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

   https://doi.org/10.5194/esurf-4-193-2016  

   Ashton, A. D.; Nienhuis, J.; Ells, K. (January 2016, Earth Surface Dynamics)

   null (Ed.)

   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. 

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2. Simulating barrier island response to sea level rise with the barrier island and inlet environment (BRIE) model v1.0

   https://doi.org/10.5194/gmd-12-4013-2019  

   Nienhuis, Jaap H.; Lorenzo-Trueba, Jorge (January 2019, Geoscientific Model Development)

   Abstract. Barrier islands are low-lying coastal landforms vulnerable toinundation and erosion by sea level rise. Despite their socioeconomic andecological importance, their future morphodynamic response to sea level riseor other hazards is poorly understood. To tackle this knowledge gap, weoutline and describe the BarrieR Inlet Environment (BRIE) model that cansimulate long-term barrier morphodynamics. In addition to existing overwashand shoreface formulations, BRIE accounts for alongshore sediment transport,inlet dynamics, and flood–tidal delta deposition along barrier islands.Inlets within BRIE can open, close, migrate, merge with other inlets, andbuild flood–tidal delta deposits. Long-term simulations reveal complexemergent behavior of tidal inlets resulting from interactions with sea levelrise and overwash. BRIE also includes a stratigraphic module, whichdemonstrates that barrier dynamics under constant sea level rise rates canresult in stratigraphic profiles composed of inlet fill, flood–tidal delta,and overwash deposits. In general, the BRIE model represents a process-basedexploratory view of barrier island morphodynamics that can be used toinvestigate long-term risks of flooding and erosion in barrier environments.For example, BRIE can simulate barrier island drowning in cases in which theimposed sea level rise rate is faster than the morphodynamic response of thebarrier island. 

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3. Cyclical shoreline erosion: The impact of a jettied river mouth on the downdrift barrier island

   Fallon, Andrew R; Hein, Christopher J; Rosen, Peter S; Gannon, Halley L. (January 2015, Proceedings of the 11th International Symposium on Coastal Engineering and Science of Coastal Sediment Processes)

   Beaches and inlets throughout the U.S. have been stabilized for purposes of navigation, erosion mitigation, and economic resilience, commonly leading to changes in shoreline dynamics and downdrift erosion/accretion patterns. The developed beach of Plum Island, Massachusetts is highly dynamic, experiencing regular complex erosion / accretion patterns along much of the shoreline. We analyzed > 100 years of high-water line positions derived from satellite imagery, t-sheets, historical maps, and aerial photography. Unlike most beaches, the river-proximal sections of Plum Island are not uniformly retreating. Rather, the beach undergoes short-term erosion, followed by periods of accretion and return to a long-term mean stable shoreline position. These cycles occur over different time frames and in different segments of the beach, creating an ephemeral erosion ‘hotspot’ lasting as briefly as one year. The highly dynamic and spatially diverse nature of erosion along Plum Island provides insight into the complex nature of coupled inlet-beach dynamics over multiple timescales. 

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4. Wave‐Influenced Deltas: Growth Through Cyclical Accretion of Barrier‐Spits and the Role of Mud

   https://doi.org/10.1029/2024JF008166  

   Broaddus, Connor M; Nienhuis, Jaap H; Edmonds, Douglas A; Foufoula‐Georgiou, Efi (August 2025, Journal of Geophysical Research: Earth Surface)

   Abstract Wave‐influenced deltas are the most abundant delta type and are also potentially the most at‐risk to human‐caused changes, owing to the effects of wave‐driven sediment transport processes and the relatively short timescales on which they operate. Despite this, the processes controlling wave‐influenced growth are poorly understood, and the role of fine‐grained cohesive sediment (mud) is typically neglected. Here we simulate idealized river deltas in Delft3D across a range of conditions to interrogate how relative wave‐influence and fluvial sediment composition impact delta evolution on decadal‐centurial timescales. Our simulations capture the barrier‐spit formation and accretion process characteristic of prograding wave‐influenced deltas, consistent with behaviors observed in natural systems. Barrier‐spit accretion exhibits multi‐decadal cyclicity driven by subaqueous accumulation of fluvial sediment near river mouths. Using a range of metrics, we quantify how waves and mud influence delta morphology and dynamics. Results show that waves stabilize and simplify channel networks, smooth shorelines, increase shoreline reworking rates, reduce mud retention in the delta plain, and rework mouth bar sediments to form barrier‐spits. Higher fluvial mud concentrations produce simpler and more stable distributary networks, rougher shorelines, and limit back‐barrier lagoon preservation. Our findings reveal distinct controls on shoreline change between river‐dominated and wave‐influenced deltas and demonstrate that mud plays a critical role in delta evolution, even under strong wave influence. These insights could enhance paleoenvironmental reconstructions and inform predictions of delta responses to climate and land‐use changes. 

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5. Comparative Metabolism and Blue Carbon Sequestration of Two Wetland-Dominated Estuaries

   https://doi.org/10.34133/olar.0091  

   Hopkinson, Charles S; Weston, Nathaniel B; Vallino, Joseph J; Garritt, Robert H; Forbrich, Inke (January 2025, Ocean-Land-Atmosphere Research)

   Coastal tidal wetlands and estuaries play important roles in the global carbon budget by contributing to the net withdrawal of CO₂from the atmosphere. We quantified the linkages between terrestrial and oceanic systems, marsh-to-bay carbon exchange, and the uptake of CO₂from the atmosphere in the wetland-dominated Plum Island Sound (MA, USA) and Duplin River (GA, USA) estuaries. The C budgets revealed that autotrophic marshes [primary production:ecosystem respiration (P:R) ~1.3:1] are tightly coupled to heterotrophic aquatic systems (P:R ~0.6:1). Levels of marsh gross primary production are similar in these systems (865 ± 39 and 768 ± 74 gC m⁻²year⁻¹in Plum Island and the Duplin, respectively) even though they are in different biogeographic provinces. In contrast to inputs from rivers and coastal oceans, tidal marshes are the dominant source of allochthonous matter that supports heterotrophy in aquatic systems. Dissolved inorganic carbon (DIC) exported from marshes to the coastal ocean was a major flux pathway in the Duplin River; however, there was no evidence of DIC export from Plum Island marshes and only minor export to the ocean. Burial was a sink for 53% of marsh net ecosystem production (NEP) on Plum Island, but only 19% of marsh NEP in the Duplin. Burial was the dominant blue carbon sequestration pathway at Plum Island, whereas in the Duplin, DIC and organic carbon export to the ocean were equally important. Regional- and continental-scale C budgets should better reflect wetland-dominated systems to more accurately characterize their contribution to global CO₂sequestration. 

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