The unintended consequence of groin construction: Forcing a coastal neighbor’s abandonment increases the overall welfare

Heavily developed coasts require maintenance to sustain beach recreation and protect property/infrastructure from erosion. One engineering approach involves the construction of immovable objects, such as shore-perpendicular groins, which slow alongshore currents to deposit sediments locally at and updrift of the object. While groins accrete sediment updrift, they also limit downdrift sediment supply, exacerbating erosion, often forcing downdrift communities to respond with new engineering measures (groins) or abandon property. Our research focuses on these local risks associated with groins. We developed a coupled geo-economic model for a two-community system to explore how community wealth and size affect welfare-driven management decisions. Benefits are a function of beach width and the number of property rows; costs are a function of groin length and maintenance. We analyze these decisions within the context of a prisoner’s dilemma, whose basic assumption is that cooperating provides the highest joint benefit, but acting selfishly affords a greater self-benefit than working together even if it is worse for the whole system. We compare how the system’s welfare changes for each combination of ‘prisoner’ choices to determine whether the dilemma’s assumption holds. Results indicate that small communities find building multiple groins most beneficial, which supports the dilemma’s assumption. Conversely, large communities find building a groin updrift and abandoning property downdrift most advantageous because the community’s size can absorb the lost properties’ marginal benefits. This contradicts the assumption that cooperating is always optimal, suggesting there is a positive incentive for coastal communities to act self-interestedly, even if it results in the loss of their neighbor.