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At least 973 records · Page 54Linked to original sources
USGS2020

Peak ground velocity spatial variability revealed by dense seismic array in southern California

Understanding and modeling variability of ground motion is essential for building accurate and precise ground motion prediction equations, which can net site‐specific characterization and reduced hazard levels. Here, we explore the spatial variability in peak ground velocity (PGV) at Sage Brush Flats along the San Jacinto Fault in southern California. We use data from a dense array (0.6 x 0.6 km 2 , 1,108 geophones, station spacings 10‐30 m) deployed in 2014 for ~1‐month. These data offer an opportunity to study small scale variability in this region. We examine 38 2≤M L ≤4.2 earthquakes within 200 km of the array. Fault strands and a small basin impact the ground motions, producing PGV variations up to 22% of the mean and a 40% reduction in P and S wave near‐surface velocities. We find along‐fault rupture directivity, source, and path effects can increase PGVs by 167%. Surface PGV measurements exceed the co‐located borehole station (depth 148 m) PGV by factors of 3‐10, confirming the impact on PGV from near surface fault structures, basins, topography, and amplifications from soft sediments. Consistently we find high PGVs within the basin structure. A pair of ~co‐located M L 2.6 events produce repeatable PGV values with similar spatial patterns. The average corner frequencies of these two events are 11‐16 Hz and viable measurements of stress drop can differ by 6.45MPa. Within this small array, the PGV values are variable implying spatial extrapolation of PGV to regions of known faults and basins, even across a small area, should be done with caution.

California↗
USGS2019

Hydrodynamic and morphologic response of a back-barrier estuary to an extratropical storm

We investigated the hydrodynamic and morphologic response of Barnegat Bay-Little Egg Harbor, New Jersey, USA to Hurricane Sandy. We implemented a three-dimensional, coupled ocean-wave-sediment transport model of the estuary and explored the role of offshore water levels, offshore waves, local winds and waves by systematically removing forcings from a series of simulations. Offshore water levels had the largest impact on water levels in the bay while waves and local wind forcing created substantial spatial variation along the longitudinal axis of the bay. The shape of the bay and its orientation relative to the storm track influenced the response to winds and restricted the maximum water levels in the northern bay and reduced the maximum volume of surge. Basin-average hydrodynamic residence time was reduced by 40%, though its typical spatial distribution remained during the storm. Wave and current-induced bed shear stress resuspended fine sediment resulting in net erosion from the shoals with ensuing net deposition over fringing low-lying land. The net sediment exchange between the bay and the ocean was several times smaller than the exchange at the peak of the storm resulting in negligible net change in the bay volume. Overall, our results suggest that water level responses are highly sensitive to the specific orientation of storm winds relative to the estuary, thereby limiting the utility of simple inundation models. The sediment transport patterns indicate that storms are an important mechanism for redistributing sediment from shoals to fringing wetlands, while net change to sediment budget can be negligible.

New Jersey↗
USGS2020

Seasonal variation in sediment delivery across the bay-marsh interface of an estuarine salt marsh

Sediment transport across bay–marsh interfaces depends on wave energy, vegetation, and marsh-edge morphology, and varies over a range of timescales. We investigated these dynamics in a tidal salt marsh with a gently-sloped, vegetated edge adjacent to northern San Francisco Bay. Spartina foliosa (cordgrass) inhabits the lower marsh and Salicornia pacifica (pickleweed) predominates on the marsh plain. We measured suspended-sediment concentration (SSC) and hydrodynamics in bay shallows and along a 100-m cross-shore transect in the marsh, during winter and summer. Four-year averaged accretion measured with marker-horizon plots was twice as great along the marsh transect as adjacent to a tidal creek, 50 m from the bay. We estimated deposition and trapping efficiency from the time-series data to assess its variation with season and wave energy. At high tide the transition zone (between cordgrass and pickleweed) was usually erosional, the pickleweed zone was depositional, and both erosion and deposition increased with wave energy, as did the landward position of maximum deposition. Erosion from the transition zone accounted for approximately one-third of the sediment flux into the pickleweed. In the pickleweed zone, SSC, the difference between flood- and ebb-tide SSC and trapping efficiency were greater in summer than winter for comparable wave conditions, which we attribute to increased sediment trapping by dense summer cordgrass. Moderate waves in summer (46%) accounted for more annual accretion in the pickleweed zone than larger waves in winter (28%), although the contribution of winter storms was diminished by the dry winter during the study.

California↗
USGS2020

Alkalinity in tidal tributaries of the Chesapeake Bay

Despite the important role of alkalinity in estuarine carbon cycling, the seasonal and decadal variability of alkalinity, particularly within multiple tidal tributaries of the same estuary, is poorly understood. Here we analyze more than 26,000 alkalinity measurements, mostly from the 1980s and 1990s, in the major tidal tributaries of the Chesapeake Bay, a large, coastal-plain estuary of eastern North America. The long-term means of alkalinity in tidal-fresh waters vary by a factor of 6 among seven tidal tributaries, reflecting the alkalinity of non-tidal rivers draining to these estuaries. At 25 stations, mostly in the Potomac River Estuary, we find significant long-term increasing trends that exceed the trends in the non-tidal rivers upstream of those stations. Box model calculations in the Potomac River Estuary indicate that the main cause of the estuarine trends is a declining alkalinity sink. The magnitude of this sink is consistent with a simple model of calcification by the invasive bivalve Corbicula fluminea. More generally, in tidal tributaries fed by high-alkalinity non-tidal rivers, alkalinity is consumed, with sinks ranging from 8 to 27% of the upstream input. In contrast, tidal tributaries that are fed by low-alkalinity non-tidal rivers have sources of alkalinity amounting to 34 to 171% of the upstream input. For a single estuarine system, the Chesapeake Bay has diverse alkalinity dynamics and can thus serve as a laboratory for studying the numerous processes influencing alkalinity among the world’s estuaries.

Maryland, Virginia↗
USGS2020

Surface methane concentrations along the mid-Atlantic bight driven by aerobic subsurface production rather than seafloor gas seeps

Relatively minor amounts of methane, a potent greenhouse gas, are currently emitted from the oceans to the atmosphere, but such methane emissions have been hypothesized to increase as oceans warm. Here, we investigate the source, distribution, and fate of methane released from the upper continental slope of the U.S. Mid-Atlantic Bight, where hundreds of gas seeps have been discovered between the shelf-break and ~1600 m water depth. Using physical, chemical, and isotopic analyses, we identify two main sources of methane in the water column: seafloor gas seeps and in situ aerobic methanogenesis which primarily occurs at 100 – 200 m depth in the water column. Stable isotopic analyses reveal that water samples collected at all depths were significantly impacted by aerobic methane oxidation, the dominant methane sink in this region, with more than 50% of the methane being oxidized, on average. Due to methane oxidation in the deeper water column, below 200 m depth, surface concentrations of methane are influenced more by methane sources found near the surface (0 – 10 m depth) and in the subsurface (10 - 200 m depth), rather than seafloor emissions at greater depths.

Atlantic margin↗
USGS2019

Assessing the seasonal dynamics of nitrate and sulfate aerosols at the South Pole utilizing stable isotopes

Atmospheric nitrate (NO 3 − = particulate NO 3 − + gas‐phase nitric acid [HNO 3 ]) and sulfate (SO 4 2− ) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO 3 − and total suspended particulate sulfate (SO 4 2− (TSP) ) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO 3 − was largely affected by snowpack emissions in which [NO 3 − ] and δ 15 N(NO 3 − ) were highest (49.3 ± 21.4 ng/m 3 , n = 8) and lowest (−47.0 ± 11.7‰, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Δ 17 O(NO 3 − ) reflected tropospheric chemistry year‐round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HO x ‐ to O 3 ‐dominated oxidation chemistry. SO 4 2− (TSP) concentrations were highest during austral summer and fall (86.7 ± 73.7 ng/m 3 , n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as δ 34 S(SO 4 2− ) (TSP) values (18.5 ± 1.0‰, n = 10) were similar to literature δ 34 S(DMS) values. The seasonal cycle of Δ 17 O(SO 4 2− ) (TSP) exhibited minima during austral summer (0.9 ± 0.1‰, n = 5) and maxima during austral fall (1.3 ± 0.3‰, n = 6) and austral spring (1.6 ± 0.1‰, n = 5), indicating a shift from HO x ‐ to O 3 ‐dominated chemistry in the atmospheric derived SO 4 2− component. Overall, the budgets of NO 3 − and SO 4 2− (TSP) at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

Journal of Geophysical Research D: Atmospheres↗
USGS2020

A lacustrine paleoenvironment recorded at Vera Rubin ridge, Gale crater: Overview of the sedimentology and stratigraphy observed by the Mars Science Laboratory Curiosity rover

For ~500 Martian solar days (sols), the Mars Science Laboratory team explored Vera Rubin ridge (VRR), a topographic feature on the northwest slope of Aeolis Mons. Here we review the sedimentary facies and stratigraphy observed during sols 1,800–2,300, covering more than 100 m of stratigraphic thickness. Curiosity's traverse includes two transects across the ridge, which enables investigation of lateral variability over a distance of ~300 m. Three informally named stratigraphic members of the Murray formation are described: Blunts Point, Pettegrove Point, and Jura, with the latter two exposed on VRR. The Blunts Point member, exposed just below the ridge, is characterized by a recessive, fine-grained facies that exhibits extensive planar lamination and is crosscut by abundant curvi-planar veins. The Pettegrove Point member is more resistant, fine-grained, thinly planar laminated, and contains a higher abundance of diagenetic concretions. Conformable above the Pettegrove Point member is the Jura member, which is also fine-grained and parallel stratified, but is marked by a distinct step in topography, which coincides with localized meter-scale inclined strata, a thinly and thickly laminated facies, and occasional crystal molds. All members record low-energy lacustrine deposition, consistent with prior observations of the Murray formation. Uncommon outcrops of low-angle stratification suggest possible subaqueous currents, and steeply inclined beds may be the result of slumping. Collectively, the rocks exposed at VRR provide additional evidence for a long-lived lacustrine environment (in excess of 10 6 years via comparison to terrestrial records of sedimentation), which extends our understanding of the duration of habitable conditions in Gale crater.

Journal of Geophysical Research: Planets↗
USGS2020

The chemostratigraphy of the Murray formation and role of diagenesis at Vera Rubin ridge in Gale crater, Mars, as observed by the ChemCam instrument

Geochemical results are presented from Curiosity’s exploration of the Vera Rubin ridge (VRR), in addition to the full chemostratigraphy of the predominantly lacustrine mudstone Murray formation up to and including VRR. VRR is a prominent ridge flanking Aeolis Mons (informally Mt. Sharp), the central mound in Gale crater, Mars, and was a key area of interest for the Mars Science Laboratory mission. ChemCam data show that VRR is overall geochemically similar to lower-lying members of the Murray formation, even though the top of VRR is characterized by strong hematite spectral signatures as observed from orbit. While overall geochemically similar, VRR is characterized by a prominent decrease in Li abundance and Chemical Index of Alteration across the ridge. This decrease follows the morphology of the ridge rather than elevation and is inferred to reflect a non-depositionally controlled decrease in clay mineral abundance in VRR rocks. Additionally, a notable enrichment in Mn above baseline levels is observed on VRR. While not supporting a single model, the results suggest that VRR rocks were likely affected by multiple episodes of post-depositional groundwater interactions that made them more erosionally resistant than surrounding Murray rocks – thus resulting in the modern-day ridge after subsequent erosion.

Journal of Geophysical Research: Planets↗
USGS2019

Combining numerical and statistical models to predict storm-induced dune erosion

Dune erosion is an important aspect to consider when assessing coastal flood risk, as dune elevation loss makes the protected areas more susceptible to flooding. However, most advanced dune erosion numerical models are computationally expensive, which hinders their application in early-warning systems. Based on a combination of probabilistic and process-based numerical modeling, we develop an efficient statistical tool to predict dune erosion during storms. The analysis focuses on Dauphin Island, AL in the northern Gulf of Mexico, where we combine synthetic sea storms with a calibrated and validated XBeach model to develop and test a range of different surrogate models for their ability to predict barrier-island geometric parameters under storm conditions. Surrogate models are developed by combining the oceanographic forcing from 100 optimally sampled sea-storm events covering the entire multivariate parameter space (used as XBeach input) and associated changes in the dune system (XBeach output). We test four surrogate models using a k-fold approach for validation. All models perform well in predicting changes in dune elevation, barrier-island area, and width, but are less accurate in predicting alterations in the cross-shore locations of dune morphological features. Multivariate adaptive regression splines are identified as the best surrogate model based on their fast development and good performance, attaining a modified Mielke index of 0.81 for dune crest height. As demonstrated at Dauphin Island, our approach shows potential to be used in an operational framework to predict dune response (in particular crest elevation change) when water level and wave forecasts are available.

Alabama↗
USGS2019

Widespread initiation, reactivation, and acceleration of landslides in the northern California Coast Ranges due to extreme rainfall

Episodically to continuously active slow-moving landslides are driven by precipitation. Climate change, which is altering both the frequency and magnitude of precipitation world21 wide, is therefore predicted to have a major impact on landslides. Here we examine the behavior of hundreds of slow-moving landslides in northern California in response to large changes in annual precipitation that occurred between 2016 and 2018. We quantify the landslide displacement using repeat-pass radar interferometry and pixel offset tracking techniques on a novel dataset from the airborne NASA/JPL Uninhabited Aerial Vehicle Synthetic Aperture Radar. We found that 312 landslides were moving due to extreme rainfall during 2017, compared to 119 during 2016, which was the final year of a historic multi-year drought. However, with a return to below average rainfall in 2018, only 146 landslides remained in motion. The increased number of landslides during 2017 was primarily accommodated by landslides that were smaller than the landslides that remained active between 2016 and 2018. Furthermore, by examining a subset of 51 landslides, we found that 49 had increased velocities during 2017 when compared to 2016. Our results show that slow-moving landslides are sensitive to large changes in annual precipitation, particularly the smaller and thinner landslides that likely experience larger basal pore water pressure changes. Based on climate model predictions for the next century in California, which include increases in annual precipitation and increases in the frequency of dry-to-wet extremes, we hypothesize that there will be an overall increase in landslide activity.

California↗
USGS2020

Causes of variability in suspended‐sand concentration evaluated using measurements in the Colorado River in Grand Canyon

Rivers commonly exhibit substantial variability in suspended‐sand concentration, even at constant water discharge. Here we derive an approach for evaluating how much of this variability arises from mean bed‐sand grain size. We apply this approach to the Colorado River in Grand Canyon, where discharge‐independent concentration of suspended sand varies by more than a factor of 23 ( N = 1.4 × 10 6 ). Theory predicts that where concentration is controlled by bed‐sand grain size, concentration and grain size in suspension will be inversely correlated (i.e., coarsening of the bed causes suspended sand to become coarser in grain size and lower in concentration). Although the observed correlation is negative, riverbed grain size accounts for only 40% of the variability in concentration. The residuals vary by an order of magnitude; they arise from other processes, such as changes in topography or distribution of sand that cause shear stress to change at constant discharge, changes in the fine tail of bed‐sand grain sizes or changing bedforms. Both bed sand and the other factors influence concentration for durations from less than 1 day to several years. Predictions of concentration based on bed‐sand grain size ( N = 4 × 10 4 ) are less accurate than predictions based on suspended‐sand grain size, probably because suspended sand is a natural integrator of sand‐transporting processes, giving more weight to those areas of the bed that exchange more sand with the flow. Although the causes of variability vary from one river to another, the approach illustrated here is applicable to any river in which concentration varies at constant water discharge.

Arizona↗
USGS2020

Water tracks enhance water flow above permafrost in upland Arctic Alaska hillslopes

Upland permafrost regions occupy approximately one third of the Arctic landscape. In upland regions, hydrologic fluxes are influenced by water tracks, curvilinear features on hillslopes that preferentially fill with and route water in response to snowmelt and rainfall when the soil above continuous permafrost thaws in the summer. As continued warming of the Arctic may alter hydrologic cycling leading to increased frequency of extreme hydrologic events like drought and flooding as well as modification of biogeochemical cycling, it is imperative to untangle the interplay between precipitation, runoff, and subsurface flow as water is routed from upland Arctic regions to the Arctic Ocean. This study quantifies how ground surface temperatures affect groundwater discharge from hillslopes with water tracks in the upland Arctic by employing a three-dimensional, physically based subsurface flow model with variable saturation and freeze and thaw capabilities that is calibrated to field measurements from the Upper Kuparuk River watershed on the North Slope of Alaska, USA. Model analysis indicates that higher ground surface temperatures along water track hillslopes promote increases in groundwater discharge where water tracks act as conduits for large recharge events and continue to discharge groundwater into the autumn after the adjacent hillslope has frozen. Simulating the conditions that distinguish water tracks from their hillslope watersheds changes subsurface water storage and ground thermal responses but does not alter the total magnitude of groundwater discharge outside of parameter uncertainty. These findings suggest that water tracks play a complex and critical role in hydrologic cycles of the upland Arctic.

Alaska↗
USGS2020

Seasonal subsurface thaw dynamics of an aufeis feature inferred from geophysical methods

Aufeis are sheets of ice unique to cold regions that originate from repeated flooding and freezing events during the winter. They have hydrological importance associated with summer flows and possibly winter insulation, but little is known about the seasonal dynamics of the unfrozen sediment layer beneath them. This layer may support perennial groundwater flow in regions with otherwise continuous permafrost. For this study, ground penetrating radar (GPR) were collected in September 2016 (maximum thaw) and April 2017 (maximum frozen) at the Kuparuk aufeis field on the North Slope of Alaska. Supporting surface nuclear magnetic resonance (NMR) data were collected during the maximum frozen campaign. These point-in-time geophysical data sets were augmented by continuous subsurface temperature data and periodic Structure-from-Motion digital elevation models (DEM) collected seasonally. GPR and difference DEM data showed maximum ice thicknesses of up to 6 m over the sediment surface. Below the ice, GPR and NMR identified regions of permafrost and regions of seasonally frozen sediment (i.e., the active layer) underlain by a substantial perennially unfrozen zone or “lateral talik” that ranged from 0 m to over 13 m thick. The seasonally frozen cobble layer above the talik was typically 3 to 5 m thick, with freezing apparently enabled by relatively high thermal diffusivity of the overlying ice and rock cobbles. The large talik beneath the aufeis and active layer suggests that year-round groundwater flow and coupled heat transport occurs beneath much of the feature. Highly permeable alluvial material and discrete zones of apparent groundwater upwelling indicated by geophysical and ground temperature data allows direct connection between the aufeis and the talik below.

Alaska↗
USGS2020

Reconstructing the velocity and deformation of a rapid landslide using multiview video

Noncontact measurements of spatially varied ground surface deformation during landslide motion can provide important constraints on landslide mechanics. Here, we present and test a new method for extracting measurements of rapid landslide surface displacement and velocity (accelerations of approximately 1 m/s 2 ) using sequences of stereo images obtained from a pair of inexpensive, stationary 4K video cameras with nominal frame rates of 29.97 Hz. The method combines elements of Structure from Motion with those of optical flow to extract data on 3‐D evolution of the ground surface during slope failure. We apply the method to an experiment at the U.S. Geological Survey debris‐flow flume in which a high‐speed, liquefying landslide was triggered by gradually adding water to a 6‐m 3 prism of loosely packed sediment on a 31° slope. Strip‐scanning lidar measurements made during the experiment corroborate our video‐based measurements, but the latter encompassed the entire landslide surface and were much lower in cost. Our video‐based measurements enabled computation of depth‐integrated landslide dilation/contraction rates. The range of computed rates was within the ranges inferred from independent measurements of evolving pore water pressures and reasonable estimates of the hydraulic permeability of the sediment. Dilation and contraction rates play a crucial role in landslide mechanics. The dilation and contraction we observe contradict the incompressible flow assumption used in many studies that have employed noncontact methods to infer landslide properties.

Oregon↗
USGS2020

The influence of frost weathering on the debris flow sediment supply in an alpine basin

Rocky, alpine mountains are prone to mass wasting from debris flows. The Chalk Cliffs study area (central Colorado, USA) produces debris flows annually. These debris flows are triggered when overland flow driven by intense summer convective storms mobilizes large volumes of sediment within the channel network. Understanding the debris flow hazard in this, and similar alpine settings, requires determining the magnitude of sed- iment accumulation between debris flow seasons, and identifying the control on sediment production. To address these knowledge gaps, we measured changes in sediment produc- tion using a sediment retention fence to quantify how sedimentation was influenced by temperature at the plot scale. These measurements were extrapolated to a larger area, where we extended the sediment fence results to explore how rockfall sedimentation con- tributed to channel refilling between active debris flow periods. This work shows debris flow channel refilling is correlated with low temperatures and time in the frost-cracking window, implicating frost weathering mechanisms as a key driver of sedimentation. This sediment production process resulted in a large amount of sediment accumulation dur- ing a single winter season in our study reach (up to 0.4 m in some locations). Using these observations, we develop a channel refilling model that generally describes the mass bal- ance of debris flow watersheds in alpine areas.

Colorado↗
USGS2020

Does channel narrowing by floodplain growth necessarily indicate sediment surplus? Lessons from sediment‐transport analyses in the Green and Colorado rivers, Canyonlands, Utah

Analyses of suspended sediment transport provide valuable insight into the role that sediment supply plays in causing geomorphic change. The sediment supply within a river system evolves depending on the discharge, flood frequency and duration, changes in sediment input, and ecohydraulic conditions that modify sediment transport processes. Changes in supply can be evaluated through analyses of coupled changes in suspended sediment concentration and grain size. The concentration of sand in transport in the Green and Colorado Rivers is most strongly controlled by discharge and the bed sand grain size distribution. Since the 1950s, sand loads have decreased in response to declines in peak discharge in the Green River and coarsening of the bed sand in the Colorado River. However, changes in the bed sand grain size distribution are associated with large changes in suspended sand concentration in both rivers; concentration varies by a factor of ~3 in the Green River and a factor of ~8 in the Colorado River, depending on the bed sand grain size distribution. Analyses of hysteresis in suspended sediment measurements show that sediment depletion during annual floods is most strongly controlled by flood duration, with peak discharge being nearly equally important in the Green River. Despite channel narrowing in both rivers, periods of bed sand coarsening and sediment depletion during annual floods indicate that these rivers are not necessarily in sediment surplus. Channel narrowing appears to be strongly controlled by short‐term declines in flood magnitude and the ecohydraulic effects of vegetation and may not be indicative of the long‐term sediment budget.

Utah↗
USGS2020

Atmospheric dust deposition varies by season and elevation in the Colorado Front Range, USA

As atmospheric dust deposition continues to increase across the southwestern United States, it has the potential to alter ecosystem productivity and structure by delivering nutrients, base cations, and pollutants to remote mountain sites. Due to the sparse distribution of dust monitoring sites, open questions remain about the spatial and temporal variability of dust fluxes and composition across mountainous terrain. We present a 1 year (November 2017 to November 2018) record of seasonal dust fluxes and composition from an elevation transect across the Colorado Front Range extending from the urban plains to the remote alpine. At all nine sites, dust was enriched in the essential nutrient phosphorus and the metals copper, zinc, lead, and cadmium, elements that are enriched in dust deposited at sites across the Rocky Mountain West. We observed a seasonal pattern in dust composition, with the highest concentrations of zinc and cadmium during the summer, when back trajectory modeling suggested a greater contribution of dust from local urban and agricultural regions to the east of the collection sites. During the summer, there was also a trend of higher dust fluxes at lower elevations; dust fluxes ranged from 18.9 ± 0.1 g m −2 yr −1 on the plains to 5.9 ± 0.2 g m −2 yr −1 in the alpine. Our results suggest that urban and agricultural land east of the Colorado Front Range is an important source of nutrients and pollutants to all elevations of the mountain range.

Colorado↗
USGS2020

Sea‐level rise will drive divergent sediment transport patterns on fore reefs and reef flats, potentially causing erosion on atoll islands

Atoll reef islands primarily consist of unconsolidated sediment, and their ocean‐facing shorelines are maintained by sediment produced and transported across their reefs. Changes in incident waves can alter cross‐shore sediment exchange and, thus, affect the sediment budget and morphology of atoll reef islands. Here we investigate the influence of sea level rise and projected wave climate change on wave characteristics and cross‐shore sediment transport across an atoll reef at Kwajalein Island, Republic of the Marshall Islands. Using a phase‐resolving model, we quantify the influence on sediment transport of quantities not well captured by wave‐averaged models, namely, wave asymmetry and skewness and flow acceleration. Model results suggest that for current reef geometry, sea level, and wave climate, potential bedload transport is directed onshore, decreases from the fore reef to the beach, and is sensitive to the influence of flow acceleration. We find that a projected 12% decrease in annual wave energy by 2100 CE has negligible influence on reef flat hydrodynamics. However, 0.5–2.0 m of sea level rise increases wave heights, skewness, and shear stress on the reef flat and decreases wave skewness and shear stress on the fore reef. These hydrodynamic changes decrease potential sediment inputs onshore from the fore reef where coral production is greatest but increase potential cross‐reef sediment transport from the outer reef flat to the beach. Assuming sediment production on the fore reef remains constant or decreases due to increasing ocean temperatures and acidification, these processes have the potential to decrease net sediment delivery to atoll islands, causing erosion.

Journal of Geophysical Research – Earth Surface↗