The Chesapeake Bay region (defined as longitudes − 78° to -74° and latitudes 36.5° to 40°) experiences the highest rates of relative sea-level rise (RSLR) on the Atlantic Coast. Regional land subsidence influences RSLR, however quantified rates of vertical land motions (VLM) are inconsistent in published solutions. For 5 years from 2019 to 2023, new Global Navigation Satellite System (GNSS) campaign data were collected at over 60 sites across the Chesapeake Bay region annually. These data were processed and combined with continuous GNSS data (120 stations) from the region covering the same time-period using GAMIT-GLOBK to produce 3D velocities and their associated uncertainties. We use the Robust Network Imaging algorithm to interpolate GNSS-derived VLM to produce a new regional VLM solution of the Chesapeake Bay region. We find that land subsidence is ubiquitous throughout the region with rates varying from − 2.97 to -0.40 mm/yr. In major cities across the Chesapeake Bay region, VLM rates are − 1.1 ± 1.6 mm/yr (1-sigma) for Washington DC, -0.8 ± 1.4 mm/yr for Baltimore, MD, -2.4 ± 0.5 mm/yr for Ocean City, MD, and − 2.3 ± 1.0 mm/yr for Hampton, VA. When we compare our VLM rates with a geodetic-based solution from 1974, we observe meaningful shifts in the locations and rates of maximum subsidence. The results of this work underscore that regular monitoring of VLM and can be used to improve projections of relative sea-level changes as well as the associated coastal hazards for communities in the Chesapeake Bay region.

The vertical bias and uncertainty of LiDAR-derived elevation data in tidal marshes is a major challenge for researchers in these critical ecosystems. Small positive bias errors in land surface elevation can lead to large underestimates of coastal inundation. Previous studies have shown that the bias and uncertainty of elevation data can be reduced using a variety of statistical techniques. However, many studies cover a relatively small extent, cover a large extent at coarse resolution, or rely on local data products that may not be widely available or are of unknown quality. This study aimed to develop and compare bias reduction approaches for digital elevation models (DEMs) using LiDAR derivatives in tidal marshes along Delaware Bay, Delaware, USA, a region that experiences high rates of relative sea-level rise and frequent coastal flooding. In this study, several statistical and machine-learning approaches were evaluated based on their ability to reduce vertical DEM error using field GPS survey data and LiDAR and terrain derivatives as inputs. The evaluated approaches for reducing DEM error included ensembles of multiple linear regressions, kernel-K Nearest Neighbors, gradient boosted regression trees, random forests, and deep neural networks (DNNs), which were compared based on statistical performance metrics. An ensemble of deep neural networks performed best at removing vertical DEM bias and reducing DEM uncertainty, though other approaches also performed well. GPS survey data indicated a mean absolute error, mean bias, and root mean square error in GPS surveys of 11.9 cm, 10.9, and 14.8 cm, respectively, in the original DEM. These error metrics were reduced to 3.5 cm, -0.2 cm, and 5.1 cm in the DNN ensemble-corrected DEM. The DNN ensemble was then applied to all tidal marshes throughout the study area. This corrected DEM clearly showed how interpretations of marsh platform inundation based on uncorrected DEM surfaces could be misguided. The approach used in this study only requires LiDAR-derived products and field surveys, so it may be applied readily in other regions.

The Chesapeake Bay is a region along the eastern coast of the United States where sea-level rise is confounded with poorly resolved rates of land subsidence, thus new constraints on vertical land motions (VLM) in the region are warranted. In this paper, we provide a description of two campaign-style Global Positioning System (GPS) datasets, explain the methods used in data collection and validation, and present the experiment designed to quantify a new baseline of VLM in the Chesapeake Bay region of eastern North America. Data from GPS campaigns in 2019 and 2020 are presented as ASCII RINEX2.11 files and logsheets for each observation from the campaigns. Data were quality checked using the open-source program TEQC, resulting in average multipath 1 and 2 values of 0.68 and 0.57, respectively. All data are archived and publicly available for open access at the geodesy facility UNAVCO to abide by Findable, Accessible, Interoperable, Reusable (FAIR) data principles.

The use of wireline coring in Delaware over the last 20 years has caused a step change in understanding the subsurface geology of the Delaware Coastal Plain. Wireline cores reveal details of sedimentology, micropaleontology, sequence stratigraphy, and age of significant stratigraphic events in important aquifer intervals and confining beds, as well as the nature of the crystalline basement rocks that lie beneath the Coastal Plain. The value of wireline cores was highlighted in a Delaware Geological Survey (DGS) project in 1999-2000 to investigate faulting and earthquake risk potential in the northern part of the Coastal Plain (New Castle). Two coreholes drilled by Gene Cobbs II and Gene Cobbs III of the United States Geological Survey (USGS) resulted in improved understanding of the stratigraphy of the non-marine Cretaceous Potomac Formation and structure of the underlying basement rocks. Later in 2000 the USGS team drilled a deep (1,470 ft) core hole along the Delaware Coast (Bethany Beach), part of International Ocean Discovery Program (IODP) Leg 174AX, that yielded an outstanding record of coastal plain sedimentation and sea-level change from the Oligocene to the Quaternary. The DGS began its own wireline coring program in 2002 using the DGS drill rig with technical assistance by the USGS drillers. Between 2004 and 2017, DGS driller Steve McCreary cored 16 sites, reaching depths of as much as 720 ft. USGS drilling by Gene Cobbs III and Jeff Grey in 2012 obtained deeper core records from two core sites in central Delaware that sampled Quaternary to Upper Cretaceous section. These coring projects have greatly improved understanding of the stratigraphy and geological characteristics of the Cretaceous (Potomac) aquifers of northern Delaware, the Miocene (Cheswold, Frederica, and others) aquifers of southern Delaware, and the Paleogene (Rancocas) aquifer of central Delaware. The cores have also provided a subsurface context for surficial geological mapping and permit the basement rocks to be tied to rock units that outcrop in Delaware’s Piedmont. The results of two decades of DGS and USGS coring in Delaware provide invaluable geological “golden spikes” that allow details of subsurface geology to be calibrated to geophysical logs and extrapolated broadly around the Delaware Coastal Plain.

A field study was conducted in man-made ditches in a tidal saltmarsh in Lewes, Delaware, USA. Ditches are prevalent throughout tidal marshes along the Atlantic US coast, and influence hydrodynamics and sediment transport. The field study focused on measuring near-bed velocity, shear stress, sediment concentration, and bed level variability at 5 stations over a 3-week period. Velocities in the ditch (2-5 m wide, 1 m deep) peaked between 0.4 and 0.6 m/s and were slightly ebb dominated. Velocity and shear stress were maximum during a storm event, with peak shear stresses of 2 N/m(2). Bed levels were estimated from acoustic amplitude return of a downward-looking velocity profiler. The bed level in the ditch at the landward locations increased similar to 0.03 in over 3 weeks, while there was similar to 0.01 m bed level decrease at the most seaward site suggesting a net import of sediment into the channel. At all sites, erosion (similar to 0.005-0.015 m) occurred during the accelerating phase of the flood tide, and accretion of a similar magnitude occurred during the decelerating phase of the ebb tide. This erosion-deposition sequence resulted in small net changes in bed level at the end of each tidal cycle. The intratidal behavior of the bed level was simulated using erosion and deposition flux equations based on shear stress, critical shear stress, and suspended sediment concentration. Erosion was predicted well with RMS errors on the order of 2.10(-3) m. The bed level during the deposition phase could not be reproduced using the simple approach. Model inaccuracies for deposition were attributed to advection and variations in fall velocity due to flocculation that were not modeled due to lack of ground-truth observations. (C) 2017 Elsevier Ltd. All rights reserved.

A field study was conducted on a tidal flat intersected by small tidal channels (depth <0.1m, width <2m) within a tidal marsh. Data were collected in the channels, and on the adjacent tidal flat that encompasses approximately 1600m2 in planform area. Hydrodynamic processes and sediment suspension between the channels and adjacent flat were compared. Shear stress and turbulent kinetic energy were computed from high frequency velocity measurements. Maximum water depth at the field site varied from 0.11m during the lowest neap high tide to 0.58m during a storm event. In the channel intersecting the tidal flat, the shear stress, turbulence and along-channel velocity were ebb dominant; e.g. 0.33m/s peak velocity for ebb compared to 0.19m/s peak velocity for flood. Distinct pulses in velocity occurred when the water level was near the tidal flat level. The velocity pulse during flood tide occurred at a higher water level than during ebb tide. No corresponding velocity pulse on the tidal flat was observed. Sediment concentrations peaked at the beginning and end of each tidal cycle, and often had a secondary peak close to high tide, assumed to be related to sediment advection. The influence of wind waves on bed shear stress and sediment suspension was negligible. Water levels were elevated during a storm event such that the tidal flat remained inundated for 4 tidal cycles. The water did not drain from the tidal flat into the channels during the storm, and no velocity pulses occurred. Along-channel velocities, turbulent kinetic energy, and shear stresses were therefore smaller in the channels during storm conditions than during non-storm conditions.

Near-bed, highly resolved velocity profiles were measured in the lower 0.03 m of the water column using acoustic Doppler profiling velocimeters in narrow tidal channels in a salt marsh. The bed shear stress was estimated from the velocity profiles using three methods: the log-law, Reynolds stress, and shear stress derived from the turbulent kinetic energy (TKE). Bed shear stresses were largest during ebbing tide, while near-bed velocities were larger during flooding tide. The Reynolds stress and TKE method gave similar results, while the log-law method resulted in smaller bed shear stress values during ebbing tide. Shear stresses and turbulent kinetic energy followed a similar trend with the largest peaks during ebbing tide. The maximum turbulent kinetic energy was on the order of 1 x 10(-2) m(2)/s(2). The fluid shear stress during flooding tide was approximately 30% of the fluid shear stress during ebbing tide. The maximum TKE-derived shear stress was 0.7 N/m(2) and 2.7 N/m(2) during flooding and ebbing tide, respectively, and occurred around 0.02 m above the bed. Turbulence dissipation was estimated using the frequency spectrum and structure function methods. Turbulence dissipation estimates from both methods were maximum near the bed (similar to 0.01 m). Both the structure function and the frequency spectrum methods resulted in maximum dissipation estimates on the order of 4 x 10(-3) m(2)/s(3). Turbulence production exceeded turbulence dissipation at every phase of the tide, suggesting that advection and vertical diffusion are not negligible. However, turbulence production and dissipation were within a factor of 2 for 77% of the estimates. The turbulence production and dissipation decreased quickly away from the bed, suggesting that measurements higher in the water column cannot be translated directly to turbulence production and dissipation estimates near the bed. Copyright (c) 2015 John Wiley & Sons, Ltd.

A procedure for estimating tidal mud flat topography from fixed-platform, long-wave infrared imagery is presented. Shallow water and low bearing capacity on many mud flats hinder traditional surveying methods by water craft, walking, or land vehicle. The approach utilizes identification of the intersection of water with the surface of the mud flat through a rising tide. Waterlines on mud flats are often indistinguishable to the naked eye and in visible-band imagery. Long-wave infrared imagery, relying on emitted radiance, provides a more distinct delineation between an exposed mud flat and water. The waterline is identified via an image intensity threshold and transferred to real-world coordinates using a geometrical transformation model that accounts for potential imager sway. Elevation estimates, interpolated to a uniform grid, show excellent agreement (absolute error generally less than 0.02 m) with ground truth elevations obtained using a sled-mounted global positioning survey system.

River surface currents are quantified from thermal and visible band imagery using two methods. One method utilizes time stacks of pixel intensity to estimate the streamwise velocity at multiple locations. The other method uses particle image velocimetry to solve for optimal two‐dimensional pixel displacements between successive frames. Field validation was carried out on the Wolf River, a small coastal plain river near Landon, Mississippi, United States, on 26–27 May 2010 by collecting imagery in association with in situ velocities sampled using electromagnetic current meters deployed 0.1 m below the river surface. Comparisons are made between mean in situ velocities and image‐derived velocities from 23 thermal and 6 visible‐band image sequences (5 min length) during daylight and darkness conditions. The thermal signal was a small apparent temperature contrast induced by turbulent mixing of a thin layer of cooler water near the river surface with underlying warmer water. The visible‐band signal was foam on the water surface. For thermal imagery, streamwise velocities derived from the pixel time stack and particle image velocimetry technique were generally highly correlated to mean streamwise current meter velocities during darkness ( r 2 typically greater than 0.9) and early morning daylight ( r 2 typically greater than 0.83). Streamwise velocities from the pixel time stack technique had high correlation for visible‐band imagery during early morning daylight hours with respect to mean current meter velocities ( r 2 > 0.86). Streamwise velocities for the particle image velocimetry technique for visible‐band imagery had weaker correlations with only three out of six correlations performed having an r 2 exceeding 0.6.

A numerical groundwater-flow model was used to characterize the source area and volume of Phillips Branch, a baseflow-dominated stream incising a highly permeable unconfined aquifer on the low relief Delmarva Peninsula, USA. Particle-tracking analyses indicate that the source area (5.51 km2) is ∼20% smaller than the topographically defined watershed (6.85 km2), and recharge entering ∼37% of the surface watershed does not discharge to Phillips Branch. Groundwater residence time within the source volume ranges from a few days to almost 100 years, with 95% of the volume “flushing” within 50 years. Artificial discharge from groundwater pumping alters the shape of the source area and reduces baseflow due to the interception of stream flow paths, but has limited impacts on the residence time of groundwater discharged as baseflow. In contrast, artificial recharge from land-based wastewater disposal substantially reduces the source area, lowers the range in residence time due to the elimination of older flow paths to the stream, and leads to increased discharge to adjacent surface-water bodies. This research suggests that, in this and similar hydrogeologic settings, the “watershed” approach to water-resource management may be limited, particularly where anthropogenic stresses alter the transport of soluble contaminants through highly permeable unconfined aquifers.