Research conducted by a team of U.S-U.K. scientists and engineers shows federal flood maps underestimate at-risk Americans by more than 27 million people.

The group is called Fathom, and they presented their research at the 2017 American Geophysical Union Meeting on Dec. 11 — research they say fills in “vast amounts of missing information” in current federal flood risk assessments.

Construction of a new spillway at the Oroville Dam in northern California—the largest dam in the U.S.—is underway and is expected to be completed sometime in 2018, according to the California Department of Water Resources. The dam replaces the previous spillway, which was damaged by heavy flooding in February.

Problems at the Oroville Dam began, when the dam’s main sluice was damaged after a winter season of record rain and snowfall, following five years of drought. Torrential rainfall caused water levels to rise so quickly that large amounts needed to be released to prevent the dam from rupturing and sending a wall of water to the communities below.