High Resolution Radar Imaging of Pyroclastic Density Currents

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Pyroclastic density currents (PDCs) are clouds of ash and rock, generated during eruptions, which propagate down volcanoes at high speed. They are the major hazard at many active volcanoes and have killed thousands of people. Our current ability to predict their behaviour and plan for their effects is limited, in part, by our incomplete knowledge of their flow dynamics. The proposed research will revolutionise our understanding of PDCs by obtaining, for the first time ever, measurements of position in time, hence velocity, of the dense core of moving PDCs using an advanced custom-built radar system (GEODAR). GEODAR has been developed and successfully used on snow avalanches, dramatically improving our knowledge of their dynamics. The project will build and deploy three GEODAR systems that have a spatial range resolution of 0.375 m and will image the dense core flow at 100 Hz: a spatial and time resolution never achieved before in studies of PDCs. GEODAR will easily penetrate the ash cloud to image the dense, destructive underflow, and can observe all particles larger than 30 mm. This novel system will be able to track PDCs along their flow paths and will allow us to image internal surges, roll-waves and flow fronts and reconstruct the velocity structure of moving PDCs. This data will enable the rigorous testing of PDC flow models and provide fundamental insights into their flow so that improved models can be developed. In addition, the flow path and deposits of the PDCs will be digitally mapped by a drone at 30 mm resolution in order to resolve the lateral extent and location of the flow. Features in the digital terrain maps will be directly matched with the features observed in the radar data and this will greatly add to the understanding of PDC emplacement mechanisms. For some flows we expect to have high resolution DTMs both before and after the event, and we will produce erosion and deposition maps. This data feeds in to the final part of the project which is the computer simulation of PDCs. The simulation code produces will be useful for predicting the path and forces of PDCs which is necessary for saving lives and protecting infrastructure. The code will be made freely available and a workshop run on its use. The DTM will be used for running the SHALTOP code and the results will be compared with GEODAR data and the erosion and deposition maps. SHALTOP is a simulation code developed, over the past fifteen years, by a French team partner in this project. It can be run with a variety of flow laws and we will determine which flow law best matches the data and from there we develop improvements. Such a detailed comparison has never been done before due to the lack of data from flowing PDCs. We have chosen Santiaguito volcano, Guatemala, as the test site. It is one of the world's most active volcanoes, which has been erupting since 1922 and dozens of PDCs are generated every year. The team has extensive experience working at this site and the local volcano observatory is an enthusiastic participant in the project. In addition, the terrain around the volcano is ideally suited for the location of GEODAR, with nearly complete sight-lines to the likely flow paths. The systems will be remotely triggered using a combination of infrasound and seismic signals. The three GEODAR systems will be stand- alone solar-powered units and communicate via a satellite-phone data link. The data storage will be on SSDs mounted in fireproof crash boxes so that they can withstand inundation.This research will produce the first ever high resolution position, and hence velocity, data for the dense core of flowing PDCs and the first ever model comparison with such data. The project will develop improved theoretical and computational models for PDCs and improve the accuracy of hazard assessments around volcanoes. The ultimate aim is to improve physical knowledge of these destructive natural hazards with the potential to save hundreds of lives.