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.
Despite the widespread appeal of the beauty of rocky coastlines—with rocky headlands jutting out beyond cliff-backed pocket beaches—how the large-scale shapes of these coastlines come about had not previously been explained. In this project we developed mathematical and computer models to help explore ideas about how headlands and pocket beaches develop over time, and how the shapes of such coastlines and the beaches that adorn them are likely to change in response to human influences such as the damming of coastal rivers, the armoring of coastal cliffs, and the stabilization of recreational beaches. The results of the models suggest that interactions between the amount of sand on beaches and the rate that cliffs erode landward (based on previous investigations) can explain how pocket beaches expand or contract over time, coincident with the expansion or contraction of rocky headlands. Previous investigations by other researchers have shown that adding a moderate amount of sand and/or gravel to an otherwise bare rocky coastline can speed up the rate of cliff erosion, as the sand grains or gravel act as abrasive ‘tools’ that the waves can use to gradually chip away the cliff. On the other hand, when a beach becomes sufficiently wide, it prevents some of the wave’s energy from reaching the cliffs, tending to slow down cliff erosion. We combined these effects with the complementary fact that the faster a cliff erodes, the faster sand and/or gravel can be added to the beach—so that while beach width affects cliff erosion, cliff erosion also affects beach width. Combined with wave processes that tend to spread sand along the shore, these interactions between beaches and cliffs, occurring over centuries to millennia, lead surprisingly to coastlines that retain their shape over time, with headlands and beaches eroding landward in unison. In this ‘steady state’, coastlines with abundant sediment input from rivers and/or tall cliffs feature few or no headlands, while coastlines with few rivers and small or cliffs consist mostly of rugged rocky shores with few or no pocket beaches. We also found that what the cliffs are made of—whether they produce sand and gravel versus mud (which does not contribute to the beach) as they erode—helps determine the prevalence or absence of rocky headlands. This work also provides predictions of the effects of damming coastal rivers, or dam removal. How much the width of regional beaches will be altered depends on the characteristics of the coastal cliffs (their height and composition). In addition, the alteration of beach widths will eventually change the shape of the cliffed coastline, causing the expansion of rocky headlands. Coastlines with tall, sand-and-gravel rich cliffs will tend to be less affected by human changes to coastal rivers. Model results also offer a potential explanation for the origin of the iconic features called ‘sea stacks’: gorgeous pillars of rock standing off of some rocky headlands. Pocket beaches with a moderate amount of sand tend to increase the erosion rate on the flanks of a headland, as the sand or gravel helps abrade and erode the rocky cliff, leading to the development of sea caves, and then an arch when the caves on each side of a headland connect, and ultimately to a sea stack when the arch collapses. All of these potential insights advance our scientific understanding, offering new potential explanations for coastal shapes and how they change over time. This project also involved analyzing coastlines on the West Coast of the US, as well as the United Kingdom, to test model predictions. Results of these tests support the validity of our potential explanations. We are contributing the computer model to the Community Surface Dynamics Modeling System, so that other researchers can use it to address other scientific questions—and so that the general public and coastal managers can use it to address practical questions about the consequences of potential actions including adding or removing dams to coastal rivers, armoring coastal cliffs, or adding sand to recreational beaches.
