Bull Volcanol (2015) 77:75 DOI 10.1007/s00445-015-0961-8
RESEARCH ARTICLE
Developing building-damage scales for lahars: application to Merapi volcano, Indonesia Susanna F. Jenkins 1 & Jeremy C. Phillips 1 & Rebecca Price 1 & Kate Feloy 1 & Peter J. Baxter 2 & Danang Sri Hadmoko 3 & Edouard de Bélizal 4
Received: 2 December 2014 / Accepted: 26 July 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Lahar damage to buildings can include burial by sediment and/or failure of walls, infiltration into the building and subsequent damage to contents. The extent to which a building is damaged will be dictated by the dynamic characteristics of the lahar, i.e. the velocity, depth, sediment concentration and grain size, as well as the structural characteristics and setting of the building in question. The focus of this paper is on quantifying how buildings may respond to impact by lahar. We consider the potential for lahar damage to buildings on Merapi volcano, Indonesia, as a result of the voluminous deposits produced during the large (VEI 4) eruption in 2010. A building-damage scale has been developed that categorises likely lahar damage levels and, through theoretical calculations of expected building resistance to impact, approximate ranges of impact pressures. We found that most weak masonry buildings on Merapi would be destroyed by dilute lahars with relatively low velocities (ca. 3 m/s) and pressures (ca. 5 kPa); however, the majority of stronger rubble stone buildings may be expected to withstand higher velocities (to 6 m/s) and pressures (to 20 kPa). We applied this preliminary damage scale to a large lahar in the Putih River on 9 January 2011, which
Editorial responsibility: G. Lube * Susanna F. Jenkins
[email protected] 1
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
2
Institute of Public Health, University of Cambridge, Cambridge, UK
3
Department of Physical Geography, Universitas Gadjah Madah, Yogyakarta, Indonesia
4
Laboratoire de Géographie Physique, Université Paris 1, Paris, France
inundated and caused extensive building damage in the village of Gempol, 16 km southwest of Merapi. The scale was applied remotely through the use of public satellite images and through field studies to categorise damage and estimate impact pressures and velocities within the village. Results were compared with those calculated independently from Manning’s calculations for flow velocity and depth within Gempol village using an estimate of flow velocity at one upstream site as input. The results of this calculation showed reasonable agreement with an average channel velocity derived from travel time observations. The calculated distribution of flow velocities across the area of damaged buildings was consistent with building damage as classified by the new damage scale. The complementary results, even given the basic nature of the tools and data, suggest that the damage scale provides a valid representation of the failure mode that is consistent with estimates of the flow conditions. The use of open-source simplified tools and data in producing these consistent findings is very promising. Keywords Lahars . Volcanic hazard and risk assessment . Damage scales . Merapi volcano . One-dimensional hydraulic flow simulation . Physical vulnerability functions
Introduction Lahars—gravity-driven flows of volcanogenic sediment and water—present a significant hazard to communities downstream of volcanoes (Pierson et al. 2014). Lahars can damage or destroy homes and community buildings resulting in a loss of home and livelihood, associated economic losses and, in some cases, fatalities. The extent to which a building impacted by a lahar is damaged depends upon both the dynamic
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characteristics of the lahar, i.e. its velocity, depth, sediment concentration and grain size, and the structural characteristics and setting of the building in question. A number of studies have been dedicated to assessing likely lahar dynamics (e.g. Rodolfo et al. 1996; Lavigne and Thouret 2003; Williams et al. 2008; Carrivick et al. 2009; Capra et al. 2010; Lube et al. 2012; Worni et al. 2012), but few have concentrated on the physical responses of buildings to lahars (e.g. Zuccaro and De Gregorio 2013). An improved understanding of how lahars cause damage can support volcanic risk assessments through better quantification of the nature and extent of likely building damage in a future lahar, which may, in turn, help identify areas where future research and mitigation actions are best directed. In this study, we focus on quantifying building vulnerability to lahar impact at Merapi volcano in central Java, Indonesia. Merapi is one of the most active and densely populated volcanoes in the world, and lahars are considered one of the most important hazards at the volcano (Lavigne et al. 2000a; Thouret et al. 2000). In 2010, Merapi produced the largest eruption of its size and style (VEI 4) in over a century, depositing more than 18 million m3 of pyroclastic material (Charbonnier et al. 2013) into the channels draining the steep volcano flanks (Fig. 1). Heavy rains subsequently remobilised the 2010 deposits to form large and damaging lahars that increased in frequency and run out distance through 2010 and 2011 (de Bélizal et al. 2013). Within 8 months of the eruption, nearly 900 buildings had been damaged or destroyed in villages up to 20 km from the volcano (de Bélizal et al. 2013). In these more distal (>15 km) areas to the southwest of Merapi, water-rich hyperconcentrated (20 to 50 or 60 % sediment concentration: Vallance 2000) lahars dominate (de Bélizal et al. 2013). This study describes the development of a lahar building-damage scale, analogous to those that exist for earthquakes (Grünthal 1998), debris flows (Toyos et al. 2003) and pyroclastic density currents (Spence et al. 2004). In applying the damage scale to a recent lahar on Merapi volcano, we estimate impact pressures and velocities (lahar dynamics) using building vulnerability calculations and compare our findings with those derived from simplified hydraulic calculations using Manning’s formulation. There are three main components to our study, each designed to test the feasibility and utility of our approach, and this is reflected in the layout of this paper: 1. Development of a preliminary lahar building-damage scale and associated vulnerability curves for two masonry-building-type characteristics of the Merapi area (BDeveloping a lahar building-damage scale^ section). Damage scales relate a number of expected damage levels to likely lahar dynamics, in this case, impact pressures. Vulnerability curves, in the form of probability distributions, are then used to give an estimate of our uncertainty
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in assigning lahar dynamics to certain levels of damage. In developing the curves for Merapi, we take a theoretical approach to calculating building vulnerability and draw from the limited available empirical data, supplemented by evidence from analogous flows such as floods and debris flows. 2. Application of the damage scale to the 9 January 2011 lahar in the Putih River, which inundated the village of Gempol 16 km southwest of the volcano; however, detailed observations and measurements of lahar dynamics are not available. Remote and field surveys of the damaged area provided information on building damage. Application of the damage scale allowed us to investigate what could be achieved in terms of relating calculated lahar dynamics to observed damage and style of building construction (BApplication to Merapi^ section). 3. Validation and comparison of the inferred dynamics of the 9 January 2011 lahar with values obtained independently using a Manning’s approximation for lahar behaviour. Limited eyewitness observations of the lahar were coupled with open-source DEM data to reconstruct the 9 January 2011 lahar using simplified tools that are freely available online (BComparison of independently inferred flow conditions^ section). This paper focuses on what can be achieved using freely available data and open-source tools remotely applied to a lahar event. We follow previous approaches to quantifying building vulnerability for flood and pyroclastic density current by incorporating engineering calculations of building strength. Our results are intended to be preliminary but robust; their uncertainty has been quantified where possible.
Developing a lahar building-damage scale Damage scales are commonly used to relate different levels and types of building damage to hazard dynamics, for example, felt intensity for earthquakes (European Macroseismic Scale: Grünthal 1998) or wind speed for tornadoes (Fujita Tornado Damage Scale: Fujita 1971). They have also been proposed for some hazardous mass flows (e.g. Alexander 1986; Toyos et al. 2003; Spence et al. 2004; Künzler et al. 2012) but are not yet available for lahars. Studies of recent lahars, e.g. the 2008–2009 Chaitén eruption, Chile (Pierson et al. 2013), the 1998 Sarno mudslides, Italy (Zanchetta et al. 2004), the 1991 Pinatubo eruption, Philippines (Major et al. 1996), and the 1985 Nevado del Ruiz eruption, Colombia (Pierson et al. 1990), provided some insight into the range of potential lahar building damage. However, detailed data on damage from lahars are scarce (Douglas 2007; Blong 1984),
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Fig. 1 Area of study around Merapi volcano showing unconsolidated deposits from the 2010 eruption and key sites along the Putih River. Inset base map provided by Google™
often because damage is total, either through structural collapse or burial, or because the redirection of a channel during or following a lahar can threaten investigator safety and inhibit detailed studies. We thus augment observations of lahar damage with reports and estimates of damage from analogous mass flows, e.g. debris flows (e.g. Hu et al. 2012), floods (e.g. Kelman and Spence 2004; Pierson et al. 2013) and, to some extent, pyroclastic density currents (e.g. Baxter et al. 2005; Jenkins et al. 2013). These studies suggest that lahar building damage may include partial or total collapse from excessive impact pressures on walls, foundation failure, undermining and structure transportation due to soil erosion and liquefaction, and/or burial. Therefore, we have categorised five distinct levels of damage that may be expected for buildings impacted by lahar (Table 1). These damage levels follow the standardised approach of other mass flows and are intended to be deliberately generic so that they
may be applicable to any lahar damaged area. We discuss impacts to buildings because of their consequences for life and livelihoods. However, issues of infrastructure vulnerability, particularly bridges, and impacts of lahars to agricultural lands and water resources are also important areas of ongoing study. At damage levels 1 and 2, damage is mostly non-structural; these levels mainly categorise damage to building openings, such as windows or doors, and contents. At damage levels 3 and 4, impact pressures associated with the flow collapse the structures. Buildings that are not destroyed, either because of lower impact pressures or because of increased structural resistance, may be buried by deposits that dry and solidify, rendering buildings uninhabitable or potentially uneconomic to recover. The closest comparative available damage levels are for debris flows (Toyos et al. 2003; Jakob et al. 2012), which are generally characterised by higher sediment concentrations
75 Table 1
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Page 4 of 17 Proposed damage levels applicable to buildings impacted by lahar
Level
Damage
Description
Consequence
0 1
None Minor
– Damage to contents of building
2
Moderate
3
Major
– Infiltration into building under door and through gaps, e.g. cracks or ventilation grills, not produced by lahar Window and door glass failure. Possible weak door and window frame failure Loss of parts of external and/or internal walls and infill panels
4
Complete
Burial by sediment Wall, frame, roof or foundation failure Burial by sediment
(>50 or 60 % sediment: Vallance 2000) and a very broad range of grain sizes, from clay to boulders (Jakob et al. 2012). Nonetheless, the assigned damage levels are similar in that they follow the European Macroseismic earthquake damage scale (Grünthal 1998) and include four or five levels ranging from ingress of the flow into the building to complete devastation. Although dynamics of the lahar strongly influence the extent and type of damage sustained, the physical characteristics and settings (e.g. the orientation, degree of sheltering) of the buildings themselves also play an influential role. For example, reinforced concrete frame buildings may be expected to resist higher impact pressures than structures in poor condition or composed of weak timber boards. The damage levels are applicable globally, but assigning appropriate lahar dynamics to the damage levels requires an understanding of the building type. Defining building types Following post-eruption damage studies carried out by Jenkins et al. (2013), we categorised buildings around Merapi into two main typologies (Fig. 2): (1) weak reinforced concrete frames with squared block masonry infill and (2) unreinforced rubble stone masonry buildings. Both types had tiled roofs supported by bamboo or timber, and most were single storey with large Fig. 2 Examples of the two masonry building types that dominate the flanks of Merapi: a squared block masonry with weak reinforced concrete frames and b rubble stone masonry sourced from local scoria and not set within a frame. Both contain timber or bamboo roof supports with clay tiles
Deposition of sediment inside building, significant damage to contents Significant internal deposits; building likely to be unsafe for occupancy Potentially irreversible damage or costly clean-up Building unsafe for occupancy; may have to be demolished Potentially irreversible damage or costly clean-up
(>1 m2) single-glazed windows and ventilation grills that allow air (and potentially lahar material) to freely enter the building. The key difference between the two masonry typologies is wall thickness (∼10 cm for squared blocks versus ∼25 cm for rubble stone blocks). Timber buildings are also found around Merapi. These types of dwellings are designed to be inexpensive to build and repair after rainy seasons and are referred to as non-permanent housing by the government (Donovan 2010). They are expected to offer negligible resistance to lahars and were therefore not considered further in our study. Progressing the damage levels developed in Table 1 towards a building-damage scale requires appropriate ranges of likely lahar dynamics to be assigned to each level. In terms of damage to the building structure (damage levels 2, 3 and 4), the horizontal impact pressure of the lahar is likely the most relevant flow property (Zuccaro and De Gregorio 2013). Relating damage to impact pressures is of additional importance because this parameter can be derived from most dynamic lahar models. In what follows, we briefly describe how we calculate impact pressure (BCalculating lahar impact pressures^ section), how we assign appropriate impact pressures to each damage level (BLimit state analysis^ section) and, in ascribing uncertainty to these estimates, how we developed failure vulnerability curves for each of the identified building types (BAccounting for variability^ section).
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Calculating lahar impact pressures The pressure imparted by a mass flow onto a building includes both a dynamic component, associated with the velocity and density of the flow (Eq. 1), and a static component associated with flow depth (Eq. 2). Pd ¼ 0:5 ρv2
ð1Þ
Ps ¼ ρ g h
ð2Þ
where ρ is the flow density, v is the velocity, g is the gravitational acceleration and h is the flow depth. Flow density (ρ) is calculated as the bulk density of the sediment concentration, using the density of water (ρw), the density of source material (ρs) and the volume fraction of water (vw): ρ ¼ ρw ⋅vw þ ρs ⋅ð1−vw Þ
ð3Þ
For well-sealed buildings and for lahars having high sediment concentrations (i.e. debris flows), the hydrostatic component of pressure can be significant (Zanchetta et al. 2004); debris flow damage scales typically refer only to the hydrostatic pressure component when assigning pressure ranges (Jakob et al. 2012). However, the tropical climate at Merapi and in many other volcanic areas means that buildings are deliberately ‘leaky’ to aid ventilation and are thus unlikely to prevent infiltration. The static pressure component is therefore expected to be negligible for the waterrich lahars being considered here, and we assume that the dynamic component of pressure (Eq. 1) is the sole contributor to lahar damage. Reapplication of the damage levels for well-sealed buildings or for debris flow lahars may require both the hydrostatic and dynamic pressure components to be incorporated when calculating likely impact pressures (Eqs. 1 and 2). Density, velocity and sediment concentration and, therefore, the impact pressure may vary spatially and with time during flow impact. As a simplification, peak pressure and a spatially uniform pressure distribution across the wall are typically assumed (Kelman and Spence 2004; Spence et al. 2004; Zuccaro and De Gregorio 2013) and we make similar assumptions. Using Eq. 1, for a dilute lahar at Merapi, with 20 % sediment concentration and a velocity of 3 m/s, calculated impact pressures are of the order 5.1 to 5.9 kPa (assuming sediment densities of between 1700 and 2600 kg/m3: de Bélizal et al. 2013; Komorowski et al. 2013 and, therefore, flow densities of 1100 to 1300 kg/m3). For a specified bulk flow density, the major influence on impact pressure is flow velocity: increasing the velocity to 4 m/s nearly doubles the impact pressures (to 9.1 to 10.6 kPa) (Fig. 3).
Fig. 3 Plot showing lahar dynamic impact pressures, calculated using Eq. 1, as a function of flow velocity and density. Flow density is calculated using Eq. 3 from the sediment concentration, grain density and water density (1000 kg/m3). At the same flow velocities, impact pressures for a higher density flow (60 % sediment concentration, 2600 kg/m3 grain density) are nearly double that of a lower density flow (20 %, 1700 kg/m3)
Buildings around Merapi are mostly located on terraces above incised river channels. On the volcano flanks, channel riverbanks can exceed 20 m high. Farther from the volcano (>15 km), channels are less incised with riverbanks more typically in the region of 2 to 10 m high. Workers quarrying deposits inside the channels are very vulnerable to lahar; however, buildings are only affected when lahars overspill the channel. We therefore consider calculated impact pressures associated with passive flow inundation into overspill areas, where buildings exist, rather than with an instantaneous wavetype impact that may occur in the flow channel where buildings do not exist. This follows the standard approach to modelling floods (Kelman and Spence 2004), rather than by analogy with instant impacts such as nuclear blast loading on buildings (Glasstone and Dolan 1977) or sea waves on coastal defences (USACE 1984). These latter approaches for instantaneous impact may be more relevant to calculating damage to bridges and for certain large, fast-flowing lahars. Missiles within a lahar flow, such as large boulders, logs, building debris and even cars, can also cause localised high pressures that puncture holes in walls and compromise the structural integrity of the building, but they are not incorporated in our model. Quantifying building vulnerability Openings in buildings (e.g. doors and windows) are typically weaker than the surrounding wall or panel and are likely to fail at lower impact pressures. Experimental testing (Zuccaro 2000) and theoretical calculations (Spence et al. 2004) of window and door failure pressures have been estimated for pyroclastic density current impact around Vesuvius, Italy. Singleglazed windows and wooden doors are approximately universal in their composition and structure, and so, little adaptation
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is required to make these findings applicable to Merapi buildings. Results suggest that pressures of around 1 to 2 kPa are required to break large single-glazed windows and more than 3 kPa to break through wooden doors typical of Merapi buildings (Table 2). This suggests that straightforward preventative action such as boarding up openings, or using wooden shutters, may limit infiltration and associated damage. Infiltration of a lahar into a building, for example, through openings, can damage the inside of the structure and its contents but may also act to decrease the pressure differential across walls, windows and doors, thereby reducing the potential for damage with any subsequent impact pressure to the exterior of the building (Spence et al. 2004). For those buildings where there are no openings or where openings resist lahar flow, impact pressure may cause wall collapse or, in the case of a framed building, failure of the wall within the frame. For the two building typologies identified at Merapi, we used theoretical calculations of likely material strength, by means of limit state analysis, to estimate at what pressure wall failure may occur (damage level 3). Limit state analysis Limit state analysis involves calculating a failure pressure, i.e. a pressure at which the resistance of a wall to impact pressure is exceeded, from a simplified collapse mechanism. Here, we consider that masonry walls and panels at Merapi follow a fracture-line failure mechanism (Fig. 4) appropriate to unreinforced or very weakly reinforced masonry. Fracture-line failure was chosen here because it is one of the most widely applicable and calculable failure methods, producing results that are consistently compatible with experimental testing. Empirical evidence suggests that the weakest interface along which the fracture will form is between the mortar and masonry, with the internal strength of masonry rarely exceeded before failure of the mortar–brick interface (Morton 1986; Kelman and Spence 2003; Jenkins et al. 2013). The horizontal failure pressure (w) can be calculated given knowledge about Table 2 Interdecile (10th to 90th percentile) and median (in brackets) estimated failure pressure ranges for upper damage levels and for the different building types
Damage level
Fig. 4 Failure mechanism for fracture-line analysis along the mortar–brick interface, following Sinha (1978)
masonry wall or panel dimensions and the flexural strength of its components (derived from Sinha 1978): w ¼ 6m=
1:5 β‐ β2 2 2 ⋅L ⋅α 2 β þ ðμα2 =K Þ
ð4Þ
where m is the moment/unit length normal to the bed joint (N/mm2). In turn, m is a function of the wall thickness (t) in millimetre and the strength of the bond between mortar and brick vertically ( fy), i.e. along the joins perpendicular to the floor; m=fy·t2/3500 where the numerical coefficient is a combination of unit conversion factors and a design factor that accounts for existing buildings of uncertain construction, age and condition. The design factor can be adjusted where more information regarding construction quality and building condition are available; here, the standard value of 3.5 is used (following BS 5628-1 2005). μ is the ratio between the moment per unit length parallel to the bed joint and m, L the span of the wall (m), α the ratio of wall height to wall span, K the ratio of the horizontal to vertical elastic moduli and β a constant that gives the location of the point where the fracture meets the top edge of the wall and is a function of the wall height and span. A fracture-line failure assumes no sheltering and that a flow acts normal to the wall; pressure estimates therefore represent
Brief description
Failure pressure (kPa) Squared masonry
2
Large window glass and frame breakage; Old wooden door failure
3
Structural wall failure
Rubble stone masonry
1.0 to 3.0 (1.5) 2.0 to 6.5 (3.5) 2.5 to 6.5 (4.2)
22 to 55 (36)
For opening failure (damage level 2), these follow the experimental and theoretical studies of Zuccaro (2000) and Spence et al. (2004) and use the probability distributions derived from expert judgement in Jenkins et al. (2014) (lognormal with a standard deviation of 0.5). For structural wall failure (damage level 3), they are informed by the theoretical calculations undertaken in this study. The estimated values and ranges are subject to significant uncertainty because of the limited data upon which they are based. Further study would be useful to supplement this scale
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conservative estimates appropriate for hazard assessments. However, wide application of the relatively simple fractureline theory in empirical and theoretical studies and the reasonable correlation with experimental results (Sinha 1978) provides a sound basis for its application here. For the two building typologies identified at Merapi, we derived a range of values for each of the inputs to Eq. 4 from published literature and from field studies following the 2010 eruption (Table 3). Failure pressures will vary for different building dimensions and estimates of material strength. Assuming a building of median dimensions and material strength and inputting the values from Table 3 into Eq. 4, rubble stone buildings may be expected to fail along fracture lines under impact pressures of 18 to 58 kPa and weak squared block masonry buildings at around 2 to 7 kPa. The variation in failure pressures for a single building type is mostly a result of varying the strength of the mortar–brick interface in the vertical direction, while the large difference in failure pressures between the two building types is mostly a result of the differing wall thicknesses and, to a lesser extent, variation in the constant β. We tested the influence of the parameters in Eq. 4 by varying them within the ranges and distributions of Table 3 and recalculating failure pressure. This sensitivity analysis suggests that the strength of the bond between mortar and block vertically (i.e. perpendicular to the floor) and the wall thickness have the largest influence on calculated failure pressures with the wall height and span of lesser importance (Fig. 5). Information on wall or panel thickness, height and span can be collected in the field. Vertical strength cannot be collected in field surveys of a building without destructive testing. The values suggested in Table 3 therefore have a relatively large
range to reflect the significant uncertainties associated with assigning a likely value. The sensitivity testing strongly indicates that further investigation and refinement of the assumed vertical strength between block and mortar would be beneficial. Accounting for variability Sensitivity tests show that estimates of likely failure pressures can vary considerably depending upon building dimensions and component strengths (Fig. 5). To account for this variability when developing vulnerability estimates, we use Monte Carlo simulation to compute probable failure pressures. We conducted 10,000 iterations using sampled building characteristics (Table 3) to produce a set of probability density functions that illustrate the probability that failure occurs as a function of the impact pressure (Fig. 6). Although these simulations cannot account for all possible sources of uncertainty, e.g. building age, construction and condition or in our assumed collapse mechanism, they offer a potential vulnerability curve. Vulnerability curves for volcanic hazards are less well established than are those for relatively frequent hazards such as earthquakes, because of the scarcity of empirical data and the complexity, diversity and relatively limited understanding of volcanic processes. However, they offer an indication of the uncertainty associated with any vulnerability estimate and, as such, are critically important to damage estimation. The squared block masonry panels and rubble stone masonry walls seen at Merapi may be expected to fail along fracture lines under impact pressures of approximately 2.5 to 6.5 and 22 to 55 kPa, respectively. Values for block masonry
Table 3 The range of input values and probability distributions used to calculate wall or panel failure pressures for the weak reinforced concrete frame buildings with squared block infill panels (SB) and the rubble stone (RS) masonry buildings typical of Merapi Input
Notation
Value at percentiles: Unit
Distribution of values
20th
50th
80th
Uniform
0.15
0.3
0.45
Horizontal mortar–brick strength (SB/RS) Vertical mortar–brick strength (SB/RS) Wall thickness (SB) Wall thickness (RS) Wall height (SB/RS)
fx
N/mm2
fy
N/mm2
0.3
0.6
0.9
t
mm
h
m
90 200 2.3
105 240 2.55
120 280 2.9
Wall span (SB/RS) Constant (SB) Constant (RS) Ratio of modulus of elasticity (SB/RS)
L β
m – – –
2.3 0.45 0.93 –
3 0.46 0.94 1.33
4.9 0.53 0.97 –
K
Triangular, with peak probability at the 50th percentile
–
Values were derived from fieldwork following the Merapi 2010 eruption
Source
Damage studies following the 2006 Java earthquake (EERI 2006) and expert judgement (Jenkins et al. 2013) Empirical data sourced from field building surveys (Jenkins et al. 2013)
Function of wall height and span (calculated following BS 5628-1 2005) Sinha (1978)
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Fig. 5 Sensitivity analysis results for the failure pressure of a wall of a squared block and b rubble stone masonry. Each input parameter shown on the plot was varied to its minimum and maximum values, with every other parameter fixed at their average value. The average value of every
parameter gives the average output value, shown by the black line. The greater the range of failure pressure, the greater the influence on the output
are in the range of those expected for a new, well-constructed masonry wall (up to 10 kPa: Sinha 1978) and less than estimated failure pressures for reinforced concrete frame masonry walls impacted by pyroclastic density currents (8 to 25 kPa: Baxter et al. 2005). Failure pressures suggested for debris flows (>18 kPa: Hu et al. 2012) and mudslides (>35 kPa: Zanchetta et al. 2004) are applicable mainly to the more robust rubble stone masonry. The significant variation within and between existing estimates and those presented here reflects the importance of incorporating building type and characteristics into any vulnerability study. The higher vulnerability of buildings at Merapi is a result of the weak strength between mortar and block, the weak reinforced concrete frame and the relatively thin wall thickness for squared block masonry buildings. The thicker rubble stone
masonry provides resistance comparable to strong reinforced concrete frame buildings assessed by Baxter et al. (2005) and Hu et al. (2012) and supports the premise that wall thickness and frame integrity are critical factors influencing building vulnerability to lahars. Variability in failure pressures highlights the importance of carrying out, and updating, building surveys in at-risk areas (e.g. Thouret et al. 2014) to better identify vulnerable areas with respect to potential impact from future lahars. We use the vulnerability estimates derived to assign approximate lahar impact pressures to the corresponding damage levels (3) (Table 2). In the following sections, we use this quantitative damage scale to categorise damage and reconstruct the impact of a damaging lahar on Merapi in January 2011.
Fig. 6 Vulnerability curves showing the exceedance probability of failure with increasing impact pressure for the range of squared block and rubble stone masonry buildings. The base curve is achieved by varying all input parameters between the ranges and probability distributions identified in Table 2. Refined vulnerability curves show the influence of wall span and thickness on resulting failure pressures
(shown by fixing each, in turn, at their 20th and 80th percentile values while varying the remaining inputs). End-member dimensions, i.e. those sourcing the upper or lower 20th percentile building dimensions, would produce unfeasible building dimensions not seen in residential buildings at Merapi
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Application to Merapi 9 January 2011 lahar The majority of lahars following the 2010 eruption of Merapi occurred in the Putih River to the southwest of Merapi (Fig. 1). Lahars probably focussed here because of the large catchment area and higher rainfalls in the west (de Bélizal et al. 2013). Approximately 16 km to the southwest of Merapi and around 200 m northeast of the village of Gempol, the Putih River splits into two channels. The larger channel turns sharply to the north–north–west before both continue under the Magelang–Jogjakarta highway and flow along the borders of Gempol. The lahar on 9 January overflowed the two channels where they split and inundated the village (Fig. 7). More than 150 buildings were destroyed,
Fig. 7 Aerial Google Earth™ images of Gempol acquired a before the lahar (22 April 2010) and b after (11 June 2011). The lahar travelled from right to left and overflowed at the point where the channels split and alter courses. The pre-lahar channels, Magelang–Jogjakarta highway and locations of buildings discussed in the text are marked. c Remotely categorised damage levels are shown for each building, with dashed lines showing the main axis of damage
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damaged or buried, leaving around 5000 people homeless (Surjono and Yufianto 2011). Studies of the observed lahar dynamics and their deposits (de Bélizal et al. 2013) found that flows in the more distal (>15 km) reaches of the Putih River were typically water-rich with flat-topped deposits characterised by massive or cross-bedded layers of gravel, coarse sand and fine sand with no large boulders. Remote application The 9 January 2011 Merapi lahar provides a good case study for applying our damage scale because satellite images were obtained the year before the lahar (22 April 2010: Fig. 7a) and 5 months (11 June 2011: Fig. 7b) and 6 months (9 July 2012) after the lahar. This imagery, when combined with geolocated photographs of buildings uploaded by the public, allowed us
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to assess structural and non-structural damage remotely and to assign approximate levels of damage to each of the impacted buildings. These initial assessments were later augmented with direct observations collected during field studies. The main axis of damage runs east–west through the northern part of the village (Fig. 7), and buildings within this zone were completely destroyed. This axis is not immediately adjacent to the channels bordering the village and reflects the straightahead inertia of the channel-confined flow as it approached the village. The northern and southern extents of the village, closer to the channels, are slightly elevated so that the flow would also have channelised more easily within this central axis. Field damage assessment Two of the authors (SFJ, PJB) visited Gempol village in August 2013 to study the damage associated with the lahar of 9 January 2011. Much of the village remained unoccupied, and many buildings were in a similar state to that observed in photographs of the days and months after the lahar, although some deposits and building materials, such as tiles, roof supporting structure and window frames, had been removed for use elsewhere. Within the principal damage axis, buildings that had not been completely removed by the flow showed failure and removal of the wall perpendicular to flow with damage or failure of the walls parallel to flow. Significant deposits (up to 2.5 m) could be found within the building remains (Figs. 8 and 9: building 1). On the periphery of the zone of total destruction (Figs. 8 and 9: buildings 2, 3 and 4), buildings had been buried in up to 2 m of relatively homogenous deposit. Deposits had been partially mined by the time of our visit, and we observed moderate to major building damage, with partial failure of some walls (building 2), broken glazing and, in some cases, broken wooden doors (buildings 3 and 4). The height of damage such as displaced roof tiles and damaged roof overhangs (e.g. buildings 2 through 5) demonstrates that the flow height exceeded the full height of the building (>4 m). Farther from the flow axis to the north and south (Figs. 8 and 9: building 5), there was still evidence that lahar flow had reached and damaged roof overhangs (Fig. 9f), but there was no damage to the building’s structural integrity, with flow infiltration into the buildings leaving deposits of 0.7 to 1.5 m. Estimating lahar dynamics Along the principal damage axis, the complete destruction of buildings suggests impact pressures potentially in excess of 10 kPa if all destroyed structures were squared masonry buildings. Beyond this zone of complete destruction, buildings are buried by deposit and show partial or no structural damage. If we assume that this damage gradient does not represent a
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transition from squared masonry or timber buildings to rubble stone buildings, then impact pressures must have attenuated rapidly with distance perpendicular to the main flow axis. Such rapid attenuation of impact pressure has been recognised following other mass flows (e.g. debris flows: Toyos et al. 2003; pyroclastic density currents: Baxter et al. 2005; Jenkins et al. 2013) and supports the notion that the greatest damage occurs along the principal flow axis as a result of flow inertia. Using estimates of impact pressure associated with failure of glazing and non-structural elements (BDeveloping a lahar building-damage scale^ section), we suggest that areas outside of the main damage axis were subject to pressures of 1 to 3 kPa. Our estimates of impact pressure assume that the peak fluid pressure was spatially uniform across the building. We therefore ignore the actions of elevated localised impact pressures associated with missiles (e.g. debris, large boulders). Field studies failed to find any evidence that missiles caused largescale damage to buildings in Gempol. Photographs taken in the days after the 9 January 2011 event show some boulders larger than 1 m within the village that may have acted as missiles, but these were reported to have been in the village prior to the lahar. If boulders were carried within the lahar, flow competencies were insufficient for them to cause damage outside of the main damage axis. In instances where the flow pressure alone is insufficient to damage buildings, the incorporation of missiles may cause damage not accounted for in our calculations, but the complete destruction of buildings within the damage axis suggests that impact pressures alone were enough to fail walls.
Comparison of independently inferred flow conditions In order to assess whether estimates of lahar flow conditions obtained from damage observations in Gempol are realistic, we made an independent estimate of the flow velocity distribution in the damaged area using a simplified flow calculation. Observations of the 9 January 2011 lahar dynamics were scarce, likely because the lahar happened at night and because instrumented lahar monitoring equipment is concentrated closer to the volcano (Lavigne et al. 2000b). Observations of flow conditions were limited to a single estimate at a fixed location upstream of Gempol, and so, we were restricted to making a steady flow calculation of the spatial distribution of flow properties. We recognise the significant difficulties in numerically describing lahar flows (Manville et al. 2013) and emphasise here that we cannot describe the lahar dynamics due to lack of observational input data. However, this situation is not unusual, as for many areas susceptible to lahar damage, the information required to reconstruct flow dynamics, such as channel geometry, elevation data and channel
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Fig. 8 a Aerial image of Gempol taken shortly after the lahar impact and before the main highway had been cleared of deposit (upper: image from Jakarta Post). b Google Earth satellite image acquired 5 months after the lahar (11 June 2011). The yellow boxes in both images contain the same buildings. The lahar flowed from right to left in both images, and the numbers refer to buildings discussed in the text and in Fig. 9
roughness, is often limited in quality and quantity, and their acquisition can be costly and rapidly outdated. Using complex dynamic models to reconstruct the 9 January lahar at Merapi would not be appropriate because of the lack of information to provide the required boundary conditions with any confidence, and results from these types of models would be highly uncertain for this event. To contextualise the damage scale, we therefore followed previous approaches (e.g. Macedonio and Pareschi 1992; Darnell et al. 2013) and applied simplified hydraulic calculations using Manning’s formulation to provide first-order estimates of steady flow conditions.
Manning’s calculations Lahars in the Putih channel at the distance from the volcano of Gempol (16 km) have been water-rich and produced predominantly fine sand (
