ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
Data acquisition and processing in a small debris flow prone catchment M. Arattano1, L. Marchi2, A.M. Deganutti2 Abstract The Moscardo Torrent is a small creek, with an area of about 4 km2, located in the Eastern Italian Alps (Fig. 1). The bedrock of the Moscardo basin is made of Carboniferous flysch, consisting of shale, slates, siltstone, sandstone and breccia that continuously outcrop in the upper portion of the basin and locally along the torrent.
Fig. 1 - The Moscardo Torrent basin and its alluvial fan. 1: debris flow initiation site; 2: raingauges; 3 and 4: instrumented channel stretches. The poor mechanical properties of the rocks, the instability caused by the presence of a deep seated gravitational slope deformation and the steepness of the upper basin slopes facilitate frequent widespread falls and toppling of rocks. The very large quantity of debris that reaches the drainage network by gravitational and erosion processes and by snow avalanches explains the strong tendency of the Moscardo Torrent to generate debris flows and hyperconcentrated flows. The debris flows of the Moscardo Torrent led to the construction of a huge fan that progressively invaded the receiving valley bottom forcing the receiving stream to flow on the opposite side of the valley floor. The 1 2
1CNR-IRPI, Strada delle Cacce 73, 10135 Torino, Italy E-mail:
[email protected]. CNR IRPI, Corso Stati Uniti 4, 35127 Padova, Italy E-mail:
[email protected]
ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
main morphological parameters of the basin closed at the fan apex are listed in Table 1; the length of the channel reach on the fan is about 1000 m (Arattano et al., 1997). The Moscardo debris flows are composed by material of a wide range of dimensions. Lateral levees and debris flow lobes are mostly formed by small and medium boulders; larger boulders of over two meters are also common. Grain size analyses have been carried out on several matrix samples (Arattano et al., 1997, Moscariello and Deganutti, 2000) from old and fresh deposits. The high percentage of fine particles in the matrix (from 13 to 35% less than 40 µm) underlines the muddy character of the Moscardo debris flows. Table 1. Main morphometric parameters of the Moscardo basin. Basin area [km2]
Maximum elevation [m]
Minimum elevation [m]
Average basin slope [%]
Channel length [km]
Average channel slope [%]
4.1
2043
890
63
2.76
37
The Moscardo debris flow material has been studied also from the rheological point of view, using a large amount of material sampled from a fresh deposit left by a debris flow occurred on July, 5 1995. Several suspensions, obtained mixing the material with water, were analysed for a range of different solid concentrations using two parallel plates rheometers and a tilting plane. The material showed a shear thinning viscous behaviour which can be well represented by a Herschel-Bulkley model (Coussot et al., 1998). This basin was chosen in 1989 by the Research Institute for Hydrogeological Protection of the Italian National Research Council (CNR IRPI) for the installation of a debris flow monitoring system. This latter consisted of two ultrasonic sensors, placed at a distance of about 300 m on the fan. A raingauge was also installed in the upper portion of the basin. In 1995, thanks to the fundings of a European Project, the ultrasonic sensors installed in 1989 were replaced by new ones and a third ultrasonic sensor was added 150 m upstream of these latter. A fixed video camera was positioned close to the intermediate of the three ultrasonic sensors and a network of four seismic detectors was set up about 1 km upstream from the ultrasonic gauging stations. Finally, in 1997, a second raingauge was installed in the centre of the basin and two new seismic sensors were set up close to the intermediate ultrasonic gauge on the fan. From 1990 to 1998, 15 debris flows occurred, 14 of which were recorded by the installed gauges. Several types of measurements have been carried out on the recorded debris flows during the years, including mean front velocity, surface velocity, volume, flow stage, hydrograph deformation, triggering rainfalls etc.. The ultrasonic sensors measure the torrent stage, making it possible to record the debris flow hydrographs. A time lag of 60 seconds was initially set between two consecutive recordings of the ultrasonic sensors. This time lag was then reduced to 10 seconds in 1990, to grant a better accuracy of the data, and further reduced to 1 second in 1995, thanks to the updating and improvement of the recording system mentioned earlier. The mean propagation velocity of the front can be calculated in the monitored reach as the ratio of the distance between the sensors to the time interval between the appearance of the peak of the debris flow surge in the two recorded hydrographs. The analysis
ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
of the hydrographs recorded by the ultrasonic sensors may show the aggradation or degradation of the channel bed at the recording sites. Flow stage measurements and topographic survey of the monitored sections make it possible to estimate peak discharges and total volume of debris flows (Arattano et al., 1997, Marchi et al., 2002). Debris flow volumes Vol have been estimated as: t
t
Vol = ∫t 0 f vA ( t ) dt = v ∫t 0 f A ( t ) dt
(1)
where v is the mean velocity of the flow, which was assumed constant for the entire debris flow wave and equal to the mean front velocity, A(t) is the cross section area occupied by the flow at the time t, known from topographic surveys and the ultrasonic data, t0 is the time of arrival of the surge at the gauging site and tf is the time at the end of the debris flow wave. Mean velocity of the main front was used for volume computation because it is the only velocity datum available for all recorded events: additional information on velocity variations during the debris flow wave have been obtained only for two events. This approach to the computation of the flowing volume assumes that the material flows through the considered section at a constant velocity during the surge. Thus computed debris flow volumes should be regarded as approximate estimates. The seismic detectors (seismometers and geophones) record ground vibrations induced by the passage of a debris flow. The purpose of the seismic sensors installation, in the initial phase of the research, was essentially to verify which information could be obtained through this type of devices on the occasion of a debris flow occurrence. However, the first results that have been obtained showed the possibility of using these detectors also as tools for velocity measurements (Arattano and Moia, 1999). The debris flow passage generates ground vibrations whose amplitude graph corresponds to the stage hydrograph. The ground vibrations peak is detectable by a seismic sensor placed at a safe distance of some tens of meters from the channel bed. The mean front velocity can be then measured placing a couple of these detectors at a known distance from each other along the torrent adopting the same procedure previously described for velocity measurements with ultrasonic sensors. A fixed video camera like that installed in 1995 on the alluvial fan of the Moscardo Torrent allows a visual interpretation of the debris flow features. The video camera shoots slantwise a straight channel reach about 80 meters long and is triggered by the upstream ultrasonic sensor by means of a triggering software that identifies abrupt increases of the stage in the torrent and starts the video recordings. The possibility was also investigated of using the video recordings for estimating debris flow surface velocity. A simple method to process the recorded images was developed that maps 2D image points on the screen and points in the 3D space (Arattano and Marchi, 2000). Average velocity of the features floating on the surface was then computed as the ratio of their traveled distance to the time elapsed between the shooting of the video frames that contained them. Average debris flow velocities estimated through image processing were consistent with measurements based on the recordings of the ultrasonic gauges; velocity variations in debris flow waves are discussed in Arattano and Marchi (2000). The instrumentation installed in the Moscardo Torrent basin (raingauges and sensors which detect debris flow passage) gives the possibility of analyzing the relations between rainstorm characteristics and debris flow occurrence more precisely than
ERB and Northern European FRIEND Project 5 Conference, Demänovská dolina, Slovakia, 2002
usually possible in alpine basins. Rainfalls recorded in the Moscardo Torrent from 1990 to 1998 were analyzed and storm characteristics of two classes, i.e. debris flow causing storms (15 cases) and storms which did not cause debris flows (58 cases) were compared (Deganutti et al., 2000). Several storm variables were taken into account, including total storm rainfall, average intensity, maximum 60 minutes intensity, antecedent precipitation: a statistical analysis showed that only total storm rainfall and maximum 60 minutes intensity were significantly different between debris flow storms and storms that did not trigger debris flows. The analysis of rainfall has shown that rainstorm characteristics and antecedent precipitation play an important role but are not sufficient to define debris flow initiation conditions. A critical combination of sediment availability and hydrologic conditions is necessary to cause debris flow formation. This is particularly true when sediment moisture is influenced by a complex groundwater flow regime and sediment availability depends on bank and slope failures, as well as on the previous occurrence of debris flows, like it occurs in the Moscardo basin. Even though debris flow monitoring in the Moscardo Torrent was intended for research purposes, the results that have been obtained provide suitable indications for the possible use of different gauges in debris flow alarm systems. In particular, the use of seismic devices as warning system, as it has been tested also for snow avalanches appears to be encouraging. It is part of the plans of CNR IRPI to continue the monitoring activities in the Moscardo Torrent through the next years. References Arattano, M., Deganutti, A.M., Marchi, L., 1997. Debris flow monitoring activities in an instrumented watershed of the Italian Alps. In: Chen, C. (Ed.), Proceedings, First International Conference on Debris-flow Hazard Mitigation: Mechanics, Prediction, and Assessment. Water Resources Engineering Division / ASCE, New York, pp. 506-515. Arattano, M., Moia, F., 1999. Monitoring the propagation of a debris flow along a torrent. Hydrological Sciences Journal 44(5), 811-823. Arattano, M., Marchi, L., 2000. Video-derived velocity distribution along a debris flow surge. Physics and Chemistry of the Earth Part B 25(8), 781-784. Coussot, Ph., Laigle, D., Arattano, M., Deganutti, A.M., Marchi, L., 1998. Direct determination of rheological characteristics of debris flow. Journal of Hydraulic Engineering ASCE 124(8), 865-868. Deganutti, A.M., Marchi, L., Arattano, M., 2000. Rainfall and debris flow occurrence in the Moscardo basin (Italian Alps). In: Wieczorek, G., Naeser, N. (Eds.), Proceedings, Second International Conference on Debris-flow Hazard Mitigation: Mechanics, Prediction, and Assessment. A.A. Balkema, Rotterdam, pp. 67-72. Marchi, L., Arattano, M., Deganutti, A.M., 2002. Ten years of debris-flow monitoring in the Moscardo Torrents (Italian Alps). Geomorphology, in press. Moscariello, A., Deganutti, A.M., 2000. Sedimentary and hydrologic processes of a debris-flow dominated alluvial fan - Moscardo Fan, Italy. In: Wieczorek, G., Naeser, N. (Eds.), Proceedings, Second International Conference on Debris-flow Hazard Mitigation: Mechanics, Prediction, and Assessment. A.A. Balkema, Rotterdam, pp. 301-310.
