2019 CSDMS meeting-114
Suboptimal beach nourishment decisions: When self-interested actions enhance a neighbor's welfare
Coastal communities facing erosion require beach maintenance for property protection and recreation. While some communities may have the means to pay for sand nourishment, others may benefit from their neighbor’s alongshore-transported sediments. If communities expect to free-ride off beach nourishment carried out by a neighbor, incentives favoring inaction may lead to narrower beaches overall. Recent work coupling human and natural systems found that coordination between neighboring communities is preferable economically to each community acting independently. Contrasting past work, we model two communities acting without knowledge of a jointly determined economically optimal nourishment program. Instead, nourishment behavior is triggered by a traditionally imposed minimum beach threshold and bounded by a predefined seaward edge. The goal is not to limit sand loss; rather, nourishment decisions are based on separate or joint benefit-cost assessments for two communities. We compare two management approaches: (1) sequential/decentralized decisions, where the updrift community chooses first and the downdrift community reacts second; and (2) simultaneous/coordinated decisions where both communities make a joint choice. We test how variable up/downdrift property values affect outcomes under these two approaches. Results suggest that communities do not always favor coordinating simultaneously. When both up- and downdrift communities have high property values, sequential/decentralized decisions are favored, leading to updrift over-nourishment to maintain beach width. This enhances alongshore sediment availability, thus providing higher marginal benefits for downdrift communities whom under-nourish. When the property values of the updrift community are low and the property values of the downdrift community are high, however, the outcome results in abandonment of property by the updrift community instead of coordinating with the downdrift community. Overall, we find that the distribution of property values across neighboring communities can be a driver for both strategy selection and the decision-making process.