Distributed Acoustic Sensing (DAS) is a new method for recording oceanographic processes using seafloor fiber-optic cables, such as telecommunication cables. DAS returns spatially distributed measurements of cable strain, which can be related to hydrodynamic pressure, turning a submarine cable into a dense sampling array. A reinforced fiber-optic cable was installed in the cross-shore from the dune toe to similar to 15-m-water depth at the USACE Field Research Facility in Duck, NC to quantitatively compare DAS strain to co-located pressure sensors. We develop a methodology for transferring DAS strain to dynamic pressure and evaluate the performance of DAS to measure shallow- and intermediate-water waves in the incident band (0.04-0.4 Hz). A frequency-dependent empirical transfer function from DAS strain to dynamic pressure at the seabed is derived from the ratio of strain and pressure power spectra. DAS-derived significant wave heights and peak periods were consistent with pressure sensors (typical rmse = 0.2 m and 1 s) over a wide range of dynamic conditions (0.24-4 m wave heights and 3-18 s periods). DAS data were input into the cBathy algorithm to calculate cross-shore bathymetric profiles and were used to calculate cross-shore wave reflection. Preliminary applications of DAS to record wave splitting and shoreline position were explored. With this field data, we demonstrate new applications of DAS for measuring nearshore processes and encourage further exploration. The promising potential of DAS lies in the near-bed data volume provided, real-time capability, and ability to sample in any weather or light.

Effective coastal management requires a fundamental understanding of the impacts of anthropogenic activities on sediment dynamics, yet it is challenging to isolate individual impacts in heavily altered regions. The Ayeyarwady Delta, Myanmar has been extensively deforested for agriculture but has few levees/polders. In this study, the relative resilience to subsidence was compared between a 45‐year‐old agricultural field and a nearby mangrove‐forest preserve. At both sites, water velocity and turbidity were measured in tidal channels, topography was mapped, and sediment cores were collected during 2018–2019. There was net sediment import at both sites due to sediment trapping by vegetation. Relative elevations were equivalent, suggesting that the field has aggraded at rates similar to the forest (0.7 cm/year). Unleveed fields may be less vulnerable to subsidence than leveed fields. However, uncertainties remain and the decision to replace mangroves with agricultural fields should weigh all the benefits provided by each environment.

Vegetated mid-channel islands play an important though poorly understood role in the sediment dynamics and morphology of tide-dominated deltas. Meinmahla Island is a mangrove-forest preserve at the mouth of the Bogale distributary channel, in the Ayeyarwady Delta, Myanmar. In this relatively unaltered mid-channel island, sediment dynamics can be directly connected to morphology. Field measurements from 2017 to 2019 provide insight into the pathways for sediment transport and resulting morphological evolution. Water depth, salinity and turbidity were monitored semi-continuously, and velocity profilers with turbidity and salinity sensors were deployed seasonally in single-entrance (dead-end/blind) and multi-entrance tidal channels of the island. The morphological evolution was evaluated using grain size, Pb-210 geochronology, remote sensing and channel surveys. The data show that ebb-dominant, single-entrance channels along the island exterior import sediment year-round to the land surface. However, these exterior channels do not deliver enough sediment to maintain the observed ca 0.8 cm/yr accretion rate, and most of the sediment import occurs via interior, multi-entrance channels. Interior channels retain water masses that are physically distinct from the water in the Bogale distributary, and estuarine processes at the tidal-channel mouths import sediment into the island. Sediment is sourced to the island from upriver in the wet season and from the Gulf of Mottoma in the dry season, as the location of the estuary shifts seasonally within the Bogale distributary. The salinity and biogeochemistry of the distributary water are affected by interactions with sediment and groundwater in the island interior. The largest interior channels have remained remarkably stable while the island has aggraded and prograded over decadal timescales. However, the studied multi-entrance channel is responding to a drainage-network change by narrowing and shoaling. Overall, mid-channel islands trap sediment and associated nutrients at the river-ocean interface, and these resilient landscape features evolve in response to changes in drainage-network connectivity.

Unlike many parts of the world, mangrove coverage has been steadily increasing in Aotearoa, New Zealand since the 1900s. Intentional mangrove removal in Aotearoa New Zealand is often motivated by a desire to remove muddy sediment from sites that were once sand-dominated. Mangrove removal can result in geomorphic and ecologic evolution over decadal timescales, yet monitoring of removal sites is often limited to a few years. This study presents the result of over a decade of monitoring in Waikaraka Estuary, a quiescent embayment where mangroves were removed between 2003 and 2012. Surface elevation was monitored between 2006 and 2019, and sediment cores and surface samples collected in 2019 were analysed for grain size, total organic content, and root biomass to assess geomorphic change. Remaining mangroves areas and benthic macrofauna were surveyed throughout the estuary to assess ecologic change. Despite initially rapid movement of mud out of cleared areas, the estuary has experienced little bed-elevation change over a decade. The upper estuary remains muddy, and much of the root biomass has not decomposed. However, the region near the mouth of the estuary has become sandier with the return of bivalve species Austrovenus stutchburyi (cockles) and Macomona liliana. These long-term measurements demonstrate the slow pace of recovery following mangrove removal in quiescent embayments. Without major changes to hydrodynamics, mud is unlikely to be flushed out and bivalve habitat is unlikely to be restored even after a decade. Site-specific characteristics should be assessed when evaluating the efficacy of mangrove removal for estuary restoration, to moderate community expectations.

Estuaries along the Amazonian coast are subjected to both a macrotidal regime and seasonally high fluvial discharge, both of which generate complex circulation. Furthermore, the Amazon River Plume (ARP) influences coastal circulation and suspended sediment concentrations (SSCs). The Gurupi estuary, located south of the mouth of the Amazon River, is relatively unstudied. This study evaluates how the Gurupi estuary dynamics respond to seasonal discharge and the varying influence of the ARP using cross-sectional and longitudinal surveys of morphology, hydrodynamics, and sediment transport. The Gurupi was classified as a tide-dominated estuary based on morphology and mean hydrodynamic conditions. However, the estuary was only partially mixed during both the wet and dry seasons. The tides propagated asymmetrically and hypersynchronously, with flood dominance during the dry season and ebb dominance during the rainy season. Seasonal variations of the ARP did not significantly affect the hydrodynamic structure of the lower Gurupi estuary. Estuarine turbidity maxima (ETM) were observed in both seasons, although the increase in fluvial discharge during the wet season attenuated and shifted the ETM seaward. Little sediment was delivered to the estuary by the river, and the SSCs were higher at the mouth in both seasons. Sediment was strongly imported during the dry season by tidal asymmetry. The morphology, hydrodynamics, and sediment dynamics all highlight the importance of considering both fluvial discharge and coastal influences on estuaries along the Amazon coast.

Mangroves are often considered integral engineers of morphologic evolution, but mangroves can also opportunistically respond to morphologic change created by abiotic sedimentary processes. Consequently, predicting the response of individual estuarine environments to changes in mangrove extent is challenging. Here, the impact of mangrove extent was explored using in-situ observations and numerical modeling of the quiescent Waikaraka Estuary in Tauranga Harbor, Aotearoa, New Zealand. Mangrove coverage expanded in the estuary from 1940 until a removal program began in 2005 with the goal of reducing fine sediment and restoring bivalve habitat. In June 2019 water velocity, turbidity, wave height, and bed-sediment grain size were measured at multiple sites to identify how sediment dynamics in the estuary were responding to removal. Flow in the predominantly sandy lower estuary was ebb dominant, resulting in net sediment export, while flow in the muddy upper estuary was weakly flood dominant, resulting in sediment import and retention. Therefore, fine sediment is unlikely to be flushed out of quiescent estuaries after mangrove removal. A Delft3D numerical model calibrated with in-situ data showed that tidal asymmetry, velocity skew, and peak ebb-tide shear stress were not significantly altered by varying the mangrove extent between the 2005 maximum coverage and complete removal. Mangroves did not significantly impact flow in this system; instead, the hydrodynamics and net sediment transport were controlled by tidal interactions with bathymetry. In model runs with mangroves covering all tidal flats, the water velocity on the intertidal flats decreased while the peak ebb-tide velocity in the main channel increased, indicating that the potential for fine-sediment export may actually decrease with mangrove removal. These results emphasize the role of mangroves as opportunistic colonizers, not engineers, and highlight the importance of considering site specific parameters when planning mangrove removals.

The largest deltas on Earth are tide‐dominated with multiple distributaries, which can experience different forcings. Yet, most observational studies of sediment dynamics in these systems focus on a single distributary. Comparison of the tidal‐to‐estuarine reaches of three representative distributaries of the Ayeyarwady Delta, Myanmar highlights the variability in sediment retention and export within a megadelta. The Ayeyarwady River has no mainstem dams, providing an opportunity to examine a relatively natural, monsoon‐dominated system. Observations were collected during high flow (September 2017) and low flow (March 2018 and 2019). The eastern Yangon distributary empties into the sheltered Gulf of Mottoma while the central Bogale and western Pathein distributaries are exposed to seasonal waves. During high flow, suspended‐sediment concentration (SSC) was consistent across all three distributaries (0.2–0.4 g/L). The upper delta retained 20%–60% of the mainstem sediment load, while the lower distributaries were tidal rivers exporting sediment. During low flow, the lower distributaries were partially mixed estuaries with little net discharge. However, SSC increased to >1 g/L in the Yangon distributary, remained constant in the Bogale distributary, and decreased in the Pathein distributary compared to high flow. This difference was driven by offshore sediment supply; fine sediment is retained near the mouth of the Yangon and advected away from the Pathein distributary. Consequently, nearshore sediment residence time in monsoon‐influenced megadeltas is controlled by coastal orientation, the timing of discharge with respect to ocean conditions, and tidal amplification. Changes to fluvial sediment delivery can cause non‐uniform coastal impacts due to these differing distributary processes.

In the original article the following disclaimer is missing.

As ice retreated from southern coastal Maine from 15,000 to 13,000 years ago, the ocean inundated coastal areas where the earth’s crust had been temporarily depressed by the weight of glacial ice. A thick veneer of glacial-marine clay and silt – the Presumpscot Formation – was deposited in coastal Maine lowlands. Due to its fine-grained character, shear waves from seismic events travel slowly through the Presumpscot Formation in comparison to other surficial sedimentary units. Low shear-wave velocities result in amplification of seismic waves, potentially increasing local seismic hazard. Prior study demonstrated that careful assignment of National Earthquake Hazards Reduction Program (NEHRP) site classifications to surficial geologic units based on shear velocities, can greatly improve earthquake loss estimations using programs like HAZUS-MH, particularly when compared with estimates using default and proxy values. The October 16, 2012 magnitude 4.0 earthquake in Hollis, Maine, provided another opportunity to assess the influence of the Presumpscot Formation on seismic hazard. Occurring in early evening, this strongest Maine earthquake in nearly 40 years was widely felt across the region. The Hollis area is near the transition between the western mountains which are mostly underlain by till, and the lowlands, mostly underlain by marine sand and mud. Using the USGS database of more than 2,000 “Did you feel it?” geolocated responses in southern Maine, we tested whether respondents experienced different intensities of ground shaking depending on substrate. When normalized by population density within each surficial unit, we found no statistical difference in respondents’ experiences. However, when normalized for areal extent of each unit, we found that more people responded in areas underlain by NEHRP class ‘E’ materials (including the Presumpscot Formation) than for other classes. Our results suggest potentially greater intensity of ground shaking and seismic hazard in areas underlain with sediment of the Presumpscot Formation.