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

A D Ashton,J Nienhuis,K Ells

doi:10.5194/esurf-4-193-2016

Abstract

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.

Highlights

Recurved barrier spits occur in a wide variety of environments, including passive sandy shorelines, delta complexes, and rocky coasts, where spits extend depositionally from a shore that is otherwise eroding
The rate of sediment input to the spit itself is determined by feedbacks between the spit and the upcoast shore even as the maximum potential sediment transport is set by the wave climate
Through a series of numerical experiments using a one-line coastal evolution model, we have explored key dynamics affecting the formation and shape of littoral spits

Summary

Introduction

Recurved barrier spits occur in a wide variety of environments, including passive sandy shorelines, delta complexes, and rocky coasts, where spits extend depositionally from a shore that is otherwise eroding. The variety of smooth, curved, wave-sculpted shapes of spits (Fig. 1) has long been of scientific interest and there are numerous studies and interpretations of spit growth and associated deposits (e.g., Schwartz, 1972); understanding of the basic controls on spit shape, evolution, and response to changes in forcing conditions remains elusive. We conduct a series of experiments with a numerical model of shoreline evolution to explore the environmental controls that influence spit growth and form. Using these model results, we present a quantitative, process-based description of key elements along a spit, including the erosional updrift neck and the depositional hook. A series of controlled model experiments suggest that even as the directional characteristics of approaching waves affect spit shape, the updrift boundary significantly affects spit growth. Spits are common on wave-dominated deltaic coasts, such as the Ebro and Rhone River deltas (Raynal et al, 2009; Sanchez-Arcilla et al, 1998), with recent research suggesting that deltaic spit extension occurs most vigorously after delta lobe abandonment

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