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Coastal communities often address shoreline erosion through beach nourishment, adding externally sourced sand to widen beaches for recreation and property protection. While nourishment enhances beachfront property values, the need for periodic maintenance creates interdependencies where the actions of neighboring communities affect local shoreline dynamics. Using a coupled model of two neighboring communities, we examine the interplay between community nourishment decisions and the redistribution of nourishment sand. We find that the value a community places on wider beaches not only influences their propensity to nourish, but also their and their neighbors' nourishment efficiency and net benefits. Communities that nourish more frequently tend to have lower nourishment efficiency, as sand is redistributed alongshore, benefiting less‐active neighbors at their expense. A 20‐year New Jersey case study confirms that communities that nourish more have lower nourishment efficiencies, including instances where less wealthy communities nourish significantly more, enabling wealthier neighbors to enjoy higher efficiencies—suggesting that such dynamics may already be shaping real‐world coastal outcomes. In future scenarios, we simulate the effects of rising sand costs and accelerated erosion due to sea‐level rise under coordinated and non‐coordinated planning methods, finding that less wealthy communities experience a higher risk of beachfront property loss under non‐coordination, exacerbating disparities in coastal management. These findings underscore the importance of inter‐community cooperation in optimizing economic and environmental outcomes in beach nourishment strategies. 

Abstract Stratigraphic interpretation generally relies upon the assumption that the fluvio‐deltaic surface responds uniformly to sea‐level changes; however, recent theoretical work suggests that changes in its relief and concavity can influence the propagation of sea‐level information upstream and result in geologically long‐lived lags in the system response. We test this theoretical result using measurements from a experimental delta subject to high and low magnitude sea‐level oscillations. In both cases, changes in relief and curvature of the fluvio‐deltaic profile result in the proximal portion of the profile being out of phase with respect to sea‐level cycles, whereas the nearshore regions remain in phase. These results underscore the importance of delayed response to sea‐level variations in the upstream portion of river deltas, often resulting in net erosion during sea‐level rise and potentially complicating the reconstruction of paleo sea‐level from deltaic deposits.