Volcanic mudflows called lahars do not stop when they hit the sea—they keep moving underwater, sometimes for hundreds of kilometres, and their behaviour is almost entirely unknown. Current disaster plans treat lahars as a purely onshore hazard, ignoring the fact that most of them enter the ocean. This blind spot matters because these underwater flows have already snapped undersea internet cables, destroyed fishing grounds, and damaged coastal infrastructure. The problem is that no field data exist to describe what happens once a lahar crosses the shoreline, so models used for hazard planning are based on an assumption—that lahars plunge straight down and stop—which recent damage patterns show is wrong. LAHAR2SEA will collect the first direct measurements of lahars moving from land into shallow water and across the deep seafloor, using offshore monitoring, sediment sampling, and repeat surveys at sites around the world. The team will build a new computer model that simulates how these flows actually behave, calibrated against real data. If successful, the work will let governments, cable-laying companies, and fisheries managers identify which coastal volcanoes pose offshore hazards, now and under future climate change. It will also reveal how much sediment volcanoes deliver to the deep ocean—a missing piece in global carbon and nutrient cycles.
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Recent damage to critical undersea internet cables, destruction of strategically-important fishery sites, and devastation of coastal infrastructure has shown that the impacts of volcanic sediment flows known as lahars can be far greater and extend further offshore than previously-thought. While lahars represent some of largest sediment flows on Earth, and as most ultimately reach the ocean, a lack of understanding of their offshore behaviour means that they are not currently included in national disaster plans, impacts on marine ecosystems and ocean biogeochemical cycles remain opaque, as does their contribution to global marine particulate budgets. Impacts of ocean-entering lahars will become more significant due to rapid-expansion of the Blue Economy, declining health of fisheries, and as climate change accelerates the rate of rainfall-triggered lahars. It is therefore critical that we understand the behaviour, sediment fluxes and hazards related to this globally-important phenomenon that occurs offshore from coastal volcanoes worldwide. The ocean is a major blind spot for lahar-related hazards and sediment fluxes. The limited utility of satellite-based remote-sensing, and challenges in monitoring from coast-deep-sea, means that linking onshore-offshore processes remains extremely challenging. We therefore rely upon models to simulate the offshore behaviour of lahars, but such models must be calibrated and validated by field-scale data which are currently completely absent. At present, we rely on a simple model that assumes when lahars reach the coast, they immediately and directly plunge beneath the sea surface; hence hazards and offshore fluxes should be instantaneous. New observations of damage to offshore infrastructure and fisheries, at sites affected by lahars, challenge this model, indicating it is not applicable in most cases. Our initial analysis reveals more complicated behaviour, with wider-reaching implications for longer-lasting and delayed hazards, impacts for seafloor ecology and sediment budgets. There is thus a pressing need to address this mismatch between the existing model and new observations, so we can understand implications and implement appropriate management and mitigation measures. LAHAR2SEA aims to address this blind spot, gathering the requisite field data to calibrate and validate a new process model, and assess the implications for offshore hazards and sediment budgets with key international industrial and government partners: Objective 1: Characterise onshore lahar events that reach the ocean at sites that span conditions that are broadly representative globally, to enable the first parameterisation of ocean-entering lahar behaviour (sediment fluxes, speeds and delivery mechanisms). Objective 2: Create the first documentation of the behaviour and impacts of lahars from shallow coastal waters to the deep sea (runout distance, pathways, velocity, rheology), based on novel offshore monitoring, sampling and repeat surveys. Objective 3: Develop a new globally-applicable model to simulate ocean-entering lahars, extending multiphase modelling across the land-ocean interface, and into the deep-sea. Calibrated by O1 and validated against O2 using exemplar sites. Objective 4: Real-world application of O1-3 to: i) assess contributions of lahars to global ocean particulate budgets; ii) determine offshore lahar-hazard-hotspots now and due to future climate change; iii) develop a globally-applicable framework for assessing and mitigating lahar-related hazards for critical seafloor infrastructure and ocean ecosystems with partners spanning subsea cable industry, fisheries, emergency management organisations and governments. This new globally-applicable model, underpinned by novel observational data, will push the frontiers of understanding of lahars in the ocean, to enable forecasting and management of the resultant hazards and impacts in the marine environment.
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