Emergent behavior in a coupled economic and coastline model for beach nourishment

E D Lazarus,M D Smith,S Gopalakrishnan,A B Murray,D E Mcnamara

doi:10.5194/npg-18-989-2011

Abstract

Abstract. Developed coastal areas often exhibit a strong systemic coupling between shoreline dynamics and economic dynamics. "Beach nourishment", a common erosion-control practice, involves mechanically depositing sediment from outside the local littoral system onto an actively eroding shoreline to alter shoreline morphology. Natural sediment-transport processes quickly rework the newly engineered beach, causing further changes to the shoreline that in turn affect subsequent beach-nourishment decisions. To the limited extent that this landscape/economic coupling has been considered, evidence suggests that towns tend to employ spatially myopic economic strategies under which individual towns make isolated decisions that do not account for their neighbors. What happens when an optimization strategy that explicitly ignores spatial interactions is incorporated into a physical model that is spatially dynamic? The long-term attractor that develops for the coupled system (the state and behavior to which the system evolves over time) is unclear. We link an economic model, in which town-manager agents choose economically optimal beach-nourishment intervals according to past observations of their immediate shoreline, to a simplified coastal-dynamics model that includes alongshore sediment transport and background erosion (e.g. from sea-level rise). Simulations suggest that feedbacks between these human and natural coastal processes can generate emergent behaviors. When alongshore sediment transport and spatially myopic nourishment decisions are coupled, increases in the rate of sea-level rise can destabilize economically optimal nourishment practices into a regime characterized by the emergence of chaotic shoreline evolution.

Highlights

Along sandy shorelines where erosion threatens waterfront property and infrastructure, beach nourishment is a commonly employed land-management strategy and engineering practice
We present results from an exploratory numerical model in which manager agents for a string of neighboring coastal towns choose economically optimal beach-nourishment intervals according to past observations of shoreline change at their beaches
Beach nourishment decisions for a given town are made by manager agents who act according to the capital accumulation model for beach nourishment in Smith et al (2009)

Summary

Introduction

Along sandy shorelines where erosion threatens waterfront property and infrastructure, beach nourishment is a commonly employed land-management strategy and engineering practice. Nourishment involves taking sand from a source site and depositing it onto an eroded beach, as a so-called “soft” alternative to permanent rock or concrete emplacements such as groynes and seawalls; groynes corral sand by locally obstructing sediment transport, while seawalls armor the shoreline outright (Pilkey and Wright, 1988). In the US, beach nourishment has cost more than $2.5 billion since 1950 (NOAA, 2006). The number and cost of nourishment projects have increased markedly in the last decade – a trend that is likely to continue given estimates for economic impacts of sea-level rise (NOAA, 2006; Smith et al, 2009; Titus et al, 1991; Sugiyama, 2007). Beach nourishment tends to occur where there is real estate development along a dynamic sandy coastline.

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Conclusion