From examination of natural events a proposal for risk mitigation of lahars by a cellular automata methodology: a case study for Vascún valley, Ecuador

Lahars are erosive floods, mixtures of water and pyroclastic deposits, they look the biggest environmental disaster as number of fatalities in the volcanic areas. Security measures have been recently adopted in the threatened territories, by 15 constructing retaining dams and embankments in key positions. Such solutions could involve a strong environmental impact for the works and the continuous accumulation of volcanic deposits, such that equilibrium conditions could lack far, triggering more disastrous events. An improved version of the Cellular Automaton model LLUNPIY for lahars simulations is presented. The growing frequency of lahars, maybe for the climatic change, in the Vascún Valley of Tungurahua Volcano, Ecuador has recently produced smaller and less dangerous events, sometimes favoured by the collapse of temporary ponds, 20 generated by small landslides. An investigation is here performed in order to reproduce such situations in controlled way by the use of LLUNPIY simulations. Using precise field data, points are individuated where dams by backfills, easy to collapse, can produce momentary ponds; LLUNPIY simulations permit to project triggering of small lahars by minor rainfall events or to project in the case of larger rainfalls the anticipation of lahar detachment, avoiding simultaneous and dangerous confluence with other lahars. 25

The use of simulation tools (from the cellular automata model LLUNPIY) needs detailed field data: DTM, depth of erodible pyroclastic stratum. It implies accurate geological investigations, including subsoil tomographies; permitting to individuate points, where dams by backfills, easy to collapse, can produce momentary ponds, whose breakdown can trigger a lahar (Machado, 2015c;Chidichimo et al., 2016).
For such points, minor rainfall events can produce small lahars, major events will anticipate the lahar detachment, avoiding 5 simultaneous confluence with other lahars. The control of the discharge channels could permit in various situations many combinations for a controlled triggering of lahars, in order to mitigate the lahar risk.
The next chapter is devoted to the geological description of the Vascún Valley, the third chapter introduces LLUNPIY, the MCA model for simulating primary and secondary lahars, together with its validation in simulating some significant lahars of Vascún Valley. In the fourth chapter, the building of dams, easy to collapse, with discharge channel is considered, then 10 favorable locations of dams are hypothesized for a controlled triggering of lahars, of which effects are simulated for possible events; at the end, conclusions and comments.

Geological setting of Vascún valley
Tungurahua is one of the most active and dangerous volcanoes in the Ecuadorian Andes (Cordillera Oriental) on the inside of the Sangay National Park; its summit, 5023 m.a.s.l., is positioned at longitude 78° 27' W, latitude 01° 28' S. It a 15 stratovolcano, whose evolution involved the succession of three major volcanic edifices (Tungurahua I, Tungurahua II, Tungurahua III) since the mid-Pleistocene over a basement of metamorphic rocks. Historical eruptions have all originated from the summit crater. The main events of eruptive activity occurred between: 1640-1641, 1773-1777, 1886-1888, 1916-1918 and from 1999 until the present (Hall et al., 1999;Ramón, 2009;Biggs et al., 2010). The average of eruptions in the last two thousand years is once per century according to the detailed studies of Le Pennec et al. (2008). They have been 20 accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. The formation of rain-induced lahar is also a cause of danger. Approximately 32,000 live within the higher risk areas, mainly in rural villages and in the touristic (thermal springs) town of Baños de Agua Santa (Mothes et al., 2015).
Baños de Agua Santa (1800 m.a.s.l.) is only 8 Km as the crow flies away from the summit; 30 Km to NW, 30 km to SW and 140 km to N are the distances respectively from the towns of Ambato, Riobamba, and from the capital Quito. 25 Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License. The small glacier of Tungurahua volcano is reduced both for the global warming phenomenon and for the intensification of the volcanic activity after the 1999, therefore the today snow cover is completely negligible for generating primary lahars, consequently triggering of lahars with significant frequency and magnitude, is subordinated both to the intensity and 5 duration of the rainfalls and the available quantity of fresh material along the slopes and within the principal canyons (Quebradas of the Rio Vascún, Juive Grade-La Pampa Valley, Achupashal Quebrada) and in a minor from other factors.
Almost all the lahars are confined to the canyons and converge into Pastaza River (Mothes and Vallance, 2015).
In fact, eruptive activity of Tungurahua volcano, during last years, has generated greater availability of pyroclastic debris that is periodically remobilized from atmospheric phenomena, often not particularly violent but prolonged for several days. 10 Between 2000 and 2011, around 900 events rain-induced lahars were triggered (Mothes and Vallance, 2015).
Generally, lahars magnitude is small and, consequently, causes limited damage. IGEPN, and its Acoustic-Flow-Monitor (AFM) station, which monitors passing of secondary lahars, detects most of lahars, while many others are traced by the Observatory of the Volcano Tungurahua (OVT), which is situated 13 km to the north-northwest of the crater, also with the observation contribution of local volunteers (vigias). 15

The 2008 secondary lahars of Vascún Valley
Rio Vascún, the most important river in the area, gave its name to the Valley on the NE flank of Tungurahua Volcano, it flows into the Rio Pastaza (Fig.1). The valley slopes exceed 35°-40°in the first steepest 3 km, while range from 20° to 6° in Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License. the last less steeply 2-3 Km. Furthermore, the path of Rio Vascún is extremely sinuous in the upper 1-2 km of terraces, the river is there characterized by a succession of tight 90° bends.
The Vascún Valley is highly susceptible to lahar flows that threaten the nearby populated areas which were inundated several times in the past years: the town of Baños, that extend in part of the depositional area, was affected several times, the thermal structure "El Salado", that is located along the river banks, had been repeatedly damaged by passage of lahars (   Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License.

LLUNPIY model for lahar simulation
LLUNPIY (Lahar modelling by Local rules based on an UNderlying PIck of Yoked processes, "llunp'iy" means flood in the Quechua language) is a model for simulating secondary and primary lahars according to MCA methodology applied to complex system, whose evolution may be mainly specified in terms of local interaction. MCA features of SCIDDICA-SS3 (Avolio et al., 2013) and SCIDDICA-SS2 (Avolio et al., 2008;Lupiano et al., 2016;Lupiano et al., 2017) are inherited by 5 LLUNPIY; LLUNPIY for secondary lahars is extensively defined in Machado et al. (2015b), here are reported only the features of the model, that were applied in the study cases (reduced version LLUNPIY/3r from SCIDDICA-SS2), where the initial phase is abrupt, it is given by the temporary pond collapse without water percolation and detachment subsequent to water inclusion, furthermore in the simulation of real and hypothesized events, all the lahars end into the Rio Pastaza, so the last phase of lahar deposition is omitted. LLUNPIY modelling of water percolation and water extrusion with pyroclastic 10 debris deposition can be found also in Machado (2015c). ℕ is the set of the natural numbers.
 Q is the finite set of states of the finite states machine (or automaton), incorporated in the cell; it is specified in terms of substates as their Cartesian product (Table 2). 25  : Q #N → Q is the deterministic transition function for each cell in T, the following "elementary" processescompose they account for the overall dynamics of the phenomenon:

The elementary processes of LLUNPIY/3r
We give in the following an outline of the of the transition function, by the elementary processes, each one updating the substates, in order to capture the mechanisms; the complete execution of the function "external influence" and all the elementary 5 processes complete a step of the MCA. Neighborhood index between square brackets, following substate specification, indicates the corresponding cell of the neighborhood. ∆ indicates variation of the sub-state . ′ indicates the new value of the substate .

Pyroclastic cover mobilization
Soil features together with the quantity of water content determine a value ptm of mobilization threshold to be compared with 10 the kinetic head of lahar debris inside the cell, when > , then the pyroclastic cover is eroded, the lahar thickness augments and altitude diminishes according to the following formula There is correspondingly a dissipation of energy with a decrease of kinetic head according to the following formula: AMD application computes the outflows, that minimize the "height" differences in the neighborhood (equation 7). An alteration of height values is introduced in the central cell for taking into account the outflow run-up; furthermore the viscosity is modelled by an adherence " ℎ" term, the lahar quantity, that cannot leave the central cell. It varies between the 5 two extreme values ℎ1 and ℎ2, which depend on the composition of the pyroclastic debris at the maximum and minimum water content (Machado, 2015c). Each moving quantity (outflow) may be considered as a "cylinder", that initially is entirely inside the cell, having a center of mass with co-ordinates of [0]and [0] and with the maximum possible radius.

15
The shift " ℎ[ ]" of [ ] is calculated according to the following formulae, where the movement of the mass center is specified as the mass movement on a constant slope with a constant coefficient of friction , the movement of [ ] is directed towards the center of cell , considering the slope angle [ ] (Fig.4).
with " " the acceleration of gravity, " " the initial velocity: 20 There are three possible outcomes: if the shifted cylinder is completely inside (

Flows Composition
Execution of the elementary process "lahar outflows" involves an updating of substates , , , by the elementary process "Flows Composition". It accounts for the loss of matter ( ) and corresponding variation of , , that is determined by the external outflows, while internal outflows don't effect a loss of matter, just a shift, that determine still 5 variations of the other previously quoted substates; inflows are determined by possible outflows of the neighbor cells towards the central cell (Machado et al., 2015).

Simulations of 2008 lahars of Vascún valley
We selected the 2005 and 2008 lahars of Vascún Valley for LLUNPIY simulations, because available (although incomplete) data of the flood phase (Machado et al., 2015b) would allow reliable simulations. Data of different sources were carefully 10 compared and analyzed (Williams et al., 2008;IGEPN, 2008) in order to reconstruct as accurately as possible the two events (Machado et al., 2014a and2014b).
The same set of LLUNPIY parameters was used in the two cases except for the parameter of progressive erosion (ppe) because of different percentages of water in the soil. The 2005 event was triggered in a higher zone of Rio Vascún, when the water concentration in the soil by rainfall reached critical values in areas, where a slope threshold was exceeded. The 2008 15 event was dissimilar, because the breaking of a temporary pond released suddenly a larger water quantity (in comparison with 2005 case) with strong turbulence, whose effects correspond to a higher value of the parameter of progressive erosion (Machado et al., 2015b).
Here, we report extensively the results of simulations of 2008 event of the Vascún Valley in Machado et al. (2015b), because the causes of this event are given by a breaking of a temporary pond, that is the same phenomenology, whose development, 20 we want to forecast. The reliability of results of the simulation in comparison with the real event permitted us to confide in the goodness of the method, the new simulations were performed with the same data precision and the same values of parameters.
2008 pre-event morphological data are founded on a DEM, 5 m cell size, furthermore a constant depth of 5 m was presumed for pyroclastic stratum, because of lack of detailed surveys. So approximations, that affect simulations, are introduced. Such 25 approximations can be reduced by an opportune survey of field data, e.g. by soil tomographies.  Table 3 compares some data of simulation by LLUNPY with corresponding field data of IGEPN (2008). It is possible to note that the data deriving from the simulations are not many different from the known measured ones. The flow velocity of 15 m/s represents an estimated value, not measured. These results demonstrate that LLUNPIY is a reliable model, if we take into account that simulation are based on incomplete, sometime very approximate data concerning the pre-event and postevent, furthermore the inevitable errors in records related to this event have to be considered. Therefore an extension of LLUNPIY/3r is promising in order to introduce secondary phenomenological features to be tested. Simulations reproduce satisfactorily the overall dynamics of the events; there is a good matching between real and simulated lahar path, velocity 5 and height of detrital flow; note that different approaches obtain always excellent results about the path because the lahar is canalized by steep faces.  Dam collapse occurs when instability conditions arise in the downstream slope. By eroding the obstruction and flowing downstream along the river bed, the initial surge of water will incorporate a dangerous volume of sediments. This can easily produce lahars with possible devastating effects for settlements in their path (Leung et al., 2003). 5 Temporary dams with a similar (but controlled) behavior can be designed and built at low cost by local backfills in order to allow the outflow of streams produced by regular rainfall events. This result is achieved by properly dimensioning the embankment according to a stability analysis. The latter is made by comparing the forces tending to cause movement of the mass of pyroclastic material (force of water) with those tending to resist the movement (soil strength) (Lambe and Whitman, 1979). The aforementioned approach is traditionally adopted to prevent dams failure, but it will be used, in this case, to 10 ensure their collapse at a fixed water level.
The Finite Element Method (FEM) was applied to perform the downstream slope stability analysis. The shear strength reduction (SSR) approach, which is one of the most popular techniques to perform FEM slope analysis, was adopted (Griffits et al., 1999). The SSR is simple in concept: it systematically reduces the shear strength envelope of material by a factor of safety ( ), and computes FEM models of the slope until deformations are unacceptably large or solutions do not converge. 15 In classical soil mechanics, the factor of safety is the ratio of the shear strength at the plane of potential failure and the shear stress acting in the same plane , namely: For the Mohr-Coulomb criterion, the previous equation reads: where is the cohesion and is the friction angle of the material, denotes the total normal stress and the pore pressure.
For the Mohr-Coulomb model, a "reduced" set of material parameters * and * is computed: The problem is then solved using this set of reduced material parameters while keeping all other parameters unchanged. If 25 convergence is obtained, the value is increased and the problem is solved again. The lowest producing nonconvergence is reported as the "factor of safety" of the problem. If the resulting FS is greater than one, for a given water level stressing the upstream slope of the dam, the structure is stable. If the iterative procedure gives back a unitary value, the limit equilibrium conditions have been reached. This means that the coupling of both the dam geometrical configuration and the water level situation are going to produce the collapse of the structure. This last condition is the one that must be 30 reached for the study purposes. The pore pressure distribution inside the dam body is a fundamental input for the strength where ℎ (x, y) represents the hydraulic head distribution within the dam body which is a function of the hydraulic 5 conductivity of the dam material. Such dependence is defined by Darcy's law: where is Darcy's velocity, is the hydraulic conductivity, is the water density, is the pore pressure and is the elevation above sea level. Several simulations were performed taking the parameters for the numerical model from the literature. Studies performed on the geotechnical properties of the volcanoclastic formation that is found in the Andes of 10 Ecuador and Colombia, known as Cangahua, reported that the dry unit weight of the material was found to range around 14 kN m -3 (Bommer et al., 2002). The strength parameters of volcanic sediments produced by recent eruptions were investigated by Orense et al., (2006), who found a value for the friction angle ( ) of these materials of about 40°, while the cohesion ( ) is close to zero. The hydraulic conductivity ( ) of pyroclastic beds was discovered to range between 10 -4 -10 -5 m s -1 (Burgisser, 2012). A middle range value was adopted to implement the numerical models. The relatively high 15 permeability of the pyroclastic material ensures the water outflow during the regular rainfall regimen. In case of extreme rainfall events (violent rains, typhoons, etc.), the volcanic material is no longer able to drain the water inflow producing the water level raising which will undermine the structure stability. The dams were thought to reach a height of 3 m and to hold out up to a maximum water level of 2.6 m. Assuming the aforementioned features, the first step started from the design of a stable dam configuration (Fig. 6a). This outcome was obtained trying different dimensioning solutions in order to avoid the 20 early collapse of the structure due, for example, to its own weight. Figure 6b shows the water velocity field in the dam cross section, while Fig. 6c shows the failure surface in the downstream slope which is generated by a factor of safety of 1.28.  Once a stable dam was obtained for the chosen working conditions, the second step was to repeat the strength reduction analysis several times by slowly increasing the inclination of the downstream slope until a unitary was reached. The 5 inclination was increased using the corner between the downstream slope and the dam crest as a pivot point. This resulted in a gradual increase of the θ angle and a consequent reduction of the dam base. Figure 7 shows the analysis final result with the sizing specifications for an easy to collapse dam built in volcanoclastic material. To ensure a greater control over the natural dam collapse timing, a discharge channel can be arranged at the dam base. The degree of openness of this channel can be adjusted according to the flow rates values observed during the extreme rainfall events recorded over time in the area, 10 in order to delay the achievement of the triggering hydraulic head. This ploy may be necessary to avoid the undesired simultaneous collapse of different dams; hazard could increase when different lahars are triggered at short time intervals and reach the confluence points almost at the same time. Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License.   (Machado et al., 2014;Machado et al., 2015b). In particular, the 2008 event is very important in order to confirm the value of the model parameters, tuned in the simulation even where the cause of the lahar was the breakdown of a temporary pond, generated by a small landslide. Those successful simulations permitted to be confident in the scenarios which could be realized by new simulations. Of course, a very accurate updating of geological data (DEM or DTM, detrital cover depth, etc.) and sufficient soil tomographies are necessary for applications in 10 order to mitigate the lahar risk. for positioning embankments as dams; three points were chosen for building temporary dams: one located into Rio Vascún (1 in Fig. 8, Fig. 9, Fig.10), the second one located in a stream, tributary from the right (2 in Fig. 8, Fig. 9, Fig. 10), the third one located in a stream, tributary from the left (3 in Fig. 8, Fig. 9, Fig. 10). Rio Vascún in turn is tributary of much broader Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License.

Preliminary hypotheses and results of simulations
Pastaza River, where lahars of Vascún Valley disperse. Simulations concern the lahars generated in the points 1, 2 and 3 ( Fig. 8, Fig. 9, Fig. 10), in short, lahar 1, 2 and 3. The same initial volume of 2008 event was selected for all the simulations except the last one. Three initial points permit to analyze an almost exhaustive set of possible conditions; we performed a sequence of simulations by LLUNPIY, of course, with the same parameters values of the successful simulation of 2008 August 29 event. We present here some selected simulations, which look interesting for many considerations, which may be 5 deduced by their analysis.  1 and 2 (a), from points 2 and 3 (b). The maximum thickness of the lahar during the conjectured events is reported in meters according the legend.
Initially, we considered two scenarios in order to investigate the effects of simultaneous and differed in time confluence of 10 two lahars, the results of such simulations induced us to consider a larger set of cases. Two possible scenarios are here considered: the former one is generated by the simultaneous triggering from the points 1 with 4875 m 3 of detachment volume and 2 with 4500 m 3 of detachment volume (Fig. 8a), the second one is generated by the simultaneous triggering from the points 2 with the same previous detachment volume and 3 with 4250 m 3 of detachment volume (Fig. 8b). The confluence of Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License. the lahars in the Rio Vascún (for the following the confluence point) is almost simultaneous in the latter scenario, because of the similar distance between the triggering points and the confluence point; the situation is diverse for the former scenario, because the distances of the triggering points from the confluence point are very different. Intuitively the former case would be less dangerous, because the flood peaks of the two lahar cannot coincide at the confluence point, but the maximum thickness of lahar in the former scenario is 27 m., while the smaller value of 21 m. is detected in the latter scenario. The 5 analysis of the two simulations showed that a very larger mass was eroded in the first part of the path from the point 1 in the former case. This unexpected result permits to plan an opportune strategy according to the degree of control for triggering lahars 1, 2 and 3 from the three respective points. If triggering can be well controlled with moderate/heavy rainfall, then the best choice is to trigger lahar 3 (smaller erosion) before lahar 1, so that lahar 3 anticipates part of the erosion process in the common path of both the lahars (1 and 3) as far as the confluence point and reduces consequently the thickness of the lahar 10 1. When the peak of the lahar 3 is gone beyond the confluence point, then the lahar 2 can be triggered before the lahar 1, which has to be generated as late as possible. Anyway, the last lahar to be triggered has to be surely lahar 1 but a further investigation needs in order to understand better the priority between lahar 2 and 3; the study of single lahars generated in the points 1, 2 and 3 could solve the question, as it may be deduced with the following simulations with triggering single lahars. 15 Lahar 1 causes the maximum erosion, with a maximum thickness of 26 m., because it follows the path of Rio Vascún, that is the largest rio in the valley (with a larger volume of pyroclastic cover to be eroded), lahar 3 shares a relevant part of the previous path and reaches a maximum thickness of 19 m., while the lahar 2, whose path is less large before its late confluence into Rio Vascún, involves the smallest erosion (maximum thickness of 16 m). Such results solve the doubt that we put forward with the first simulations. 20 An operation of "cleaning" of pyroclastic cover could be projected by triggering initially lahar 2, with a first mobilization of the detrital cover as well for the area related to last part of the Rio Vascún from the confluence point of lahar 2 (maximum thickness 16 m, Fig.9b); when the lahar 2 dissolve into Pastaza river, lahar 3 could be triggered with a first erosion of the detrital cover between the confluence points of the lahars 2 and 3 into Rio Vascún, its maximum thickness does not overcome 16 m, (Fig.9c), then the most dangerous lahar 1 of Rio Vascún could start at the exhaustion of lahar 3, minimizing 25 the hazard; the maximum thickness before the confluence of lahar 3 into Rio Vascún doesn't overcome 22 m ( Fig.8a and   Fig.9a).
We tested successfully the outcomes of this strategy by simulating the triggering of the three lahars in successive times, each one immediately after the exhaustion of the previous one; the first phase concerns the lahar 2 (Fig.9b), the maximum thickness doesn't overcome 14 m in the last part of the path, from north of El Salado Bath to south of Baños. 30 Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License. The erosion depth of pyroclastic cover after the lahar 2 exhaustion prevents that the maximum thickness of the successive lahar 3 overcomes 10-12 m after the confluence point with lahar 2 (Fig.10a) because of the reduced pyroclastic cover, while 5 19 m are reached with triggering only the lahar 3 (Fig.9c).
Finally, the most dangerous lahar 1 doesn't overcome 4-8 m in the inhabited zones (Fig.10b), while it reaches 26 m of maximum thickness in the last part of the path (Fig.9a), when the other lahars 2 and 3 are not generated.
This last result points out the importance of "cleaning" of pyroclastic cover according to an opportune strategy, that can be deduced by the outcomes of simulations, that explore all the possible significant cases. 10 Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License. Last simulations concern two cases of simultaneous triggering of all the lahars with the same detachment volumes from points 1, 2 and 3 of the previous simulations (Fig. 11a) and the double detachment volumes from the same points (Fig. 11b) 5 in order to understand how triggering larger volume could increase the lahar dangerousness. Results show that the maximum thickness of the lahar in the former case (Fig. 11a) is 28 m, while the maximum thickness of the lahar in the latter case ( Fig.   11b) is 29 m, just a meter more. A double initial volume does not involve a much larger erosion in this context, the joint effect of a larger volume and erosion doesn't increase in dramatic way the high hazard of the situation.
The overall results confirm the goodness of the strategy of triggering lahars at different times according to an accurate 10 analysis of simulations after a precise knowledge of the geological features of the area of application. We remember that these simulations were obtained without sufficient data about the pyroclastic cover (it is obviously overestimated) because of Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License.
the lack of an adequate number of soil tomographies, that would have led to results with the desired precision, anyway, this problem doesn't compromise the reliability and validity of the proposed methodology.

Conclusions and comments
Our case study starts from examination of some significant natural events in the Vascún Valley, an area that is heavily exposed to lahar risk. Momentary ponds form along watercourses (rios) in the canyons of this volcanic zone, when landslides of volcanic deposits impede the normal flow. The most frequent cause of breakout of such natural ponds is the overflow of 10 water across the newly formed dam and subsequent erosion and rapid down cutting into the loose rock debris. By eroding the Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org /10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License.
blockage and flowing out watercourse downstream, the initial surge of water will incorporate part of volcanic sediments and will generate lahars. The hazard related to these lahars depends both on the features of the temporary pond and the volcanic cover along the lahar path; a larger frequency of lahars produces smaller (shorter accumulation periods) and therefore less dangerous events.
We explored the possibility to induce artificially lahars and performed many simulations for analyzing possible different 5 scenarios for extremely complex situations; positive results of this case study permit us to settle a methodology and encourage us to continue this investigation. We propose for risk mitigation a controlled generation of small lahars by the collapse of temporary ponds at different times in order to avoid the superposition of different lahars with the same final path.
Such a proposal is out of standard and is based on observations and study of favorable situations together with the usage of a robust and well validated model of simulation in order to choose the best procedures of intervention. 10 The computational paradigm of Cellular Automata permitted to develop a reliable model for simulating the complex dynamics of the lahars. Reliable simulation tools permit to test various hypotheses and to create related scenarios to be analyzed.
The possibility to simulate different scenarios permits to forecast the thickness of lahars, their velocity, times of their peaks, to operate the best choice as potential hazard with more efficient and reliable alert procedures. Applications of LLUNPIY 15 need a thorough geological study of the area of interest, especially regarding morphology (DEM and DTM), pyroclastic soil cover, the composition of the erodible layer, also specified by soil tomography at the strategic points. Furthermore, it is also important to conduct a hydrological study of watercourses, where most likely the lahar are channeled.
Feasibility studies confirmed the previous hypotheses of building weak dams with significant cost containment. Unexpected (and sometime dangerous) situations were evidenced by simulation results, which permit to evaluate the hazard of possible 20 choices. Furthermore, more efficient and efficacious early warning protocols may be produced in such a context, social impact for partial evacuation could be mitigated. Interventions, that solve provisionally the lahar hazard, but involve future risks, can be avoided. The complexity of the objective presupposes a multidisciplinary (or perhaps transdisciplinary) approach, which implies an even greater effort to ensure those competences of different types (which are reflected by the various scientific extractions of the authors): geology, physics, mathematics, engineering, computer science can cooperate in 25 achieving common goals. Protocols for mitigating the lahar risk can be developed in such a context, involving also social and political sciences (Leung et al., 2003).
A pilot project at least with a temporary pond needs in order to experiment in safe conditions the triggering of a small lahar. This is a preliminary step for standard applications of this new strategy for reducing the risk of lahars.
A further achievement is the extension of LLUNPIY for modelling the flows in the urbanized area as in the last versions of 30 SCIDDICA (Lupiano et al., , 2017. Nat. Hazards Earth Syst. Sci. Discuss., https://doi.org/10.5194/nhess-2018-406 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Discussion started: 28 January 2019 c Author(s) 2019. CC BY 4.0 License.