Resilience of socio-ecological systems in volcano risk-prone areas, but how much longer? Assessment of adaptive water governance in Merapi volcano, Central Java, Indonesia

International audienc

The potential contribution of cultural ecological knowledge to resources management in a Volcanic River Basin

This study examined the question "What are the main elements of water-related cultural ecological knowledge (CEK) in a volcanic river basin?" The CEK is defined as a body of knowledge, both written and orally transmitted, produced by many generations' observation on the human-nature relationship. This study illustrates that the water-related CEK is interrelated with other resources, the volcano and the lahar (debris flow), in the hydrological cycle. This research applied a qualitative methodology for the case study of the Opak sub-basin at Mt. Merapi volcano, within the unique setting of Yogyakarta, the only ruling kingdom in the Republic of Indonesia. Previous research on traditional ecological knowledge and indigenous technical knowledge provided the conceptual framework. The results provide the main elements of CEK and its formulation patterns. The main elements were historical experiences, legitimatization process, and mechanism of transfer. Although the formulation process stemmed from historical experiences of natural conditions and how humans managed the environment, a legitimatization process is necessary before it is formed as mechanisms of transfer for the future generation. These mechanisms were divided into the following five forms: (1) philosophy and values as the core and more practical forms such as (2) internalization (rituals and myth), (3) resource management practices, (4) artifacts, and (5) five-senses wisdom

Rainfall-runoff properties of tephra: Simulated effects of grain-size and antecedent rainfall

Rain-triggered lahars (RTLs) are a significant and often persistent secondary volcanic hazard at many volcanoes around the world. Rainfall on unconsolidated volcaniclastic material is the primary initiation mechanism of RTLs: the resultant flows have the potential for large runout distances (> 100 km) and present a substantial hazard to downstream infrastructure and communities. RTLs are frequently anticipated in the aftermath of eruptions, but the pattern, timing and scale of lahars varies on an eruption-by-eruption and even catchment-by-catchment basis. This variability is driven by a set of local factors including the grain size distribution, thickness, stratigraphy and spatial distribution of source material in addition to topography, vegetation coverage and rainfall conditions. These factors are often qualitatively discussed in RTL studies based on post-eruption lahar observations or instrumental detections. Conversely, this study aims to move towards a quantitative assessment of RTL hazard in order to facilitate RTL predictions and forecasts based on constrained rainfall, grain size distribution and isopach data. Calibrated simulated rainfall and laboratory-constructed tephra beds are used within a repeatable experimental set-up to isolate the effects of individual parameters and to examine runoff and infiltration processes from analogous RTL source conditions. Laboratory experiments show that increased antecedent rainfall and finer-grained surface tephra individually increase runoff rates and decrease runoff lag times, while a combination of these factors produces a compound effect. These impacts are driven by increased residual moisture content and decreased permeability due to surface sealing, and have previously been inferred from downstream observations of lahars but not identified at source. Water and sediment transport mechanisms differ based on surface grain size distribution: a fine-grained surface layer displayed airborne remobilisation, accretionary pellet formation, rapid surface sealing and infiltration-excess overland flow generation whilst a coarse surface layer demonstrated exclusively rainsplash-driven particle detachment throughout the rainfall simulations. This experimental protocol has the potential to quantitatively examine the effects of a variety of individual parameters in RTL initiation under controlled conditions

Main issues of an evacuation in case of volcanic crisis: social stakes in Guadeloupe (Lesser Antilles Arc)

International audienc

Probabilistic analysis of rain-triggered lahar initiation at Tungurahua volcano

Semi-continuous production of pyroclastic material by intermittent strombolian, vulcanian and sub-plinian eruptions at Volcán Tungurahua, Ecuador has created a persistent rain-triggered lahar hazard during the 1999-present eruptive episode. Lahars threaten the city of Baños, which lies approximately 8 km from the crater, as well as other villages and vital infrastructure situated in close proximity to the dense radial drainage network of the volcano. This study analyses the initiation of rain-triggered lahars and the influence of antecedent rainfall on this process in two northern instrumented drainages, La Pampa and the Vazcun. Analysis of lahar-triggering rainfall intensity and duration between March 2012 and June 2013 yields a power-law relationship, while Receiver Operating Characteristic (ROC) analysis indicates that peak rainfall intensity (10 minute, 30 minute and 60 minute) is the most effective single predictor of lahar occurrence. The probability of a lahar exceeding a pre-defined magnitude increases with peak rainfall intensity. Incorporation of antecedent rainfall (24 hour, 3 day, 5 day and 7 day) as a secondary variable significantly impacts lahar probabilities, particularly during moderate-high intensity rainfall events. The resultant two and three-dimensional lahar probability matrices are applied to rainfall data between 1st July and 31st December 2013 with the aim of predicting lahar occurrence. Composite lahar indicators comprised from the mean lahar probability estimates of individual matrices are shown to perform this task most effectively. ROC analysis indicates a probability >80% that these composite indicators will generate a higher estimated lahar probability for a randomly selected lahar event than a randomly selected non-lahar event. This method provides an average of 24 minutes of additional warning time compared with the current Acoustic Flow Monitors (AFMs) used for lahar detection, effectively doubling warning times for key downstream infrastructure in the two drainages. Ultimately, this method of lahar analysis could be used to construct real-time probabilistic rain-triggered lahar forecasts as an aid to current lahar hazard mitigation techniques at any location with a significant rain-triggered lahar hazard and a basic instrumental set-up

The pre-eruptive magma plumbing system of the 2007–2008 dome-forming eruption of Kelut volcano, East Java, Indonesia

Kelut volcano, East Java, is an active volcanic complex hosting a summit crater lake that has been the source of some of Indonesia’s most destructive lahars. In November 2007, an effusive eruption lasting approximately 7 months led to the formation of a 260-m-high and 400-m-wide lava dome that displaced most of the crater lake. The 2007–2008 Kelut dome comprises crystal-rich basaltic andesite with a texturally complex crystal cargo of strongly zoned and in part resorbed plagioclase (An47–94), orthopyroxene (En64–72, Fs24–32, Wo2–4), clinopyroxene (En40–48, Fs14–19, Wo34–46), Ti-magnetite (Usp16–34) and trace amounts of apatite, as well as ubiquitous glomerocrysts of varying magmatic mineral assemblages. In addition, the notable occurrence of magmatic and crustal xenoliths (meta-basalts, amphibole-bearing cumulates, and skarn-type calc-silicates and meta-volcaniclastic rocks) is a distinct feature of the dome. New petrographical, whole rock major and trace element data, mineral chemistry as well as oxygen isotope data for both whole rocks and minerals indicate a complex regime of magma-mixing, decompression-driven resorption, degassing and crystallisation and crustal assimilation within the Kelut plumbing system prior to extrusion of the dome. Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are postulated, with a deeper zone at lower crustal levels or near the crust-mantle boundary (>15 km depth), a second zone at mid-crustal levels (~10 km depth) and several magma storage zones distributed throughout the uppermost crust (<10 km depth). Plagioclase-melt and amphibole hygrometry indicate magmatic H2O contents ranging from ~8.1 to 8.6 wt.% in the lower crustal system to ~1.5 to 3.3 wt.% in the mid to upper crust. Pyroxene and plagioclase δ18O values range from 5.4 to 6.7 ‰, and 6.5 to 7.6 ‰, respectively. A single whole rock analysis of the 2007–2008 dome lava gave a δ18O value of 7.6 ‰, whereas meta-basaltic and calc-silicate xenoliths are characterised by δ18O values of 6.2 and 10.3 ‰, respectively. Magmatic δ18O values calculated from individual pyroxene and plagioclase analyses range from 5.7 to 7.0 ‰, and 6.2 to 7.4 ‰, respectively. This range in O-isotopic compositions is explained by crystallisation of pyroxenes in the lower to mid-crust, where crustal contamination is either absent or masked by assimilation of material having similar δ18O values to the ascending melts. This population is mixed with isotopically distinct plagioclase and pyroxenes that crystallised from a more contaminated magma in the upper crustal system. Binary bulk mixing models suggest that shallow-level, recycled volcaniclastic sedimentary rocks together with calc-silicates and/or limestones are the most likely contaminants of the 2007–2008 Kelut magma, with the volcaniclastic sediments being dominant