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

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.

Two new conceptual models for the formation and degradation of baymouth spits by longshore drift and fluvial discharge (Iguape, SE Brazil)

“…Numerical models can reproduce the growth and elongation of linear and curved spits under a predominant longshore transport (Kraus, 1999;Palalane et al, 2014;Ashton et al, 2016). Simulation of a wave-dominated coast with a river mouth resulted in the development of both a sandy spit and a submarine terrace, the latter was related to the fluvial sediment input, but fluvial blockage was not identified (Kuroiwa et al, 2014).…”

Mechanics and rates of tidal inlet migration: Modeling and application to natural examples

“…For high tidal range and narrow barriers, the updrift barrier spit curves inward forming a drumstick-like barrier updrift of the inlet (Figure 9) [Hayes, 1980]. The deposited barrier becomes less affected by the retreating barrier and attains a shape resembling that of a recurving, free spit [e.g., Ashton et al, 2016].…”

Correlation Between Shoreline Change and Planform Curvature on Wave‐Dominated, Sandy Coasts

“…Given that extensive reaches of the U.S. Atlantic and Gulf Coasts feature low curvatures with local wave climates tending to be low-angle dominated (e.g., Ashton & Murray, 2006b;Johnson et al, 2015), a diffusive, smoothing signal should be apparent over large spatial and long timescales across a broad span of locations. In numerical modeling experiments, even where regional high-angle wave climates (relative to the regional coastline trend) have shaped large-scale, emergent coastline features, such as cuspate capes or free spits, wave-shadowing effects, and local shoreline reorientation, result in diffusive prevailing conditions everywhere but near the cape tip or spit terminus (Ashton et al, 2016;Ashton & Murray, 2006a, 2006b). Thus, model results and observations (or hindcasts) of local wave climates lead us to expect a coastline-smoothing signal, that is, positive diffusivity, in almost all locations (Ashton & Murray, 2006b).…”