Adjusting design floods for urbanization across groundwater-dominated watersheds of Long Island, NY

•Peak flows have increased in both magnitude and variability for most watersheds on Long Island.•Urbanization shifts flood frequency estimates more than precipitation trends in isolation.•Management and remediation of stormwater have contributed to nonmonotonic trends in peak flows for two of 17 gaged watersheds.•Long term water table drawdowns contribute to an increase in peak flow variability.•Using 95% confidence intervals, nonstationary estimates are distinct from stationary estimates for smaller floods.