A. Brad Murray
Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences/
Center for Nonlinear and Complex Systems,
Duke University, USA

Humans increasingly inhabit coastal environments, and in urbanized countries many coastline are being developed at a rapid pace. The spatial dynamics of natural sandy coastlines involve areas of erosion (landward movement of the shoreline), and other areas of accretion, and thus affect human development. In addition, the patterns of human manipulations of the coastal system will also affect the spatial pattern of coastline change. The local effects of relatively small-scale coastal structures designed to affect the position of the shoreline, such as groins and seawalls, are fairly well known. However, recent research suggests that human efforts to affect the position of the shoreline on larger scales (e.g. large beach nourishment programs or pervasive seawall construction over large areas) will affect the coastal system on temporal and spatial scales that have not been previously considered. Modeling work concerning Large-Scale Coastal Behavior (LSCB) suggests that changing how the coastline position moves (or doesn’t move, in the case of a coastal town) at one alongshore location can directly affect the patterns of accretion and erosion in areas far removed (Ashton, Murray and Arnoult, Nature 414, 2001).

Recent analysis shows that, because of the nonlinear relationship between alongshore sediment flux and the angle between deep water wave crests and local shoreline orientation, in some wave climates a straight coastline is unstable (Ashton et al., 2001). When deep-water waves approach from angles greater than the one that maximizes alongshore flux, in concave-seaward shoreline segments sediment flux will diverge, causing erosion. Similarly, convex regions such as the crests of perturbations on an otherwise straight shoreline will experience accretion; perturbations will grow. Numerical modeling work to explore the long-term effects of this instability operating over a spatially extended alongshore domain, and with temporally varying wave directions, has shown that as perturbations grow to finite amplitude and interact with each other, large-scale coastline structures can emerge. The character of the local and non-local interactions, and the resulting emergent structures, depends on the wave climate. The 100-km scale capes and cuspate forelands that form much of the coast of the Carolinas, USA—a partially developed popular recreational region—provides one possible natural example. ‘Sandwaves’ (such as those on the Dutch and Danish coasts—migrating patterns of erosion and accretion with alongshore scales of up to 10s of kms), and ‘cuspate spits’ (striking protrusions extending at an angle from the coastline trend, such as those on the Azov Sea, Ukraine, and parts of the developed shoreline of Lake Erie, USA) provide others. This modeling suggests that when such features exhibit a significant aspect ratio (cross-shore extent/alongshore length), they affect both the flux of sediment into/out of adjacent regions, and the wave climate felt by more remote segments of the coastline.

Robust model predictions compare favorably with observations of natural coastlines: 1) wave climates where natural cuspate spits, cuspate forelands, sandwaves, and smooth coastlines exist match those predicted by the model; and 2) general patterns of coastline change on human time scales on the Dutch and SE US coasts, including some enigmatic erosion ‘hot spot’ behaviors, can be explained by the simple model (Ashton, List, Murray and Ferris, Coastal Sediments ’03, 2003; Ashton, Murray and Ruessink, 3rd River, Coastal, and Estuarine Morphodynamics Symposium, in prep.). While the model is not designed to reproduce as accurately as possible the details of any particular geographic location, these tests imply that the behaviors in the model are relevant to natural coastline evolution.

This model of coastline-position change suggests that: 1) every point on an undeveloped sandy shoreline will be mobile in the long term, eroding at some times and accreting at others; and 2) the shape and position of the coastline at one location will affect the evolution of other parts of the coastline pattern. Coastal management policies and strategies often involve fixing the position of the shoreline where communities, highways, or other infrastructure exist. To the extent that such policies are successful, each human manipulation of this type will affect the erosion/accretion at other locations, up to many tens of kilometers distant, over time scales of decades to centuries and longer. The collection of human manipulations must already be affecting the way developed coastlines behave. While the effects of a single human effort (e.g. beach nourishment at the location of one town) can be predicted conceptually or analytically over relatively short time scales, how the new human/natural coastal system as a whole will behave in coming centuries—especially in light of sea level rise—requires numerical investigation.

Using this new, unique model of Large-Scale Coastal Behavior that can treat arbitrarily complicated coastline shapes and wave climates (Ashton et al., 2001), with the effects of sea level rise and human manipulations incorporated, I plan to address questions including: 1) How do the effects of a single human manipulation depend on the position of the manipulation relative to coastline features (e.g. towns located in the back of cuspate bays vs. near the extremities of capes or cuspate spits)? 2) What configurations of multiple human developments and activities will minimize or maximize negative socio-economic impacts? 3) What configurations of human manipulations will minimize or maximize negative environmental impacts? (For example, will some combination of human activities enhance the likelihood that sea level rise will lead to the disappearance of barrier-island chains, with the consequent loss of estuary and wetland environments and ecosystems?) Such basic understanding of the interplay between human practice and the natural system will allow for the planning of optimal human development patterns.

Although the model of coastline change is well developed, the investigation of the interplay between human manipulations and natural responses is just beginning. To address the behavior of this coupled system in a meaningful way will require addressing the way human policies and practices are likely to adjust in the future. Policies and patterns of development are likely to change as the configurations of open-ocean coastlines evolve, in response to changing impacts on: 1) coastal property and infrastructure; and 2) ecosystems and environments including those in estuaries, lagoons, and wetlands behind barrier islands. To this end, a collaboration within Duke University including expertise on climate change, coastal policy and coastal ecosystems is forming. We are seeking interactions with social scientists, land use planners, and policy makers to both move this work forward and ultimately to disseminate the findings that could facilitate long-term planning.