Economic comparison of sea-level rise adaptation solutions along the green-gray infrastructure continuum: a case study from an estuary on the U.S. Pacific Coast

Compound flooding in urban coastal areas is expected to become an increasingly costly problem due to projected sea-level rise throughout the 21^st century. The emergence, and increasingly widespread acceptance, of “green infrastructure solutions” in recent years provides a wider range of adaptation measures compared to traditional gray infrastructure alone but comes with additional challenges. First, the impact of green infrastructure on flood risk is less straightforward to quantify relative to the augmentation of hard structures. Second, the net economic impact of green vs gray infrastructure in the form of flood reduction and associated ecosystem co-benefits is difficult to compare. Here we present a cost-benefit analysis of different sea-level rise adaptation options for Coos Bay, Oregon, U.S., which each incorporate different degrees of wetland restoration (green infrastructure) and levee heightening (gray infrastructure). For each scenario, continuous water level predictions are produced over the period 2020-2100 by pairing a physically constrained hybrid harmonic tidal water level model with stochastically modeled storm surges and a simplified wind wave runup model. Property damages and transportation delay costs are then calculated for each flooding event. This novel workflow produces temporally granular flood damage quantification which incorporates evolving hydrodynamic and meteorologic conditions. We hypothesize that wetland restoration will be cost-competitive with levee heightening once ecosystem services are financialized along with avoided flood losses.