Coastal Community Couplets: How Alongshore Wealth Disparities Can Lead to Suboptimal Beach Nourishment Programs and Increased Erosion Risks in the Long Term

Abstract:

Coastal communities facing erosion maintain their beaches for recreation and property protection. One form of maintenance is nourishment, the placement of externally sourced sand to increase cross-shore beach width, forming an ephemeral seaward protrusion that requires periodic re-nourishment. These projects add value to beachfront properties, thus affecting future management choices and forming an alongshore-connected system influenced by multi-community dynamics. Prior work suggests that coordinated beach nourishment programs between adjacent communities were economically preferred to independent projects, however the long-term consequences of not coordinating, particularly each community’s financial capacity to continue to nourish, have not been fully examined. Furthermore, many real-world communities have historically failed to coordinate. To understand the effects of coordination, we investigate a coupled geomorphic-economic model with two neighboring communities (i.e. couplets) and adjacent natural coastline, where those that act alone choose nourishment frequencies to maximize their independent net benefits, and those that coordinate choose frequencies to maximize their joint net benefits. We examine how community-scale property values could affect the nourishment choices produced by coordinated and uncoordinated schemes, and whether certain economic conditions can explain inter-community nourishment policies and the aggregate geomorphic outcomes. The model suggests that wealth-disparate couplets that fail to coordinate tend to over-nourish in the poorer community compared to the optimal nourishment plan. Evidence of this dynamic exists in field couplets where nourishment fluxes are similar between such communities, resulting in wider beaches system-wide, and suggesting that communities may be ignoring the benefits of regional coordination. With future increases in both material costs and background erosion due to sea-level rise, poorer communities might be unable to over-nourish—or even to nourish at all—causing forced property abandonment. To avoid that fate, a coordinated strategy of slow retreat that delays (via infrequent nourishment) but results in the removal of vulnerable beachfront properties will likely be the optimal form of adaptation.