Human activities increasingly influence landscape change in many environments, both directly through construction and agricultural activities and indirectly through changes to the natural processes that shape landscapes. In turn, the processes that shape landscapes affect humans, often posing natural hazards. In coastal environments these two-way interactions involve coastal erosion, which threatens coastal communities, and shoreline-stabilization efforts, which affect the evolution of the surrounding coastline. Previous numerical modeling has shown that localized shoreline stabilization efforts, such as nourishing beaches by adding sand, can alter shoreline erosion rates even in distant parts of a coastline. Thus, a coastal community that chooses to stabilize its shoreline inadvertently affects other communities, so that the economies and management of coastal communities are linked. This research will use numerical modeling to address the kinds of coupled environmental and economic patterns that emerge under different decision-making regimes. An economic component to the numerical modeling, based on an empirical relationship between property values and beach width, determines the beach replenishment strategy that optimizes the net benefits to an individual community. Coupling this model to a coastline-change model reveals the unexpected ways that communities unwittingly interact with one another, and the feedbacks that induce some communities to shoulder more of the shoreline stabilization effort than others. In contrast, a different economic-model approach will analyze what pattern of beach replenishment would maximize the net benefits of a stretch of coastline more holistically. This project will investigate the different patterns of coastline change and economic benefits these approaches would produce under different scenarios for: 1) sea-level rise; 2) changing storm climate; 3) coastline physical and economic attributes; and 4) diminishing common-pool sand resources and the associated increase in the price of beach replenishment.
Changes in coastal environments can no longer be understood by considering either physical or economic processes in isolation; this research provides a necessary step toward understanding the dynamics of developed coastlines?what causes the patterns of shoreline erosion and economic impacts under various possible futures (given uncertainties in climate change and economic driving factors including sand resources). The results of computer-model experiments testing how coordinated planning for shoreline stabilization could increase net wealth will not only increase basic knowledge about how coupled human/landscape systems work, but it could lead to improvements in coastal management strategies.
On most developed coastlines, humans react to physical processes in coastal environments by stabilizing shorelines against chronic erosion and by taking measures to prevent destruction of coastal infrastructure during storms. Shoreline stabilization measures, though often funded non-locally (e.g. by the federal government), are planned in a decentralized and uncoordinated manner. Our research shows that this pattern of decision-making leads to unintentional geo-engineering of whole coastlines (1). For example, beach nourishment–the practice of rebuilding beaches with sand dredged from offshore or from inlets—affects the beach profiles of nearby communities. These physical changes, in turn, can affect economic incentives of nearby communities to nourish their beaches and ultimately lead to beach widths that are narrower than those that would result from coordinated actions, leaving coastal communities more at risk (2). Local shoreline stabilization can also influence shoreline changes over large distances. Local interventions, including beach nourishment and the building of seawalls, can propagate shoreline changes as far as 10s of kilometers away (3, Fig. 1). Decentralized coastal management can even alter coastal erosion patterns along whole coastlines, causing new emergent coastline shapes and behaviors (e.g. 4, Fig. 2). A more purposeful geo-engineering of coastlines requires a richer understanding of the two-way couplings between physical and human coastline dynamics (Fig. 3), including efforts to reduce uncertainties in forecasting future scenarios for the coupled human-environment system. Shoreline stabilization and associated federal funding are part of a broad suite of policies to combat coastal erosion and mitigate coastal storm risks. However, these policies can have unintended consequences by influencing the ways in which humans interact with the environment. Federal subsidies for beach nourishment inflate coastal real estate values. Exposed to more climate change (e.g. increased storm frequency and/or sea-level rise), the difference between the fundamental property value and the policy-influenced market value grows (5). Over the very long term, subsidies serve to delay the inevitable; in the face of sea-level rise, subsidized beach nourishment delays the time to abandonment due to coastal inundation (6). When decentralized beach nourishment is practiced along complex coastline shapes, such as the cuspate shape of the Carolinas, property values can become divorced from underling spatially varying erosion signals (7, Fig. 4). Decentralized shoreline stabilization has important equity implications as coastal communities develop climate adaptation strategies. Decentralized management can drive differences between coastal property values along straight and complex coastlines (2,7-8). Higher rates of sea-level rise further accentuate these differences (2). Decentralized management can also lead to an emergent pattern of communities that shoulder the burden of maintaining the beach and communities that free ride on others (7). Steering toward preferred outcomes for our coastlines and coastal economies will involve coordination across local, state, and federal jurisdictions to mitigate spatial externalities that extend beyond local communities. This project mentored one MS student, two PhD students, and one post-doctoral fellow. References (1) Smith, M.D., A.B. Murray, S. Gopalakrishnan, A.G. Keeler, C.E. Landry, D. McNamara, and L. Moore. "Geoengineernig Coastlines? From accidental to intentional." In Review (2013). (2) Gopalakrishnan, S., D. McNamara, M.D. Smith, and A.B. Murray. "Decentralized Management Hinders Coastal Climate Adaptation: The Spatial-dynamics of Beach Nourishment." In Review (2014). (3) Ells, Kenneth, and A. Brad Murray. "Long?term, non?local coastline responses to local shoreline stabilization." Geophysical Research Letters 39, no. 19 (2012). (4) Lazarus, E. D., D. E. McNamara, M. D. Smith, S. Gopalakrishnan, and A. B. Murray. "Emergent behavior in a coupled economic and coastline model for beach nourishment." Nonlinear Processes in Geophysics 18, no. 6 (2011): 989-999. (5) McNamara, D.E., S. Gopalakrishnan, M.D. Smith, and A.B. Murray. "Climate Adaptation and Policy-Induced Inflation of Coastal Property Value." In Review (2014). (6) McNamara, Dylan E., and Andrew Keeler. "A coupled physical and economic model of the response of coastal real estate to climate risk." Nature Climate Change 3, no. 6 (2013): 559-562. (7) Williams, Zachary C., Dylan E. McNamara, Martin D. Smith, A. Brad Murray, and Sathya Gopalakrishnan. "Coupled economic?coastline modeling with suckers and free riders." Journal of Geophysical Research: Earth Surface 118, no. 2 (2013): 887-899. (8) McNamara, Dylan E., A. Brad Murray, and Martin D. Smith. "Coastal sustainability depends on how economic and coastline responses to climate change affect each other." Geophysical Research Letters 38, no. 7 (2011).
