Earth Surface Processes and Landforms Earth Surf. Process. Landforms 27, 759–772 (2002) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/esp.351 VOLUME ESTIMATION OF THE 1998 FLANK COLLAPSE AT CASITA VOLCANO, NICARAGUA: A COMPARISON OF PHOTOGRAMMETRIC AND CONVENTIONAL TECHNIQUES NORMAN KERLE* † Department of Geography, Volcano Remote Sensing Group, University of Cambridge, UK Received 18 April 2001; Revised 6 February 2002; Accepted 11 February 2002 ABSTRACT In October 1998 a precipitation-triggered flank collapse occurred at Casita volcano, Nicaragua, leading to a devastating lahar. In this paper the failure volume was calculated using a range of methods. Several pre- and post-failure digital elevation models (DEMs) were created, based on photogrammetric, cartometric and surveying data. The wide range in resulting volumes prompted an assessment of the accuracies and potential problems associated with each of the datasets and techniques used. The best estimate for the failure volume is 1Ð6 ð 10 6 m 3 . It is based on a vegetation-corrected pre-failure DEM, generated using automated digital photogrammetry, and a post-failure surface based on a field survey carried out with a Total Station. The volume figure is approximately an order of magnitude higher than values reported in previous publications, all of which are based solely on field estimates. This demonstrates that values reported in the literature, if they are not based on rigorous quantitative analysis, must be regarded with caution. Copyright  2002 John Wiley & Sons, Ltd. KEY WORDS: DEM; flank collapse; landslide; failure volume; photogrammetry INTRODUCTION When Hurricane Mitch passed over Central America in October 1998, it triggered a small flank collapse at the inactive Casita volcano in northwestern Nicaragua. The collapse material directly transformed into a debris avalanche, and subsequently into a lahar that obliterated two towns and caused more than 2500 fatalities (Figure 1; Kerle and van Wyk de Vries, 2001). Survivors’ reports and field evidence allow the estimation of some basic flow parameters, such as a maximum flow width of approximately 1200 m, and a maximum wave height of 3–6 m (Scott et al., in press) where the towns were destroyed. However, the size of the flank failure that led to the disaster is more difficult to determine. Initial volume estimates range from 1Ð3 ð 10 5 m 3 (Carre˜ no Collatupa, 1998) to 2Ð0 ð 10 5 m 3 (Sheridan et al., 1999). The difficulty and uncertainty result primarily from lack of knowledge of the pre-disaster topography, but also from limited access to the site. Accurate knowledge of the failure volume is critical, as it is the principal input for lahar modelling and the assessment of potential future hazards. Although flow models such as LAHARZ (Iverson et al., 1998) are based on a range of hypothetical input volumes, simulations of the 1998 disaster begin with parameterization of the failure volume. The volume is also a critical parameter for sedimentological work, where the total material budget of the flow, including erosion and deposition, is assessed. Scott et al. (in press) estimated that the lahar bulked to at least 2Ð6 times the contributing failure material as it descended down the flank. However, any bulking factor is only meaningful if associated with an accurate initial volume figure. Van Wyk de Vries et al. (2000) identified edifice deformation and associated sector collapse potential at Casita. The 1998 flank failure was linked to the same southeastward deformation, which had generated steep and unstable slopes along a horseshoe-shaped rim (Kerle and van Wyk de Vries, 2001). Knowledge of the * Correspondence to: N. Kerle, Department of Geography, Volcano Remote Sensing Group, University of Cambridge, Cambridge CB2 3EN, UK. E-mail: nk220@cam.ac.uk † Present address: International Institute for Geoinformation Science and Earth Observation (ITC), Enschede, The Netherlands. E-mail: kerle@itc.nl Copyright  2002 John Wiley & Sons, Ltd.