One of the most comprehensive studies ever of flood risk has determined that more than 17 million people in eight cities along the Atlantic and Gulf coasts are at the highest risk of being affected by flooding. Using machine learning, historic flood data and 16 factors representing hazards, exposure and vulnerability, the study published Wednesday in the journal Science Advances has come up with what is likely the most comprehensive model of coastal cities’ risk of flood damage. Researchers are alarmed by the findings, including that more than 4 million people are at risk in New York City, alone, and that 99% of people and buildings in New Orleans are vulnerable.