Development and Testing of a Numerical Model for the Evolution of Rocky Coastlines

Duke University, Durham NC

Investigators

Abstract

Many of the world's rocky coastlines exhibit planform roughness in the form of alternating headlands and embayments. Along wave-dominated cliffed coasts, it is often assumed that headlands consist of rock that is more resistant to wave attack than in neighboring bays, because of either structural or lithologic variations. Bays would then retreat landward faster than headlands, creating the undulating planform profiles characteristic of a rocky coastal landscape. While interplay between alongshore rock strength and nearshore wave energy is certainly often a fundamental control on coastline shape, sediment also plays a key role. Beach sediment, in small volumes, can act as an abrasive tool to encourage sea cliff retreat. In large volumes, however, beach sediment discourages wave attack on the cliff face, acting as a protective barrier. This suggests a new, unexplored mechanism for headland persistence, even in the absence of alongshore variations in rock strength: bare-rock headlands could retreat more slowly than, or at the same rate as, neighboring sediment-filled embayments because of alongshore variations in the availability of beach sediment. Accordingly, nearshore sediment dynamics (i.e. sediment production from sea cliff retreat, alongshore sediment transport, and beach retreat) could promote the development of stable and persistent planform rocky coastline features. To explore these ideas, this project will undertake numerical and analytical modeling of large-scale (> one kilometer) and long-term (millennial-scale) planform rocky coastline evolution, and compare model predictions with real landscapes. Approximately 80% of the world's coastline is rocky, and coastline change impacts individuals, corporations, and governments. Although some research has been done on the physical, biological and chemical processes that influence the erosion of cliffs and beaches, very little is known about how rocky coastlines behave in the long-term when the alongshore dimension is included. This project, therefore, will initiate a new line of research. The aspects of the proposed work exploring how human manipulations (e.g. river damming and cliff stabilization), affect long-term, large-scale coastline evolution will therefore attract broad interest. The results, and the model itself, will be readily available for use by coastal managers and planners, and results will also be useful to the general public, teachers, and students.