Reversal of the levee effect towards sustainable floodplain management

Levees constrain roaring floodwater but are blamed for reducing people’s perception of flood risks and promoting floodplain human settlements unprepared for high-consequence flood events. Yet the interplay between levee construction and floodplain development remains poorly quantified, obscuring an objective assessment of human–water relations. Here, to quantitatively assess how floodplain urban expansion is linked to levee construction, we develop a multiscale composite analysis framework leveraging a national levee database and decades of annual land-cover maps. We find that in the contiguous United States, levee construction is associated with a 62% acceleration in floodplain urban expansion, outpacing that of the county (29%), highlighting a clear change in risk perception after levees are built. Regions historically lacking strong momentum for population growth while experiencing frequent floods tend to rely more strongly on levees and we suggest these areas to develop a more diversified portfolio to cope with floods. Temporally, the positive levee effect is found to have weakened and then reversed since the late 1970s, reflecting the role of legislative regulations to suppress floodplain urban expansion. Our quantitative framework sheds light on how structural and non-structural measures jointly influence floodplain urban growth patterns. It also provides a viable framework to objectively assess the floodplain management strategies currently in place, which may provide useful guidance for managing flood risks towards sustainable development goals.