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Much of the world's population lives close to coastlines and this proximity is becoming increasingly impactful because of sea-level rise (SLR). Barrier islands and backbarrier saltmarshes, which comprise >10% of these coasts, are particularly susceptible. To better understand this risk, we model backbarrier morphologic and hydrodynamic evolution over a 200-year period of SLR, incorporating an erodible bed and a range of grain sizes. Here, we show that reduction in intertidal area creates negative feedback, shifting transport of coarse sediment (silt and sand) through the inlet from net export to net import. Imposing a modest marsh vertical accretion rate decreases the period of silt and sand import to 40 years (years 90 to 130) before being exported again. Clay is continuously exported thereby decreasing inorganic deposition on marshes and threatening their sustainability. Simulated marsh loss increases tidal prism and the volume of sand contained in ebb deltas, depleting coastal sand resources. Barrier islands and backbarrier saltmarshes are particularly threatened by sea level rise. Here, the authors show how reduction in intertidal areas create negative feedback, shifting transport of coarse sediment through the inlet from net export to net import. [ABSTRACT FROM AUTHOR]

When longshore transport systems encounter tidal inlets, complex mechanisms are involved in bypassing sand to downdrift barriers. Here, this process is examined at Plum Island Sound and Essex Inlets, Massachusetts, USA. One major finding from this study is that sand is transferred along the coast—especially at tidal inlets—by parcels, in discrete steps, and over decadal-scale periods. The southerly orientation of the main-ebb channel at Plum Island Sound, coupled with the landward migration of bars from the ebb delta to the central portion of the downdrift Castle Neck barrier island, have formed a beach protuberance. During the constructional phase, sand is sequestered at the protuberance and the spit-end of the barrier becomes sediment starved, leading to shoreline retreat and a broadening of the spit platform at the mouth to Essex Bay (downdrift side of Castle Neck). Storm-induced sand transport from erosion of the spit and across the spit platform is washed into Essex Bay, filling channels and enlarging flood deltas. This study illustrates the pathways and processes of sand transfer along the shoreline of a barrier-island/tidal-inlet system and provides an important example of the processes that future hydrodynamic and sediment-transport modeling should strive to replicate. [ABSTRACT FROM AUTHOR]

Louisiana barrier islands, such as the chain surrounding the southeast region of the state, are experiencing rapid loss of land area, shoreline erosion, and landward migration due to transgression and in-place drowning, and the landfall of several major hurricanes in the last decade. Observations of migration rates and overall impacts to these barrier islands are poorly understood since they do not respond in a traditional way, such as barrier rollover. This paper aims to verify how wave energy and potential longshore sediment transport trends have influenced the recent evolution of the Chandeleur Islands, by direct comparison with recent observations of migration and erosion trends. The Chandeleur Islands are characterized by a bidirectional transport system, with material moving from the central arc to the flanks. The longshore sediment transport along the barrier islands was calculated after propagation and transformation of waves to breaking (generated using observed winds), and through the use of a common longshore sediment transport formula. Seasonal variations in wind climate produced changes in the transport trends and gradients that agree with migration and rotation patterns observed for this barrier island system. Results suggest that wind dominance produces seasonal oscillations that cause an imbalance in the resulting transport gradients that over time are responsible for higher rates of transport in the northward direction. These results and data from other works verify the evolutionary model previously suggested, and qualitatively confirm the recent observations in asymmetric shoreline erosion. [ABSTRACT FROM AUTHOR]

This paper depicts a simulation-based assessment of sediment quality on the performance of dedicated dredging projects for barrier island restoration in coastal Louisiana, USA. The research involved the development and integration of two sub-models. In the first, geomorphic modeling was used to simulate sediment transport dynamics within a proxy barrier island template over a 50-year trajectory. The template was assumed to be nourished with one of two sources of dredged material: nearshore (NS) sediments of lower quality (smaller grain diameter, higher organic fines); or higher quality sediments from distal sources located on the Outer Continental Shelf (OCS). In the second model, agency project records and commercial bids were used to estimate project construction costs as a function of dredge material quantity, transport distance, and project target elevation. These sub-models were coupled within a net present value framework from which average annual break-even values for ecosystem services (EBEV) were derived as an efficiency metric for comparing the economic performance of NS- and OCS-sourced projects. Results indicate that in some cases, the physical resiliency afforded by even small increases in sand diameter (+4 μm d50) can translate to greater long-term economic viability (lower EBEV) for OCS-sourced sediment transported over longer distances. Moreover, projects constructed with much higher diameter OCS sediment (+44 μm d50) with low fines and transported over relatively long distances (200 μm, 5% fines, 15–20 miles) were found to be more cost-effective than all comparably-sized projects constructed with lower quality NS sediments obtained from proximal sources (156 μm, 20% fines, 3–5 miles). For some comparisons, this quality advantage yielded a lower EBEV for OCS-sourced projects with transport distances exceeding 30 miles. Under storm-punctuated simulations, these quality advantages were more pronounced, with greater physical and economic implications for earlier (Y 5) versus later (Y 20) occurring storms. Budgeting for dedicated dredging projects has traditionally centered on the value of sediment as a commodity, with a focus on material placement cost. The findings of this study, however, indicate that a more comprehensive accounting of sediment quality and performance is required to maximize the economic efficiency of coastal restoration spending. • This paper examines the economic performance of nearshore and offshore sediments for the renourishment of barrier islands and shorelines. • Geomorphic and economic sub-models are combined to simulate the benefits and costs of a large-scale, dedicated dredging project. • Ecosystem break-even values are derived as an efficiency metric for examining the economic tradeoffs of source material attributes over time. • In many case comparisons, the performance advantages of larger diameter sands are found to offset their higher costs for extraction and transport. [ABSTRACT FROM AUTHOR]

The Great Marsh in northern Massachusetts and southern New Hampshire is the largest salt marsh in New England and includes the marsh surrounding Plum Island Sound, one of the most studied coastal wetlands in the United States. A primary goal in marsh research today is predicting their resilience in a regime of sea-level rise, which has become increasingly based on modeling studies with sparse or site-specific field data input. Here, we present the results of a vertical accretion study using 210Pb and 137Cs geochronology developed from 50-cm long auger cores taken at 15 locations throughout the PIS marsh. Two significant outliers were omitted from the final analyses due to their unusual depositional settings. The 137Cs spike produced the shortest-term record and lowest average accretion rate (2.5 mm/yr). Of the various 210Pb accretion models (Constant Flux-Constant Sedimentation: CF-CS, Constant Rate of Supply: CRS, Constant Initial Concentration: CIC), we chose the CF-CS model accretion rate for comparison to other environmental parameters and future marsh projections due to the robust R2 linear regression values and its wide use. The 13 stations exhibited an average accretion rate of 3.13 mm/yr, which is 0.6 mm/yr greater than the average rate reported elsewhere in New England. Individual accretion rates show no correlation with geography, elevation, bulk density, loss on ignition, distance to tidal channels, or peat thickness, indicating the complexity of the interacting factors that control marsh growth. Using moderate sea-level rise predictions, we show that large portions of the high marsh are likely to be converted to low marsh by 2050, and that the entire marsh high platform will become low marsh prior to 2070. This is the first time that wide spread field data are used to project a major transformation of the Plum Island Sound platform marsh that undoubtedly will lead to dramatic changes to marsh drainage, backbarrier hypsometry, tidal current dominance, and redistribution of sediment reservoirs, that will include the fronting barrier islands. [ABSTRACT FROM AUTHOR]