Articles | Volume 4, issue 1
Research article
03 Feb 2016
Research article |  | 03 Feb 2016

On a neck, on a spit: controls on the shape of free spits

A. D. Ashton, J. Nienhuis, and K. Ells

Abstract. We investigate the controls upon the shape of freely extending spits using a one-contour-line model of shoreline evolution. In contrast to existing frameworks that suggest that spits are oriented in the direction of alongshore sediment transport and that wave refraction around the spit end is the primary cause of recurving, our results suggest that spit shoreline shapes are perhaps best understood as graded features arising from a complex interplay between distinct morphodynamic elements: the headland updrift of the spit, the erosive "neck" (which may be overwashing), and the depositional "hook". Between the neck and the hook lies a downdrift-migrating "fulcrum point" which tends towards a steady-state trajectory set by the angle of maximum alongshore sediment transport. Model results demonstrate that wave climate characteristics affect spit growth; however, we find that the rate of headland retreat exerts a dominant control on spit shape, orientation, and progradation rate. Interestingly, as a spit forms off of a headland, the rate of sediment input to the spit itself emerges through feedbacks with the downdrift spit end, and in many cases faster spit progradation may coincide with reduced sediment input to the spit itself. Furthermore, as the depositional hook rests entirely beyond the maximum in alongshore sediment transport, this shoreline reach is susceptible to high-angle wave instability throughout and, as a result, spit depositional signals may be highly autogenic.

Short summary
Depositional shoreline spits are found along many of the Earth's modern and paleo-shorelines. Here we use a numerical model of shoreline evolution to develop a comprehensive model of the controls on spit shape, demonstrating that spits are graded shapes that arise from feedbacks between the headland, neck, and hook through the migration of a "fulcrum point". Our results suggest that spit form is controlled not only by the directional wave climate but also by the rate of headland recession.