Tratamiento estadístico del refinanciamiento de la deuda

Due to their rapid emplacement and highly destructive potential, volcanic debris flows, or lahars, represent a great hazard to the infrastructure and populations of towns and cities around volcanoes. Between 1950 and 2001, lahars occurred at 31 volcanoes, claimed more than 28,000 lives, and cost governments many millions of dollars in damages (Major et al. (2005);Witham (2005)). With detailed knowledge of the travel times, nature, and depositional processes of such flows, we can now more accurately model their flow paths, volumes, and destructive potential. This will allow for the improvement of public safety and emergency planning for such hazards.

Lahars are flowing mixtures of sediment and water that, irrespective of their trig- gering mechanisms, originate from a volcano (Smith and Fritz (1989)). Volcanic debris flows can vary widely in both size and extent; as such, the hazard posed by them also varies considerably. Flows that originate from snow-covered volcanoes are generally larger than those from non-glaciated edifices, especially when triggered by eruptions (Major et al.(2005)). A catastrophic example occurred on 13th_{November 1985 with the}

eruption of Nevado del Ruiz, Columbia, where small volume pyroclastic flows eroded and entrained snow and ice to bulk up and descend into stream valleys. These flows continued down the valleys and buried the town of Armero, situated on a fan c. 74 km downstream, killing more than 23,000 people, destroying more than 5,000 homes, and causing damage valued at millions of dollars (US) (e.g., Janda et al. (1986);Lowe et al. (1986)).

Difficulties arise in understanding and accurately modelling the physics, motion, and impacts of lahars, primarily due to the presence of sediment within the flows. Within the flow, sediment may be variably incorporated and deposited as a lahar travels down a channel under a range of slope, geology, and confinement conditions (e.g., Pierson and Scott (1985); Cronin et al. (1997); Procter et al. (2009)). By un- derstanding the nature of sediment entrainment and deposition, it will be possible to more accurately describe and model the transport and physical properties of moving lahars, including velocity, frictional contact, basal interaction, erosion, sedimentation etc. Current models developed (e.g., Pierson (1995); Iverson (1997); Pierson (1998);

O’Brien (1999); Doyle et al. (2009)) that attempt to describe the basic properties of a debris flow struggle to replicate the fine-scale process within the flow, and do not take into account any variations in rheology that may occur along the flow in space and time. Continuous observations and measurements of debris flows in motion are necessary if our fundamental understanding of lahars is to grow. Development of new instrumentation and further analysis of results from current instrumentation will help to more realistically define and model debris flows.

The physics of the processes of sediment transport, erosion, and deposition of lahars is very poorly understood (e.g., O’Brien (1999); Doyle et al. (2009)). More empirical information is needed about the way in which lahars carry sediment, both spatially and temporally. Models of inundation are increasingly realistic due to better constrained topographic descriptions and improved computational hardware and software engineer- ing. However, these are still limited by a lack of basic understanding of the effects of topography, primary driving forces, and other variable factors, such as channel width, depth, and sediment content on lahar dynamics (e.g., Iverson and Denlinger (2001);

Fagents and Baloga (2006);Carrivick et al.(2008)). It is important to use observations and geophysical measurements of debris flows to further qualify, as well as quantify,

numerical models to predict possible future lahars.

Much of the current research on the internal properties, flow, and emplacement mechanisms of lahars has been the result of artificial experiments (e.g., Major and Iverson (1999)) or from observations of repeated seasonal rain-triggered mass-flows (e.g., Lavigne and Thouret (2002)). Limited real-world scientific observations have been possible (e.g., Cronin et al.(1997); Cronin et al.(2000b);Cronin et al. (2000a)), due to the unpredictable onsets of such flows.

Even in situations when conventional scientific observation methods have been pos- sible, it remains very difficult to see ‘into’ and monitor the changing properties of moving lahars. Due to their high sediment concentrations, particularly in fines (mud), visual observations are limited to the surface of the flow. Velocity estimates are also typically limited to surface measurements. Direct sampling of flows is generally carried out by tossing sampling containers into the channel from steep-edged banks or bridges; hence they also cannot be taken as a complete representation of the entire flow depth (e.g., Cronin et al. (1999)). None of these measurements, therefore, have produced any convincing account of vertical stratification in the velocity and sediment profiles of moving lahars.

Despite these difficulties, a number of instrumental techniques to record a lahar have been developed and possible vertical stratifications have been tested. These include load cells, to measure the weight and, with independent measurements of flow depth, the concentration of the flow above (e.g., Genevois et al. (2000)), scour chains buried below the channel floor to measure the depths to which bedload transport reaches (e.g.,

Berger et al.(2010)), and bedload traps to capture samples of the sediment concentra- tions of the lower parts of the flow (e.g., Sear et al. (2000)). Problems arise, however, due to the violence and scale of lahars, where instruments are buried, scoured out completely, or destroyed. Bedload traps are able to capture parts of the passing lahar; unfortunately, once they are filled they cannot collect any more samples, because they are unable to be excavated until after the flow has passed completely. The samples are also collected over an unknown length of time, which means that they are repre- sentative of a period within the flow, rather than a true measurement of a particular moment.

the internal properties of flows with less potential for damage, because it does not rely on actual contact with them. Non-contact methods ensure the equipment will survive the passing of a flow, while also being able to record its entire passage. Instruments such as seismometers and acoustic flow monitors can be installed in a variety of places near to the channel to record the flow; others, including stage gauges, can be installed on bridges or other structures above the flow path (e.g., Marcial et al. (1996); Cole et al.(2009)). New forms of load cells and pore pressure transducers that have internal memory capability and do not, therefore, require connection to a separate datalogger can be used where erosion at banks and channel sides are high. By bolting these instruments into a hard rock bed, they resist erosion by the flow (e.g., Doyle et al.

(2009)).

This study aims to progress research about lahar flow by (1) developing geophysical methods for empirical and field-based flow observations in order to (2) test current hypotheses of lahar horizontal and vertical structure and flow transformation that have been derived from past studies (e.g., Hodgson and Manville (1999);Cronin et al.

(2000a)) and (3) to provide new lahar warning and monitoring methods. Primarily data recorded by seismometers and acoustic flow monitors will be used, as they show promise for the most robust non-invasive measurement technique that provides a rich depth of multicomponent data. Additional techniques such as pore pressure transducers, load cells, stage gauges, direct flow sampling, and visual observations made during lahar flows will be used to develop and interpret the seismic-based tools.