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XII Congreso Nacional de Geotecnia CONGEO 2015

Barrera para Flujo de Detritos: Estrategia y Abordaje de Proyecto

Giorgio Giacchetti Alpigeo SC. – Belluno – Italy

Alberto Grimod

France Maccaferri

Alessio Savioli, Maccaferri Ldt - UK

Marco Cerro,

Maccaferri USA – Sacramento – Ca.

Ing. Oldemar Bermúdez Campos Maccaferri de Centro América Ltda. – Costa Rica

ABSTRACT Debris flows are highly mobile flows of mixed material and are trigged by the rapid build-up of water within the slope, saturating the ground. Since they can travel at high speeds and transport huge volumes of material, they pose a high risk to human life, infrastructures; roads and railway are particularly exposed to the risk as they cannot avoid to across gullies and channels. In these situations the deformable debris flow barriers are one of more often used remedial solutions because they can be easily installed within the path of the debris flow (or shallow landslide). Such barriers are composed by a ring-net interception structure, which restrained to the channel sided by mean of longitudinal cables generally coupled with energy dissipators (brakes). If the length of the barrier is longer than 15-20 m, posts may be adopted to hold the ring-net and the cables. Upon the impact by the debris flow, the barrier progressively deform with the compression brakes and systems absorbing the energy. The hydrostatic pressure within the flow rapidly dissipates once the debris flow has been arrested, leaving the accumulated volume within the fence. Moreover, these barriers are environmentally friendly. If on one hand the debris flow barriers mitigate the risk (at least they slow down the motion and give longer escaping time), on the other one they pose severe problems for the maintenance.

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Therefore the designer has to face two basic problems: first of all the global design strategy aimed at getting a cost effective remedial barrier and then the calculation of the structure. The paper recaps the main criteria to be considered to evaluate the effectiveness the barrier, and a simplified model to design the structure based on the experiences and the researches carried out by Officine Maccaferri with the University of Parma, allow to design all the components of the fence: type of interception structure, diameter of the cables and their vertical space, length of the energy brakes and the length of the lateral anchors. The model returns restraining forces and cable stresses that can be used for an appropriate barrier design. RESUMEN Los flujos de escombros son flujos altamente móviles de material mezclado y son disparados por la rápida acumulación de agua en el talud, saturando el suelo. Puesto que pueden viajar a altas velocidades y transportar grandes volúmenes de material, que suponen un alto riesgo para la vida humana e infraestructuras; las carreteras y vías de ferrocarril están particularmente expuestos a este riesgo ya que no pueden esquivar barrancos y canales en su trazado. En estas situaciones las barreras de flujo de detritos deformables son una de las soluciones correctoras utilizadas con más frecuencia, ya que pueden ser instalados fácilmente dentro de la trayectoria del flujo de escombros (o deslizamiento de tierra superficial). Estas barreras están compuestas por una estructura de intercepción mediante redes de anillos, sujetadas al canal de lado a lado por medio de cables longitudinales generalmente junto con disipadores de energía (frenos). Si la longitud de la barrera es mayor que 15-20 m, deben incorporarse postes metálicos para sujetar la red de anillos y cables. Posterior al impacto del flujo de escombros, la barrera se deforma progresivamente con los frenos de compresión y sistemas de absorción de la energía. La presión hidrostática dentro del flujo se disipa rápidamente una vez que el flujo de escombros ha sido detenido, dejando el volumen acumulado dentro de la valla. Por otra parte, estas barreras son amigables con el medio ambiente. Por un lado estas barreras funcionan para mitigar el riesgo (al menos disminuyen la velocidad del movimiento y dan mayor tiempo de escape), en el otro plantean graves problemas para el mantenimiento. Por lo tanto, el diseñador tiene que enfrentarse a dos problemas básicos: en primer lugar la estrategia de diseño global dirigida a conseguir una barrera correctivas rentable y luego el cálculo de la estructura. El documento resume los principales criterios a tener en cuenta para evaluar la eficacia de la barrera, y un modelo simplificado para diseñar la estructura sobre la base de las experiencias y las investigaciones llevadas a cabo por Officine Maccaferri con la Universidad de Parma, permiten diseñar todos los componentes de la valla: tipo de estructura de intercepción, diámetro de los cables y su espacio vertical, longitud de los frenos de energía y la longitud de los anclajes laterales.

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PRELIMINARY REMARKS The debris flow is a flow of sediments and water mixture in a manner as if it was a flow of continuous fluid driven by gravity, and it attains large mobility from the enlarged void space saturated with water of slurry (Takahashi, 2007). The nature of such mass movement mainly depends on the rainfall type, amplitude and orography of the rainfall basin, morphological aspects, lithology and instabilities phenomenon of the basin. Actually, according to Hungr et al. (2001) the following main categories of flows may be distinguished: debris flow, earth flow, debris avalanches, mud flow. The velocity of the flow can vary depending on the water ratio content, grain size and slope gradient. The rheology of the debris flow varies with the time: the increasing of the debris front raises the discharge that drives the flow downstream. Thus, the peak discharge rises as well. Generally the biggest particles constitute the forward-face of the debris flow, whereas the small ones create the central core. The tail of the flow is basically composed of water and very small particles (Figure 1). The movement expands along preferential ways, such as natural draining systems, creeks, etc., that allow the flow traveling for miles, therefore their negative effect can be perceived far away from the starting zone (Figure 2). Due to these characteristics, they are ascribed among the most dangerous and catastrophic events.

Figure 1. Evolution of the debris flow (Source: Pierson, 1986).

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Figure 2. Origin and evolution of a debris flow (Source: Natural Resources Canada). The total volume of material moved toward the accumulation zone during an event defines the magnitude of the debris flow. The magnitude is rarely related to the volume of the initial landslide. Often, the initiating slide is small and the bulk of the volume transported to the deposition area results from entrainment of material along the path. Thus, it is the flow mechanism that primarily determines the total volume of a debris flow. Such aspect is extremely important to scale the event and allow correlating it to the run-out distance and maximum discharge (Hungr et al., 2005). MITIGATION SYSTEMS Intervention strategy As the debris flow develops with features while running down the slope (figure 2), different remedial strategy should be adopted to face the problem accordingly. Considering the typical section of the debris flow (Figure 3) the following remedial measures can be adopted: Starting zone: characterized by erosion and landslide movements. The protection

measures must be able to prevent the triggering of the phenomena. Thus, in this zone, erosion control systems, drainages, bio-engineering techniques, soil nailing and superficial stabilizations may be adopted; the remedial solution is effective for

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the long term, but usually a huge fund effort is required since the intervention area is wide and the technical difficulties are relevant.

Transit zone: characterized by the passage of the flow with different velocity and volumes. The protection measures must be able to control the flow by reducing its speed and contain its solid transportation in order to limit the erosion. Therefore, in this area, erosion control systems, channel lining, weirs, selective check dams and channel debris flow barriers may be designed; such remedial measures are quite cost effective but they must be accurately evaluated for the maintenance.

Deposition zone: characterized by the accumulation of the material transported by the flow. The protection measure must be able to mitigate the negative effects of the event. Thus, deviation (i.e. embankments, channeling) and/or accumulation basins and structures (i.e. selective check dam, embankments, accumulation dam, and open-slope debris flow barriers) should be designed. Such remedial solutions require a reliable evaluation of their effectiveness, as frequently they are last chance of defense.

Figure 3. Areas characterizing a debris flow channel. Three areas can be identified: starting zone

(orange), transit zone (red), and deposition zone (blue).

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Design approach of the debris barriers The design of all the above mentioned remedial solutions requires a good knowledge of the debris flow and of the hydrological basin, included the geological and geotechnical features. Many times, despite the lack of reliable information, designer are forced to find out practical solution that supplies a minimum of safety. The debris flow barriers hereby presented are one among the most common solutions; they are constituted by the following main components (figure 4): (a) transversal steel ropes that horizontally across the channel; (b) Energy dissipaters that are inserted into the transversal ropes for dissipating the impact pressure of the debris flow; (c) interception structure that is a mesh aimed at catching the material floating on riverbed; it is constituted by a primary mesh (ring net – mandatory) and a secondary mesh (double twist wire mesh - optional); (d) lateral anchors aimed at restraining the transversal ropes into the ground.

Figure 4- Typical configuration of the debris flow barrier for channel.

The effectiveness of the barrier should always be evaluated even if the computation of the forces is not possible for lack of data. The criteria of evaluation are suggested by the following questions: The location is suitable for the maintenance of the barrier? The removal and the transportation of the debris far away in a safe position is feasible? Is the place easily reachable for visual inspections? How the barrier can be founded? Can the ordinary flow and sediments pass below the barrier and keep cleaned the barrier screen? Which is the total quantity of debris that could be kept? What about the exceeding quantity of sediments? Which is the sustainable frequency of maintenance? If the barriers are used for, which is their life span and what happens after?

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Basically considering the required maintenance and the possible location of the barriers, the best use of these structures should be as ultimate defense, especially for emergency situation. Practically these barriers can slow down a little bit or delay few seconds the debris flow, just for giving the time to escape, or hold a certain quantity of sediments and limit the damages on the valley side infrastructures. In these terms, the goal of the barrier should be smoothing as much as possible of cutting off the peak flow of the solid transportation (figure 5).

Figure 5. Correlation between the flow-discharge vs. the duration of the debris flow event. Debris flow

barriers should mainly cut the peak of the solid transportation. Designers should take into account the following aspects based on laboratory and field experience: - Generally the maximum span should not exceed 15-17 m. Larger spans require

stronger ropes and deep anchors, thus the introduction of one or more intermediate post may be more cost-effective;

- As the debris flow barrier should be intended as emergency remedial measure, the followings implications come out accordingly:

(a) The riverbed should always be free, so that the ordinary flow and solid transportation can pass without filling the barrier. In these terms a gap should be preserved at the bottom (i.e. lower rope placed 1.m or more above the river bed level);

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(b) The primary mesh (ring net) is able to catch the debris flow, thus a secondary mesh (i.e. double twist) is unnecessary. The goal of the interception structure is to allow the passage of the water and small particles, and contain the debris; by the way, the higher the flow velocity is, the greater the impact force is. Moreover, the smaller the grain size is, the higher the velocity is.

(c) As soon as the barrier screen is filled with the material carried by the event, maintenance works must be foreseen. If the gap between the lower longitudinal steel rope and the creek bed (or slope) is properly designed, and the sediments do not include boulders the barrier can be self-cleaned. The concept of “self cleaning” is analogous in the slit dams (Shun et al., 1997) where the gaps in the screen are aimed at allowing harmless non-flood sediments to descend rather than accumulate necessarily, and restoring the flood deposition level by discharging the debris after the flood period thanks to progressive erosion throughout the large ring net of the screen and the bottom gap.

SIMPLIFIED DESIGN OF DEBRIS FLOW BARRIERS Debris flow modeling In order to design a debris flow barrier all the morphological, geotechnical, hydrological and hydraulic parameters of the analyzed area must be known. Numerical models can be used to analyze and describe the phenomena (Lo, 2000). Several mechanisms suitable to describe the impact and the accumulation of the debris, as well as the procedures to estimate the effect of the debris flow impact, can be found in literature. Therefore designers can use different approaches (Canelli et al., 2012; Sun et al., 2012; Ferrero et al., 2010). According to the experience of the authors, the most reliable model seems to be the “run up” mechanism defined by Geo Hong Kong Office (Sun et al., 2011) (Figure 6).

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Figure 6. Flowing and deposition sequence of debris and loads acting on the debris flow barrier (after Sun et al., 2011). Symbols: d = distance between the top surface of the accumulated material against the barrier and the analyzed point; v = velocity of the debris flow at the impact; h = constant height of the

friction angle of the debris flow surge.

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According to this model, the loads acting on the debris flow barrier can be divided in:

1. Dynamic load: due to the force of the flow wave that impact against the fence (Figure 6.i);

2. Static load: due to the debris accumulated against the barrier (Figure 6.ii); 3. Drag load: due to the motion of the flow passing over the top of the structure once

this one is completely filled of material upstream (Figure 6.iii). Preliminary assumptions The debris flow barrier can be designed using a simplified method based on the following hypothesis:

- The barrier is placed on a vertical plane perpendicular to the flow direction; - Longitudinal ropes absorb all the forces developed by the debris flow (expressed

in: Pressure x Screen surface); - During the calculation, the plastic and elastic behavior of the ropes is neglected in

order to increase the overall safety factor of the structure; - The brakes are able to dissipate a considerable part of the energy developed by

the impact. Thanks to these additional elements is possible to reduce the stress acting on the ropes;

- The energy dissipation due to the deformation of the interception structure (ring net) is neglected;

- The interferences between longitudinal ropes due to the stiffness of the interception structure is neglected (Canelli et al., 2012);

- The interception structure (ring net) is fit to withstand the impact of the debris, as demonstrate by the behavior of deformable rockfall barriers (Cantarelli et al., 2008; Grimod et al., 2013);

- Ropes are stressed by homogenous distributed loads; - Longitudinal ropes are modeled considering an arch deformation shape.

Calculation approach The calculation of a debris flow barrier is related to:

1. The action (pressure) acting against the barrier; 2. The resistance of the structure.

The pressure acting against the barrier can be estimated in accordance with the principles previously described. This part of the design is the most uncertain, and it may introduce several errors in the calculation.

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The barrier resistance can be estimated according to structural principles, which are totally independent from the rheological and hydraulic aspects of the debris flow. The barrier is designed considering the dynamic, the static and the drag load acting on it (Figure 6). These loads can be developed by any kind of body as long as the body is deformable. Such consideration on one hand clarifies that the full scale tests on these fences could be carried out with any deformable body with or without water; on the other one, it evidences that these structures are suitable to withstand the motion of bodies constituted by debris, shallow landslide, mad flow, debris flow and so on. On contrary, the impact of a rigid body (as per ETAG 027 guideline) that develops higher stress than the deformable body being equal the energy level, cannot help to predict the deformations of the barriers. However, the structural design has to consider that the pressure is not uniformly distributed on the screen, because it depends on the history of the static and dynamic loads moving from the bottom to the top of the barrier (GEO, 2011; Huang et al, 2007; Lo, 2000; Sun et al., 2011; Sun et al., 2012). In other terms, the calculation must assume that each longitudinal rope is subjected to a distributed load that varies its intensity during the time of the debris flow event. Therefore, the calculation process results to be quite complex. The conceptual solution is represented by the flow chart of Figure 7, where the results get for each rope level are stored and processed in order to define the maximum axial load acting on each transversal rope.

Figure 7. Conceptual solution for calculating the forces on the transversal ropes.

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Figure 8. Load versus deformation on the transversal rope. The calculation has to minimize force-gap

max depends on fmax, which is function of Tmax. Therefore an iterative process is needed to solve the problem. When the elongation of

the brake reaches the maximum elongation, the force grows vertically up to the max rope resistance. The solution considers that each longitudinal transversal rope of the barrier is composed by coupling one deformable element (energy dissipaters) and one non-deformable element (steel ropes). Due to the deformation of the brake, the stress on the ropes is reduced. According to the graph of figure 8, the energy dissipater device constantly deforms until the axial force on the rope reaches the maximum force applied on the energy dissipater. The maximum load Tmax acting on the generic rope ith can be calculated: Tmax(i) = (V(i)

2 + H(i)2)1/2 [1]

Where: Vi and Hi are the maximum loads acting on the generic rope ith respectively in the directions parallel and transversal to flow direction (Figure 9).

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Figure 9. Plan view with the with the distribution of the forces acting on the generic deformed rope-ith; qd =

pressure of the debris flow; LI = width of the generic rope before deforming; fmax = maximum sag of the generic rope.

The force are defined as following: V(i) = qd(i) . L(i) / 2 [2] H(i) = qd(i) . L(i)

2 / (8 . fmax(i)) [3] A non-linear analysis must be performed in order to define the axial force developed on the different longitudinal rope levels. At each stage of the run-up mechanism, an interactive model evaluates the axial force acting on the rope-brake system (Figure 10). The process ends when the force acting in the rope reaches the equilibrium with the deformation of the energy dissipater. At any time of the event (t) (figure 7), the static pressure acting at the depth d, measured from to the top free surface of the debris flow, can be assessed through the following relation (Figure 11) (Segalini et al., 2013; Canelli et al, 2012): qs(d) = k . d(t) . d . g = k . (h0 + h(t) - z) . d . g [4] The spacing between the ropes can be described as: p = hB / (n-1) [5] Where hB is the height of the debris flow barrier and n is the number of rope levels. Furthermore, the pressure load acting on the i-th longitudinal rope depends on the load sequence according to figures 6 and 11: zi = hB . (i – 1) / (n - 1) ≥ h0 [6]

Cable i

qd

fmax

H

V

Tmax

H

VTmax

Li

Deformed Cable i

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Where (figure 11) i = number of analyzed level; zi = level of the generic rope, h0 = constant height of the debris flow, hb = total height of the barrier.

Figure 10. Conceptual solution solving the non-linear problem of the equilibrium between forces of rope

(Li/2R), Lb = brake elongation (see also figure 8).

Figure 11. Debris accumulation against the debris flow barrier and corresponding loads at the generic

instant time (Segalini et al., 2012; Canelli et al., 2012)

HypothesisSag fi=0,1m

CalculationDTmax=Tmax-Tb

DT < 10kN

DT > 10kN

fi = fmaxTfune = Tmax

SOLUTION

fi = fi + 0,1m

START

Calculation

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Therefore, it is possible to define the distributed load (qd) acting against the barrier qt(zi,t) = qt(d,t) = { p . q(zi,t) / 2 i = 1, n [7] p . q(zi,t) 2 ≤ i ≤ n-1 Where p = vertical distance between the horizontal cables according to figure 11 (a). Once the distributed load (qd) for each rope (i) has been calculated, the resulting maximum tensile load (Tmax) has to be assessed for each rope level. This value strictly depends on the length of the longitudinal rope (Li) and its maximum displacement (fmax) (Figures 9 and 11). In conclusion, the different level of ropes and lateral anchors can be designed using the force Tmax(i). The elements of the debris flow barrier must be fit to guarantee a defined minimum safety factor. All the process is depending on the time of the pile up mechanism that can be appreciated with t = zi

2 / (2 v0 h0 tg ) (case of the generic rope above the lower one) [8] t = (zi – h0)2 / (2 v0 h0 tg ) (case of the bottom rope) [9] Where zi = generic vertical coordinate of the transversal rope; h0 = constant height of the debris flow surge; v0 = arrival velocity of the debris flow surge; = inclination angle of the creek. Calibration of the calculation model The calculation model has been calibrated based on the researches carried out by the University of Parma and Maccaferri (Segalini et al., 2013; Canelli et al., 2012; Ferrero et al., 2010). These studies were based on laboratory (figure 12) and full-scale tests (Figure 13), as well as analytical and numerical models.

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Figure 12. Laboratory test carried out on the brake system (upper pictures) and ring net components

(lower pictures).

Figure 13. Lateral view of the flexible debris flow barrier impacted during the full scale test carried out by the University of Parma in cooperation with Maccaferri.

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Figure 14. Results of an impact analysis.: maximum load acting on the longitudinal transversal ropes for

the static and dynamic case.

It was possible to underline the following aspects:

- The energy dissipaters assume a basic function for efficiency of the debris flow barrier. In fact, they are suitable to: maintain the longitudinal ropes properly aligned, reduce the stresses acting on the ropes, and allow the barrier deforming during the event. The functionality of these elements can be taken into account through the design process only if there is a proper knowledge of their behaviour. Therefore, the brakes must be always manufactured and tested in order to guarantee high performances (Figures 8 and 12) and be able to describe their behaviour with Force - Displacement diagrams (an example in Figure 14 and followings).

- The barrier does not homogenously deform along its height (Figure 14). During the history of the loading process different displacement can occur depending on the rope level analyzed (Figure 16). The intensity of these deformations are related to structural factors (length of longitudinal transversal ropes, properties of the energy dissipaters devices) and to the debris flow history, which is basically described by the height of the debris flow waves, and by the rheological properties.

- According to the previous point, it is not possible to define a direct proportion between the load acting on the barrier and its deformation (Figure 15 - 16).

- The dynamic load is almost always greater than the static one (Figure 14). A small increasing of the debris flow velocity gives large increasing of the load on the ropes.

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- According to the previous point, it is not possible to define a direct proportion between the load acting on the barrier and its deformation.

Figure 15. Results of an impact analysis: maximum elongation of the brake system on each level of the

screen

Figure 16. Maximum distributed load (kN/m) acting on different levels of the transversal ropes while the

debris flow rising up, from the first impact (left line) to the filling of the barrier (right line).

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CONCLUSION The flexible barriers represents a cost effective solution that can solve or reduce the debris flow hazard, since they can be easily implemented on creeks. Their design requires a careful evaluation of several factors mainly related to the maintenance of the structure. In the designing the main factor of uncertainty affecting the barrier design concerns the estimation of the pressure of the debris flow on the screen. The simplified analytical solution presented to design flexible debris flow barrier synthetizes the pragmatic approach developed by Officine Maccaferri in cooperation the University of Parma. These studies, based on laboratory and full-scale tests, as well as analytical and numerical models, were performed in order to understand the behavior of a flexible barrier impacted by a debris flow. Thanks to these investigations, the simplified calculation approach here presented has been defined and validated. The simplified analytical approach solves the non-linear problem of the load-displacement on the transversal ropes and allows quick reliable results without time-consuming numerical approaches (i.e. FEM). The implemented algorithm shows that the design of the longitudinal ropes, and consequently of the lateral anchors, requires the good knowledge of each component of the barrier. REFERENCES Canelli, L., Ferrero, A.M., Migliazza, M., Segalini, A. 2012. Debris flow risk mitigation by the means of rigid and flexible barriers–experimental tests and impact analysis. Nat. Hazards Earth Syst. Sci. 12: 1–7 (www.nat-hazards-earth-syst-sci.net/12/1/2012/doi:10.5194/nhess-12-1-2012). Cantarelli, G., Giani, G.P., Gottardi G., and Govoni L. 2008. Modeling rockfall fences. The First World Landslide Forum: 103-108. United Nations University. Tokyo. Ferrero, A.M., Giani, G.P., Segalini, A. (2010). Numerical and experimental analysis of debris flow protection fence efficiency. Proc. European Rock Mechanics Symposium (Eurock 2010), Lausanne, CH. Balkema, Rotterdam: 578–578. GEO. 2011. Design Requirements for Flexible Debris-resisting Barriers. Geotechnical Engineering Office, Civil Engineering and Development Department. The Government of the Hong Kong Special Administrative Region (Draft as at 12.10.2011). Grimod A. and Giacchetti G. 2013. High energy rockfall barriers: a design procedure for different applications. World Mining Congress, Montreal, August 2013.

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Huang, K., Yang, C., and S.W. Lai, S.W. 2007. Impact force of debris flow on filter dam, Abstracts, Vol. 9, 03218, 2007 S Ref-ID: 1607-7962 /gra / EGU2007-A-03218 European Geosciences Union 2007. Hungr, O., Evans, S.G., Bovis, M., and Hutchinson, J.N. 2001. Review of the classification of landslides of the flow type. Environmental and Engineering Geoscience, VII, 221-238. Hungr, O., McDougall S., and Bovis, M. 2005. Debris-Flow Hazards and Related Phenomena. Jakob, M. and Hungr, O., Eds.,. Praxis-Springer, Berlin. Heidelberg, 2005. Chapter 7. Lo, D.O.K. 2000. Review of Natural Terrain Landslide Debris-resisting Barrier Design. GEO Report No. 104. Geotechnical Engineering Office, 91 p. Pierson, T.C.,1986. Flow behaviour of channelized debris flow, Mount. St. Helens, Washington. In: A.D. Abhrams (ed). Hillslope process(pp.269-296). Allen Unwin, Boston. Segalini, A., Brighenti, R., Ferrero, A.M., Umili, G. 2013. Comparison between the mechanical behavior of barriers against rock fall vs debris flows. Rock Mechanics for Resources, Energy and Environment – Kwasniewski & Lydzba (eds) - 2013 Taylor & Francis Group, London, ISBN 978-1-138-00080-3. Shun, O., Hiroshi, I., Yshiharum, I., Takaahi, Y., 1997. Development of new methods for countermeasures against debris flow. In: - Recent developments on debris flows Armanini and Masanori (eds.). Sun, H.W. and Law, R.P.H. 2011. Design Requirements of Flexible Debris-resisting Barrier. A Note on the Key Design Requirements of Force Approach. Geotechnical Engineering Office, Civil Engineering and Development Department. The Government of the Hong Kong Special Administrative Region Sun, H.W. and Law, R.P.H. 2012. A preliminary study on impact of landslide debris on flexible barriers. Geotechnical Engineering Office. Standard and Testing Divison. Technical Note 1/2012. The Government of Hong Kong Special Administrative Region Takahashi, T. 1991. Debris flow. IAHR Monograph, A.A. Balkema, Rotterdam, 165 pp. Takahashi, T. 2007. Debris flow, Mechanics, prediction and countermeasures – A.A. Balkema, Rotterdam, 448 pp.