The levee effect revisited: Processes and policies enabling development in Yuba County, California

Abstract

Prolonged, heavy rain in Northern California led to the evacuation of over 180,000 residents on February 12, 2017 after the capacity of the Oroville Dam, which spills into the Feather River, came into question. This paper examines the development of the floodplain along the Yuba and Feather rivers and identifies changes in risk distribution resulting from increased urbanisation. The levee system in this area was extended, and additional embankments were erected between 2004 and 2011. Olivehurst has particularly experienced high population growth and increased housing values despite a history of flooding, partly due to failed mitigation structures. Increased development stemming from a false sense of security associated with mitigation projects, termed the levee effect, has been well documented. Analyses of census data, land cover change, Federal Emergency Management Agency (FEMA) flood zone designations, and county development plans indicate that several factors other than available land have precipitated expansion: (a) population pressure, (b) recertification of the levees that now confine the official floodplain, and (c) the fact that flood insurance is no longer a requirement for homeowners. Development trends from 1980 to 2015 put communities built in anticipation of the upgraded levee system that are completely reliant upon it for flood protection at risk.