The sensitivity of a volcanic flow model to digital elevation model accuracy: experiments with digitised map contours and interferometric SAR at Ruapehu and Taranaki volcanoes, New Zealand

A growing trend in the field of volcanic hazard assessment is the use of computer models of a variety of flows to predict potential areas of devastation. These can be compared against historic and geologic evidence of past events for model calibration, or used to construct hazard zone maps for mitigation and planning purposes. The accuracy of these computer models depends on two factors, the nature and veracity of the flow model itself, and the accuracy of the topographic data set over which it is run. All digital elevation models (DEMs) contain innate errors. The nature of these depends on the accuracy of the original measurements of the terrain, and on the method used to build the DEM. In this paper we investigate the effect that these errors have on the performance of a volcanic flow model designed to delineate areas at risk from lahar inundation. The model was run over two DEMs of southern Ruapehu volcano derived from (1) digitised 1:50 000 topographic maps, and (2) airborne C-band synthetic aperture radar (SAR) interferometry obtained using the NASA AIRSAR system. On steep slopes of ∼4° or more, drainage channels are more likely to be incised deeply, and flow paths predicted by the model are generally in agreement for both DEMs despite the differing nature of the source data. Over shallow slopes (∼<4°), where channels are less deep and are more likely to meander, problems were encountered with flow path prediction in both DEMs due to interpolation errors between contours, and due to forestry. The predicted lateral and longitudinal extent of deposit inundation was also sensitive to the type of DEM used, most likely in response to the differing degrees of surface texture preserved in the DEMs. A technique to refine contour-derived DEMs and reduce the error in predicted flow paths was tested to improve the reliability of the modelled flow path predictions. In areas where high-resolution topographic maps are unavailable, forthcoming topographic measurements acquired by a single-pass space-borne instrument, the NASA Shuttle Radar Topography Mission (SRTM) are likely to prove invaluable.