Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova A.A

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1 GRANULAR FLUXES IN LABORATORY AND IN THE FIELD: 3-DIMENSIONAL SURVEY BY TERRESTRIAL PHOTOGRAMMETRIC TECHNIQUES Ph.D. candidate: DANIELE DALLAVALLE Tutor: Prof. PAOLO SCOTTON Cycle: XXVI Abstract The work is inserted in an ongoing experimental researchdirected to describe, with photogrammetric techniques the surface evolution of granular masses moving down a slope. The obtained experimental data, derived from time evolution of the fluxes, can contribute to the description of the rheology and to the calibration of physical-mathematical-numerical models, aimed to improve the understanding of dry and wet granular phenomena: dense snow avalanches, rock avalanches, debris flows. The experimental apparatus is equipped with a system for fast image acquisition from eight digital cameras. The synchronization of the frames has been obtained a National Instrument hardware control system and a proper software written in LabView. The image transfer velocity was optimized using Raid technology. The system developed at the laboratory will be soon applied in a debris flow site (Acquabona, Belluno Province, Italy), where a new monitoring system has been recently. Full Report laboratory activity - The calibration of physical-mathematical-numerical models, that try to simulate the dynamics of wet and dry granular masses, is carried by physical models based on the similarity theory, together with the direct observation on the field. Numerical models are widely used in the process of definition of hazard maps of several phenomena: among them dense snow avalanches, rock avalanches and debris flows. In order to provide new experimental data useful for the calibration of numerical models and to test rheological laws, an experimental apparatus has been built at the Geosciences Department. The apparatus has the purpose of reproducing, three-dimensionally, the outer surface of a moving granular mass from the initiation to the end of motion. The technique that we have chosen to fulfil the goal is the photogrammetry, also considering the relatively low cost of the necessary equipment and the possibility of application in the field. As a final product of the research we expect to realize a procedure, as automatic as possible, able to describe, moment by moment, the three-dimensional distribution of the granular mass, to define the precision of the obtained topographic data and to establish the limits of the technique. An experimental flume, consisting of two planes with adjustable inclination, has been used. The upstream plane, with slope ranging from 15% to 60%, simulates the flowing zone and the downstream plane, with slopes ranging from 0% to 30%, simulates the deposition area. The experiments are being performed using a maximum of eight industrial non metric digital videocameras. A full photogrammetric camera calibration process has been first conducted 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 for 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. 1 Figure 1: The experimental apparatus used in the tests.

2 In order to obtain the better image quality for the photogrammetric process, shutter speed and lens aperture have to be carefully chosen. The shutter speed of the Basler Ace Cameras can be selected in the range from 25 µs to µs. The use of high power, high frequency illuminators allowed a quite small exposure time of 300 µs (Figure 2), with lens aperture almost completely open. Figure 2: On the left an image of the moving mass taken with normal illumination at a shutter speed of 0.01 s. On the right an image taken at a shutter speed of s, using two high frequency illuminators (Lupo quadrilight). The frame synchronization is achieved by a NI PCI-6602, which can generate a voltage trigger signal. The eight cameras are connected to a computer with the protocol 2 GigE NI PCIE-8235 (Quad GigE). The computer is equipped with seven hard disks managed with Raid 0 mode which improves the potential of the whole system. The maximum transmission and storage rate is approximately 3 Gb/s, with an acquisition frequency of 20 Hz, at the maximum resolution obtainable by the cameras. In order to verify the synchronization of the frames an analogical stopwatch has been used (Figura 3). Figure 3: The camera synchronization has been verified through the simultaneous observation of an analogical stopwatch. Experimental tests are being carried out by varying the following physical parameters: - type of flow (channeled and not channeled). In Figure 1 is showed a channeled example; - type of granular material (well sorted sub-angular material, diameter 0.7 cm and 1.5 cm, density 2600 kg/m 3 ; synthetic zeolite material, mean diameter 1.0 mm, density 1080 kg/m 3 ); - type of retarding works (mounds and flow diverters). In Figure 4 are showed some results of a channeled test. On the right is presented the 3-D reconstruction, 1 s after the release, of 7 litres of sub-angular material, diameter 0.7 cm. On the left is showed the longitudinal section at the middle of the channel. In Figure 5 is showed the comparison between the slope of the two planes measured by photogrammetry and by a clinometer. As can be seen the agreement between the measures is quite high. The experimental apparatus has proved to give sufficient high quality frames for the photogrammetric elaborations, also under dynamic conditions. The precision of the photogrammetric reconstruction, determined statically at the moment, appears to be quite high. A dynamic confirmation is under execution. The experimental apparatus has proved to give sufficient high quality frames for the photogrammetric elaborations, also in a dynamic configuration of the material. 2

3 Figure 4: Three-dimensional vision obtained by the photogrammetric analysis on a canalized sliding zone. The orthophoto was overlapped on the digital terrain model. The contour lines are 20 mm spacing. On the left, the longitudinal section referred to the picture on the right, is rapresented. Slope ( ) Experimental Software measurements measurements Upstream floor 30 ± ±0.5 Figure 5: Comparison between the downstream and upstream angles obtained experimentally with a clinometers and via software. Downstream floor 7.5 ± ±0.5 A good arrangement direct-photogrammetric in a static configuration. A confirmation about the dynamic precision will be soon take in account as well as a three dimensional reproduction of the experimental tests. The final step is the comparison between numerical simulations and photogrammetrical derived experiments. Full report - field activity - Among the goals of the field-activity is to transfer the technology developed in the laboratory. The design of a new station at the monitoring site of Acquabona is now under study and will be, likely, active starting from the next spring. Working on this direction we hope to be able to collect dynamic metric field-data through which highlight the dynamic properties of real debris-flows. In the meanwhile, some field data have been collected during the last season, the first season of full activity of the station. Three debris-flow events have been registered. Some data recorded during the event occurred on May, 12, are displayed in Figure 6 (rainfall and depth of the debris flow at the downstream station). The overall duration of the flow was approximately 12 min. The flow presented two main surges with solid concentration decreasing from the bouldery front to the watery turbulent tail. The flow mobilized mainly the loose debris that filled the bottom of the channel, though a significant contribution came also from the bank failures caused by severe undercutting. The final deposit was lobe-shaped. The deposition took place in the artificial final basin, where the inclination angle is The maximum dynamic thickness, measured at the downstream station, was 0.6 m. The segregation of the coarse solid elements was clear on the top and at the front of the deposit. The time interval elapsed between the end of rainfall and the first surge arrival at the downstream station suggests a very slow velocity due to the low water content, high viscosity of the interstitial fluid and low slope. 3

4 Figure 6: a) The total cumulated rainfall recorded by the upstream weather station is mm. The first part of the storm had a duration of 45' with a hourly average intensity of mm/h (0.466 mm/min.). The second part had a comparable duration of 37 with a hourly average intensity of mm /h (0.569 mm /min). b) Debris flow height measured by the ultrasonic sensor at the downstream station. Concluding remarks A new experimental system, equipped with the necessary instrumentation was this year designed and built. An industrial acquisition system was installed, to ensure frames synchronization. Three-dymensionally dynamic analysis of granular mass moving down a slope was undertaken. Numerical modeling and comparison with photogrammetric data is one of the next year objectives. Data collection at the Acquabona site supports researches on the hydrologic factors controlling debris flow initiation, entrainment, and flow dynamics. The structure of the monitoring system being developed is simpler than previous systems. The stability of the base system allows for future enrichment of the complexity, both downstream and upstream. The 3G universal mobile telecommunication system (UMTS) provides high speed data transmission so that it is ensured continuous downloading of large amounts of data. Getting results from a debris flow monitoring system requires of a constant and continuous service and money to maintain and improve the whole system too. Next year the installation of a third monitoring station between the two previous will begin. References SCOTTON, P., GENEVOIS, R., MORO, F., ZORZI, L., GIRARDI, G., PRATICELLI, N. The new Debris Flows monitoring system of Acquabona torrent (Cortina d Ampezzo, Bl, Italy) Proceedings of the 5th International Conference on Debris Flow Hazards Mitigation: Mechanics, Prediction and Assesment June 14 17, 2011 University of Padua. TECCA, PIA R., ANTONIO GALGARO, RINALDO GENEVOIS, ANDREA M. DEGANUTTI 2003 Development of a remotely controlled debris flow monitoring system in the Dolomites (Acquabona, Italy). Hydrological processes (2003). GENEVOIS, R., BERTI, M., GHIROTTI, M., SIMONI, A. and TECCA, P.R Debris flow monitoring and analysis in the Dolomitic region (Upper Boite Valley, Italian Alps). BERTI, M.,SIMONI, A., Debris flow initiation in an highly-conductive soil. BERTI, M. GENEVOIS, R., LAHUSEN, R., SIMONI, A., TECCA, PIA R Debris flow monitoring in the Acquabona watershed on the Dolomites (Italian Alps). Phys. Chem. Earth (B), Vol. 25, No. 9, pp BERTI, M., GENEVOIS, R., SIMONI, A., TECCA, PIA R Debris-flow in the Dolomites: 4

5 Experimental data from a monitoring system. Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment, Wieczorek & Naeser (eds) 2000 Balkema, Rotterdam, ISBN X. ARATTANO, M., MARCHI, L., GENEVOIS, R., BERTI, M., SIMONI, A., TECCA, P. R., BONTE, M Field monitoring and real time management of debris flows. BERTI, M., GENEVOIS, R., SIMONI, A., TECCA, P. R., Field observations of a debris flow event in the Dolomites. Geomorphology 29 (1999) SUMMARY OF ACTIVITY IN THIS YEAR Courses: P.SCOTTON: Sistemazioni idraulico forestali (parte numerica), Dipartimento di Geoscienze, Università degli Studi di Padova. CALANDRUCCIO E. Corso di Inglese parlato, Dipartimento di Geoscienze, Università degli Studi di Padova. GULIK L. Corso avanzato di Inglese scientifico, Dipartimento di Geoscienze, Università degli Studi di Padova. Communications: SCOTTON P., DALLAVALLE D. Rilievo dinamico tridimensionale di ammassi granulari in movimento con tecniche fotogrammetriche. Posters: DALLAVALLE D., SCOTTON P., GENEVOIS R. The debris flow monitoring system of Acquabona torrent (Cortina d Ampezzo, BL, Italy). European Geosciences Union - General Assembly 2012 Vienna, Austria, April DALLAVALLE D., SCOTTON P., TECCA P. R., BOCCANEGRA V. Three-dymensional dynamic topographic survey of granular flows using photogrammetric techniques. European Geosciences Union - General Assembly 2012 Vienna, Austria, April Publications: SCOTTON P., DALLAVALLE D. Rilievo dinamico tridimensionale di ammassi granulari in movimento con tecniche fotogrammetriche. Teaching activities: Co-supervisor of the MSc Thesis Rilievo fotogrammetrico dinamico di ammassi granulari, indagine sperimentale (by Valentina Boccanegra). Anno Accademico: 2011/2012. Workshops and Congresses Monitoring bedload and debris flows in mountain basins International workshop, Bozen-Bolzano (Italy), October 2012 EGU General Assembly, Wien, April