As recently as 2010, Bristol came tenth in the list of cities in England and Wales most at risk of flooding. Now it’s fifth, and the number of homes at risk has shot up from 12,800 to 29,000. But this has got nothing to do with any physical changes in the city itself – it’s solely down to how the risk is measured. The council has invested in an extremely detailed model of flood risk across the city, the first of its kind in the world. The results have not only challenged perceptions of Bristol’s own flood risk, but scientists’ understanding of flooding itself. Flood modelling is a highly specialised discipline combining environmental science, applied mathematics, hydrology, surveying and advanced IT, not a combination that usually captures the public imagination. But 2013 is likely to see a great deal more demand for accurate models, when a gentlemen’s agreement between insurers and the government expires – and the owners of 200,000 high-risk properties find their cover rockets in price or is withdrawn completely, rendering their homes worthless. For this article for Modus magazine, I spoke to the world’s leading flood risk engineers to find out how they do it, and what their latest discoveries could mean for us all.