Self-organized pattern formation in coastal barrier washover deposits

Abstract

7Storm-driven overwash is a sediment-transport process fundamental to the 8 evolution of low-lying coastal environments. Physical insight into overwash 9 morphodynamics is crucial for improved risk assessment and hazard forecasting in 10 vulnerable coastal zones. Spatially extended observations of washover deposits have 11shown that back-barrier shoreline planforms can be quasi-periodic. These rhythmic 12 patterns have been attributed to the influence of a forcing template in bathymetry or 13 topography, or inherent in the forcing itself. With an alternative to this prevailing 14 explanation, we present results of a physical experiment and numerical model in which 15 quasi-periodic patterns in washover deposits are self-organized, arising from interactions 16 between barrier topography, routing of overwash flow, and sediment flux. 17

Scaling laws for coastal overwash morphology

“…For barrier islands with back‐barrier bays or lagoons, back‐barrier depth may drop or surge with storm‐driven “wind tides” (setup) typical of shallow basins [ Fagherazzi and Wiberg , ], changing the effective back‐barrier base level “felt” by washover deposition and thus affecting longer or shorter washover intrusion lengths [ Shaw et al ., ]. Hydrodynamic characteristics of the storm impact scale (wave height, setup, tide, and surge) [ Sallenger , ] and storm duration also inform morphological pattern formation: the former forces barrier response, and the latter allows time for morphology to organize [ Perron and Fagherazzi , ; Lazarus and Armstrong , ].…”

“…Although these scale‐invariant empirical relationships are a necessary step toward resolving the dynamics of alongshore pattern formation in overwash morphology [ Lazarus and Armstrong , ], they do not demonstrate mechanism. How extreme sea levels translate into morphological change through feedbacks between fluid flow and sediment transport on the time scale of a storm event remains poorly understood [ Leatherman and Zaremba , ; Donnelly et al ., ; McCall et al ., ; Lazarus and Armstrong , ; Shaw et al ., ].…”

“…Although these scale‐invariant empirical relationships are a necessary step toward resolving the dynamics of alongshore pattern formation in overwash morphology [ Lazarus and Armstrong , ], they do not demonstrate mechanism. How extreme sea levels translate into morphological change through feedbacks between fluid flow and sediment transport on the time scale of a storm event remains poorly understood [ Leatherman and Zaremba , ; Donnelly et al ., ; McCall et al ., ; Lazarus and Armstrong , ; Shaw et al ., ]. The implication that a variety of erosional and depositional landscapes, characterized by different sediment transport processes, may be related by common allometric properties (Figure ) suggests that geomorphic behaviors at advection‐diffusion transitions (and vice versa) [ Perron et al ., ; Haff , ; Houssais and Jerolmack , ] could be the key to disentangling mechanistic causality from acausality in physical landscape patterns [ Dodds and Rothman , ; Paola et al ., ].…”

“…Competition for available flow meant pairs of throats and lobes grew at different rates relative to, and at the expense of, other pairs. The development of throats and lobes in series alongshore, starting from a uniform initial condition under steady forcing, reinforces recent work proposing that alongshore patterns in overwash morphology can self‐organize [ Lazarus and Armstrong , ]. Morphometric traits of throats and lobes (centroid position, length l , width w , area A , and volume V ) were measured from topographic scans of final barrier topography (Figures d–g, S1, and S2).…”

Lateral vegetation growth rates exert control on coastal foredune hummockiness and coalescing time

“…Changes in storminess may also impact the hummockiness of coastal foredunes, with an increase in storm intensity or frequency leading to a greater tendency for dunes to be hummocky and therefore to provide less protection to habitats behind them. Here, we have focused on the development of hummocky dunes from an initially flat condition, but Lazarus and Armstrong (2015) discuss the potential for storm events to create regularly spaced overwash throats (via self-organization) that could also set up hummocky dune topography. Although beyond the scope of this effort, observational work aimed at assessing the relationships among storm frequency/magnitude, species composition of dune-building vegetation and dune development (e.g., van Puijenbroek et al, 2017a, b) will be useful in addressing the future implications of model results presented here as climate change is anticipated to alter each of these factors.…”