Geyer W. Rockwell

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Geyer

First Name

W. Rockwell

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0000-0001-9030-1744

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Sources, mechanisms, and timescales of sediment delivery to a New England salt marsh

(American Geophysical Union, 2022-02-23) Baranes, Hannah E. ; Woodruff, Jonathan D. ; Geyer, W. Rockwell ; Yellen, Brian ; Richardson, Justin B. ; Griswold, Frances

he availability and delivery of an external clastic sediment source is a key factor in determining salt marsh resilience to future sea level rise. However, information on sources, mechanisms, and timescales of sediment delivery are lacking, particularly for wave-protected mesotidal estuaries. Here we show that marine sediment mobilized and delivered during coastal storms is a primary source to the North and South Rivers, a mesotidal bar-built estuary in a small river system impacted by frequent, moderate-intensity storms that is typical to New England (United States). On the marsh platform, deposition rates, clastic content, and dilution of fluvially-sourced contaminated sediment by marine material all increase down-estuary toward the inlet, consistent with a predominantly marine-derived sediment source. Marsh clastic deposition rates are also highest in the storm season. We observe that periods of elevated turbidity in channels and over the marsh are concurrent with storm surge and high wave activity offshore, rather than with high river discharge. Flood tide turbidity also exceeds ebb tide turbidity during storm events. Timescales of storm-driven marine sediment delivery range from 2.5 days to 2 weeks, depending on location within the estuary; therefore the phasing of storm surge and waves with the spring-neap cycle determines how effectively post-event suspended sediment is delivered to the marsh platform. This study reveals that sediment supply and the associated resilience of New England mesotidal salt marshes involves the interplay of coastal and estuarine processes, underscoring the importance of looking both up- and downstream to identify key drivers of environmental change.
High and variable drag in a sinuous estuary with intermittent stratification

(American Geophysical Union, 2021-09-29) Bo, Tong ; Ralston, David K. ; Kranenburg, Wouter M. ; Geyer, W. Rockwell ; Traykovski, Peter A.

In field observations from a sinuous estuary, the drag coefficient C based on the momentum balance was in the range of 5-20 X10-3, much greater than expected from bottom friction alone. C also varied at tidal and seasonal timescales. CD was greater during flood tides than ebbs, most notably during spring tides. The ebb tide CD was negatively correlated with river discharge, while the flood tide CD showed no dependence on discharge. The large values of CD are explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of CD, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated with CD during flood tides. While CD generally increased with water depth, CD decreased for the highest water levels that corresponded with overbank flow. The decrease in CD may be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence of CD during ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature-induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source.
Using tracer variance decay to quantify variability of salinity mixing in the Hudson River Estuary

(American Geophysical Union, 2020-11-12) Warner, John C. ; Geyer, W. Rockwell ; Ralston, David K. ; Kalra, Tarandeep S.

The salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.
Mechanisms of exchange flow in an estuary with a narrow, deep channel and wide, shallow shoals

(American Geophysical Union, 2020-10-29) Geyer, W. Rockwell ; Ralston, David K. ; Chen, Jia‐Lin

Delaware Bay is a large estuary with a deep, relatively narrow channel and wide, shallow banks, providing a clear example of a “channel‐shoal” estuary. This numerical modeling study addresses the exchange flow in this channel‐shoal estuary, specifically to examine how the lateral geometry affects the strength and mechanisms of exchange flow. We find that the exchange flow is exclusively confined to the channel region during spring tides, when stratification is weak, and it broadens laterally over the shoals during the more stratified neap tides but still occupies a small fraction of the total width of the estuary. Exchange flow is relatively weak during spring tides, resulting from oscillatory shear dispersion in the channel augmented by weak Eulerian exchange flow. During neap tides, stratification and shear increase markedly, resulting in a strong Eulerian residual shear flow driven mainly by the along‐estuary density gradient, with a net exchange flow roughly 5 times that of the spring tide. During both spring and neap tides, lateral salinity gradients generated by differential advection at the edge of the channel drive a tidally oscillating cross‐channel flow, which strongly influences the stratification, along‐estuary salt balance, and momentum balance. The lateral flow also causes the phase variation in salinity that results in oscillatory shear dispersion and is an advective momentum source contributing to the residual circulation. Whereas the shoals make a negligible direct contribution to the exchange flow, they have an indirect influence due to the salinity gradients between the channel and the shoal.
Unprecedented summer hypoxia in southern Cape Cod Bay: an ecological response to regional climate change?

(European Geosciences Union, 2022-07-28) Scully, Malcolm E. ; Geyer, W. Rockwell ; Borkman, David ; Pugh, Tracy L. ; Costa, Amy ; Nichols, Owen C.

In late summer 2019 and 2020 bottom waters in southern Cape Cod Bay (CCB) became depleted of dissolved oxygen (DO), with documented benthic mortality in both years. Hypoxic conditions formed in relatively shallow water where the strong seasonal thermocline intersected the sea floor, both limiting vertical mixing and concentrating biological oxygen demand (BOD) over a very thin bottom boundary layer. In both 2019 and 2020, anomalously high sub-surface phytoplankton blooms were observed, and the biomass from these blooms provided the fuel to deplete sub-pycnocline waters of DO. The increased chlorophyll fluorescence was accompanied by a corresponding decrease in sub-pycnocline nutrients, suggesting that prior to 2019 physical conditions were unfavorable for the utilization of these deep nutrients by the late-summer phytoplankton community. It is hypothesized that significant alteration of physical conditions in CCB during late summer, which is the result of regional climate change, has favored the recent increase in sub-surface phytoplankton production. These changes include rapidly warming waters and significant shifts in summer wind direction, both of which impact the intensity and vertical distribution of thermal stratification and vertical mixing within the water column. These changes in water column structure are not only more susceptible to hypoxia but also have significant implications for phytoplankton dynamics, potentially allowing for intense late-summer blooms of Karenia mikimotoi, a species new to the area. K. mikimotoi had not been detected in CCB or adjacent waters prior to 2017; however, increasing cell densities have been reported in subsequent years, consistent with a rapidly changing ecosystem.
North River estuary 2017 dataset

(Woods Hole Oceanographic Institution, 2021-03-07) Geyer, W. Rockwell ; Ralston, David K. ; Kranenburg, Wouter M. ; Garcia, Adrian Mikhail P. ; Bo, Tong

These are the observational data collected in 2017 from the North River estuary. Data files include the long-term (LT) CTD and Aquadopp measurements from April to July, the short-term (STI from April to May and STII in late July) CTD measurements, eight shipboard CTD and ADCP surveys in April, May and July, the ADV measurements in late July, the North River mid-estuary region bathymetry, and the North River discharge (from USGS measurements).
Southern Cape Cod Bay hypoxia data

(Woods Hole Oceanographic Institution, 2022-01-14) Scully, Malcolm E. ; Pugh, Tracy L. ; Geyer, W. Rockwell ; Costa, Amy ; Nichols, Owen C.

This project investigated the distribution of low dissolved oxygen bottom waters (hypoxia) in southern Cape Cod Bay. Hypoxia was documented for the first time in late summer 2019 and 2020 despite extensive monitoring for the past decade. The data include: 1) measurements of bottom dissolved oxygen collected in 2019 by the Massachusetts Division of Marine Fisheries (MDMF) and the Center for Coastal Studies (CCS) ; 2) full water column profiles of temperature, salinity, chlorophyll fluorescence, dissolved oxygen concentration and optical backscatter collected in late summer 2020 by the Woods Hole Oceanographic Institution (WHOI); 3) monthly water quality data including CTD with dissolved oxygen and chlorophyll fluorescence and discrete bottom samples analyzed for dissolved nutrients collected by the CCS for the period 2011-2020; 4) inorganic nutrients from discrete surface and bottom samples collected monthly for the period 2006-2020; 5) bottom temperature data collected the Wreck of Mars location by the MDMF over the period 1991-2021. There are four separate data sets included: 1) MDMF and CCS bottom dissolved oxygn from 2019; 2) CTD and ancillary data collected by WHOI in 2019; 3) CCS monthly survey data from 2011-2020; and 4) bottom temperature data collected by MDMF for 1991-2021. 1) MDMF/CCS dissolved oxygen data was collected from ship-based surveys using an YSI 6920 V2-2 data sonde; 2) WHOI CTD data was collected from vertical casts made from a small research vessel using an RBR CTD; 3) CCS CTD data was collected from vertical casts made from a small research vessel using a SeaBird Electronics CTD; 4) MDMF temperature data was collected from a bottom mounted temperature logger. Related Publications: Scully, M.E., W.R. Geyer, D. Borkman, T.L. Pouch, A. Costa, and O.C. Nichols, in press. Unprecedented summer hypoxia in southern Cape Cod Bay: An ecological response to regional climate change? Biogeosciences.
Mechanisms of exchange flow in an estuary with a narrow, deep channel and wide, shallow shoals

(Woods Hole Oceanographic Institution, 2020-01-31) Geyer, W. Rockwell ; Ralston, David K. ; Chen, Jia-Lin

Delaware Bay is a large estuary with a deep, relatively narrow channel and wide, shallow banks, providing a clear example of a “channel-shoal” estuary. This numerical modeling study addresses the exchange flow in this channel-shoal estuary, specifically to examine how the lateral geometry affects the strength and mechanisms of exchange flow. We find that the exchange flow is exclusively confined to the channel region during spring tides, when stratification is weak, and it broadens laterally over the shoals during the more stratified neap tides, but still occupies a small fraction of the total width of the estuary. Exchange flow is relatively weak during spring tides, resulting from oscillatory shear dispersion in the channel augmented by weak Eulerian exchange flow. During neap tides, stratification and shear increase markedly, resulting in a strong Eulerian residual shear flow, with a net exchange flow roughly 5 times that of the spring tide. During both spring and neap tides, lateral salinity gradients generated by differential advection at the edge of the channel drive a tidally oscillating cross-channel flow, which strongly influences the stratification, along-estuary salt balance and momentum balance. The lateral flow also causes the phase variation in salinity that results in oscillatory shear dispersion during both spring and neap tides and is a significant advective momentum source driving the residual circulation. Thus, although the shoals make a negligible direct contribution to the exchange flow, the salinity gradients between the channel and the shoal are critical to the stratification and exchange flow within the estuarine channel.
Salt marsh response to inlet switch‐induced increases in tidal inundation

(American Geophysical Union, 2022-12-22) Yellen, Brian ; Woodruff, Jonathan D. ; Baranes, Hannah E. ; Engelhart, Simon E. ; Geywer, W. Rockwell ; Randall, Noa ; Griswold, Frances R.

There is widespread concern that rapidly rising sea levels may drown salt marshes by exceeding the rate at which these important ecosystems can build elevation. A significant fraction of marshes reside within backbarrier estuaries, yet little attention has been paid to how changes in inlet geometry influences estuarine tides and marshes. In 1898, a coastal storm eroded a new inlet through the barrier beach that fronts the North‐South Rivers Estuary in Massachusetts, USA. The new inlet shortened the North River by 5.6 km and lengthened the South River channel by the same amount. Modern measurements of tidal attenuation suggest that channel shortening abruptly increased mean high tide along the North River by at least 30 cm. Foraminifera communities within North River marsh sediments indicated an environmental change from infrequent to frequent inundation at the time of the 1898 switch in inlet location, which supports this hypothesis. Increased mineral sediment deposition after the inlet switch played a dominant role in allowing marshes along the North River channel to adjust to greater inundation. Following the inlet switch, sediment accreted in North River marshes at 2–5 times the rate of sea level rise (SLR). The North River channel widened by an average of 18% relative to pre‐1898 conditions to accommodate the increased tidal prism. The role of mineral sediment accretion in making this marsh resilient to an abrupt increase in inundation depth highlights the importance of maintaining adequate sediment supplies in coastal regions as SLR accelerates.
Tidal dispersion in short estuaries

(American Geophysical Union, 2023-02-03) Garcia, Adrian Mikhail P. ; Geyer, W. Rockwell

The salinity distribution of an estuary depends on the balance between the river outflow, which is seaward, and a dispersive salt flux, which is landward. The dispersive salt flux at a fixed cross‐section can be divided into shear dispersion, which is caused by spatial correlations of the cross‐sectionally varying velocity and salinity, and the tidal oscillatory salt flux, which results from the tidal correlation between the cross‐section averaged, tidally varying components of velocity and salinity. The theoretical moving plane analysis of Dronkers and van de Kreeke (1986) indicates that the oscillatory salt flux is exactly equal to the difference between the “local” shear dispersion at a fixed location and the shear dispersion which occurred elsewhere within a tidal excursion; therefore, they refer to the oscillatory salt flux as “nonlocal” dispersion. We apply their moving plane analysis to a numerical model of a short, tidally dominated estuary and provide the first quantitative confirmation of the theoretical result that the spatiotemporal variability of shear dispersion accounts for the oscillatory salt flux. Shear dispersion is localized in space and time due to the tidal variation of currents and the position of the along‐channel salinity distribution with respect to topographic features. We find that dispersion near the mouth contributes strongly to the salt balance, especially under strong river and tidal forcing. Additionally, while vertical shear dispersion produces the majority of dispersive salt flux during neap tide and high flow, lateral mechanisms provide the dominant mode of dispersion during spring tide and low flow.
Hudson River estuary 2004 field experiment

(Woods Hole Oceanograpic Institution, 2023-08-08) Lerczak, James A. ; Ralston, David K. ; Geyer, W. Rockwell ; Conley, Margaret M.

This dataset includes data from moorings and shipboard observations in the Hudson River estuary during the spring and summer of 2004. The moorings were deployed in the thalweg at 7 sites for 108 days and included a combination of bottom temperature, conductivity, and pressure measurements as well as upward-looking ADCPs. Each mooring site also had near-surface temperature and conductivity measurements. Shipboard CTD surveys were carried out along the estuary on 7 days just after the deployment and just before the recovery of the moorings.
Tidal dispersion in short estuaries

(Woods Hole Oceanographic Institution, 2022-04-25) Garcia, Adrian Mikhail P. ; Geyer, W. Rockwell

The salinity distribution of an estuary depends on the balance between the river outflow, which is seaward, and a dispersive salt flux, which is landward. The dispersive salt flux at a fixed cross-section can be divided into shear dispersion, which is caused by spatial correlations of the cross-sectionally varying velocity and salinity, and the tidal oscillatory salt flux, which results from the tidal correlation between the cross-section averaged, tidally varying components of velocity and salinity. The theoretical moving plane analysis of Dronkers and van de Kreeke (1986) indicates that the oscillatory salt flux is exactly equal to the difference between the “local” shear dispersion at a fixed location and the shear dispersion which occurred elsewhere within a tidal excursion – therefore, they refer to the oscillatory salt flux as “nonlocal” dispersion. We apply their moving plane analysis to a numerical model of a short, tidally dominated estuary and provide the first quantitative confirmation of the theoretical result that the spatiotemporal variability of shear dispersion accounts for the oscillatory salt flux. Shear dispersion is localized in space and time and is most pronounced near regions of flow separation. Notably, we find that dispersive processes near the mouth contribute significantly to the overall salt balance, especially under strong river and tidal forcing. Furthermore, while mechanisms of vertical shear dispersion produce the majority of the dispersive salt flux during neap tide and high river flow, lateral mechanisms associated with flow separation provide the dominant mode of dispersion during spring tide and low flow. Dataset used in support of manuscript "Tidal dispersion in short estuaries". The dataset includes the model output from the idealized estuary for 16 different forcing conditions, corresponding to 4 tidal conditions (weak
Sources of drag in estuarine meanders: momentum eedistribution, bottom atress wnhancement, and bend-scale form drag

(American Meteorological Society, 2023-07-01) Bo, Tong ; Ralston, David K. ; Geyer, W. Rockwell

Curvature can create secondary circulation and flow separation in tidal channels, and both have important consequences for the along-channel momentum budget. The North River is a sinuous estuary where drag is observed to be higher than expected, and a numerical model is used to investigate the influence of curvature-induced processes on the momentum distribution and drag. The hydrodynamic drag is greatly increased in channel bends compared to that for straight channel flows. Drag coefficients are calculated using several approaches to identify the different factors contributing to the drag increase. Flow separation creates low-pressure recirculation zones on the lee side of the bends and results in form drag. Form drag is the dominant source of the increase in total drag during flood tides and is less of a factor during ebb tides. During both floods and ebbs, curvature-induced secondary circulation transports higher-momentum fluid to the lower water column through vertical and lateral advection. Consequently, the streamwise velocity profile deviates from the classic log profile and vertical shear becomes more concentrated near the bed. This redistribution by the lateral circulation causes an overall increase in bottom friction and contributes to the increased drag. Additionally, spatial variations in the depth-averaged velocity field due to the curvature-induced flow are nonlinearly correlated with the bathymetric structure, leading to increased bottom friction. In addition to affecting the tidal flow, the redistributed momentum and altered bottom shear stress have clear implications for channel morphodynamics.
Mobile Bay 2021 synthetic aperture radar images

(Woods Hole Oceanographic Institution, 2024-02-02) Ralston, David K. ; Geyer, W. Rockwell ; Wackerman, Christopher

This dataset includes satellite synthetic aperture radar images (SAR) that were obtained for the region around the mouth of Mobile Bay during the period April-June 2021. Images were collected to identify the location of oceanographic fronts associated with the outflow plume of Mobile Bay. SAR images show density fronts as regions of increased radar cross-section, i.e. image brightness.
Hudson River estuary 2002 field experiment: moorings

(Woods Hole Oceanographic Institution, 2023-09-20) Geyer, W. Rockwell ; Chant, Robert J. ; Houghton, Robert ; Lerczak, James A. ; Hunter, Elias J. ; Conley, Margaret

This dataset includes data from moorings deployed in the Hudson River estuary during the spring of 2002. The moorings were deployed at Spuyten Duyvil for 43 days and included a cross-channel array of temperature and conductivity sensors as well as 4 upward-looking ADCPs and 2 pressure sensors flanking the channel.

Geyer W. Rockwell

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