Wave-Dominated Coastlines Responding to Climate Change: Large-Scale Morphodynamics, Human Involvement, and Possible Path Dependence
A. Brad Murray1, Andrew D Ashton2, Andrew Barkwith3, Michael A Ellis4, Kenneth D Ells5, Martin D Hurst6, Dylan McNamara5, Chris Thomas6 and James Wood7, (1)Duke University, Nicholas School of the Environment, Durham, NC, United States, (2)Woods Hole Oceanographic Institution, Geology and Geophysics, Woods Hole, MA, United States, (3)British Geological Survey Keyworth, Nottinghamshire, United Kingdom, (4)British Geological Survey Keyworth, Nottinghamshire, NG12, United Kingdom, (5)University of North Carolina - Willmington, Physics and Physical Oceanography, Willmington, NC, United States, (6)British Geological Survey, Nottinghamshire, United Kingdom, (7)North Carolina School of Science and Math, Durham, NC, United States
Abstract:
The flux toward shore of alongshore momentum, which drives alongshore sediment flux, varies with local coastline orientation, and with local degree of exposure to waves. Coastline shape therefore influences the alongshore patterns of alongshore sediment flux. Gradients in this flux, in turn, alter coastline shape—a morphodynamic feedback. Modeling studies show that such feedbacks lead ultimately to dynamic-equilibrium coastline shapes, including sandwaves, capes, and spits (e.g. Ashton and Murray, 2006; Ashton et al., 2015); spiral bays on rocky coastlines (e.g. Barkwith et al., 2014); and convex, spit-bounded coastlines (Ells et al., in prep.). One conclusion arises in each of these studies: Coastline shape depends sensitively on the wave climate, defined as the angular distribution of wave influences on alongshore sediment transport.
Given this sensitive dependence, shifts in wave climate, as can be expected from shifts in storm statistics, will tend to change coastline shape—involving decadal-scale changes in the location and intensity of coastal erosion (or accretion) zones. Such changes, likely related to changing influence from hurricane-generated waves, have been detected along undeveloped large-scale cuspate capes (NC, USA; Moore et al., 2013). On a developed cape nearby, shoreline stabilization through beach nourishment has prevented an equivalent change in erosion rates. Combined observations and modelling indicate that the signal of wave climate change can be detected in the human component of the system, in the form of increased nourishment rates on one flank of the cape (Johnson et al., 2015). Finally, these recent works involved the implicit assumption that coastline response to changing forcing occurs in a quasi-equilibrium manner. However, new modeling shows that in some cases coastline responses can exhibit long-term memory and path dependence, complicating potential detection and forecasting of climate change signals in some human/coastline systems.
