STRUCTURAL AND NON-STRUCTURAL STRATEGIES FOR FLOOD CONTROL: CASE STUDIES FROM JAPAN AND THE US

Widespread flooding across the US this year has brought renewed attention to long-term strategies for flood protection. Press coverage and public discussion have explored options for both structural protection and managed retreat. Case studies from past US flooding and from post-tsunami recovery in Japan provide tangible lessons for both structural and non-structural pathways for future flood resilience.

US flooding in 2019 has resulted in calls for expansion of and investment in levee protection. Levees remain the primary structural tool for riverine flood control, including a prevailing US policy of "levees only" until 1927. New and enlarged US levee proposals today are scrutinized primarily on a positive cost-benefit balance, despite political pressure to circumvent the cost-benefits test. In Japan, after the 2011 Tohoku earthquake and tsunami, the country began construction on a $8 billion tsunami barrier system along 457 km of the coastline, financed by a 1.2% national income tax surcharge slated to last 25 years. The coastal barrier is part of a multi-layered system, which includes additional barriers, land-use controls, and evacuation planning.

Managed retreat refers to non-structural approaches for managing coastal and riverine flooding, often focusing on withdrawal of infrastructure and population from at-risk areas. In Japan, after the 2011 tsunami, residential structures have been removed from the highest-risk portion of the Tohoku coastal margin. Displaced residents are being resettled in approximately 145,000 homes, including whole new towns built outside the hazard zone. In the US, in contrast, few managed retreat solutions have been widely embraced. Exceptions include ~20-30 communities–generally small and rural–partially or entirely moved off US floodplains. Hard-won lessons from past investments in structural and non-structural flood protection should be incorporated into a broad toolkit to address river flooding, coastal flooding, and sea-level rise.