The Transport Capacity of Pyroclastic Flows: Experiments and Models of Substrate-Flow Interaction
One of the more distinctive features of many ignimbrites is the presence of large lithics (some greater than meter scale) and pumices that have been transported great distances (>10 km) from the eruptive vent, sometimes over steep terrain and expanses of water. In many cases, these particles have been transported much further than can be explained by aerodynamic forces and ballistic trajectories. We examine the forces responsible for transport of large clasts and examine in detail the momentum transfer occurring when particles interact with their boundaries. We performed a suite of experiments and numerical simulations to quantify the mass and momentum transfer that occurs when particles interact with a pumice-bed substrate and with water substrate, two geologically motivated flow end-members. We find that clasts transported in dilute currents are particularly sensitive to the nature of the boundary, and while large particles can skip several times on a water substrate they travel less far than particles that impact pumice-bed substrates. All else being equal, large particles in dense pyroclastic flows are themselves relatively insensitive to the details of their boundaries; however, one of the most important ways boundary conditions influences large particles is not through direct interaction, but by changing the local concentration of fine particles. Momentum transfer from fine particles to large particles appears to be required to transport large clasts great distances. If initially dense flows become dilute during transport, than the transport capacity of large particles in the flow is substantially decreased.
Evaluation of Geophysical Mass Flow Models Using the 2006 Block-and-ash Flows of Merapi Volcano, Indonesia
The dynamics and depositional processes associated with block-and-ash flows (BAFs) are most commonly inferred to be a function of granular or inertial grain flow, similar to debris flows and cold rock avalanches. Existing geophysical mass flow models are either based on frictional (Mohr-Coulomb) behavior (the Titan2D model developed at the University of Buffalo, USA) or another rheological law (i.e., a constant retarding stress), eventually adding some viscous and turbulent components (the VolcFlow model developed at the Laboratoire Magmas et Volcans, Clermont-Ferrand, France). The 2006 BAFs of Merapi have been used to test the validity of these two well-established models. Our work has allowed the definition of key flow parameters that will lead to more reliable predictions of the areas and levels of hazards associated with such flows. We first show that with the incorporation of spatially varying bed friction angles, Titan2D is capable of reproducing the paths, runout distance, areas covered and deposited volumes of the 2006 Merapi flows over highly complex topography. However, some discrepancies with field data are noted and the velocity and travel time of the flows do not match entirely. Using a single free parameter (a constant retarding stress), simulations obtained with the VolcFlow model also reproduce the morphology and distribution of the natural deposits as well as the time of emplacement and velocities of the flows. The results suggest that the performance of these models in simulating actual events is critically dependent on (1) the calibration of the model by using extensive field- based data such as deposit distribution, processes of flow generation, transport and deposition, (2) the incorporation of a suitable empirical law into the model (spatially varying bed friction angles or constant retarding stress) and (3) the choice of input parameters, such as location and volume of the initial pile of material and source characteristics (single or multiple dome-collapse, dome-collapse duration and total volume of collapsed material). The model evaluations presented here provide an invaluable tool for guiding hazard assessement of BAFs during future eruptive crises at Merapi.
Using Ground-Penetrating Radar to Unravel the Dynamics and Hazards of Block-and-ash Flows at Merapi Volcano, Indonesia
Assessing the dynamics and depositional processes of block-and-ash flows (BAFs) can be extremely difficult when based on traditional field-based investigations alone, as these are often complicated by a combination of poor exposure, rapid lateral facies variations of associated deposits and unknown paleotopography. However, when combined with other techniques such as ground-penetrating radar (GPR), a method that allows the internal architecture of BAF deposits to be mapped to a depth of several meters at high resolution, many of the limitations of traditional field studies can be overcome. Merapi volcano in Central Java is one of the most frequently erupting volcanoes in Indonesia and a classic site for the study of BAFs. In a case study in the application of non-invasive geophysical investigations of BAF deposits, GPR was used to evaluate the dynamics and hazards of BAFs generated during the 2006 Merapi eruption. During the later eruption phases, BAFs affected areas on the volcano's southern flank when they spilled over the sides of existing river valleys to create overbank flows that extended across adjacent, densely populated interfluve (non-valley) regions. A GPR facies-based approach has been adopted in order to interpret radar sections and correlate the observed features to process-related information gained from outcrop descriptions and granulometric analysis. Across the whole deposit, distinct variations are evident with relatively uniform high-energy facies passing into more irregular, locally varying facies in the medial and overbank regions. These variations at larger (deposit) and smaller (intra-deposit) scale, which cannot be detected by traditional field-based studies alone, show that GPR is capable of being an important non-invasive tool for developing new, improved interpretations of BAF transport and emplacement dynamics and, ultimately, the hazards associated with such flows.
Ash Production in Eruptive Flows: Comminution in Conduits and Pyroclastic Flows
Processes occurring at the grain scale, termed microphysical processes, can exert strong control of explosive eruption dynamics. In this talk we illustrate the importance of particle-particle interaction on the mass and momentum balance of eruptive flows. In particular we examine the break-up and transport of clasts during particle-particle interactions for two high-energy flow environments: pyroclastic and conduit flows. Abrasion and comminution of pumice clasts during the propagation of pyroclastic flows and post-fragmentation conduit flow have long been recognized as a potential source for the enhanced production of volcanic ash, however its relative importance has eluded quantification. The amount of fine-material produced in-situ can potentially affect runout distance, deposit sorting, the volume of ash introduced in the upper atmosphere, and internal pore pressure in pyroclastic flows. We conduct a series of laboratory experiments on the collisional production of ash that may occur during different regimes of pyroclastic flow transport and conduit flow. Using these laboratory experiments we develop a subgrid model for ash production that can be included in analytical and multiphase numerical procedures to estimate the total volume of ash produced during transport. We find that for most pyroclastic flow conditions, 10-20% of the initially 1 cm clasts comminutes into ash with the percentage increasing as a function of initial flow energy. Most of the ash is produced in the high-energy regions near the flow inlet, although flow acceleration on steep slopes can produce ash far from the vent. On level terrain, collisionally and frictionally produced ash generates gravity currents that detach from the main flow. Ash produced at the frictional base of the flow and in the collisional upper regions of the flow can be redistributed through the entirety of the flow, although frictionally produced ash accumulates preferentially near its source in the bed load. Flows that descend steep slopes produce the majority of their ash in the collisonally dominant flow head, and flow snouts likely develop sub-angular to rounded pumice during this process. Compared to pyroclastic flows, conduit conditions typically produce higher energy collisions; based on experimental measurements, these conditions are more likely to produce fragmentation of larger pieces of pumice compared to a smaller fraction of fine ash produced in lower energy collisions. We analyze the morphology of pumices during both conduit collision and pyroclastic flow processes and compare them to recent field measurements of rounding of Mount St. Helens pumice.
Decoupling Processes in Block-and-ash Flows
Lava dome collapse and collapse of lava flow fronts generate short-lived, highly mobile block-and-ash flows, usually comprising three components, a high-density, ground-hugging basal avalanche, a low-density ash cloud surge and a more dilute ash cloud on top. Generally the basal avalanche is valley-confined whereas the overlying surge has the ability to decouple from the dense basal part, overtop topographic barriers and affect greater areas than the basal avalanche, hence posing a larger hazard to the population. These processes have been observed at several volcanoes, e.g. Unzen (Japan), Merapi (Indonesia) and Soufriere Hills (Montserrat), resulting often in the loss of lives. Decoupling is often accentuated by topographic obstructions causing blocking or deflecting of the basal avalanche. Currently laboratory flume experiments are being undertaken to examine the factors initiating and influencing the decoupling processes of block-and-ash flows. A better understanding of the flow dynamics of a moving block-and-ash flow and of the interaction with the underlying substrate and topographic irregularities will provide better hazard zone delineation maps for the future. The experimental results will be used to investigate the emplacement of the block-and-ash flows at Mount Tarawera, New Zealand, where the deposits show indicators of possible decoupling processes during the AD 1305 Kaharoa eruptive episode.
Titan2D Based Pyroclastic Flows Hazard Maps for Santa Ana Volcano, El Salvador
Santa Ana Volcano is located in the Apaneca Volcanic Field located to the west of El Salvador, Central America. It is one the six active volcanoes monitor by the Servicios Nacionales de Estudios Territoriales (SNET) in El Salvador, out of twenty that are considered active in this small country by Smithsonian definition. The Santa Ana Volcano is surrounded by rural communities in its proximal areas and in its close distal areas by the second largest city of the country. On October 1st 2005, after a few months of increased fumarolic and seismic activity, it erupted generating a 10 km high steam and ash plume, reportedly seen by some aircraft and estimated using photography by SNET members. Ash was deposited to the west, north-west part of the country, following typical wind pattern for the region, as well as small pyroclastic flows and major lahars in its eastern part. Coffee plantations were lost, as was some crop of coffee in the following season. However, to the west the ash fertilized the land and resulted in an enhanced harvest of coffee beans. Only 2 people were killed from the Blast, thanks to the auto evacuation of proximal communities. Whilst the last eruption had a relatively low human life toll, a stronger eruption spells havoc almost certainly for the region. At this moment no exhaustive study and understanding exists of the pyroclastic flows generated by the Santa Ana Volcano nor a map for this particular hazard. This study proposes the use of Titan2D for those two purposes, using a DEM generated by the SNET using topographic maps as well as DEMs generated using Advanced Spaceborne Thermal Emission and Reflection Radiometer Images (ASTER).
Towards a new approach for generating probabilistic hazard maps for pyroclastic flows during lava dome eruptions.
It is increasingly being understood that development of mathematical models of a geophysical phenomena, while a fundamental step, is only part of the process of modeling and predicting inundation limits for natural hazards. In this work we combine data from hundreds of observed pyroclastic flows at the Soufriere Hills Volcano, Montserrat, a geophysical flow model, and statistical modeling to derive a new methodology for generating probabilistic hazard maps. The initial step consists of estimating probabilities of inundation at particular discrete points of interest (e.g. airport and Plymouth). The methodology starts with a computer model of the geophysical process, in this case the TITAN2D model that has been developed for modeling geophysical mass flows. A key input to the computer model is the probability distribution for the initial volume and direction of the flows based on observed data. An important limitation is that for modeling purposes, the observations represent relatively scarce datasets, while from a volcanological perspective datasets such as those from the prolonged and relatively well-monitored eruption of the Soufriere Hills Volcano, are as complete as can be realistically obtained. By combining flow event data, probability modeling and statistical methods, a probability distribution of severity and frequency of flow events is derived. Understanding and predicting the effects of volcanic hazards involves understanding the extreme event tail (the largest flow events) but this is notoriously difficult, especially with the limited data and prohibitively expensive to compute.. Instead a statistical emulator (or surrogate of the computer model) is used, a computationally cheap response surface approximating the output of the flow simulations, which is constructed based on carefully chosen computer model runs. The speed of the emulator then allows to 'solve the inverse problem': that is, to determine regions of inputs values (characteristics of the flow) which result in a events of interest (such as one that that reaches a given critical point). The flow frequency distribution is then used to determine the probability of this region, that is, the probability that an event of a given magnitude will occur at a particular site. Using quantitative measures like these to solve for the probabilities across an area, zoned maps could be generated from which civil protection authorities can make more informed decisions about hazard mitigation.
Combining hydraulic and granular flow extremes for density currents by depth averaging two phase flow equations.
Ground-hugging particle-laden flows constitute some of the most dangerous
natural phenomena on Earth. Such currents, in the form of snow avalanches,
pyroclastic flows, debris flows, lahars, and landslides, are among the most
destructive processes in nature. Humans tend to settle in areas near rich
soils, volcanoes, or watercourses, all of which could be strongly affected
by these dangerous flows.
In order to improve risk preparedness and site management in the affected zones, an appropriate knowledge of these natural hazardous phenomena is required. Their evolution in time, flow dynamics and run out distance are key aspects that help in the planning for hazardous events, development of hazardous regions and design of management policy to prepare in advance of potential natural disasters.
This paper describes a depth-averaged model for two phase flows that is currently in develop at the University at Buffalo. It is presently implemented within the TITAN2D framework to improve the version that currently simulates dry geophysical mass flows over natural-scale terrains. The initial TITAN2D code was developed to simulate granular flow. But because the introduction of an interstitial fluid strongly modifies the dynamics of the flow, a new, more general, two-phase model was developed to account for a broad range in volume fraction of solids. The proposed mathematical model depth-integrates the Navier-Stokes equations for each phase, solid and fluid. The solid phase is modeled assuming a Coulomb constitutive behavior (at the theoretical limit of pure solids), whereas the fluid phase conforms to a typical hydraulic approach (at the limit of pure fluid). The linkage for compositions between the pure end-member phases is accommodated by the inclusion of a phenomenological-based drag coefficient. The model is capable of simulating particle volumetric fractions as dilute as 0.001 and as concentrated as 0.55.