of material along the flow channel. Field estimates of the rheological properties based on open channel flow of Bingham fluid indicate a yield strength of 5000 ± 400 Pa and relatively low viscosity (60-326 Pa s), probably due to a high percentage of fines (approx. 30%)

Three-dymensional dynamic topographic survey of granular flows using photogrammetric techniques.

In order to better characterize the behavior of fast dry granular mass movements, such as dense snow or rock avalanches, laboratory analyses have been undertaken in a model scale (Froude similarity, geometrical scale of the order of 50:1 - 100:1). To this end, an experimental flume, consisting of two planes with adjustable inclination, has been used: the upstream plane, with slope varying from 15% to 60%, simulates the flowing zone and the downstream plane, with slopes ranging from 0% to 30%, simulates the deposition zone. The experimental apparatus has been completed in order to obtain a three-dimensional dynamic topographic survey of the sliding free surface, using photogrammetric techniques. The experiments are being performed using a maximum of eight industrial digital video-cameras. A full photogrammetric camera calibration process has been first conducted in order to define the parameters of inner orientation of the cameras and of the objective lenses distortion, in order to reduce the uncertainties in the collinearity equations. The recording time is digitally triggered at the same time to all the cameras. A dedicated acquisition code, based on LabView software, has been realized to achieve the best accuracy in the frames synchronization. The surface is reconstructed, at different times, using the frames taken at the same instant from the different video-cameras. The photogrammetric analysis has being performed by means of commercial dedicated software. As a final product of the research it is expected the tuning of an automatic procedure for the photogrammetric analysis of the series of frames taken in order to describe the dynamic evolution of the motion of a granular mass driven by the gravity and the limits of the proposed techniques. The dynamic three-dimensional reconstruction of the free surface of the sliding granular mass will be used in the calibration process of granular mathematical-numerical models. The comprehension and the estimation of the rheological parameters to be used in the numerical simulations is one of the most important aspect of the research, in order to allow greater reliability in the evaluation of the avalanche and landslide risk at prototype scale

Debris flow monitoring in the Acquabona watershed on the Dolomites (Italian Alps)

In 1997 a field monitoring system was installed in Acquabona Creek in the Dolomites (Eastern Italian Alps) to observe the hydrologic conditions for debris flow occurrence and some dynamic properties of debris flow. The monitoring system consists of three remote stations: an upper one located at the head of a deeply-incised channel and two others located downstream. The system is equipped with sensors for measuring rainfall, pore pressures in the mobile channel bottom, ground vibrations, debris flow depth, total normal stress and fluid pore-pressure at the base of the flow. Two video cameras record events at the upper channel station and one video is installed at the lowermost station. During summer 1998, three debris flows (volumes from less than 1000 m3 up to 9000 m3) occurred at Acquabona. The following results were obtained from a preliminary analysis of the data: 1) All of the flows were triggered by rainfalls of less than 1 hour duration, with peak rainfall intensities ranging from 4.8 to 14.7 mm / 10 minute. 2) Debris flows initiated in several reaches of the channel, including the head of the talus slope. 3) The initial surges of the mature flows had a higher solid concentration and a lower velocity (up to 4 m/s) than succeeding, more dilute surges (more than 7 m/s). 4) Total normal stress and pore fluid pressures measured at the base of the flow (mean depth about 1.1 m) were similar (about 15 kPa), indicating a completely liquefied flow. 5) Peak flows entrained debris at a rate of about 6 m3/m of channel length and channel bed scouring was proportional to the local slope gradient and was still evident in the lower channel where the slope was 7°. © 2000 Elsevier Science Ltd. All rights reserved

Failure mechanisms and runout behaviour of three rock avalanches in thye North-Eastern Italian Alps

The nature of the physical processes that trigger rockslope insatbility is, today, understood even if with some difficulties and simplifying hypothesis, but the striking mobility of the flow of these rock masses remains in a large part inexplicable. Three major rock avalanches in the Italian alps have been studied focusing in particular on aspects relating to the fragmentation and deposition processes. Studies have focused on grain size distribution of samples taken at different distances and elevation and on the morphology of the ground surface of the deposits. Image analysis technique was useful to complete the cumulative grain size curves to include larger particles. In none of the three cases the significant parameters of the grain size distribution and of the particles moprphology have shown any relationships with the distance from the center of the failed mass and with the height of the taken sample on the base of the deposits. The roundness of particles is practically the same within the whole deposit and even between different depositsfor which travel distances and thicknesses are different. The morphology of the three deposits gave some information on the dynamics of the phenomenon and, in particular, on the progressive thinning of the moving debris