CORE Metadata, citation and similar papers at core.ac.uk Provided by RERO DOC Digital Library Bull Volcanol (2012) 74:309–324 DOI 10.1007/s00445-011-0522-8 RESEARCH ARTICLE Challenges of modeling current very large lahars at Nevado del Huila Volcano, Colombia Raphael Worni & Christian Huggel & M. Stoffel & B. Pulgarín Received: 12 April 2010 /Accepted: 21 June 2011 /Published online: 9 August 2011 # Springer-Verlag 2011 Abstract Nevado del Huila, a glacier-covered volcano in modeling. Measured hydrographs, geophone seismic sensor the South of Colombia’s Cordillera Central, had not data and calculated peak discharges served as input data for experienced any historical eruptions before 2007. In 2007 the reconstruction of flow hydrographs and for calibration and 2008, the volcano erupted with phreatic and phreato- of the models. For model validation, results were compared magmatic events which produced lahars with flow volumes with field data collected along the Páez and Simbola of up to about 300 million m3 causing severe damage to Rivers. Based on the results of the 2007 lahar simulation, infrastructure and loss of lives. The magnitude of these we modeled lahar scenarios with volumes between 300 lahars and the prevailing potential for similar or even larger million and 1 billion m3. The approach presented here events, poses significant hazards to local people and makes represents a feasible solution for modeling high-magnitude appropriate modeling a real challenge. In this study, we flows like lahars and allows an assessment of potential analyze the recent lahars to better understand the main future events and related consequences for population processes and then model possible scenarios for future centers downstream of Nevado del Huila. events. We used lahar inundation depths, travel duration, and flow deposits to constrain the dimensions of the 2007 Keywords Lahar . Nevado del Huila Volcano . Hydrograph event and applied LAHARZ and FLO-2D for lahar reconstruction . Model calibration/validation . Lahar modeling . FLO-2D . LAHARZ Editorial responsibility: H. Delgado Granados R. Worni : C. Huggel : M. Stoffel Introduction Institute for Environmental Sciences, University of Geneva, Geneva, Switzerland Ice- and snow-covered volcanoes may produce large and devastating water-sediment flows (lahars) because of C. Huggel e-mail: [email protected] possible interactions between volcanic activity and the subsequent and rapid melting of snow and ice (Major and M. Stoffel Newhall 1989). Lahars have occurred in many settings and e-mail: [email protected] they are typically highly destructive. R. Worni (*) : M. Stoffel As historical events have shown, even small volcanic Institute of Geological Sciences, University of Berne, eruptions may produce large-scale water-sediment floods 3012 Bern, Switzerland when interacting with snow and ice. This was the case at e-mail: [email protected] the world’s largest historic volcano-glacier disaster at C. Huggel Nevado del Ruiz Volcano in the Colombian Andes. In Department of Geography, University of Zurich, 1985, this glaciated volcano produced a small Plinian 8057 Zurich, Switzerland eruption during which a density current entrained snow and ice, resulting in rapid melt. The estimated volume of ice, B. Pulgarín 3 INGEOMINAS, firn, and snow lost during the eruption was 60 million m , Popayán, Colombia corresponding to a water equivalent of approximately 43 e-mail: [email protected] million m3 (Thouret 1990). Only 11–12 million m3 of released 310 Bull Volcanol (2012) 74:309–324 water formed the deadly set of lahars that claimed more than improved understanding of ongoing processes and potential 23,000 lives in the town of Armero (Pierson et al. 1990). future hazards will further improve the planning and In consideration of the serious hazard potential emanat- implementation of prevention measures that could protect ing from large-magnitude water-driven processes such as local people and their assets. lahars and outburst floods, it is of great importance to The purpose of this paper is to (1) reconstruct flow improve the understanding of processes involved and their dynamics of recent lahars at Nevado del Huila, including dynamics. However, due to the difficulty of directly hydrographs and a number of flow parameters and to (2) measuring large-magnitude mass flows, there is an unsur- use these data for simulations of past (retrospective prising lack of quantitative information. Studies of well- modeling) and (3) potential future events (scenario-based characterized events are an important basis for enhancing modeling) with the two-dimensional flow model FLO-2D knowledge on flow physics and flow parameters. Flow and the semi-empirical model LAHARZ. We are aware of parameters and dynamics of real flows need to be included several uncertainties that can neither be avoided nor in numerical modeling, which provides a valuable tool for a necessarily be overcome with such an approach. Neverthe- quantitative prediction of large-scale floods. For example, less, the objective is to constrain interpretations of the Carrivick (2006) analyzed fluvial landforms, which are dimensions of the different flow parameters to a level seen as records of hydraulic processes at a specific place acceptable and useful in hazard assessment and mitigation. and time during a flood. Cross-sectional geometry, hydrau- lic roughness, deposited products, and the altitude of scour lines are important indicators to identify flow mechanism Study site (Baker 2000; Carrivick 2006). Geophone recordings (Arattano 1999; Van Westen and Daag 2005) and gauging The Nevado del Huila Volcano stations can help to reconstruct the flow hydrograph. With such information, a simple modeling approach can link cross- Nevado del Huila is a stratovolcano with predominantly sectional areas, run-out distance or velocities of the flow with effusive activity. No historic eruptions are known before 2007 its total volume using semi-empirical relationships (Iverson et when eruptive activity from Nevado del Huila was first al. 1998; Pierson 1998; Berti and Simoni 2007;Muñoz- reported. The volcano has an elliptical form with approximate Salinas et al. 2007). More sophisticated models assume that basal axes of 16 km in the north–south and 11 km in the east– flows propagate as kinematic waves (Weir 1982; Vignaux west directions. The steep flanks have average slopes ranging and Weir 1990) and calculations are based on the fully from 13° to 27° and the four glaciated peaks, named North, dynamic wave momentum equation (O’Brien et al. 1993). Crest, Central and South are aligned on a longitudinal axis, Even though the precise physical behavior of large with Central peak forming the highest summit of the sediment-laden flows may not be completely predictable, Colombian Andes at 5,364 masl (Pulgarín et al. 2004). the application of existing simulation programs can be Although the glacier area on Nevado del Huila has shrunk meaningful and helpful. Flow parameters need to be from 19.1 km2 in 1965 (Pulgarín et al. 1996) to a surface of defined and calibrated for these models, and perhaps even about 10.7 km2 in 2007, the estimated volume of the glacier more importantly, models have to be fully validated with is still 450 million m3, corresponding to a water equivalent adequate field studies (Carrivick et al. 2009). Worldwide, of ∼400 million m3. Following the 2007 and 2008 eruptions, few study sites exist where recent lahars with volumes of tens the glacier area was further reduced. to hundreds of millions of cubic meters could be observed Runoff from Nevado del Huila drains into the Páez (Major and Newhall 1989). This is not only true for lahars (western slopes) and Simbola (eastern slopes) Rivers, but also for other comparable water-sediment flows caused which merge 2 km above Belalcázar, the biggest town in by failures of natural or artificial dams (Cenderelli and Wohl the valley (3,500 inhabitants). Belalcázar is located 46 km 2003; Pulgarín et al. 2004;Capra2007). downstream from the volcano summit. Other important In 1994, 2007, and 2008, Nevado del Huila volcano riverine villages are Tóez, Talaga, Ricaurte and Paicol. produced lahars with volumes of up to 320 million m3 and Some 120 km downstream from the volcano (Central peak), run-out distances of up to 160 km, killing up to 1,000 the Páez River discharges into the Magdalena River and people and causing severe damage to infrastructure. The 30 km further downstream into the Betania reservoir remoteness, limited accessibility and armed conflicts in the (Fig. 1). area have made investigations on Nevado del Huila and the affected drainage basins difficult. Nevertheless, this volcano Recent lahars at Nevado del Huila Volcano offers a unique opportunity to gain knowledge about complex and interactive roles of ground-water release and On 6 June 1994, after a period of heavy rainfall, a tectonic glacier melting that led to formation of these lahars. An earthquake at the base of Nevado del Huila triggered over Bull Volcanol (2012) 74:309–324 311 Fig. 1 Study area of the Nevado del Huila Volcano with the Páez and Simbola River valleys south of the volcano in the south of Colombia's Cordillera Central. (Quickbird image: Digital Globe—Copyright 2007) 3,000 shallow landslides, which coalesced into a massive On November 20, 2008, Nevado del Huila produced a lahar flowing down the Páez River (Martínez et al. 1995). phreatomagmatic eruption that generated yet another lahar. Almost 1,000 people were killed and 28,000 persons were A crater with an approximate diameter of 400 m and a directly affected by the disaster. In the communities of dome were formed. Due to the threat of a possible collapse Irlanda and Tóez, devastation was especially high, as they of the dome, detailed monitoring was performed by seismic were buried almost completely with lahar deposits (Ávila et surveillance, aerial inspections and a web-linked camera al. 1995; Scott et al. 2001).