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Analysis of the ground vibration generated by debris flows and other torrential processes at the Rebaixader monitoring site (Central Pyrenees, Spain)

Analysis of the ground vibration generated by debris flows and other torrential processes at the... Open Access Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 Natural Hazards www.nat-hazards-earth-syst-sci.net/14/929/2014/ and Earth System doi:10.5194/nhess-14-929-2014 © Author(s) 2014. CC Attribution 3.0 License. Sciences Analysis of the ground vibration generated by debris flows and other torrential processes at the Rebaixader monitoring site (Central Pyrenees, Spain) C. Abancó, M. Hürlimann, and J. Moya Geotechnical Engineering and Geosciences Department, Technical University of Catalonia (UPC), Jordi Girona 1–3 (D2),08034 Barcelona, Spain Correspondence to: C. Abancó ([email protected]) Received: 25 July 2013 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 30 August 2013 Revised: 8 January 2014 – Accepted: 26 February 2014 – Published: 17 April 2014 Abstract. Monitoring of debris flows using ground vibration Although debris-flows monitoring has strongly improved sensors has increased in the last two decades. However, the during the last decades and several torrential catchments in correct interpretation of the signals still presents ambiguity. the world have been instrumented with different types of In the Rebaixader monitoring site (Central Pyrenees, Spain) sensors and techniques (Itakura et al., 2005), this is still a two different ground vibration stations are installed. At the challenging topic in debris-flow research. Apart from de- first station the ground velocity signal is transformed into bris flows, monitoring is also used for the analysis of other an impulses-per-second signal (low frequency, 1 Hz). The types of rapid mass movements like snow avalanches or rock- analysis of the data recorded at this station show that the falls (Suriñach et al., 2005; Bessason et al., 2007; Vilajosana shape of the impulses signal is one of the key parameters to et al., 2008) and bedload transport in rivers and torrents describe the evolution of the event. At the second station the (Rickenmann et al., 1998, 2012). Torrential processes, espe- ground velocity signal is directly recorded at high frequency cially debris flows, generate seismic waves in the ground, (250 Hz). The results achieved at this station show that the originated by the collision between boulders or between differences in time series and spectral analysis are helpful boulders and the bedrock. These vibrations can be measured to describe the temporal evolution of the events. In addition, by several seismic and sonic devices such as geophones, some general outcomes were obtained: the attenuation of the seismographs or infrasounds (Itakura et al., 2005; Kogel- signal with the distance has been identified as linear to ex- nig et al., 2011a). Geophones are the most common seismic ponential; and the assembly of the geophones to the terrain sensors used in debris-flow monitoring because of their ro- has an important effect on the amplification of the signal. All bustness and low power consumption. These features make these results highlight that the definition of ground vibration them also very suitable not only for monitoring, but also thresholds for debris-flow detection or warning purposes is a for warning purposes. All over the world, several sites have difficult task; and that influence of site-specific conditions is been instrumented with geophones: Illgraben in Switzerland notable. (Hürlimann et al., 2003), Lattenbach in Austria (Kogelnig et al., 2011a), Moscardo (Arattano et al., 2012), Acquabona (Berti et al., 2000) or Gadria (Marchi et al., 2012) in Italy, 1 Introduction Manival or Réal in France (Navratil et al., 2011), Mount St. Helens in USA (LaHusen, 2005b), Houyenshan (Chou et al., Debris flows are one of the most hazardous geomorphologic 2010), Fong-Ciou Creek or Ai-Yu-Zi Creek (Huang et al., processes. In order to improve the understanding of debris- 2007; Fang et al., 2011) in Taiwan, and Jiangjia in China flow mechanisms, torrents are being instrumented with an (Cui et al., 2005) are some examples. increasing variety of sensors. The data collected are not only needed to calibrate numerical models, but also to develop and adjust warning systems. Published by Copernicus Publications on behalf of the European Geosciences Union. 930 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Some analyses of geophone signals induced by debris and a sensibility analysis of the threshold values. The out- flows have been published during the last decades (Arattano comes of this research improve the knowledge on some cur- and Moia, 1999; Berti et al., 2000; Hürlimann et al., 2003; rent issues (i.e. process differentiation, geophone location, Huang et al., 2007; Chou et al., 2010; Arattano et al., 2012). recording method or threshold assessment) and should help All these studies have substantially increased our knowledge for the set-up of future debris-flow monitoring or warning on the dynamic behaviour of debris flows and the ground vi- systems. bration they induce. However, there are still many open ques- tions, such as the use of the ground vibration for the defini- 2 Debris flow characterisation by ground vibration tion of thresholds; in particular, because a threshold is key monitoring information not only for the detection and warning of events, but also for the distinction between different flow types (e.g. 2.1 Debris-flow features debris flows vs. debris floods). The ground velocity signal can be recorded by two dif- Debris flows are rapid landslides formed by water and solid ferent approaches: (a) continuously (e.g. in Moscardo tor- material poorly sorted, from boulder to clay (Iverson, 1997). rent; Arattano and Moia, 1999); and, (b) by switching from Pierson (1986) describes a typical debris flow in three parts: a no-event mode into an event-mode (e.g. in the Swiss tor- the front, the fully developed debris flow (also called “body”) rents; Hürlimann et al., 2003). The latter approach needs the and the tail. The front carries the biggest boulders and is fol- incorporation of a trigger into the recording algorithm and lowed by the debris flow body; both of them are characterised the correct definition of its value. Different types of trig- by a high sediment concentration and a turbulent regime. At gers can be found in the literature, such as (a) level trig- last, there is the tail with much less solid material concen- gers: fixed value of the ground velocity (LaHusen, 2005a) tration, which can also be described as a hyperconcentrated or fixed values of a transformed signal (Badoux et al., 2009; flow. Many debris-flow events occur in a series of surges, Hürlimann et al., 2013); or (b) more sophisticated thresh- each of them showing a front, a body and a tail (Pierson, olds based on the frequency content of the signal (Bessa- 1986; Johnson and Rodine, 1984). son et al., 2007). The type of trigger applied mainly depends The coexistence of torrential processes has been noted in on the data recording system implemented at the site. Sev- the Rebaixader site. Debris floods can be defined as episodes eral systems are employed, including (a) analogical record- of massive bedload transport characterised by a limited max- ing (Arattano and Moia, 1999); (b) digital sampling (Arat- imum grain size (Aulitzky, 1982). Debris floods are also de- tano, 2000; Kogelnig et al., 2011b); and, (c) transformations scribed as very rapid surging flows of water in a steep chan- of ground vibration velocity signal (Navratil et al., 2011; nel heavily charged with debris (Hungr et al., 2001). A de- Abancó et al., 2012). bris flood may transport quantities of sediment comparable Normally, the threshold value (level triggers) is established to a debris flow, in the form of massive surges. However, the combining an empirical analysis of the signals of past events transport is carried out by the tractive forces of water over- and expert criteria (Kurihara et al., 2007). The threshold has lying the debris. As a result, the peak discharge of a debris to be defined at each geophone, as there are several site- flood is comparable to that of a water flood (perhaps mul- specific factors that influence the vibration recorded at the tiplied by a factor up to 2). This fact clearly contrasts with seismic sensors. An accurate assessment of the threshold is of the peak discharges of debris flows, which are tens of times crucial importance, especially in warning systems, when the greater than major water floods (VanDine, 1985; Hungr et al., detection of events triggers some kind of alarm process, such 2001). Another important difference between debris flows as the closing of traffic lines or messages to the stakeholders. and debris floods is the absence of the bouldery front. However, there are only very few studies dealing with the Sediment concentration and boulder content alters the en- influence of the site-specific factors affecting the vibration ergy transmitted to the ground. Thus, on one hand debris induced by debris-flow events (Huang et al., 2007; Navratil flows can be distinguished from other torrential processes et al., 2011). and, on the other hand, the different phases of a debris flow In this paper the features of the ground vibration signals can be detected (e.g. Huang et al., 2007; Navratil, 2013). registered at two monitoring stations located in the Rebaix- ader monitoring site are analysed. The main difference be- 2.2 Monitoring of debris-flow induced ground vibration tween the two stations is the data recording system, but also some other aspects regarding the mounting and the location Velocity of ground movement is transduced by a geophone to of the geophones. The major purpose of this work is to define a voltage that is (generally linearly) related to the ground ve- the main characteristics of debris flows and other torrential locity. The digital measuring of the geophone output is done processes using the seismic signal recorded at the two sta- by sampling the signal at a fixed frequency. To avoid aliasing tions (each with a different data recording system). Other ob- problems, the sampling rate must be greater than the Nyquist jectives are the analysis of the influence of some site-specific frequency, which is twice the highest frequency of the sig- factors on the ground vibration signal by means of field tests nal. Digital sampling is used to record the signal from the Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 931 geophones, but other techniques based on the transformation a) of the original signal into simpler data have also been devel- oped (Arattano, 2000; Navratil et al., 2011; Abancó et al., 2012). These data recording systems are widely described in the following sections. Several features of moving debris flows have been deter- mined due to the analyses of the ground vibrations time se- ries. For instance, the correspondence between the flow stage and the ground velocity signal (Arattano and Moia, 1999), or the increase of the amplitude of the ground vibration as the flow front approaches to the seismic sensor (Arattano et al., 1999). Furthermore, the flow volume was correlated with the time integral of the acceleration amplitude (Suwa et al., 2000). Other authors found some general patterns in the fre- quency domain. For instance, LaHusen (1996) described the typical peak frequency range of the debris flows between 30 and 80 Hz, or Huang et al. (2007) suggested this range from 50 to 100 Hz. 2.3 Ground vibration site-specific factors b) Both the amplitude and the frequency of the signal measured by the geophones depend on several site-specific factors (Yin et al., 2007; Navratil et al., 2013). The influencing factors considered herein are the distance between the sensor and the debris-flow path, the material in the channel and in the channel banks and the assembly of the geophone. Geophones are generally installed outside the channel bed, in a protected location, to avoid damage when a torren- tial event occurs. However, waves are attenuated with the distance and they do not travel long distances (LaHusen, 2005b). For this reason, the distance between sensor and flow path is a crucial factor and geophones are commonly installed not further than a few tens of metres from the active Fig. 1. (a) The Rebaixader torrent, its fan and source area. Seis- 2 Figure 1: a) The Rebaixader torrent, its fan and source area. Seismic stations (FLOW-WR and mic stations (FLOW-WR and FLOW-SPI) and the corresponding channel or directly on its lateral banks. 3 FLOW-SPI) and the corresponding geophones are indicated and labelled. The ultrasonic geophones are indicated and labelled. The ultrasonic device is rep- The attenuation of the seismic waves depends on the prop- resented by a black line in the middle of the channel reach. Inset 4 device is represented by a black line in the middle of the channel reach. Inset shows the erties of the material the wave travels through. Depending on shows the location of the Rebaixader site; (b) detailed location of the material, the absorption of the energy by the ground is 5 location of the Rebaixader site; b) detailed location of the sensors at the channel section. the sensors at the channel section. higher or lower (Itakura et al., 2000; Suriñach et al., 2001; Biescas et al., 2003). Also the physical properties of the transmission medium affect the velocity of the waves. For 3 Description of the Rebaixader site −1 example, P wave velocity ranges from about 350 m s in −1 alluvium up to 700 m s in bedrock (Arattano and Moia, 3.1 General setting 1999). When geophones cannot be buried in soil, the sensors must The Rebaixader catchment is a first-order basin with an area be fixed to the bedrock, big boulders or existing concrete 2 of 0.53 km , which is located in the Central Pyrenees near structures (e.g. check dams). In this case, the method of fix- the village of Senet (Fig. 1). The catchment has the typical ing the geophones to these hard surfaces controls the transfer morphology of a torrential basin formed by three zones (ero- of vibrations to the sensor; and, consequently, has a strong sional source area, channel zone and fan). The source area influence on the signal recorded. Since the surfaces are of- ◦ ◦ has a steep slope (average of 29 , but up to 50 ), an area ten irregular, different assembly systems are designed in the 2 of 0.09 km and it is located between 1425 and 1710 m a.s.l. existing monitoring stations (Abancó et al., 2012). ◦ (Fig. 1). The channel zone has an average slope of 21 , is 250 m long and about 20 m wide and is located between 1425 and 1350 m a.s.l. Downstream of the channel zone, there is a 2 ◦ fan with an area of 0.082 km and a mean slope of 17 . The www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 932 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Fig. 2. (a) Downstream view inside the channel indicating the places where the geophones are placed. Pictures of the detailed assemblies are shown in (b) to (d). Picture (d) was taken during the installation of Geo6, before being covered. Noguera Ribagorçana River defines the lower boundary of stations regarding the detection and characterisation of the the fan. There is no protection works in the Rebaixader tor- flow dynamics. Further details about the instrumentation and rent. the events recorded can be found in Hürlimann et al. (2013). The geology of the source area consists of a thick till Herein, we focus on the ground vibration recorded at the two deposit over a bedrock of slates and phyllites of Devonian flow dynamic stations (FLOW-WR and FLOW-SPI in Fig. 1). age. The bedrock crops out only locally in the source and The geophones of both stations are 1-D vertical, moving coil forms the margins of the channel zone. The till corresponds geophones (Geospace 20-DX) with a natural frequency of to a lateral moraine of the glacier that occupied the Noguera 8 Hz and a spurious frequency of 200 Hz. The main differ- Ribargorçana Valley during the last glacial cycle (Vilaplana, ence between the stations is the data recording system. The 1983). data acquisition in station FLOW-WR is based on a low sam- The meteorological conditions of the site are affected by pling rate of a transformed signal (Abancó et al., 2012), while the proximity of the Mediterranean Sea, the influence of in station FLOW-SPI the high sampling rate provides data on the northern Atlantic winds and the orographic effects of the original ground velocity signal. the Pyrenees. The annual precipitation ranges from 800 to The station FLOW-WR includes five geophones, an ultra- 1200 mm. The debris flows and debris floods analysed in this sonic device for stage measurements and a video camera. The study are mostly triggered by convective storms in the sum- sampling frequency for the geophones and the ultrasonic de- mer, which are characterised by short and intense rainfalls. vice when an event is detected is 1 Hz. The sensors are con- However, it has also been observed that rainfalls of lower in- nected by wires and controlled by a Campbell CR1000 data tensities accompanied by snowmelt can also trigger events in logger, which is powered by a 12 V 24 Ah battery, charged by spring (Hürlimann et al., 2013). a 30W solar panel. The data are transmitted via GSM modem to our server in Barcelona. The geophones are distributed 3.2 Monitoring network along 175 m at the right side of the torrent, between 1415 and 1345 m a.s.l (Figs. 1 and 2a). The distances between geo- The monitoring system installed in the Rebaixader torrent in- phones are up to 75 m, and the distances between the sensors cludes, on one side, four stations measuring the meteorologi- and the active channel range from 8 to 25 m (Table 1). Four cal and hydrological conditions in the catchment for the anal- of the five geophones are mounted by a metal sheet box to ysis of the debris-flow initiation and, on the other side, two Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 933 Table 1. Summary of main characteristics of the geophones analysed in this paper. IS stands for “impulses per second” and GVS stands for “ground velocity signal”. Geophone Mounting Station Distance to active Material (abbreviation) (data recording channel (planimetric at the system) distance in m) cross section Geophone 3 (Geo3) Metal sheet box attached to bedrock FLOW-WR (IS) 25 Colluvium and bedrock Geophone 3b (Geo3b) Bedrock FLOW-WR (IS) 25 Colluvium and bedrock Geophone 4 (Geo4) Metal sheet box attached to bedrock FLOW-WR (IS) 8 Bedrock Geophone 5 (Geo5) Bedrock FLOW-SPI (GVS) 3 Bedrock Geophone 6 (Geo6) Buried into soil FLOW-SPI (GVS) 3 Colluvium and bedrock Geophone 7 (Geo7) Buried into soil FLOW-SPI (GVS) 5 Colluvium and bedrock the bedrock (geophones Geo1 to Geo4 in Fig. 2b). Each box correspond to seismic noise of the site. Second, the voltage is protected by a plastic structure in order to avoid the im- exceeding a certain threshold is transformed into an impulses pact of raindrops or hail on it. The fifth geophone (Geo3b) is signal. This filtering and transformation is made analogi- fixed directly on the bedrock without a metal box. It is also cally by a set of electrical resistors in the conditioning circuit protected by a plastic structure like the other geophones. board, which acts like a threshold voltage. Since at Rebaix- The station FLOW-SPI was set up in June 2012 in order to ader site, two types of geophone assemblies have been ap- record the ground vibration at high frequency (250 Hz). The plied, and two values of “ground velocity threshold” (GVth) station contains three geophones, which are located at the left have been defined. For the geophones mounted in a metal side of the channel (Figs. 1 and 2a). The geophones are lo- sheet box (Geo1, Geo2, Geo3, Geo4), the threshold corre- −1 cated between 3 and 5 m from the active channel, thus much sponds to a velocity of 0.17 mm s . The other geophone closer than those of the station FLOW-WR (Table 1). At this (Geo3b), which was fixed directly to the bedrock and no station, all the geophones are fixed directly to the ground. resonance effect of the metal box is expected, the velocity −1 Two of them (geophones Geo6 and Geo7) are buried in the threshold is much lower (GVth= 0.019 mm s ). After this soil (granular colluvium) at a depth of about 20 cm (Fig. 2d), filtering, the signal is transformed into an impulse signal by while the third one (Geo5) is fixed to the bedrock (Fig. 2c) the conditioning circuit (for further details, see Abancó et and protected by a plastic structure as in the FLOW-WR sta- al., 2012). Finally, the signal is sent to the data logger, which tion. Data logging is carried out by a 24 bits broadband seis- counts the number of impulses each second. mic recording unit (Spider, manufactured by WorldSensing The frequency of measuring is controlled by the CR1000 s.l.), powered by a battery of 12 V, 22 Ah, and charged by data logger, which was programmed to scan the geophones of a 50 W solar panel. The Spider sends the data to a gateway, the station every second. To avoid high power consumption where they are resent to our server via GSM modem. and to optimise the memory management of the data files, an algorithm was introduced into the CR1000 data logger and the recording is not carried out continuously, but only when the number of impulses per second exceeds a threshold. This 4 Analysis of transformed ground velocity signal threshold is called “event mode threshold” and is based on the number of impulses per second cumulated during a cer- 4.1 Methods tain time span (Fig. 3). Therefore, this “event mode thresh- The data recording system at the FLOW-WR station is based old” (Eth) includes two components: (a) the number of im- on the transformation of the original signal, which corre- pulses of the Eth (Ethi); and, (b) the duration of the time span sponds to a voltage signal proportional to the ground veloc- in which Ethi is exceeded (Ethd). The event mode thresh- ity, into a signal consisting of impulses per second (Abancó old was defined progressively by analysing the data of the first year of the monitoring period. Since August 2010, the et al., 2012). The signal transformation is carried out by −1 Ethi has been fixed at 20 impulses per second (IMP s ) with an electronic conditioning circuit board that is connected to the Ethd established as three consecutive seconds. When the each geophone (Fig. 2b). The aim of the transformation is threshold is exceeded in any of the geophones of the station, twofold: (a) it filters and deletes the ground vibration noise; the “event mode” is triggered by the data logger code and the and, (b) the impulses per second (IS) data constitute a sim- signal is recorded each second. Event mode is deactivated af- ple discretised signal, which can be analysed more easily and ter 2 minutes, with vibration smaller than Ethi scanned in any with lower memory requirements. of the geophones. The recording is also carried out during the The signal transformation consists of two parts. First, the “no event mode” to monitor the noise and the performance of original voltage delivered by the geophone is filtered in or- the system; although at a much lower frequency (each hour). der to remove low ground velocities, which are assumed to www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 934 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Fig. 3. Flow chart of the event detection of FLOW-WR system in the Rebaixader. In italics, the value of the parameters used nowadays in the Rebaixader. Fig. 4. Typical shapes of the IS signal registered during a debris flow (a), debris flood (b), and rockfall (c). Horizontal and vertical scales are the same in the three cases. For each process, a snapshot from the video camera is shown. As it is shown below, several types of events (debris flows, after most of the events to identify geomorphic changes in debris floods and rockfalls) were recorded in the Rebaixader the torrent. torrent. The analysis of the IS times series revealed different 4.2 Results types of responses (IS curve morphologies). Finally, these IS curve morphologies were assigned to different types of For the whole monitoring period, 21 torrential events have torrential processes by means of cross-checking the vibration been recorded by the station since its installation in summer gathered in the five geophones, the flow depth measured by 2009: 6 debris flows, 11 debris floods and 4 rockfalls. Re- the ultrasonic device, the video images (available only for garding the shape of the IS time series curves, three types of 10 events) and periodic field trips (31 campaigns) carried out curves were distinguished (Fig. 4). Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 935 A Type A curve is characterised by three phases (Fig. 4a): Table 2. Characteristics of the events analysed in this work. Vol- umes were estimated using the records gathered at the different sen- (a) a first phase of stationary level of no or very low IS values, sors (geophones, ultrasonic device video-camera) and field observa- (b) an abrupt increase of the impulses, reaching values over −1 tions (see Hürlimann et al., 2013 for further information). 100 IMP s in less than 5 s, followed by (c) a slow (mostly exponential) decrease. Date (dd/mm/yyyy) Type Volume (m ) A Type B curve consists of a first phase of gradual in- crease of IS-values, which is followed by a gradual decrease 04/07/2012 Debris flow 16200 (Fig. 4b). 11/07/2010 Debris flow 12500 A Type C curve is defined by a very short duration (2 to 27/06/2012 Debris flow 4000 05/08/2011 Debris flood 2500 5 s fast increase of the IS-values, with a high maximum (up −1 25/03/2010 Debris flow 2100 to 190 IMP s ; Fig. 4c). 07/06/2012 Debris flood 750 Video images and geomorphological reconnaissance clearly showed that A-curves were recorded during debris- flow events (Fig. 5b, d, f and h), the B-curves was associ- ated with debris floods or immature phases of debris flows, 5 Analysis of original ground velocity signal and C-curves were related to rockfalls (Hürlimann et al., 2012). However, only Geo4 recorded A-curves for all the de- 5.1 Methods bris flows. The time series recorded at the upper geophones show B-curves, especially during the “small-magnitude” de- The station FLOW-SPI records the geophone signal directly bris flows (Fig. 5a and e). These facts suggest that some de- as a voltage and represents the vertical velocity of ground vi- bris flows may not be fully developed with a well-defined bration. The data provided by FLOW-SPI station differ from front until they reach the location of Geo4. This geophone the FLOW-WR station in two main points: (a) the record- is placed in the most downstream position (close to the fan ing of the ground velocity signal (GVS) is continuous with- apex) of the network. This interpretation is supported by the out distinction between “event” and “no event” modes; and observations of Arattano (2003) in Moscardo, where in some (b) the signal is recorded without filtering the noise. The data events the proper debris-flow front was only visible down- are stored in “mseed” files, a typically seismological format. stream of the fan apex. In addition, it should be noted that Each of these files contain approximately 30 min of data sam- geophones 1 to 3 are located at greater distances from the pled at 250 Hz (250 samples per second). active channel (15 to 25 m) than Geo4 (8 m), and that the at- The sampling frequency depends on the nature of the pro- tenuation of the vibration with distance may play a role in the cess and the site-specific characteristics of the geophones recordings of debris flows, as it is shown below. Besides the and their placement. Preliminary spectral analyses of some −1 shape of the curve, the peak of IMP s time series at Geo4 is flow events in the Rebaixader catchment indicated frequency useful to distinguish between debris flows and debris floods. ranges between 30 and 100 Hz. Therefore, a sampling fre- The values of peak vibration in this geophone never exceeded quency of 250 Hz is sufficient in our case. −1 100 IMP s for debris floods, while the values are from 130 FLOW-SPI station was installed in summer 2012, and for −1 up to 211 IMP s for debris flows. In contrast, the highest this reason only three events were recorded. Due to the small values and the shortest durations of vibration were recorded number of events, the distinction between types of events or in Geo1, the uppermost geophone, and correspond to rock- their features by a detailed GVS analysis (as performed for falls. the IS time series) was not possible. Video images were avail- Most of the recorded debris flows and debris floods present able only for one of the events because the first two events similar durations (Fig. 5), though they show a wide range occurred at night and the infrared spot lights were damaged of volume (Table 2). In general, these flow events last sev- by an unexpected large debris flow. Thus, the interpretation eral hundreds of seconds, around 10 min. Exceptionally, the of the GVS signals recorded during the events was carried debris-flow event registered on the 11 July 2010 lasted ap- out mainly by cross-checking the data from both stations proximately 10 times longer. An unusually long-lasting and (FLOW-WR and FLOW-SPI), by analysing the flow depth −1 high-intensity rainfall event (∼ 50 mm h as peak hourly recorded at the ultrasonic device, which is located very close rainfall intensity and more than 3 h of duration) accompanied to the three geophones of FLOW-SPI station (Fig. 1), and by this debris flow and generated many surges. Therefore, ex- information obtained from the field reconnaissance. cept for this July 2010 event, the registers suggest that there are no differences between debris flows and debris floods in 5.2 Results terms of duration of the IS signal. The GVS recorded during the events shows differences in amplitude and frequency according to the progression of the flowing mass over time. In Fig. 6, data from geophone Geo5 is shown for the three different events: (a) debris flow www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 936 C. Abancó et al.: Analysis of the ground vibration generated by debris flows −1 Fig. 5. Plots of the ground vibration (time vs IMP s ) during some debris flows and debris floods occurred in the Rebaixader monitoring site. Left column (a, c, e, g, i, k) corresponds to Geo3 and the right column (b, d, f, h, j, l) to Geo4. 4 July 2012, (b) debris flow 27 June 2012; and (c) debris window (Fig. 6a2 and b2). A significant increase of flood 5 July 2012. The spectral analysis for each event was energy is concentrated near 40–50 Hz. The response performed over windows of 2.3 min. This time interval was is similar in the phase of the debris flood, especially selected after evaluating different options and noticing that it when the sediment concentration reaches the maxi- was optimal for the observation of the process evolution for mum. However, the amplitude of the time series and these events. the spectra (Fig. 6c2) is lower than for the debris flows. The evolution of the seismic signal along the duration of 3. After the main front of the debris flow passes, the am- the events can be noticed by significant differences between plitude decreases again, as it does the energy. For de- the following three time windows: bris flows, the power spectra show a similar pattern as in the second window, but with lower energy (Fig. 6a3 1. The first window corresponds to the signal recorded and b3). For debris floods, the spectrum is small again, before the debris-flow front arrived at Geo 5 (Fig. 6a, as it was in the first window, indicating low energy b) or the maximum discharge is achieved in the debris (Fig. 6c3). flood (Fig. 6c). Time series show constant low ampli- tude of the ground velocity. Any significant peaks of Besides the evolution of the signal over time in each time amplitude exceed the background noise. The spectral series, global differences can be observed by comparing the analysis performed over the first window (Fig. 6a1, ground vibration data of the three different events. Fig. 6a b1 and c1) show very small energies associated to this and Fig. 6c refer to the big and small debris flows respec- phase. tively. The big event (Fig. 6a) reaches amplitudes of the time −1 2. When the debris-flow front reaches the location of series greater than 1 mm s during the pass of the flow front Geo5, the amplitude of the signal strongly increases. (Fig. 6a4). In contrast, the maximum values achieved dur- The spectral behaviour in this second window also ing the second (smaller) debris flow (Fig. 6b) are only up to −1 changes. A wider spectrum characterises this time ∼ 0.5 mm s (Fig. 6b4). Although the peaks are in a similar Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 937 Fig. 6. Ground vibration signals from Geo5 recorded previously and during the debris flow occurred on the 4 July 2012 (a), 27 June 2012 (b) and the debris flood occurred on the 5 July 2012 (c). Time series (a4, b4 and c4) and power spectra (a1 to a3, b1 to b3 and c1 to c3) are shown respectively for the three events. Red dashed lines indicate the limits of the 2.3 min intervals. Each power spectra corresponds to the time interval below, respectively. Note differences in vertical scales. frequency range (40–50 Hz), the energy associated is more 6 Effects of site-specific factors than 7 times greater in the big event than in the smaller one. In contrast, the debris flood event achieves maximum ampli- The ground vibration signal detected by the geophones in −1 tude values of ∼ 0.4 mm s (Fig. 6c4). The spectrum shows both seismic stations of the Rebaixader torrent is affected by a peak in 50–60 Hz and an energy three orders of magnitude site-specific conditions of the geophones. Some factors such lower than for the debris flows. as the distance to the flow path, the underground material, the assembly of the geophones or the ground vibration threshold www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 938 C. Abancó et al.: Analysis of the ground vibration generated by debris flows at three different geophones (Geo3, Geo3b and Geo5) were compared. These three geophones were selected, because they are installed approximately at the same cross-section of the channel (Fig. 1). Geophones Geo3 and Geo3b are located at the right side of the channel and very close together (they are only 50 cm apart). Geophone Geo5 is placed at the left side of the channel, 35 m upstream from Geo3 and Geo3b. All of them are mounted on bedrock. Geo5 and Geo3b are fixed directly on bedrock and Geo3 is mounted in a metal sheet box, which is fixed to the bedrock. As it was mentioned above, the signal at Geo5 is recorded Fig. 7. Distance vs. peak of the ground vibration signals recorded directly as GVS. Thus, the Geo5 data were transformed into during field tests. Geophones Geo6 and Geo7 are installed in collu- IS in order to be comparable with the data measured at Geo3 vium and Geo5 is installed in bedrock. and Geo3b, which were recorded as IS signal. This trans- formation was carried out as a post-process by a MATLAB code (MATLAB, 2009). The code applies the same trans- used in FLOW-WR will be studied and discussed in this sec- formations to the GVS (digitally) that is done by the sig- tion. nal conditioner of the FLOW-WR station (analogically). As a preliminary stage, a baseline correction was performed to 6.1 Underground material and distance between flow avoid offsets derived from the analogue-to-digital converter path and geophone (field tests) (ADC). Then, the GVS below a certain threshold GVth is filtered and the GVS over the threshold is transformed into In summer 2012, we carried out some field tests at station an IS signal. The GVth is applied by means of electrical re- FLOW-SPI in order to record the GVS under specific con- sistors in the signal conditioner for Geo3 and Geo3b, but as ditions. We released a 9 kg sledgehammer from a height of an input variable of the MATLAB code for Geo5. The GVth 1.5 m at different distances (0 to 20 m) from the three geo- −1 values used for the transformation into IS are: 0.17 mm s in phones Geo5, Geo6 and Geo7 along the corresponding cross- −1 Geo3 and 0.019 mm s at Geo3b. The reason for choosing section of the torrent. We performed the tests mostly twice to a 10 times higher GVth at Geo3b (fixed directly to bedrock) improve data quality. Similar tests have also been performed than at Geo3 (mounted in a metal sheet box) is given below. in other studies (Navratil et al., 2011; Kogelnig et al., 2011b). IS time series from Geo5 were obtained for both threshold The results showed that the highest amplitudes were −1 values (0.17 and 0.019 mm s ). The comparison of the re- recorded in geophone Geo6, which is buried into a thin layer sulting IS time series shows the influence of the distance and (< 50 cm) of colluvium (Fig. 7). Geophone Geo5 (fixed to the effect of the metal sheet box. In Fig. 8 the ground vi- the bedrock) shows larger amplitudes than Geo7 (buried into bration of the 4 July 2012 debris flow is presented for the thicker soil layer, > 2 m), however they are more than one three geophones: Geo3 (Fig. 8a), Geo3b (Fig. 8c) and Geo5 order of magnitude lower than in Geo6. In terms of attenua- (Fig. 8b and d). tion with distance, Geo5 and Geo6 show similar exponential The results show that the metal sheet box has a strong am- trends, while Geo7 shows a considerably lower attenuation, plification effect on the signal. The significant difference of following a linear trend. the records of Geo3 and Geo3b can only be explained by The comparison of these results with the tests carried out at the effect of the metal sheet box, which works as a reso- the Réal torrent show similarities (Navratil et al., 2011). Al- nant structure magnifying the vibration registered by the geo- though at the Réal torrent the higher amplitudes were found phone. The influence of the metal sheet box produces an in- when the geophone was fixed on a big boulder embedded crease of the values of the IS signal at Geo3, up to 10 times in a gravel deposit (situation not present at the Rebaixader), higher than the ones measured at Geo3b. In fact, the higher these were followed by the geophones placed inside the soil, −1 value of GVth was set for the Geo3 (0.17 mm s , instead which is similar to Geo6. These results can be considered as −1 of 0.019 mm s as in Geo3b) to filter the amplification of experimental results that demonstrate the variations of simi- the vibration caused by the metal box. The effect of the dis- lar signals recorded at geophones with different underground tance to the flow path can be noticed by comparing the data conditions. The attenuation with distance is evident and can from Geo3b (Fig. 8c) and Geo5 (Fig. 8d). Both geophones be observed at the three geophones. are directly mounted on bedrock and the velocity threshold −1 is the same in both cases (GVth= 0.019 mm s ), while the 6.2 Assembly of geophone and distance between flow distance between the geophones and the active channel is path and geophone greatly different (25 m at Geo3b and 3 m at Geo5). The ef- In order to identify the influence of the assembly of the geo- fect of the distance in Geo3b almost produces the loss of the −1 phone and the distance to the flow path, the signals recorded signal, as the records do not exceed the 20 IMP s . Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 939 Fig. 8. Comparison of the IS signal observed during 4 July 2012 debris flow. Data registered at Geo3 (a) and Geo3b (c) and the signal obtained from the transformation of data from Geo5 into IS time series (b and d). To summarise, the influence of the box assembly and the The most important point regarding the development of distance greatly affect the IS signal registered at the geo- a reliable warning system is the definition of the detection phones. Although an exact quantification of the effects is dif- threshold, in such a way that false alarms can be reduced ficult, it can be stated that the metal sheet box amplifies the to a minimum. In the FLOW-WR station at the Rebaixader signal stronger than the attenuation caused by 20 m distance. monitoring test site, we defined a “detection threshold” (Dth) for the monitoring system, calibrated for research purposes. The Dth is based on two thresholds: on one side the GVth, 6.3 Threshold definition for debris-flow detection and on the other side the Eth, which is formed by Ethd and Ethi (see Sect. 4.1. and Fig. 3). As suggested by the results Between August 2009 and December 2012, the “event mode” in previous sections, the site-specific factors influence the vi- was triggered 363 times. The triggers were mostly (216 bration recorded at each sensor, and the values recorded can times) caused by malfunctions in one of the geophones, be widely different from one geophone to another. For this which was caused by a rockfall in 2010 (Hürlimann et al., reason, the values of GVth and Eth should be defined for 2012). Another 126 triggers were attributed to small mass each specific geophone, according to its placement and as- movements at the lower part of the scarp area, that did not sembly. This calibration has a crucial importance for warn- progress downstream. This hypothesis is supported by the ing systems, but since in the Rebaixader site the installation observations obtained during the periodic field reconnais- was intended for research purposes, the thresholds have been sance, which indicated no apparent geomorphic changes in maintained constant and low for all the geophones. the channel reach after many of these triggers. Consequently, Using the data from the debris flows that occurred on 342 of the 363 events were not considered as significant tor- 27 June 2012 and 4 July 2012, a sensibility analysis of the rential events and were classified as “other triggers”, includ- three Dth parameters was carried out. Different values of ing both the malfunctions and the small movements that trig- GVth and Eth were tested using data recorded by the geo- gered the system. Indeed, the Eth was calibrated and adapted phones of FLOW-SPI station, where the complete register during the first monitoring year to minimise the recording of of the ground velocity signal was available (Geo5, Geo6, this type of trigger of the event mode. Geo7). First, the data were transformed into impulses using a MATLAB code and 10 different values of GVth. Then, two www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 940 C. Abancó et al.: Analysis of the ground vibration generated by debris flows −1 Fig. 9. Influence of the three parameters of the detection threshold (Dth): ground velocity threshold (GVth) vs. time over the IMP s −1 −1 threshold of the event mode (Ethi). The value of the Ethi is 10 IMP s for (a and c) and 20 IMP s for (b and d). values of Ethi (10 and 20) were chosen and the number of In conclusion, a reliable threshold should detect the de- seconds over it was calculated for each GVth value and Ethi. sired events as early as possible, but filter the ground velocity If the duration threshold Ethd is greater than the number of that does not correspond to a torrential event. The definition seconds over Ethi, no data would be recorded. Therefore the of an incorrect combination of the three threshold parameters number of seconds over Ethi corresponds to the maximum (Ethd, Ethi and GVth) could suppose missing an event, such value of Ethd that could be defined for each combination of as it can be observed for the data from the event of 27 June Ethi and GVth in order to detect the debris flow. (Fig. 9a and b), where Ethi was almost never exceeded. The results of this analysis show two major outcomes Thus, we propose that best configuration at the Rebaixader (Fig. 9). First, the number of consecutive seconds with a GVS site, for the detection including small events, would be a −1 exceeding the Ethi exponentially decreases with increasing GVth from 0.1 to 0.2 mm s , an Ethi of 20 and an Ethd of 3 GVth. This exponential decrease can be seen for both values to 5 s for the geophones with box. For the geophones directly of Ethi (10 and 20) and in both events. Second, the change of fixed at bedrock, the same Eth parameters and a much lower −1 Ethi from 10 to 20 IMP s does not influence significantly, GVth are proposed. The GVth-value depends on the distance which suggests that the most important factor for debris flow of the geophone to the active channel and should range be- −1 detection is the GVth. tween 0.005 and 0.03 mm s . For the implementation of an It is worth noting that any of the debris flows would alarm system in the future, all these threshold values must be not have been detected by the Dth parameters used tested applying the following methods: (a) a calibration of the for most of the geophones of the station FLOW-WR parameters in the field during a testing period of the system −1 (GVth= 0.17 mm s ; Ethi= 20; Ethd= 3). This fact en- (including additional field tests), or (b) a detailed sensibility forces the outcomes of the previous section on the effect of analysis of the three parameters applied over events recorded the metal sheet box, which strongly amplifies the ground vi- in FLOW-SPI and transformed into impulses using different −1 bration. Assuming a GVth – value of 0.019 mm s (as used values. However, for the second option, a greater database of at Geo3b, where no box is added), the big event (4 July) events recorded in FLOW-SPI station should be available. would have been detected by the three geophones, while the small event (27 June) would only have been detected by Geo5 and Geo6. Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 941 7 Conclusions at bedrock with another one mounted in a metal sheet box, which is attached to the bedrock. The results suggest that Monitoring torrents prone to debris flows is an increasing the metal sheet box amplifies the signal. At Rebaixader, this activity all over the world. The efficiency of the geophones amplification was useful for the detection of events, because to monitor the occurrence of torrential processes has been the geophones with a metal box were not placed close to the widely proved, and so it is their convenience for warning active channel. However, another amplification system (like purposes (Suwa and Okuda, 1985; Arattano and Moia, 1999; an electronic amplifier in the circuit board) would be more LaHusen, 2005b; Bessason et al., 2007; Huang et al., 2007; appropriate, because the exact amplification factor could be Badoux et al., 2009). However, there is a great variety of data known and controlled. recording systems, highly conditioned by the technical de- Finally, the choice of a correct detection threshold (Dth) tails of each monitoring station and many site-specific factors is fundamental, since it could produce the loss of an event that affect the ground vibration measured. or a great number of system triggers not related to torrential In this work, two different recording systems have been flows (which can result in false alarms in an alarm system). compared, both of them installed in the Rebaixader torrent In this study a sensibility analysis of the parameters of the (Central Pyrenees). One data recording system consists of Dth was carried out. The results point out that the number −1 −1 collecting the entire ground velocity signal (GVS), digitised of seconds over the IMP s threshold (10 or 20 IMP s ) at a high frequency rate (250 Hz), while the other is a sim- decreases exponentially with the ground velocity threshold plified system, which records a transformed signal (IS) at (GVth). From the sensibility analysis of the parameters it was low frequency (1 Hz). Both recording systems demonstrated noted that the ground velocity threshold GVth is the most their efficiency of recording the typical debris-flow features important of the three parameters of the Dth. For the same including the different phases of the events. Thus, both tech- reason, a too high value of GVth could induce a loss of an niques should be considered as suitable for debris-flow mon- event, which would be fatal for an alarm system. In order itoring. On the one hand, the GVS recording technique pro- to avoid the false alarms, the option would be to verify the vides more information about the signal generated by the propagation of the flowing mass by cross-checking different debris-flow passing, but it generates a large amount of data geophones. and subsequently consumes more electric power and time for Although many uncertainties are still remaining and addi- analysis. On the other hand, the IS recording technique pro- tional data must be gathered and analysed, the outcomes of vides less information on the signal, but it has been demon- this research improve the knowledge on the use of seismic strated that it is reliable for detection. Moreover, it requires sensors for the detection of debris flow and other torrential less power and simplifies the data collecting and gathering. processes and help on the design of an alarm system using These latter issues make the transformed signal especially geophones as key sensors. useful for a warning system. The data analysis showed that the differences between de- bris flows and debris floods can be observed by both record- Acknowledgements. This research has been funded by the ing techniques (GVS and IS). The differences are mainly Spanish Ministry MINECO contract CGL2011-23300 (project based on the shape of the signal and the values of the ground DEBRISTART). We would like to thank Ignasi Vilajosana from velocity. The results point out that the geophones that better Worldsensing s.l., Emma Suriñach from Barcelona University and Lluís Pujades from Technical University of Catalonia for their show the debris-flow features are the ones installed closest to collaboration on the geophysical analysis and interpretation. We the active channel, as can be expected. It is also worthwhile are grateful to Massimo Arattano and Oldrich Navratil for their that the active channel runs over bedrock on these cross- valuable comments and suggestions during the reviewing process, sections. The geophones located far from the active channel which helped to improve the manuscript. show less clearly the characteristics of debris flows. All these results suggest that the optimum position for a geophone to Edited by: B. D. Malamud obtain reliable records of debris flows would be as closest as Reviewed by: M. Arattano and O. Navratil possible to the active channel, but in a safe position to avoid potential damage by flows, and preferably where it runs over bedrock. References The site-specific factors that influence the ground vibra- tion measured at the geophones were evaluated by field tests Abancó, C., Hürlimann, M., Fritschi, B., Graf, C., and Moya, and the comparison of the GVS registered at three geo- J.: Transformation of Ground Vibration Signal for Debris-Flow phones. Two major conclusions were obtained: (a) the dis- Monitoring and Detection in Alarm Systems, Sensors, 12, 4870– tance produces a linear to exponential attenuation of the sig- 4891, 2012. nal; and (b) the assembly of the geophone can strongly condi- Arattano, M.: On the use of seismic detectors as monitoring and tion the amplification of the signal. This last conclusion was warning systems for debris flows, Nat. Hazards, 20, 197–213, clearly observed by comparing one geophone directly fixed 2000. www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 942 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Arattano, M.: Monitoring the presence of the debris flow front and Itakura, Y., Inaba, H., and Sawada, T.: A debris-flow monitoring its velocity through ground vibration detectors. Proc. 3rd Inter- devices and methods bibliography, Nat. Hazards Earth Syst. Sci., national Conference on Debris-flow Hazard Mitigation: Mechan- 5, 971–977, doi:10.5194/nhess-5-971-2005, 2005. ics, Prediction and Assessment, Millpress, Rotterdam, 719–730, Iverson, R. M.: The physics of debris flows, Rev. Geophys., 35, 2003. 245–296, 1997. Arattano, M. and Moia, F.: Monitoring the propagation of a debris Johnson, A. M. and Rodine, J. R.: Debris flow, in: Slope Stability, flow along a torrent, Hydrol. Sci. J., 44, 811–823, 1999. edited by: D. Brunsden, and Prior, D. B., John Wiley and Sons, Arattano, M., Marchi, L., and Cavalli, M.: Analysis of debris- New York, 257–361, 1984. flow recordings in an instrumented basin: confirmations and Kogelnig, A., Hübl, J., Suriñach, E., Vilajosana, I., and new findings, Nat. Hazards Earth Syst. Sci., 12, 679–686, McArdell, B.: Infrasound produced by debris flow: propaga- doi:10.5194/nhess-12-679-2012, 2012. tion and frequency content evolution, Nat. Hazards, 70, 1–21, Aulitzky, H.: Preliminary two-fold classification of torrents, doi:10.1007/s11069-011-9741-8, 2011a. Mitteilungen der Forstlichen Bundesversuchsanstalt, 243–256, Kogelnig, A., Hübl, J., Suriñach, E., Vilajosana, I., Zhang, S., 1982. Yun, N., and McArdell, B.: A study of infrasonic signals of de- Badoux, A., Graf, C., Rhyner, J., Kuntner, R., and McArdell, B.: bris flow, 5th International Conference on Debris-Flow Hazards A debris-flow alarm system for the Alpine Illgraben catchment: “Mitigation, Mechanics, Prediction and Assessment”, Padua, design and performance, Nat. Hazards, 49, 517–539, 2009. 563–572, 2011b. Berti, M., Genevois, R., La Husen, R., Simoni, A., and Tecca, P. Kurihara, J., Takezawa, N., Yamakoshi, T., and Yanagimachi, T.: R.: Debris flow monitoring in the Acquabona watershed on the Study on method of setting threshold of ground vibration sen- Dolomites (Italian Alps), Phys. Chem. Earth, 25, 707–715, 2000. sor for detecting debris flow; 4th International Conference on Bessason, B., Eiriksson, G., Thorarinsson, O., Thorarinsson, A., Debris-Flow Hazards Mitigation, edited by: Chen, C. and Ma- and Einarsson, S.: Automatic detection of avalanches and debris jor, J., Millpress, 603–611, 2007. flows by seismic methods, J. Glaciol., 53, 461–472, 2007. LaHusen, R.: Detecting debris flows using ground vibrations, 1996. Biescas, B., Dufour, F., Furdada, G., Khazaradze, G., and Suriñach, LaHusen, R.: Acoustic Flow Monitor System – User Manual, US E.: Frequency content evolution of snow avalanche seismic sig- Geological Survey, Open-File Report 02-429, 16 pp., 2005a. nals, Surv. Geophys., 24, 447–464, 2003. LaHusen, R.: Debris-flow instrumentation, in: Debris-flow Hazards Chou, H.-T., Chang, Y.-L., and Zhang, S.-C.: Acoustic signals and and Related Phenomena, edited by: Jakob, M. and Hungr, O., geophone response induced by stony-type debris flows, Inter- Springer, Berlin, 291–304, 2005b. praevent, Taipei (Taiwan), 712–720, 2010. Marchi, L., Comiti, F., Arattano, M., Cavalli, M., Macconi, P., Cui, P., Chen, X., Waqng, Y., Hu, K., and Li, Y.: Jiangjia Ravine and Penna, D.: A new debris-flow monitoring system in an debris flows in south-western China, in: Debris-flow Hazards Alpine catchment, Geophys. Res. Abstr., 14, EGU2012-6104, and Related Phenomena, edited by: Jakob, M. and Hungr, O., EGU General Assembly 2012, Vienna, Austria, 2012. Springer, Berlin, 565–594, 2005. Navratil, O., Liébault, F., Bellot, H., Theule, J., Ravanat, X., Ousset, Fang, Y. M., Huang, T. M., Lee, B. J., Chou, H. T., and Yin, F., Laigle, D., Segel, V., and Fiquet, M.: Installation d’un suivi en H. Y.: Analysis of debris flow underground sound by Wavelet continu des crues et laves torrentielles dans les Alpes françaises, Transform- A case study of events in Aiyuzih River, 5th Interna- Journée de Rencontre sur les Dangers Naturels, Lausanne, 8 pp., tional Conference on Debris-Flow Hazards Mitigation, Mechan- 2011. ics, Prediction and Assessment, Padua, 545–551, 2011. Navratil, O., Liébault, F., Bellot, H., Travaglini, E., Theule, Huang, C., Yin, H., Chen, C., Yeh, C. H., and Wang, C. L.: Ground J., Chambon, G., and Laigle, D.: High-frequency moni- vibrations produced by rock motions and debris flows, J. Geo- toring of debris-flow propagation along the Réal Torrent, phys. Res., 112, F02014, doi:10.1029/2005JF000437, 2007. Southern French Prealps, Geomorphology, 201 157–171, Hungr, O., Evans, S. G., Bovis, M. J., and Hutchinson, J. N.: A re- doi:10.1016/j.geomorph.2013.06.017, 2013. view of the classification of landslides of the flow type, Environ. Pierson, T. C.: Flow behavior of channelized debris flows, Mount St. Eng. Geosci., 7, 221–238, 2001. Helens, Washington, in: Hillslope Processes, The Binghamton Hürlimann, M., Rickenmann, D., and Graf, C.: Field and monitoring symp. in geomorphology, edited by: Abraham, A. D., Allen & data of debris-flow events in the Swiss Alps, Can. Geotech. J., 40, Unwin, Boston, 269–296, 1986. 161–175, 2003. Rickenmann, D., D’Agostino, V., Dalla Fontana, G., Lenzi, M., and Hürlimann, M., Abancó, C., and Moya, J.: Rockfalls detached from Marchi, L.: New results from sediment transport measurements a lateral moraine during spring season. 2010 and 2011 events in two Alpine torrents, Hydrology, Water Resources and Ecology observed at the Rebaixader debris-flow monitoring site (Central in Headwaters, Merano (Italy), 248, 283–289, 1998. Pyrenees, Spain), Landslides, 3, 385–393, doi:10.1007/s10346- Rickenmann, D., Turowski, J. M., Fritschi, B., Klaiber, A., and 011-0314-4, 2012. Ludwig, A.: Bedload transport measurements at the Erlenbach Hürlimann, M., Abancó, C., Moya, J., and Vilajosana, I. Results and stream with geophones and automated basket samplers, Earth experiences gathered at the Rebaixader debris-flow monitoring Surf. Proc. Landf., 37, 1000–1011, doi:10.1002/esp.3225, 2012. site, Central Pyrenees, Spain, Landslides, doi:10.1007/s10346- Suriñach, E., Furdada, G., Sabot, F., Biescas, B., and Vilaplana, J. 013-0452-y, online first, 2013. M.: On the characterization of seismic signals generated by snow Itakura, Y., Fujii, N., and Sawada, T.: Basic characteristics of avalanches for monitoring purposes, Ann. Glaciol., 32, 268–274, ground vibration sensors for the detection of debris flow, Phys. 2001. Chem. Earth, Part B, 25, 717–720, 2000. Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 943 Suriñach, E., Vilajosana, I., Khazaradze, G., Biescas, B., Furdada, Vilajosana, I., Suri´ nach, E., Abellán, A., Khazaradze, G., Garcia, G., and Vilaplana, J. M.: Seismic detection and characterization D., and Llosa, J.: Rockfall induced seismic signals: case study in of landslides and other mass movements, Nat. Hazards Earth Montserrat, Catalonia, Nat. Hazards Earth Syst. Sci., 8, 805–812, Syst. Sci., 5, 791–798, doi:10.5194/nhess-5-791-2005, 2005. doi:10.5194/nhess-8-805-2008, 2008. Suwa, H. and Okuda, S.: Measurement of debris flows in Japan Vilaplana, J. M.: Quaternary Glacial Geology of Alta Ribagorça IV International Conference and Field Workshop on Landslides, Basin (Central Southern Pyrenees), Acta Geológica Hispánica, Tokyo, 391–400, 1985. 18, 217–233, 1983. Suwa, H., Yamakoshi, T., and Sato, K.: Relationship between Yin, H. Y., Huang, C. J., Chen, C. Y., Yeh, C. H., Lee, B. J., Fang, debris-flow discharge and ground vibration, 2nd International Y. M., and Chang, Y. H.: Monitoring ground vibrations generated Conference on Debris-Flow Hazards Mitigation, Taipei, 311– by debris flows, 4th Int. Conf. on Debris-Flow Hazards Mitiga- 318, 2000. tion, Chengdu, China, 625–633, 2007. VanDine, D. F.: Debris flows and debris torrents in the southern Canadian Cordillera, Can. Geotech. J., 22, 44–68, 1985. www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Natural Hazards and Earth System Sciences Unpaywall

Analysis of the ground vibration generated by debris flows and other torrential processes at the Rebaixader monitoring site (Central Pyrenees, Spain)

Abancó, C.; Hürlimann, M.; Moya, J.
Natural Hazards and Earth System Sciences – Apr 17, 2014
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Open Access Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 Natural Hazards www.nat-hazards-earth-syst-sci.net/14/929/2014/ and Earth System doi:10.5194/nhess-14-929-2014 © Author(s) 2014. CC Attribution 3.0 License. Sciences Analysis of the ground vibration generated by debris flows and other torrential processes at the Rebaixader monitoring site (Central Pyrenees, Spain) C. Abancó, M. Hürlimann, and J. Moya Geotechnical Engineering and Geosciences Department, Technical University of Catalonia (UPC), Jordi Girona 1–3 (D2),08034 Barcelona, Spain Correspondence to: C. Abancó ([email protected]) Received: 25 July 2013 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 30 August 2013 Revised: 8 January 2014 – Accepted: 26 February 2014 – Published: 17 April 2014 Abstract. Monitoring of debris flows using ground vibration Although debris-flows monitoring has strongly improved sensors has increased in the last two decades. However, the during the last decades and several torrential catchments in correct interpretation of the signals still presents ambiguity. the world have been instrumented with different types of In the Rebaixader monitoring site (Central Pyrenees, Spain) sensors and techniques (Itakura et al., 2005), this is still a two different ground vibration stations are installed. At the challenging topic in debris-flow research. Apart from de- first station the ground velocity signal is transformed into bris flows, monitoring is also used for the analysis of other an impulses-per-second signal (low frequency, 1 Hz). The types of rapid mass movements like snow avalanches or rock- analysis of the data recorded at this station show that the falls (Suriñach et al., 2005; Bessason et al., 2007; Vilajosana shape of the impulses signal is one of the key parameters to et al., 2008) and bedload transport in rivers and torrents describe the evolution of the event. At the second station the (Rickenmann et al., 1998, 2012). Torrential processes, espe- ground velocity signal is directly recorded at high frequency cially debris flows, generate seismic waves in the ground, (250 Hz). The results achieved at this station show that the originated by the collision between boulders or between differences in time series and spectral analysis are helpful boulders and the bedrock. These vibrations can be measured to describe the temporal evolution of the events. In addition, by several seismic and sonic devices such as geophones, some general outcomes were obtained: the attenuation of the seismographs or infrasounds (Itakura et al., 2005; Kogel- signal with the distance has been identified as linear to ex- nig et al., 2011a). Geophones are the most common seismic ponential; and the assembly of the geophones to the terrain sensors used in debris-flow monitoring because of their ro- has an important effect on the amplification of the signal. All bustness and low power consumption. These features make these results highlight that the definition of ground vibration them also very suitable not only for monitoring, but also thresholds for debris-flow detection or warning purposes is a for warning purposes. All over the world, several sites have difficult task; and that influence of site-specific conditions is been instrumented with geophones: Illgraben in Switzerland notable. (Hürlimann et al., 2003), Lattenbach in Austria (Kogelnig et al., 2011a), Moscardo (Arattano et al., 2012), Acquabona (Berti et al., 2000) or Gadria (Marchi et al., 2012) in Italy, 1 Introduction Manival or Réal in France (Navratil et al., 2011), Mount St. Helens in USA (LaHusen, 2005b), Houyenshan (Chou et al., Debris flows are one of the most hazardous geomorphologic 2010), Fong-Ciou Creek or Ai-Yu-Zi Creek (Huang et al., processes. In order to improve the understanding of debris- 2007; Fang et al., 2011) in Taiwan, and Jiangjia in China flow mechanisms, torrents are being instrumented with an (Cui et al., 2005) are some examples. increasing variety of sensors. The data collected are not only needed to calibrate numerical models, but also to develop and adjust warning systems. Published by Copernicus Publications on behalf of the European Geosciences Union. 930 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Some analyses of geophone signals induced by debris and a sensibility analysis of the threshold values. The out- flows have been published during the last decades (Arattano comes of this research improve the knowledge on some cur- and Moia, 1999; Berti et al., 2000; Hürlimann et al., 2003; rent issues (i.e. process differentiation, geophone location, Huang et al., 2007; Chou et al., 2010; Arattano et al., 2012). recording method or threshold assessment) and should help All these studies have substantially increased our knowledge for the set-up of future debris-flow monitoring or warning on the dynamic behaviour of debris flows and the ground vi- systems. bration they induce. However, there are still many open ques- tions, such as the use of the ground vibration for the defini- 2 Debris flow characterisation by ground vibration tion of thresholds; in particular, because a threshold is key monitoring information not only for the detection and warning of events, but also for the distinction between different flow types (e.g. 2.1 Debris-flow features debris flows vs. debris floods). The ground velocity signal can be recorded by two dif- Debris flows are rapid landslides formed by water and solid ferent approaches: (a) continuously (e.g. in Moscardo tor- material poorly sorted, from boulder to clay (Iverson, 1997). rent; Arattano and Moia, 1999); and, (b) by switching from Pierson (1986) describes a typical debris flow in three parts: a no-event mode into an event-mode (e.g. in the Swiss tor- the front, the fully developed debris flow (also called “body”) rents; Hürlimann et al., 2003). The latter approach needs the and the tail. The front carries the biggest boulders and is fol- incorporation of a trigger into the recording algorithm and lowed by the debris flow body; both of them are characterised the correct definition of its value. Different types of trig- by a high sediment concentration and a turbulent regime. At gers can be found in the literature, such as (a) level trig- last, there is the tail with much less solid material concen- gers: fixed value of the ground velocity (LaHusen, 2005a) tration, which can also be described as a hyperconcentrated or fixed values of a transformed signal (Badoux et al., 2009; flow. Many debris-flow events occur in a series of surges, Hürlimann et al., 2013); or (b) more sophisticated thresh- each of them showing a front, a body and a tail (Pierson, olds based on the frequency content of the signal (Bessa- 1986; Johnson and Rodine, 1984). son et al., 2007). The type of trigger applied mainly depends The coexistence of torrential processes has been noted in on the data recording system implemented at the site. Sev- the Rebaixader site. Debris floods can be defined as episodes eral systems are employed, including (a) analogical record- of massive bedload transport characterised by a limited max- ing (Arattano and Moia, 1999); (b) digital sampling (Arat- imum grain size (Aulitzky, 1982). Debris floods are also de- tano, 2000; Kogelnig et al., 2011b); and, (c) transformations scribed as very rapid surging flows of water in a steep chan- of ground vibration velocity signal (Navratil et al., 2011; nel heavily charged with debris (Hungr et al., 2001). A de- Abancó et al., 2012). bris flood may transport quantities of sediment comparable Normally, the threshold value (level triggers) is established to a debris flow, in the form of massive surges. However, the combining an empirical analysis of the signals of past events transport is carried out by the tractive forces of water over- and expert criteria (Kurihara et al., 2007). The threshold has lying the debris. As a result, the peak discharge of a debris to be defined at each geophone, as there are several site- flood is comparable to that of a water flood (perhaps mul- specific factors that influence the vibration recorded at the tiplied by a factor up to 2). This fact clearly contrasts with seismic sensors. An accurate assessment of the threshold is of the peak discharges of debris flows, which are tens of times crucial importance, especially in warning systems, when the greater than major water floods (VanDine, 1985; Hungr et al., detection of events triggers some kind of alarm process, such 2001). Another important difference between debris flows as the closing of traffic lines or messages to the stakeholders. and debris floods is the absence of the bouldery front. However, there are only very few studies dealing with the Sediment concentration and boulder content alters the en- influence of the site-specific factors affecting the vibration ergy transmitted to the ground. Thus, on one hand debris induced by debris-flow events (Huang et al., 2007; Navratil flows can be distinguished from other torrential processes et al., 2011). and, on the other hand, the different phases of a debris flow In this paper the features of the ground vibration signals can be detected (e.g. Huang et al., 2007; Navratil, 2013). registered at two monitoring stations located in the Rebaix- ader monitoring site are analysed. The main difference be- 2.2 Monitoring of debris-flow induced ground vibration tween the two stations is the data recording system, but also some other aspects regarding the mounting and the location Velocity of ground movement is transduced by a geophone to of the geophones. The major purpose of this work is to define a voltage that is (generally linearly) related to the ground ve- the main characteristics of debris flows and other torrential locity. The digital measuring of the geophone output is done processes using the seismic signal recorded at the two sta- by sampling the signal at a fixed frequency. To avoid aliasing tions (each with a different data recording system). Other ob- problems, the sampling rate must be greater than the Nyquist jectives are the analysis of the influence of some site-specific frequency, which is twice the highest frequency of the sig- factors on the ground vibration signal by means of field tests nal. Digital sampling is used to record the signal from the Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 931 geophones, but other techniques based on the transformation a) of the original signal into simpler data have also been devel- oped (Arattano, 2000; Navratil et al., 2011; Abancó et al., 2012). These data recording systems are widely described in the following sections. Several features of moving debris flows have been deter- mined due to the analyses of the ground vibrations time se- ries. For instance, the correspondence between the flow stage and the ground velocity signal (Arattano and Moia, 1999), or the increase of the amplitude of the ground vibration as the flow front approaches to the seismic sensor (Arattano et al., 1999). Furthermore, the flow volume was correlated with the time integral of the acceleration amplitude (Suwa et al., 2000). Other authors found some general patterns in the fre- quency domain. For instance, LaHusen (1996) described the typical peak frequency range of the debris flows between 30 and 80 Hz, or Huang et al. (2007) suggested this range from 50 to 100 Hz. 2.3 Ground vibration site-specific factors b) Both the amplitude and the frequency of the signal measured by the geophones depend on several site-specific factors (Yin et al., 2007; Navratil et al., 2013). The influencing factors considered herein are the distance between the sensor and the debris-flow path, the material in the channel and in the channel banks and the assembly of the geophone. Geophones are generally installed outside the channel bed, in a protected location, to avoid damage when a torren- tial event occurs. However, waves are attenuated with the distance and they do not travel long distances (LaHusen, 2005b). For this reason, the distance between sensor and flow path is a crucial factor and geophones are commonly installed not further than a few tens of metres from the active Fig. 1. (a) The Rebaixader torrent, its fan and source area. Seis- 2 Figure 1: a) The Rebaixader torrent, its fan and source area. Seismic stations (FLOW-WR and mic stations (FLOW-WR and FLOW-SPI) and the corresponding channel or directly on its lateral banks. 3 FLOW-SPI) and the corresponding geophones are indicated and labelled. The ultrasonic geophones are indicated and labelled. The ultrasonic device is rep- The attenuation of the seismic waves depends on the prop- resented by a black line in the middle of the channel reach. Inset 4 device is represented by a black line in the middle of the channel reach. Inset shows the erties of the material the wave travels through. Depending on shows the location of the Rebaixader site; (b) detailed location of the material, the absorption of the energy by the ground is 5 location of the Rebaixader site; b) detailed location of the sensors at the channel section. the sensors at the channel section. higher or lower (Itakura et al., 2000; Suriñach et al., 2001; Biescas et al., 2003). Also the physical properties of the transmission medium affect the velocity of the waves. For 3 Description of the Rebaixader site −1 example, P wave velocity ranges from about 350 m s in −1 alluvium up to 700 m s in bedrock (Arattano and Moia, 3.1 General setting 1999). When geophones cannot be buried in soil, the sensors must The Rebaixader catchment is a first-order basin with an area be fixed to the bedrock, big boulders or existing concrete 2 of 0.53 km , which is located in the Central Pyrenees near structures (e.g. check dams). In this case, the method of fix- the village of Senet (Fig. 1). The catchment has the typical ing the geophones to these hard surfaces controls the transfer morphology of a torrential basin formed by three zones (ero- of vibrations to the sensor; and, consequently, has a strong sional source area, channel zone and fan). The source area influence on the signal recorded. Since the surfaces are of- ◦ ◦ has a steep slope (average of 29 , but up to 50 ), an area ten irregular, different assembly systems are designed in the 2 of 0.09 km and it is located between 1425 and 1710 m a.s.l. existing monitoring stations (Abancó et al., 2012). ◦ (Fig. 1). The channel zone has an average slope of 21 , is 250 m long and about 20 m wide and is located between 1425 and 1350 m a.s.l. Downstream of the channel zone, there is a 2 ◦ fan with an area of 0.082 km and a mean slope of 17 . The www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 932 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Fig. 2. (a) Downstream view inside the channel indicating the places where the geophones are placed. Pictures of the detailed assemblies are shown in (b) to (d). Picture (d) was taken during the installation of Geo6, before being covered. Noguera Ribagorçana River defines the lower boundary of stations regarding the detection and characterisation of the the fan. There is no protection works in the Rebaixader tor- flow dynamics. Further details about the instrumentation and rent. the events recorded can be found in Hürlimann et al. (2013). The geology of the source area consists of a thick till Herein, we focus on the ground vibration recorded at the two deposit over a bedrock of slates and phyllites of Devonian flow dynamic stations (FLOW-WR and FLOW-SPI in Fig. 1). age. The bedrock crops out only locally in the source and The geophones of both stations are 1-D vertical, moving coil forms the margins of the channel zone. The till corresponds geophones (Geospace 20-DX) with a natural frequency of to a lateral moraine of the glacier that occupied the Noguera 8 Hz and a spurious frequency of 200 Hz. The main differ- Ribargorçana Valley during the last glacial cycle (Vilaplana, ence between the stations is the data recording system. The 1983). data acquisition in station FLOW-WR is based on a low sam- The meteorological conditions of the site are affected by pling rate of a transformed signal (Abancó et al., 2012), while the proximity of the Mediterranean Sea, the influence of in station FLOW-SPI the high sampling rate provides data on the northern Atlantic winds and the orographic effects of the original ground velocity signal. the Pyrenees. The annual precipitation ranges from 800 to The station FLOW-WR includes five geophones, an ultra- 1200 mm. The debris flows and debris floods analysed in this sonic device for stage measurements and a video camera. The study are mostly triggered by convective storms in the sum- sampling frequency for the geophones and the ultrasonic de- mer, which are characterised by short and intense rainfalls. vice when an event is detected is 1 Hz. The sensors are con- However, it has also been observed that rainfalls of lower in- nected by wires and controlled by a Campbell CR1000 data tensities accompanied by snowmelt can also trigger events in logger, which is powered by a 12 V 24 Ah battery, charged by spring (Hürlimann et al., 2013). a 30W solar panel. The data are transmitted via GSM modem to our server in Barcelona. The geophones are distributed 3.2 Monitoring network along 175 m at the right side of the torrent, between 1415 and 1345 m a.s.l (Figs. 1 and 2a). The distances between geo- The monitoring system installed in the Rebaixader torrent in- phones are up to 75 m, and the distances between the sensors cludes, on one side, four stations measuring the meteorologi- and the active channel range from 8 to 25 m (Table 1). Four cal and hydrological conditions in the catchment for the anal- of the five geophones are mounted by a metal sheet box to ysis of the debris-flow initiation and, on the other side, two Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 933 Table 1. Summary of main characteristics of the geophones analysed in this paper. IS stands for “impulses per second” and GVS stands for “ground velocity signal”. Geophone Mounting Station Distance to active Material (abbreviation) (data recording channel (planimetric at the system) distance in m) cross section Geophone 3 (Geo3) Metal sheet box attached to bedrock FLOW-WR (IS) 25 Colluvium and bedrock Geophone 3b (Geo3b) Bedrock FLOW-WR (IS) 25 Colluvium and bedrock Geophone 4 (Geo4) Metal sheet box attached to bedrock FLOW-WR (IS) 8 Bedrock Geophone 5 (Geo5) Bedrock FLOW-SPI (GVS) 3 Bedrock Geophone 6 (Geo6) Buried into soil FLOW-SPI (GVS) 3 Colluvium and bedrock Geophone 7 (Geo7) Buried into soil FLOW-SPI (GVS) 5 Colluvium and bedrock the bedrock (geophones Geo1 to Geo4 in Fig. 2b). Each box correspond to seismic noise of the site. Second, the voltage is protected by a plastic structure in order to avoid the im- exceeding a certain threshold is transformed into an impulses pact of raindrops or hail on it. The fifth geophone (Geo3b) is signal. This filtering and transformation is made analogi- fixed directly on the bedrock without a metal box. It is also cally by a set of electrical resistors in the conditioning circuit protected by a plastic structure like the other geophones. board, which acts like a threshold voltage. Since at Rebaix- The station FLOW-SPI was set up in June 2012 in order to ader site, two types of geophone assemblies have been ap- record the ground vibration at high frequency (250 Hz). The plied, and two values of “ground velocity threshold” (GVth) station contains three geophones, which are located at the left have been defined. For the geophones mounted in a metal side of the channel (Figs. 1 and 2a). The geophones are lo- sheet box (Geo1, Geo2, Geo3, Geo4), the threshold corre- −1 cated between 3 and 5 m from the active channel, thus much sponds to a velocity of 0.17 mm s . The other geophone closer than those of the station FLOW-WR (Table 1). At this (Geo3b), which was fixed directly to the bedrock and no station, all the geophones are fixed directly to the ground. resonance effect of the metal box is expected, the velocity −1 Two of them (geophones Geo6 and Geo7) are buried in the threshold is much lower (GVth= 0.019 mm s ). After this soil (granular colluvium) at a depth of about 20 cm (Fig. 2d), filtering, the signal is transformed into an impulse signal by while the third one (Geo5) is fixed to the bedrock (Fig. 2c) the conditioning circuit (for further details, see Abancó et and protected by a plastic structure as in the FLOW-WR sta- al., 2012). Finally, the signal is sent to the data logger, which tion. Data logging is carried out by a 24 bits broadband seis- counts the number of impulses each second. mic recording unit (Spider, manufactured by WorldSensing The frequency of measuring is controlled by the CR1000 s.l.), powered by a battery of 12 V, 22 Ah, and charged by data logger, which was programmed to scan the geophones of a 50 W solar panel. The Spider sends the data to a gateway, the station every second. To avoid high power consumption where they are resent to our server via GSM modem. and to optimise the memory management of the data files, an algorithm was introduced into the CR1000 data logger and the recording is not carried out continuously, but only when the number of impulses per second exceeds a threshold. This 4 Analysis of transformed ground velocity signal threshold is called “event mode threshold” and is based on the number of impulses per second cumulated during a cer- 4.1 Methods tain time span (Fig. 3). Therefore, this “event mode thresh- The data recording system at the FLOW-WR station is based old” (Eth) includes two components: (a) the number of im- on the transformation of the original signal, which corre- pulses of the Eth (Ethi); and, (b) the duration of the time span sponds to a voltage signal proportional to the ground veloc- in which Ethi is exceeded (Ethd). The event mode thresh- ity, into a signal consisting of impulses per second (Abancó old was defined progressively by analysing the data of the first year of the monitoring period. Since August 2010, the et al., 2012). The signal transformation is carried out by −1 Ethi has been fixed at 20 impulses per second (IMP s ) with an electronic conditioning circuit board that is connected to the Ethd established as three consecutive seconds. When the each geophone (Fig. 2b). The aim of the transformation is threshold is exceeded in any of the geophones of the station, twofold: (a) it filters and deletes the ground vibration noise; the “event mode” is triggered by the data logger code and the and, (b) the impulses per second (IS) data constitute a sim- signal is recorded each second. Event mode is deactivated af- ple discretised signal, which can be analysed more easily and ter 2 minutes, with vibration smaller than Ethi scanned in any with lower memory requirements. of the geophones. The recording is also carried out during the The signal transformation consists of two parts. First, the “no event mode” to monitor the noise and the performance of original voltage delivered by the geophone is filtered in or- the system; although at a much lower frequency (each hour). der to remove low ground velocities, which are assumed to www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 934 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Fig. 3. Flow chart of the event detection of FLOW-WR system in the Rebaixader. In italics, the value of the parameters used nowadays in the Rebaixader. Fig. 4. Typical shapes of the IS signal registered during a debris flow (a), debris flood (b), and rockfall (c). Horizontal and vertical scales are the same in the three cases. For each process, a snapshot from the video camera is shown. As it is shown below, several types of events (debris flows, after most of the events to identify geomorphic changes in debris floods and rockfalls) were recorded in the Rebaixader the torrent. torrent. The analysis of the IS times series revealed different 4.2 Results types of responses (IS curve morphologies). Finally, these IS curve morphologies were assigned to different types of For the whole monitoring period, 21 torrential events have torrential processes by means of cross-checking the vibration been recorded by the station since its installation in summer gathered in the five geophones, the flow depth measured by 2009: 6 debris flows, 11 debris floods and 4 rockfalls. Re- the ultrasonic device, the video images (available only for garding the shape of the IS time series curves, three types of 10 events) and periodic field trips (31 campaigns) carried out curves were distinguished (Fig. 4). Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 935 A Type A curve is characterised by three phases (Fig. 4a): Table 2. Characteristics of the events analysed in this work. Vol- umes were estimated using the records gathered at the different sen- (a) a first phase of stationary level of no or very low IS values, sors (geophones, ultrasonic device video-camera) and field observa- (b) an abrupt increase of the impulses, reaching values over −1 tions (see Hürlimann et al., 2013 for further information). 100 IMP s in less than 5 s, followed by (c) a slow (mostly exponential) decrease. Date (dd/mm/yyyy) Type Volume (m ) A Type B curve consists of a first phase of gradual in- crease of IS-values, which is followed by a gradual decrease 04/07/2012 Debris flow 16200 (Fig. 4b). 11/07/2010 Debris flow 12500 A Type C curve is defined by a very short duration (2 to 27/06/2012 Debris flow 4000 05/08/2011 Debris flood 2500 5 s fast increase of the IS-values, with a high maximum (up −1 25/03/2010 Debris flow 2100 to 190 IMP s ; Fig. 4c). 07/06/2012 Debris flood 750 Video images and geomorphological reconnaissance clearly showed that A-curves were recorded during debris- flow events (Fig. 5b, d, f and h), the B-curves was associ- ated with debris floods or immature phases of debris flows, 5 Analysis of original ground velocity signal and C-curves were related to rockfalls (Hürlimann et al., 2012). However, only Geo4 recorded A-curves for all the de- 5.1 Methods bris flows. The time series recorded at the upper geophones show B-curves, especially during the “small-magnitude” de- The station FLOW-SPI records the geophone signal directly bris flows (Fig. 5a and e). These facts suggest that some de- as a voltage and represents the vertical velocity of ground vi- bris flows may not be fully developed with a well-defined bration. The data provided by FLOW-SPI station differ from front until they reach the location of Geo4. This geophone the FLOW-WR station in two main points: (a) the record- is placed in the most downstream position (close to the fan ing of the ground velocity signal (GVS) is continuous with- apex) of the network. This interpretation is supported by the out distinction between “event” and “no event” modes; and observations of Arattano (2003) in Moscardo, where in some (b) the signal is recorded without filtering the noise. The data events the proper debris-flow front was only visible down- are stored in “mseed” files, a typically seismological format. stream of the fan apex. In addition, it should be noted that Each of these files contain approximately 30 min of data sam- geophones 1 to 3 are located at greater distances from the pled at 250 Hz (250 samples per second). active channel (15 to 25 m) than Geo4 (8 m), and that the at- The sampling frequency depends on the nature of the pro- tenuation of the vibration with distance may play a role in the cess and the site-specific characteristics of the geophones recordings of debris flows, as it is shown below. Besides the and their placement. Preliminary spectral analyses of some −1 shape of the curve, the peak of IMP s time series at Geo4 is flow events in the Rebaixader catchment indicated frequency useful to distinguish between debris flows and debris floods. ranges between 30 and 100 Hz. Therefore, a sampling fre- The values of peak vibration in this geophone never exceeded quency of 250 Hz is sufficient in our case. −1 100 IMP s for debris floods, while the values are from 130 FLOW-SPI station was installed in summer 2012, and for −1 up to 211 IMP s for debris flows. In contrast, the highest this reason only three events were recorded. Due to the small values and the shortest durations of vibration were recorded number of events, the distinction between types of events or in Geo1, the uppermost geophone, and correspond to rock- their features by a detailed GVS analysis (as performed for falls. the IS time series) was not possible. Video images were avail- Most of the recorded debris flows and debris floods present able only for one of the events because the first two events similar durations (Fig. 5), though they show a wide range occurred at night and the infrared spot lights were damaged of volume (Table 2). In general, these flow events last sev- by an unexpected large debris flow. Thus, the interpretation eral hundreds of seconds, around 10 min. Exceptionally, the of the GVS signals recorded during the events was carried debris-flow event registered on the 11 July 2010 lasted ap- out mainly by cross-checking the data from both stations proximately 10 times longer. An unusually long-lasting and (FLOW-WR and FLOW-SPI), by analysing the flow depth −1 high-intensity rainfall event (∼ 50 mm h as peak hourly recorded at the ultrasonic device, which is located very close rainfall intensity and more than 3 h of duration) accompanied to the three geophones of FLOW-SPI station (Fig. 1), and by this debris flow and generated many surges. Therefore, ex- information obtained from the field reconnaissance. cept for this July 2010 event, the registers suggest that there are no differences between debris flows and debris floods in 5.2 Results terms of duration of the IS signal. The GVS recorded during the events shows differences in amplitude and frequency according to the progression of the flowing mass over time. In Fig. 6, data from geophone Geo5 is shown for the three different events: (a) debris flow www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 936 C. Abancó et al.: Analysis of the ground vibration generated by debris flows −1 Fig. 5. Plots of the ground vibration (time vs IMP s ) during some debris flows and debris floods occurred in the Rebaixader monitoring site. Left column (a, c, e, g, i, k) corresponds to Geo3 and the right column (b, d, f, h, j, l) to Geo4. 4 July 2012, (b) debris flow 27 June 2012; and (c) debris window (Fig. 6a2 and b2). A significant increase of flood 5 July 2012. The spectral analysis for each event was energy is concentrated near 40–50 Hz. The response performed over windows of 2.3 min. This time interval was is similar in the phase of the debris flood, especially selected after evaluating different options and noticing that it when the sediment concentration reaches the maxi- was optimal for the observation of the process evolution for mum. However, the amplitude of the time series and these events. the spectra (Fig. 6c2) is lower than for the debris flows. The evolution of the seismic signal along the duration of 3. After the main front of the debris flow passes, the am- the events can be noticed by significant differences between plitude decreases again, as it does the energy. For de- the following three time windows: bris flows, the power spectra show a similar pattern as in the second window, but with lower energy (Fig. 6a3 1. The first window corresponds to the signal recorded and b3). For debris floods, the spectrum is small again, before the debris-flow front arrived at Geo 5 (Fig. 6a, as it was in the first window, indicating low energy b) or the maximum discharge is achieved in the debris (Fig. 6c3). flood (Fig. 6c). Time series show constant low ampli- tude of the ground velocity. Any significant peaks of Besides the evolution of the signal over time in each time amplitude exceed the background noise. The spectral series, global differences can be observed by comparing the analysis performed over the first window (Fig. 6a1, ground vibration data of the three different events. Fig. 6a b1 and c1) show very small energies associated to this and Fig. 6c refer to the big and small debris flows respec- phase. tively. The big event (Fig. 6a) reaches amplitudes of the time −1 2. When the debris-flow front reaches the location of series greater than 1 mm s during the pass of the flow front Geo5, the amplitude of the signal strongly increases. (Fig. 6a4). In contrast, the maximum values achieved dur- The spectral behaviour in this second window also ing the second (smaller) debris flow (Fig. 6b) are only up to −1 changes. A wider spectrum characterises this time ∼ 0.5 mm s (Fig. 6b4). Although the peaks are in a similar Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 937 Fig. 6. Ground vibration signals from Geo5 recorded previously and during the debris flow occurred on the 4 July 2012 (a), 27 June 2012 (b) and the debris flood occurred on the 5 July 2012 (c). Time series (a4, b4 and c4) and power spectra (a1 to a3, b1 to b3 and c1 to c3) are shown respectively for the three events. Red dashed lines indicate the limits of the 2.3 min intervals. Each power spectra corresponds to the time interval below, respectively. Note differences in vertical scales. frequency range (40–50 Hz), the energy associated is more 6 Effects of site-specific factors than 7 times greater in the big event than in the smaller one. In contrast, the debris flood event achieves maximum ampli- The ground vibration signal detected by the geophones in −1 tude values of ∼ 0.4 mm s (Fig. 6c4). The spectrum shows both seismic stations of the Rebaixader torrent is affected by a peak in 50–60 Hz and an energy three orders of magnitude site-specific conditions of the geophones. Some factors such lower than for the debris flows. as the distance to the flow path, the underground material, the assembly of the geophones or the ground vibration threshold www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 938 C. Abancó et al.: Analysis of the ground vibration generated by debris flows at three different geophones (Geo3, Geo3b and Geo5) were compared. These three geophones were selected, because they are installed approximately at the same cross-section of the channel (Fig. 1). Geophones Geo3 and Geo3b are located at the right side of the channel and very close together (they are only 50 cm apart). Geophone Geo5 is placed at the left side of the channel, 35 m upstream from Geo3 and Geo3b. All of them are mounted on bedrock. Geo5 and Geo3b are fixed directly on bedrock and Geo3 is mounted in a metal sheet box, which is fixed to the bedrock. As it was mentioned above, the signal at Geo5 is recorded Fig. 7. Distance vs. peak of the ground vibration signals recorded directly as GVS. Thus, the Geo5 data were transformed into during field tests. Geophones Geo6 and Geo7 are installed in collu- IS in order to be comparable with the data measured at Geo3 vium and Geo5 is installed in bedrock. and Geo3b, which were recorded as IS signal. This trans- formation was carried out as a post-process by a MATLAB code (MATLAB, 2009). The code applies the same trans- used in FLOW-WR will be studied and discussed in this sec- formations to the GVS (digitally) that is done by the sig- tion. nal conditioner of the FLOW-WR station (analogically). As a preliminary stage, a baseline correction was performed to 6.1 Underground material and distance between flow avoid offsets derived from the analogue-to-digital converter path and geophone (field tests) (ADC). Then, the GVS below a certain threshold GVth is filtered and the GVS over the threshold is transformed into In summer 2012, we carried out some field tests at station an IS signal. The GVth is applied by means of electrical re- FLOW-SPI in order to record the GVS under specific con- sistors in the signal conditioner for Geo3 and Geo3b, but as ditions. We released a 9 kg sledgehammer from a height of an input variable of the MATLAB code for Geo5. The GVth 1.5 m at different distances (0 to 20 m) from the three geo- −1 values used for the transformation into IS are: 0.17 mm s in phones Geo5, Geo6 and Geo7 along the corresponding cross- −1 Geo3 and 0.019 mm s at Geo3b. The reason for choosing section of the torrent. We performed the tests mostly twice to a 10 times higher GVth at Geo3b (fixed directly to bedrock) improve data quality. Similar tests have also been performed than at Geo3 (mounted in a metal sheet box) is given below. in other studies (Navratil et al., 2011; Kogelnig et al., 2011b). IS time series from Geo5 were obtained for both threshold The results showed that the highest amplitudes were −1 values (0.17 and 0.019 mm s ). The comparison of the re- recorded in geophone Geo6, which is buried into a thin layer sulting IS time series shows the influence of the distance and (< 50 cm) of colluvium (Fig. 7). Geophone Geo5 (fixed to the effect of the metal sheet box. In Fig. 8 the ground vi- the bedrock) shows larger amplitudes than Geo7 (buried into bration of the 4 July 2012 debris flow is presented for the thicker soil layer, > 2 m), however they are more than one three geophones: Geo3 (Fig. 8a), Geo3b (Fig. 8c) and Geo5 order of magnitude lower than in Geo6. In terms of attenua- (Fig. 8b and d). tion with distance, Geo5 and Geo6 show similar exponential The results show that the metal sheet box has a strong am- trends, while Geo7 shows a considerably lower attenuation, plification effect on the signal. The significant difference of following a linear trend. the records of Geo3 and Geo3b can only be explained by The comparison of these results with the tests carried out at the effect of the metal sheet box, which works as a reso- the Réal torrent show similarities (Navratil et al., 2011). Al- nant structure magnifying the vibration registered by the geo- though at the Réal torrent the higher amplitudes were found phone. The influence of the metal sheet box produces an in- when the geophone was fixed on a big boulder embedded crease of the values of the IS signal at Geo3, up to 10 times in a gravel deposit (situation not present at the Rebaixader), higher than the ones measured at Geo3b. In fact, the higher these were followed by the geophones placed inside the soil, −1 value of GVth was set for the Geo3 (0.17 mm s , instead which is similar to Geo6. These results can be considered as −1 of 0.019 mm s as in Geo3b) to filter the amplification of experimental results that demonstrate the variations of simi- the vibration caused by the metal box. The effect of the dis- lar signals recorded at geophones with different underground tance to the flow path can be noticed by comparing the data conditions. The attenuation with distance is evident and can from Geo3b (Fig. 8c) and Geo5 (Fig. 8d). Both geophones be observed at the three geophones. are directly mounted on bedrock and the velocity threshold −1 is the same in both cases (GVth= 0.019 mm s ), while the 6.2 Assembly of geophone and distance between flow distance between the geophones and the active channel is path and geophone greatly different (25 m at Geo3b and 3 m at Geo5). The ef- In order to identify the influence of the assembly of the geo- fect of the distance in Geo3b almost produces the loss of the −1 phone and the distance to the flow path, the signals recorded signal, as the records do not exceed the 20 IMP s . Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 939 Fig. 8. Comparison of the IS signal observed during 4 July 2012 debris flow. Data registered at Geo3 (a) and Geo3b (c) and the signal obtained from the transformation of data from Geo5 into IS time series (b and d). To summarise, the influence of the box assembly and the The most important point regarding the development of distance greatly affect the IS signal registered at the geo- a reliable warning system is the definition of the detection phones. Although an exact quantification of the effects is dif- threshold, in such a way that false alarms can be reduced ficult, it can be stated that the metal sheet box amplifies the to a minimum. In the FLOW-WR station at the Rebaixader signal stronger than the attenuation caused by 20 m distance. monitoring test site, we defined a “detection threshold” (Dth) for the monitoring system, calibrated for research purposes. The Dth is based on two thresholds: on one side the GVth, 6.3 Threshold definition for debris-flow detection and on the other side the Eth, which is formed by Ethd and Ethi (see Sect. 4.1. and Fig. 3). As suggested by the results Between August 2009 and December 2012, the “event mode” in previous sections, the site-specific factors influence the vi- was triggered 363 times. The triggers were mostly (216 bration recorded at each sensor, and the values recorded can times) caused by malfunctions in one of the geophones, be widely different from one geophone to another. For this which was caused by a rockfall in 2010 (Hürlimann et al., reason, the values of GVth and Eth should be defined for 2012). Another 126 triggers were attributed to small mass each specific geophone, according to its placement and as- movements at the lower part of the scarp area, that did not sembly. This calibration has a crucial importance for warn- progress downstream. This hypothesis is supported by the ing systems, but since in the Rebaixader site the installation observations obtained during the periodic field reconnais- was intended for research purposes, the thresholds have been sance, which indicated no apparent geomorphic changes in maintained constant and low for all the geophones. the channel reach after many of these triggers. Consequently, Using the data from the debris flows that occurred on 342 of the 363 events were not considered as significant tor- 27 June 2012 and 4 July 2012, a sensibility analysis of the rential events and were classified as “other triggers”, includ- three Dth parameters was carried out. Different values of ing both the malfunctions and the small movements that trig- GVth and Eth were tested using data recorded by the geo- gered the system. Indeed, the Eth was calibrated and adapted phones of FLOW-SPI station, where the complete register during the first monitoring year to minimise the recording of of the ground velocity signal was available (Geo5, Geo6, this type of trigger of the event mode. Geo7). First, the data were transformed into impulses using a MATLAB code and 10 different values of GVth. Then, two www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 940 C. Abancó et al.: Analysis of the ground vibration generated by debris flows −1 Fig. 9. Influence of the three parameters of the detection threshold (Dth): ground velocity threshold (GVth) vs. time over the IMP s −1 −1 threshold of the event mode (Ethi). The value of the Ethi is 10 IMP s for (a and c) and 20 IMP s for (b and d). values of Ethi (10 and 20) were chosen and the number of In conclusion, a reliable threshold should detect the de- seconds over it was calculated for each GVth value and Ethi. sired events as early as possible, but filter the ground velocity If the duration threshold Ethd is greater than the number of that does not correspond to a torrential event. The definition seconds over Ethi, no data would be recorded. Therefore the of an incorrect combination of the three threshold parameters number of seconds over Ethi corresponds to the maximum (Ethd, Ethi and GVth) could suppose missing an event, such value of Ethd that could be defined for each combination of as it can be observed for the data from the event of 27 June Ethi and GVth in order to detect the debris flow. (Fig. 9a and b), where Ethi was almost never exceeded. The results of this analysis show two major outcomes Thus, we propose that best configuration at the Rebaixader (Fig. 9). First, the number of consecutive seconds with a GVS site, for the detection including small events, would be a −1 exceeding the Ethi exponentially decreases with increasing GVth from 0.1 to 0.2 mm s , an Ethi of 20 and an Ethd of 3 GVth. This exponential decrease can be seen for both values to 5 s for the geophones with box. For the geophones directly of Ethi (10 and 20) and in both events. Second, the change of fixed at bedrock, the same Eth parameters and a much lower −1 Ethi from 10 to 20 IMP s does not influence significantly, GVth are proposed. The GVth-value depends on the distance which suggests that the most important factor for debris flow of the geophone to the active channel and should range be- −1 detection is the GVth. tween 0.005 and 0.03 mm s . For the implementation of an It is worth noting that any of the debris flows would alarm system in the future, all these threshold values must be not have been detected by the Dth parameters used tested applying the following methods: (a) a calibration of the for most of the geophones of the station FLOW-WR parameters in the field during a testing period of the system −1 (GVth= 0.17 mm s ; Ethi= 20; Ethd= 3). This fact en- (including additional field tests), or (b) a detailed sensibility forces the outcomes of the previous section on the effect of analysis of the three parameters applied over events recorded the metal sheet box, which strongly amplifies the ground vi- in FLOW-SPI and transformed into impulses using different −1 bration. Assuming a GVth – value of 0.019 mm s (as used values. However, for the second option, a greater database of at Geo3b, where no box is added), the big event (4 July) events recorded in FLOW-SPI station should be available. would have been detected by the three geophones, while the small event (27 June) would only have been detected by Geo5 and Geo6. Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 941 7 Conclusions at bedrock with another one mounted in a metal sheet box, which is attached to the bedrock. The results suggest that Monitoring torrents prone to debris flows is an increasing the metal sheet box amplifies the signal. At Rebaixader, this activity all over the world. The efficiency of the geophones amplification was useful for the detection of events, because to monitor the occurrence of torrential processes has been the geophones with a metal box were not placed close to the widely proved, and so it is their convenience for warning active channel. However, another amplification system (like purposes (Suwa and Okuda, 1985; Arattano and Moia, 1999; an electronic amplifier in the circuit board) would be more LaHusen, 2005b; Bessason et al., 2007; Huang et al., 2007; appropriate, because the exact amplification factor could be Badoux et al., 2009). However, there is a great variety of data known and controlled. recording systems, highly conditioned by the technical de- Finally, the choice of a correct detection threshold (Dth) tails of each monitoring station and many site-specific factors is fundamental, since it could produce the loss of an event that affect the ground vibration measured. or a great number of system triggers not related to torrential In this work, two different recording systems have been flows (which can result in false alarms in an alarm system). compared, both of them installed in the Rebaixader torrent In this study a sensibility analysis of the parameters of the (Central Pyrenees). One data recording system consists of Dth was carried out. The results point out that the number −1 −1 collecting the entire ground velocity signal (GVS), digitised of seconds over the IMP s threshold (10 or 20 IMP s ) at a high frequency rate (250 Hz), while the other is a sim- decreases exponentially with the ground velocity threshold plified system, which records a transformed signal (IS) at (GVth). From the sensibility analysis of the parameters it was low frequency (1 Hz). Both recording systems demonstrated noted that the ground velocity threshold GVth is the most their efficiency of recording the typical debris-flow features important of the three parameters of the Dth. For the same including the different phases of the events. Thus, both tech- reason, a too high value of GVth could induce a loss of an niques should be considered as suitable for debris-flow mon- event, which would be fatal for an alarm system. In order itoring. On the one hand, the GVS recording technique pro- to avoid the false alarms, the option would be to verify the vides more information about the signal generated by the propagation of the flowing mass by cross-checking different debris-flow passing, but it generates a large amount of data geophones. and subsequently consumes more electric power and time for Although many uncertainties are still remaining and addi- analysis. On the other hand, the IS recording technique pro- tional data must be gathered and analysed, the outcomes of vides less information on the signal, but it has been demon- this research improve the knowledge on the use of seismic strated that it is reliable for detection. Moreover, it requires sensors for the detection of debris flow and other torrential less power and simplifies the data collecting and gathering. processes and help on the design of an alarm system using These latter issues make the transformed signal especially geophones as key sensors. useful for a warning system. The data analysis showed that the differences between de- bris flows and debris floods can be observed by both record- Acknowledgements. This research has been funded by the ing techniques (GVS and IS). The differences are mainly Spanish Ministry MINECO contract CGL2011-23300 (project based on the shape of the signal and the values of the ground DEBRISTART). We would like to thank Ignasi Vilajosana from velocity. The results point out that the geophones that better Worldsensing s.l., Emma Suriñach from Barcelona University and Lluís Pujades from Technical University of Catalonia for their show the debris-flow features are the ones installed closest to collaboration on the geophysical analysis and interpretation. We the active channel, as can be expected. It is also worthwhile are grateful to Massimo Arattano and Oldrich Navratil for their that the active channel runs over bedrock on these cross- valuable comments and suggestions during the reviewing process, sections. The geophones located far from the active channel which helped to improve the manuscript. show less clearly the characteristics of debris flows. All these results suggest that the optimum position for a geophone to Edited by: B. D. Malamud obtain reliable records of debris flows would be as closest as Reviewed by: M. Arattano and O. Navratil possible to the active channel, but in a safe position to avoid potential damage by flows, and preferably where it runs over bedrock. References The site-specific factors that influence the ground vibra- tion measured at the geophones were evaluated by field tests Abancó, C., Hürlimann, M., Fritschi, B., Graf, C., and Moya, and the comparison of the GVS registered at three geo- J.: Transformation of Ground Vibration Signal for Debris-Flow phones. Two major conclusions were obtained: (a) the dis- Monitoring and Detection in Alarm Systems, Sensors, 12, 4870– tance produces a linear to exponential attenuation of the sig- 4891, 2012. nal; and (b) the assembly of the geophone can strongly condi- Arattano, M.: On the use of seismic detectors as monitoring and tion the amplification of the signal. This last conclusion was warning systems for debris flows, Nat. Hazards, 20, 197–213, clearly observed by comparing one geophone directly fixed 2000. www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 942 C. Abancó et al.: Analysis of the ground vibration generated by debris flows Arattano, M.: Monitoring the presence of the debris flow front and Itakura, Y., Inaba, H., and Sawada, T.: A debris-flow monitoring its velocity through ground vibration detectors. Proc. 3rd Inter- devices and methods bibliography, Nat. Hazards Earth Syst. Sci., national Conference on Debris-flow Hazard Mitigation: Mechan- 5, 971–977, doi:10.5194/nhess-5-971-2005, 2005. ics, Prediction and Assessment, Millpress, Rotterdam, 719–730, Iverson, R. M.: The physics of debris flows, Rev. Geophys., 35, 2003. 245–296, 1997. Arattano, M. and Moia, F.: Monitoring the propagation of a debris Johnson, A. M. and Rodine, J. R.: Debris flow, in: Slope Stability, flow along a torrent, Hydrol. Sci. J., 44, 811–823, 1999. edited by: D. Brunsden, and Prior, D. B., John Wiley and Sons, Arattano, M., Marchi, L., and Cavalli, M.: Analysis of debris- New York, 257–361, 1984. flow recordings in an instrumented basin: confirmations and Kogelnig, A., Hübl, J., Suriñach, E., Vilajosana, I., and new findings, Nat. Hazards Earth Syst. Sci., 12, 679–686, McArdell, B.: Infrasound produced by debris flow: propaga- doi:10.5194/nhess-12-679-2012, 2012. tion and frequency content evolution, Nat. Hazards, 70, 1–21, Aulitzky, H.: Preliminary two-fold classification of torrents, doi:10.1007/s11069-011-9741-8, 2011a. Mitteilungen der Forstlichen Bundesversuchsanstalt, 243–256, Kogelnig, A., Hübl, J., Suriñach, E., Vilajosana, I., Zhang, S., 1982. Yun, N., and McArdell, B.: A study of infrasonic signals of de- Badoux, A., Graf, C., Rhyner, J., Kuntner, R., and McArdell, B.: bris flow, 5th International Conference on Debris-Flow Hazards A debris-flow alarm system for the Alpine Illgraben catchment: “Mitigation, Mechanics, Prediction and Assessment”, Padua, design and performance, Nat. Hazards, 49, 517–539, 2009. 563–572, 2011b. Berti, M., Genevois, R., La Husen, R., Simoni, A., and Tecca, P. Kurihara, J., Takezawa, N., Yamakoshi, T., and Yanagimachi, T.: R.: Debris flow monitoring in the Acquabona watershed on the Study on method of setting threshold of ground vibration sen- Dolomites (Italian Alps), Phys. Chem. Earth, 25, 707–715, 2000. sor for detecting debris flow; 4th International Conference on Bessason, B., Eiriksson, G., Thorarinsson, O., Thorarinsson, A., Debris-Flow Hazards Mitigation, edited by: Chen, C. and Ma- and Einarsson, S.: Automatic detection of avalanches and debris jor, J., Millpress, 603–611, 2007. flows by seismic methods, J. Glaciol., 53, 461–472, 2007. LaHusen, R.: Detecting debris flows using ground vibrations, 1996. Biescas, B., Dufour, F., Furdada, G., Khazaradze, G., and Suriñach, LaHusen, R.: Acoustic Flow Monitor System – User Manual, US E.: Frequency content evolution of snow avalanche seismic sig- Geological Survey, Open-File Report 02-429, 16 pp., 2005a. nals, Surv. Geophys., 24, 447–464, 2003. LaHusen, R.: Debris-flow instrumentation, in: Debris-flow Hazards Chou, H.-T., Chang, Y.-L., and Zhang, S.-C.: Acoustic signals and and Related Phenomena, edited by: Jakob, M. and Hungr, O., geophone response induced by stony-type debris flows, Inter- Springer, Berlin, 291–304, 2005b. praevent, Taipei (Taiwan), 712–720, 2010. Marchi, L., Comiti, F., Arattano, M., Cavalli, M., Macconi, P., Cui, P., Chen, X., Waqng, Y., Hu, K., and Li, Y.: Jiangjia Ravine and Penna, D.: A new debris-flow monitoring system in an debris flows in south-western China, in: Debris-flow Hazards Alpine catchment, Geophys. Res. Abstr., 14, EGU2012-6104, and Related Phenomena, edited by: Jakob, M. and Hungr, O., EGU General Assembly 2012, Vienna, Austria, 2012. Springer, Berlin, 565–594, 2005. Navratil, O., Liébault, F., Bellot, H., Theule, J., Ravanat, X., Ousset, Fang, Y. M., Huang, T. M., Lee, B. J., Chou, H. T., and Yin, F., Laigle, D., Segel, V., and Fiquet, M.: Installation d’un suivi en H. Y.: Analysis of debris flow underground sound by Wavelet continu des crues et laves torrentielles dans les Alpes françaises, Transform- A case study of events in Aiyuzih River, 5th Interna- Journée de Rencontre sur les Dangers Naturels, Lausanne, 8 pp., tional Conference on Debris-Flow Hazards Mitigation, Mechan- 2011. ics, Prediction and Assessment, Padua, 545–551, 2011. Navratil, O., Liébault, F., Bellot, H., Travaglini, E., Theule, Huang, C., Yin, H., Chen, C., Yeh, C. H., and Wang, C. L.: Ground J., Chambon, G., and Laigle, D.: High-frequency moni- vibrations produced by rock motions and debris flows, J. Geo- toring of debris-flow propagation along the Réal Torrent, phys. Res., 112, F02014, doi:10.1029/2005JF000437, 2007. Southern French Prealps, Geomorphology, 201 157–171, Hungr, O., Evans, S. G., Bovis, M. J., and Hutchinson, J. N.: A re- doi:10.1016/j.geomorph.2013.06.017, 2013. view of the classification of landslides of the flow type, Environ. Pierson, T. C.: Flow behavior of channelized debris flows, Mount St. Eng. Geosci., 7, 221–238, 2001. Helens, Washington, in: Hillslope Processes, The Binghamton Hürlimann, M., Rickenmann, D., and Graf, C.: Field and monitoring symp. in geomorphology, edited by: Abraham, A. D., Allen & data of debris-flow events in the Swiss Alps, Can. Geotech. J., 40, Unwin, Boston, 269–296, 1986. 161–175, 2003. Rickenmann, D., D’Agostino, V., Dalla Fontana, G., Lenzi, M., and Hürlimann, M., Abancó, C., and Moya, J.: Rockfalls detached from Marchi, L.: New results from sediment transport measurements a lateral moraine during spring season. 2010 and 2011 events in two Alpine torrents, Hydrology, Water Resources and Ecology observed at the Rebaixader debris-flow monitoring site (Central in Headwaters, Merano (Italy), 248, 283–289, 1998. Pyrenees, Spain), Landslides, 3, 385–393, doi:10.1007/s10346- Rickenmann, D., Turowski, J. M., Fritschi, B., Klaiber, A., and 011-0314-4, 2012. Ludwig, A.: Bedload transport measurements at the Erlenbach Hürlimann, M., Abancó, C., Moya, J., and Vilajosana, I. Results and stream with geophones and automated basket samplers, Earth experiences gathered at the Rebaixader debris-flow monitoring Surf. Proc. Landf., 37, 1000–1011, doi:10.1002/esp.3225, 2012. site, Central Pyrenees, Spain, Landslides, doi:10.1007/s10346- Suriñach, E., Furdada, G., Sabot, F., Biescas, B., and Vilaplana, J. 013-0452-y, online first, 2013. M.: On the characterization of seismic signals generated by snow Itakura, Y., Fujii, N., and Sawada, T.: Basic characteristics of avalanches for monitoring purposes, Ann. Glaciol., 32, 268–274, ground vibration sensors for the detection of debris flow, Phys. 2001. Chem. Earth, Part B, 25, 717–720, 2000. Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014 www.nat-hazards-earth-syst-sci.net/14/929/2014/ C. Abancó et al.: Analysis of the ground vibration generated by debris flows 943 Suriñach, E., Vilajosana, I., Khazaradze, G., Biescas, B., Furdada, Vilajosana, I., Suri´ nach, E., Abellán, A., Khazaradze, G., Garcia, G., and Vilaplana, J. M.: Seismic detection and characterization D., and Llosa, J.: Rockfall induced seismic signals: case study in of landslides and other mass movements, Nat. Hazards Earth Montserrat, Catalonia, Nat. Hazards Earth Syst. Sci., 8, 805–812, Syst. Sci., 5, 791–798, doi:10.5194/nhess-5-791-2005, 2005. doi:10.5194/nhess-8-805-2008, 2008. Suwa, H. and Okuda, S.: Measurement of debris flows in Japan Vilaplana, J. M.: Quaternary Glacial Geology of Alta Ribagorça IV International Conference and Field Workshop on Landslides, Basin (Central Southern Pyrenees), Acta Geológica Hispánica, Tokyo, 391–400, 1985. 18, 217–233, 1983. Suwa, H., Yamakoshi, T., and Sato, K.: Relationship between Yin, H. Y., Huang, C. J., Chen, C. Y., Yeh, C. H., Lee, B. J., Fang, debris-flow discharge and ground vibration, 2nd International Y. M., and Chang, Y. H.: Monitoring ground vibrations generated Conference on Debris-Flow Hazards Mitigation, Taipei, 311– by debris flows, 4th Int. Conf. on Debris-Flow Hazards Mitiga- 318, 2000. tion, Chengdu, China, 625–633, 2007. VanDine, D. F.: Debris flows and debris torrents in the southern Canadian Cordillera, Can. Geotech. J., 22, 44–68, 1985. www.nat-hazards-earth-syst-sci.net/14/929/2014/ Nat. Hazards Earth Syst. Sci., 14, 929–943, 2014

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