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Richard P. Signell

Publications and source records attributed to Richard P. Signell.

50 records · Page 3Linked to original sources
USGS2000

Circulation and sediment transport in the vicinity of the Hudson Shelf Valley

Sediment transport in the Hudson Shelf Valley and on the adjacent Long Island Shelf are evaluated using available data along with a three-dimensional wind-driven circulation model and a one-dimensional sediment transport model. Winds from the northwest drive currents up the Hudson Shelf Valley, while winds from the east produce weaker currents directed down the valley. Consistent with previous studies, sediment transport on the Long Island Shelf is dominated by resuspension during energetic wave events that are correlated with strong winds from the northeast, and net sediment flux is predicted to be towards the southwest along bathymetric contours. Transport of muddy sediments in the Hudson Shelf Valley, however, does not appear to be wave-dominated. These sediments are most likely to be resuspended by energetic currents driven by strong winds from the northwest that are not associated with energetic waves. The strong up-valley flows associated with these winds implies that net sediment flux along the Hudson Shelf Valley is up-valley.

Proceedings of the International Conference on Est↗
USGS2000

Modeling sand bank formation around tidal headlands

Sandbanks are often found in the vicinity of coastal headlands around which tidal flows are strong enough to generate significant tidally-forced residual eddies, typically with scales of 2-10 km. One popular hypothesis is that these sandbanks are generated by a 'tidal stirring' mechanism in which the inward-directed pressure gradient associated with these residual eddies produces an inward-directed movement of sand near the seabed. This hypothesis predicts asymmetric sandbank formation when planetary vorticity is significant compared to the relative vorticity of the residual eddies. This mechanism is tested with a numerical sediment transport model, using idealized symmetrical coastline geometry and tidal forcing that represents conditions similar to regions where these tidal headland sandbanks are known to occur. For both suspended and bedload simulations, we find that nearly symmetric sandbanks form, and that the sediment transport patterns that are responsible for building and maintaining the banks are due more the patterns of shear stress and sediment flux that occur over the course of the tidal cycle rather than to the characteristics of the tidally-averaged residual fields. We also find that sediment supply can be an important factor in controlling the nature of the resulting sandbanks.

Proceedings of the International Conference on Est↗
USGS2000

Modeling surface trapped river plumes: A sensitivity study

To better understand the requirements for realistic regional simulation of river plumes in the Gulf of Maine, we test the sensitivity of the Blumberg-Mellor hydrodynamic model to choice of advection scheme, grid resolution, and wind, using idealized geometry and forcing. The test case discharges 1500 m3/s of fresh water into a uniform 32 psu ocean along a straight shelf at 43?? north. The water depth is 15 m at the coast and increases linearly to 190 m at a distance 100 km offshore. Constant discharge runs are conducted in the presence of ambient alongshore current and with and without periodic alongshore wind forcing. Advection methods tested are CENTRAL, UPWIND, the standard Smolarkiewicz MPDATA and a recursive MPDATA scheme. For the no-wind runs, the UPWIND advection scheme performs poorly for grid resolutions typically used in regional simulations (grid spacing of 1-2 km, comparable to or slightly less than the internal Rossby radius, and vertical resolution of 10% of the water column), damping out much of the plume structure. The CENTRAL difference scheme also has problems when wind forcing is neglected, and generates too much structure, shedding eddies of numerical origin. When a weak 5 cm/s ambient current is present in the no-wind case, both the CENTRAL and standard MPDATA schemes produce a false fresh- and dense-water source just upstream of the river inflow due to a standing two-grid length oscillation in the salinity field. The recursive MPDATA scheme completely eliminates the false dense water source, and produces results closest to the grid-converged solution. The results are shown to be very sensitive to vertical grid resolution, and the presence of wind forcing dramatically changes the nature of the plume simulations. The implication of these idealized tests for realistic simulations is discussed, as well as ramifications on previous studies of idealized plume models.

Maine↗
USGS1997

Modeling waves and circulation in Lake Pontchartrain, Louisiana

The U.S. Geological Survey is conducting a study of storm-driven sediment resuspension and transport in Lake Pontchartrain, Louisiana. Two critical processes related to sediment transport in the lake are (1) the resuspension of sediments due to wind-generated storm waves and (2) the movement of resuspended material by lake currents during storm wind events. The potential for sediment resuspension is being studied with the wave prediction model which simulates local generation of waves by wind and shallow-water effects on waves (refraction, shoaling, bottom friction, and breaking). Long-term wind measurements are then used to determine the regional "climate" of bottom orbital velocity (showing the spatial and temporal variability of wave-induced currents at the bottom). The circulation of the lake is being studied with a three-dimensional hydrodynamic model. Results of the modeling effort indicate that remote forcing due to water levels in Mississippi Sound dominate the circulation near the passes in the eastern end of the lake, while local wind forcing dominates water movement in the western end. During typical storms with winds from the north-northeast or the south-southeast, currents along the south coast near New Orleans generally transport material westward, while material in the central region moves against the wind. When periods of sustained winds are followed by a drop in coastal sea level, a large amount of suspended sediment can be flushed from the lake.

Louisiana↗
USGS1996

Seafloor environments in Cape Cod Bay, a large coastal embayment

Cape Cod Bay is a glacial, semi-enclosed embayment that has a patchy distribution of modern seafloor sedimentary environments of erosion or nondeposition, deposition, and sediment reworking. Sidescan-sonar records and supplemental bathymetric, sedimentary, subbottom, and physical-oceanographic data indicate that the characteristics and distribution of these three categories of bottom environments are controlled by a combination of geologic and oceanographic processes that range from episodic to long-term and from regional to local. (1) Environments of erosion or nondeposition comprise exposures of bedrock, glacial drift, and coarse lag deposits that contain sediments (where present) ranging from boulder fields to gravelly coarse-to-medium sands. These environments are dominant on the shallow margins of the bay (water depths < 30 m) where they reflect sediment resuspension, winnowing, and transport during modern northerly storms. (2) Environments of deposition are blanketed by fine-grained sediments ranging from muds to muddy fine sands. These environments are dominant across the floor of the central basin (water depths = 30–60 m) where fine-grained sediments (derived from regional and local sources and emplaced primarily during episodic wind- and density-driven flow) settle through the water column and accumulate under weak bottom currents during nonstorm conditions. (3) Environments of sediment reworking contain patches with diverse textures ranging from gravelly sands to muds. These environments occupy much of the transitional slopes between the margins and the basin floor and reflect a combination of erosion and deposition. The patchy distribution of sedimentary environments within the bay reflects not only regional changes in processes between the margins and the basin but local changes within each part of the bay as well. Small-scale patchiness is caused by local changes in the strengths of wave- and wind-driven currents and (on the margins) by local variations in the supply of fine-grained sediments. This study indicates areas within Cape Cod Bay where fine-grained sediments and associated contaminants are likely to be either moved or deposited. It also provides a guide to the locations and variability of benthic habitats.

Cape Cod Bay↗
USGS1996

Circulation and effluent dilution modeling in Massachusetts Bay : model implementation, verification and results

A three-dimensional hydrodynamic model was developed as part of a cooperative U.S. Geological Survey/Massachusetts Water Resources Authority program to study contaminated sediment accumulation and transport in Massachusetts Bay. This report details the development of the model and assesses how well the model represents observed currents and water properties in the bay. It also summarizes circulation and comparative effluent dilution simulations from existing and future Boston sewage outfalls over a three-year period from October 1, 1989 to December 31, 1992. The ECOM-si model, a semi-implicit version of the Blumberg and Mellor (1987) Estuarine, Coastal and Ocean Model, is shown to reproduce many of the important hydrodynamical features of Massachusetts Bay: the seasonal evolution of the pycnocline, the mean flow pattern, and the strength of sub-tidal current fluctuations. Throughout the simulation period, during both vertically well-mixed and stratified conditions, the seasonal statistics of observed currents are well-represented by the model. The model is therefore appropriate for studying the average dilution of sewage effluent and other continuously discharged substances over seasonal time scales. The ability of the model to reproduce individual flow events varies with season and location within the bay. Flow events during unstratified conditions in western Massachusetts Bay are particularly well-represented, indicating that the model is appropriate for studying processes such as the transport of suspended material from the future outfall site due to winter storms. Individual flow events during stratified conditions and in the offshore Stellwagen Bank region, however, are less well-represented due to small length scales (caused by upwelling and river discharge events) coupled with insufficient data to specify open boundary forcing from the Gulf of Maine. Thus while the model might be used to answer issues such as the frequency with which Gulf of Maine river plumes visit the new outfall site, attempting to predict whether a particular plume would visit the outfall site could be problematic. Comparative simulations of effluent discharged from the existing and future Boston outfalls show that the region of relatively high effluent concentrations (1 part effluent to 200 parts sea water) is significantly smaller with the future outfall and is limited to Western Massachusetts Bay during both unstratified and stratified seasons. The region of even higher concentration (1 part effluent to 50 parts sea water) that covers much of Boston Harbor with the existing outfall is non-existent in the future outfall simulation. Additional simulations of chlorination plant failure predict that the offshore location of the future outfall will lead to dramatically lower levels of pathogens at area beaches.

Open-File Report↗
USGS1994

Modeling the seasonal circulation in Massachusetts Bay

An 18 month simulation of circulation was conducted in Massachusetts Bay, a roughly 35 m deep, 100??50 km embayment on the northeastern shelf of the United States. Using a variant of the Blumberg-Mellor (1987) model, it was found that a continuous 18 month run was only possible if the velocity field was Shapiro filtered to remove two grid length energy that developed along the open boundary due to mismatch in locally generated and climatologically forced water properties. The seasonal development of temperature and salinity stratification was well-represented by the model once ??-coordinate errors were reduced by subtracting domain averaged vertical profiles of temperature, salinity and density before horizontal differencing was performed. Comparison of modeled and observed subtidal currents at fixed locations revealed that the model performance varies strongly with season and distance from the open boundaries. The model performs best during unstratified conditions, and in the interior of the bay. The model performs poorest during stratified conditions and in the regions where the bay is driven predominantly by remote fluctuations from the Gulf of Maine.

Conference Paper↗
USGS1992

A reassessment of the role of tidal dispersion in estuaries and bays

The role of tidal dispersion is reassessed, based on a consideration of the relevant physical mechanisms, particularly those elucidated by numerical simulations of tide-induced dispersion. It appears that the principal influence of tidal currents on dispersion occurs at length scales of the tidal excursion and smaller; thus the effectiveness of tidal dispersion depends on the relative scale of the tidal excursion to the spacing between major bathymetric and shoreline features. In estuaries where the typical spacing of topographic features is less than the tidal excursion, tidal dispersion may contribute significantly to the overall flushing. In estuaries and embayments in which the typical spacing between major features is larger than the tidal excursion, the influence of tidal dispersion will be localized, and it will not markedly contribute to overall flushing. Tidal dispersion is most pronounced in regions of abrupt topographic changes such as headlands and inlets, where flow separation occurs. The strong strain rate in the region of flow separation tends to stretch patches of fluid into long filaments, which are subsequently rolled up and distorted by the transient eddy field. The dispersion process accomplished by the tides varies strongly as a function of position and tidal phase and thus does not lend itself to parameterization by an eddy diffusion coefficient.

Estuaries↗
USGS1992

Physical oceanographic investigation of Massachusetts and Cape Cod Bays

This physical oceanographic study of the Massachusetts Bays (fig. 1) was designed to provide for the first time a bay-wide description of the circulation and mixing processes on a seasonal basis. Most of the measurements were conducted between April 1990 and June 1991 and consisted of moored observations to study the current flow patterns (fig. 2), hydrographic surveys to document the changes in water properties (fig. 3), high-resolution surveys of velocity and water properties to provide information on the spatial variability of the flow, drifter deployments to measure the currents, and acquisition of satellite images to provide a bay-wide picture of the surface temperature and its spatial variability. A longterm objective of the Massachusetts Bays program is to develop an understanding of the transport of water, dissolved substances and particles throughout the bays. Because horizontal and vertical transport is important to biological, chemical, and geological processes in Massachusetts and Cape Cod Bays, this physical oceanographic study will have broad application and will improve the ability to manage and monitor the water and sediment quality of the Bays. Key results are: There is a marked seasonal variation in stratification in the bays, from well mixed conditions during the winter to strong stratification in the summertime. The stratification acts as a partial barrier to exchange between the surface waters and the deeper waters and causes the motion of the surface waters to be decoupled from the more sluggish flow of the deep waters. During much of the year, there is weak but persistent counterclockwise flow around the bays, made up of southwesterly flow past Cape Ann, southward flow along the western shore, and outflow north of Race Point. The data suggest that this residual flow pattern reverses in fall. Fluctuations caused by wind and density variations are typically larger than the long-term mean. With the exception of western Massachusetts Bay, flushing of the Bays is largely the result of the mean throughflow. Residence time estimates of the surface waters range from 20-45 days. The deeper water has a longer residence time, but its value is difficult to estimate. There is evidence that the deep waters in Stellwagen Basin are not renewed between the onset of stratification and the fall cooling period. Current measurements made near the new outfall site in western Massachusetts Bay suggest that water and material discharged there are not swept away in a consistent direction by a well-defined steady current but are mixed and transported by a variety of processes, including the action of tides, winds, and river inflow. One-day particle excursions are typically less than 10 km. The outfall is apparently located in a region to the west of the basin-wide residual flow pattern. Observations in western Massachusetts Bay, near the location of the future Boston sewage outfall, show that the surficial sediments are episodically resuspended from the seafloor during storms. The observations suggest onshore transport of suspended material during tranquil periods and episodic offshore and southerly alongshore transport of resuspended sediments during storms. The spatial complexity of the flow in the Massachusetts Bays is typical of nearshore areas that have irregular coastal shorelines and topography and currents that are forced locally by wind and river runoff as well as by the flow in adjacent regions. Numerical models are providing a mechanism to interpret the complex spatial flow patterns that cannot be completely resolved by field observations and to investigate key physical processes that control the physics of water and particle transport.

Massachussetts↗
USGS1989

Flow separation of currents in shallow water

Flow separation of currents in shallow coastal areas is investigated using a boundary layer model for two-dimensional (depth-averaged) tidal flow past an elliptic headland. If the shoaling region near the coast is narrow compared to the scale of the headland, bottom friction causes the flow to separate just downstream of the point where the pressure gradient switches from favoring to adverse. As long as the shoaling region at the coast is well resolved, the inclusion of eddy viscosity and a no-slip boundary condition have no effect on this result. An approximate analytic solution for the pressure gradient along the boundary is obtained by assuming the flow away from the immediate vicinity of the boundary is irrotational. On the basis of the pressure gradient obtained from the irrotational flow solution, flow separation is a strong function of the headland aspect ratio, an equivalent Reynolds number, and a Keulegan-Carpenter number.

Conference Paper↗