Abstract. On March 19, 2021, the first eruption in ca. 800 years took place in Fagradalsfjall on the Reykjanes Peninsula, in the backyard of the capital Reykjavík. This effusive eruption was the most visited eruption in Iceland to date and needed intense lava flow hazard assessment and became a test case for hazard assessment for future eruptions on the Peninsula, which can issue lava into inhabited areas or inundate essential infrastructure. In this study we documented how lava flow modelling

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... trategies were implemented using the stochastic code MrLavaLoba, evaluating hazards during the 6-month long effusive event. Overall, the purposes were three-fold; (a) Pre-eruption simulation to investigate potential infrastructure at danger for lava flow inundation (b) Syn-eruptive simulations for short-term (two weeks' time frame) lava flow hazard assessment and (c) Syn-eruptive simulations for long-term hazard assessments (months to years). Furthermore, strategies for lava barrier testing were developed and incorporation of near-real time syn-eruptive topographic models were implemented. During the crisis the code was updated to increase functionalites such as considering multiple active vents as well as code optimization that led to a substantial decrease in the computational time required for the simulations, speeding up the delivery of final products.