Subaqueous and aeolian bedforms are ubiquitous on Earth and other planetary environments. However, it is still unclear which hydrodynamical mechanisms lead to the observed variety of morphologies of self-organized natural patterns such as ripples, dunes or compound bedforms. Here we present simulations with a coupled hydrodynamic and sediment transport model that resolve the initial and mature stages of subaqueous and aeolian bedform evolution in the limit of large flow thickness. We identify two types of bedforms consistent with subaqueous ripples and dunes, and separated by a gap in wavelength. This gap is explained in terms of an anomalous hydrodynamic response in the structure of the inner boundary layer that leads to a shift of the position of the maximum shear stress from upstream to downstream of the crest. This anomaly gradually disappears when the bed becomes hydrodynamically rough. By also considering the effect of the spatial relaxation of sediment transport we provide a new unifying framework to compare ripples and dunes in planetary environments to their terrestrial counterparts.Bedforms like aeolian dunes, found in arid regions and along shorelines, and subaqueous ripples and dunes in riverine and marine environments, result from the same instability mechanism, related to the lag between bed shear stress, sediment transport and bed elevation 1-7 . Analogous bedforms are also observed in other planetary environments as diverse as Mars, Venus, Titan or comet Churyumov-Gerasimenko 8-10 . They can display, however, rather different characteristic lengths and formation time scales. In order to deduce a reliable interpretation of these atmospheric conditions from remote photos and measurements, an obstacle is to determine relevant terrestrial analogues. Martian meter scale bedforms were for instance first understood as aeolian ripples superimposed on barchan dunes 11, 12 , but have been recently reinterpreted as being more 1 arXiv:1911.12300v1 [physics.geo-ph] 25 Nov 2019 akin to subaqueous current ripples 13,14 . But, how could aeolian-like bedforms (dunes and impact ripples 12 ) and subaqueous-like bedforms (current ripples) coexist in the same Martian environment? Another source of confusion in the literature results from the analysis of the pattern wavelength -defined as the average crest to crest distance. Ripples usually refer to bedforms of small dimension, typically scaling 15, 16 with the grain size d, whereas dunes are associated with larger dimensions, with a wavelength on the order of the river depth in the subaqueous case 7,16 or that can reach the thickness of the convective atmospheric boundary layer in the aeolian one 17 . However, this classification must be revisited when planetary bedforms are investigated: 'small' or 'large' can mean different actual sizes when the hydrodynamic parameters take unusual values as compared to terrestrial aeolian or subaqueous environments. How should we name, for instance, the decimeter scale bedforms emerging in the wind tunnel reproducing the high CO 2 ...

Interactions between backbarrier marshes and barrier islands will likely play an important role in determining how low-lying coastal systems respond to sea level rise and changes in storminess in the future. To assess the role of couplings between marshes and barrier islands under changing conditions, we develop and apply a coupled barrier island-marsh model (GEOMBEST+) to assess the impact of overwash deposition on backbarrier marsh morphology and of marsh morphology on rates of island migration. Our model results suggest that backbarrier marsh width is in a constant state of change until either the backbarrier basin becomes completely filled or backbarrier marsh deposits have completely eroded away. Results also suggest that overwash deposition is an important source of sediment, which allows existing narrow marshes to be maintained in a long-lasting alternate state (~500 m wide in the Virginia Barrier Islands) within a range of conditions under which they would otherwise disappear. The existence of a narrow marsh state is supported by observations of backbarrier marshes along the eastern shore of Virginia. Additional results suggest that marshes reduce accommodation in the backbarrier bay, which, in turn, decreases island migration rate. As climate change results in sea level rise, and the increased potential for intense hurricanes resulting in overwash, it is likely that these couplings will become increasingly important in determining future system behavior.

Barrier islands represent about 10% of the world's coastline 1 , sustain rich ecosystems, host valuable infrastructure and protect mainland coasts from storms. Future climate-change-induced increases in the intensity and frequency of major hurricanes 2 and accelerations in sea level rise 3,4 will have a significant impact on barrier islands 5,6 -leading to increased coastal hazards and flooding-yet our understanding of island response to external drivers remains limited 1,7-8 . Here, we find that island response is intrinsically bistable and controlled by previously unrecognized dynamics: the competing, and quantifiable, effects of storm erosion, sea level rise, and the aeolian and biological processes that enable and drive dune recovery. When the biophysical processes driving dune recovery dominate, islands tend to be high in elevation and vulnerability to storms is minimized. Alternatively, when the effects of storm erosion dominate, islands may become trapped in a perpetual state of low elevation and maximum vulnerability to storms, even under mild storm conditions. When sea level rise dominates, islands become unstable and face possible disintegration. This quantification of barrier island dynamics is supported by data from the Virginia Barrier Islands, U.S. and provides a broader context for considering island response to climate change and the likelihood of potentially abrupt transitions in island state.Barrier islands respond to rising sea level by migrating landward or drowning 7,9-10 . Landward migration is driven mostly by storms and is controlled by island elevation. Extensive measurements of dune elevation performed at the Virginia Barrier Islands 11 , a relatively undisturbed barrier system including 12 islands, show a bimodal distribution of barrier island elevation with two well-defined island types: low-elevation and highelevation islands (Fig. 1, Supplementary Fig. 1). Low islands lacking vegetated dunes are relatively narrow and prone to frequent overwash, resulting in rapid landward migration (Fig. 1a,b,g) and low biodiversity (as in the case of the islands associated with the Mississippi Delta, e.g., The Chandeleur Islands). In contrast, high islands with well-developed dunes resist storm impacts, are wider and migrate slowly (if at all, Fig. 1c,d,g) and support a rich ecosystem and/or human development. In this way, barrier island evolution is fundamentally linked to dune dynamics. However, because vegetated dunes both protect islands from storm impacts and are themselves

We investigate the formation of multiple dunes using a >15 yr record of dune growth from Long Beach Peninsula, Washington State (USA), and a recently published coastal dune model modified to include a feedback between vegetation growth and local dune slope. In the presence of shoreline progradation, we find that multiple dune ridge formation can be autocyclic, arising purely from internal dune dynamics rather than requiring variations in external conditions. Our results suggest that the ratio of the shoreline progradation rate and the lateral dune growth rate is critical in determining the height, number, and form of multiple dunes, allowing the development of testable predictions. Our findings are consistent with observations and imply that caution is required when using dune ridges as proxies for past changes in climate, sea level, land use, and tectonic activity because the relationship between external events and the formation of multiple dunes may not be one to one as previously thought.

Abstract. Coastal foredunes form along sandy, low-sloped coastlines and range in shape from continuous dune ridges to hummocky features, which are characterized by alongshore-variable dune crest elevations. Initially scattered dune-building plants and species that grow slowly in the lateral direction have been implicated as a cause of foredune "hummockiness". Our goal in this work is to explore how the initial configuration of vegetation and vegetation growth characteristics control the development of hummocky coastal dunes including the maximum hummockiness of a given dune field. We find that given sufficient time and absent external forcing, hummocky foredunes coalesce to form continuous dune ridges. Model results yield a predictive rule for the timescale of coalescing and the height of the coalesced dune that depends on initial plant dispersal and two parameters that control the lateral and vertical growth of vegetation, respectively. Our findings agree with previous observational and conceptual work -whether or not hummockiness will be maintained depends on the timescale of coalescing relative to the recurrence interval of high-water events that reset dune building in low areas between hummocks. Additionally, our model reproduces the observed tendency for foredunes to be hummocky along the southeast coast of the US where lateral vegetation growth rates are slower and thus coalescing times are likely longer.

Coastal landscape change represents aggregated sediment transport gradients from spatially and temporally variable marine and aeolian forces. Numerous tools exist that independently simulate subaqueous and subaerial coastal profile change in response to these physical forces on a range of time scales. In this capacity, coastal foredunes have been treated primarily as wind-driven features. However, there are several marine controls on coastal foredune growth, such as sediment supply and moisture effects on aeolian processes. To improve understanding of interactions across the land-sea interface, here the development of the new Windsurf-coupled numerical modeling framework is presented. Windsurf couples standalone subaqueous and subaerial coastal change models to simulate the co-evolution of the coastal zone in response to both marine and aeolian processes. Windsurf is applied to a progradational, dissipative coastal system in Washington, USA, demonstrating the ability of the model framework to simulate sediment exchanges between the nearshore, beach, and dune for a one-year period. Windsurf simulations generally reproduce observed cycles of seasonal beach progradation and retreat, as well as dune growth, with reasonable skill. Exploratory model simulations are used to further explore the implications of environmental forcing variability on annual-scale coastal profile evolution. The findings of this work support the hypothesis that there are both direct and indirect oceanographic and meteorological controls on coastal foredune progradation, with this new modeling tool providing a new means of exploring complex morphodynamic feedback mechanisms.

Salt marshes are valuable but vulnerable coastal ecosystems that adapt to relative sea level rise (RSLR) by accumulating organic matter and inorganic sediment. The natural limit of these processes defines a threshold rate of RSLR beyond which marshes drown, resulting in ponding and conversion to open waters. We develop a simplified formulation for sediment transport across marshes to show that pond formation leads to runaway marsh fragmentation, a process characterized by a self-similar hierarchy of pond sizes with power-law distributions. We find the threshold for marsh fragmentation scales primarily with tidal range and that sediment supply is only relevant where tides are sufficient to transport sediment to the marsh interior. Thus the RSLR threshold is controlled by organic accretion in microtidal marshes regardless of the suspended sediment concentration at marsh edge. This explains the observed fragmentation of microtidal marshes and suggests a tipping point for widespread marsh loss.

Coastal dunes protect beach communities and ecosystems from rising seas and storm flooding and influence the stability of barrier islands by preventing overwashes and limiting barrier migration. Therefore, the degree of dune recovery after a large storm provides a simple measure of the short-term resiliency (and potential long-term vulnerability) of barrier islands to external stresses. Dune recovery is modulated by low-intensity/high-frequency high-water events (HWEs), which remain poorly understood compared to the low-frequency extreme events eroding mature dunes and dominating the short-term socio-economic impacts on coastal communities. Here, we define HWEs and analyze their probabilistic structure using time series of still-water level and deep-water wave data from multiple locations around the world. We find that HWEs overtopping the beach can be modeled as a marked Poisson process with exponentially distributed sizes or marks and have a mean size that varies surprisingly little with location. This homogeneity of global HWEs is related to the distribution of the extreme values of a wave-runup parameter,HR=HsL0, defined in terms of deep-water significant wave heightHsand peak wavelengthL0. Furthermore, the characteristic beach elevation at any given location seems to be tied to a constant HWE frequency of about one event per month, which suggests a stochastic dynamics behind beach stabilization. Our results open the door to the development of stochastic models of beach, dune, and barrier dynamics, as well as a better understanding of wave-driven nuisance flooding.