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Richard P. Signell

Publications and source records attributed to Richard P. Signell.

At least 37 records · Page 2Linked to original sources

Transient eddy formation around headlands

Eddies with length scales of 1-10 km are commonly observed in coastal waters and play an important role in the dispersion of water-borne materials. The generation and evolution of these eddies by oscillatory tidal flow around coastal headlands is investigated with analytical and nu merical models. Using shallow water depth-averaged vorticity dynamics, eddies are shown to form when flow separation occurs near the tip of the headland, causing intense vorticity generated along the headland to be injected into the interior. An analytic boundary layer model demonstrates that flow separation occurs when the pressure gradient along the boundary switches from favoring (ac celerating) to adverse (decelerating), and its occurrence depends principally on three parameters: the aspect ratio [b/a], where b and a are characteristic width and length scales of the headland; [H/CDa], where H is the water depth, CD is the depth-averaged drag coefficient; and [Uo/aa], where Uo and a are the magnitude and frequency of the far-field tidal flow. Simulations with a depth-averaged numerical model show a wide range of responses to changes in these parameters, including cases where no separation occurs, cases where only one eddy exists at a given time, and cases where bottom friction is weak enough that eddies produced during successive tidal cycles coexist, interacting strongly with each other. These simulations also demonstrate that in unsteady flow, a strong start-up vortex forms after the flow separates, leading to a much more intense patch of vorticity and stronger recirculation than found in steady flow.

Journal of Geophysical Research

Modeling tidal exchange and dispersion in Boston Harbor

Tidal dispersion and the horizontal exchange of water between Boston Harbor and the surrounding ocean are examined with a high-resolution (200 m) depth-averaged numerical model. The strongly varying bathymetry and coastline geometry of the harbor generate complex spatial patterns in the modeled tidal currents which are verified by shipboard acoustic Doppler surveys. Lagrangian exchange experiments demonstrate that tidal currents rapidly exchange and mix material near the inlets of the harbor due to asymmetry in the ebb/flood response. This tidal mixing zone extends roughly a tidal excursion from the inlets and plays an important role in the overall flushing of the harbor. Because the tides can only efficiently mix material in this limited region, however, harbor flushing must be considered a two step process: rapid exchange in the tidal mixing zone, followed by flushing of the tidal mixing zone by nontidal residual currents. Estimates of embayment flushing based on tidal calculations alone therefore can significantly overestimate the flushing time that would be expected under typical environmental conditions. Particle-release simulations from point sources also demonstrate that while the tides efficiently exchange material in the vicinity of the inlets, the exact nature of dispersion from point sources is extremely sensitive to the timing and location of the release, and the distribution of particles is streaky and patchlike. This suggests that high-resolution modeling of dispersion from point sources in these regions must be performed explicitly and cannot be parameterized as a plume with Gaussian-spreading in a larger scale flow field.

Massachusetts

Effect of wave-current interaction on wind-driven circulation in narrow, shallow embayments

The effect of wind waves on the steady wind-driven circulation in a narrow, shallow bay is investigated with a two-dimensional ( y , z ) circulation model and the Grant and Madsen [1979] bottom-boundary layer model, which includes wave-current interaction. A constant wind stress is applied in the along-channel x direction to a channel with a constant cross-sectional profile h ( y ). The wind-induced flushing of shallow bays is shown to be sensitive to both the shape of the cross section and the effects of surface waves. The flushing increases with increasing , where h ′ is the standard deviation of cross-channel depth and is the mean depth. This is consistent with the findings of Hearn et al. [1987]. The flushing decreases, however, with the inclusion of surface wave effects which act to increase the bottom drag felt by the currents. Increasing effective bottom friction reduces the strength of the circulation, while the along-bay surface slope, bottom stress and the structure of current profiles remain nearly unchanged. An implication of the circulation dependence on wave-current interaction is that low-frequency oscillatory winds may drive a mean circulation when the wave field changes with wind direction. x

Journal of Geophysical Research C: Oceans

Science strategy for Core Science Systems in the U.S. Geological Survey, 2013-2023

Core Science Systems is a new mission of the U.S. Geological Survey (USGS) that grew out of the 2007 Science Strategy, “Facing Tomorrow’s Challenges: U.S. Geological Survey Science in the Decade 2007–2017.” This report describes the vision for this USGS mission and outlines a strategy for Core Science Systems to facilitate integrated characterization and understanding of the complex earth system. The vision and suggested actions are bold and far-reaching, describing a conceptual model and framework to enhance the ability of USGS to bring its core strengths to bear on pressing societal problems through data integration and scientific synthesis across the breadth of science. The context of this report is inspired by a direction set forth in the 2007 Science Strategy. Specifically, ecosystem-based approaches provide the underpinnings for essentially all science themes that define the USGS. Every point on earth falls within a specific ecosystem where data, other information assets, and the expertise of USGS and its many partners can be employed to quantitatively understand how that ecosystem functions and how it responds to natural and anthropogenic disturbances. Every benefit society obtains from the planet—food, water, raw materials to build infrastructure, homes and automobiles, fuel to heat homes and cities, and many others, are derived from or effect ecosystems. The vision for Core Science Systems builds on core strengths of the USGS in characterizing and understanding complex earth and biological systems through research, modeling, mapping, and the production of high quality data on the nation’s natural resource infrastructure. Together, these research activities provide a foundation for ecosystem-based approaches through geologic mapping, topographic mapping, and biodiversity mapping. The vision describes a framework founded on these core mapping strengths that makes it easier for USGS scientists to discover critical information, share and publish results, and identify potential collaborations that transcend all USGS missions. The framework is designed to improve the efficiency of scientific work within USGS by establishing a means to preserve and recall data for future applications, organizing existing scientific knowledge and data to facilitate new use of older information, and establishing a future workflow that naturally integrates new data, applications, and other science products to make it easier and more efficient to conduct interdisciplinary research over time. Given the increasing need for integrated data and interdisciplinary approaches to solve modern problems, leadership by the Core Science Systems mission will facilitate problem solving by all USGS missions in ways not formerly possible. The report lays out a strategy to achieve this vision through three goals with accompanying objectives and actions. The first goal builds on and enhances the strengths of the Core Science Systems mission in characterizing and understanding the earth system from the geologic framework to the topographic characteristics of the land surface and biodiversity across the nation. The second goal enhances and develops new strengths in computer and information science to make it easier for USGS scientists to discover data and models, share and publish results, and discover connections between scientific information and knowledge. The third goal brings additional focus to research and development methods to address complex issues affecting society that require integration of knowledge and new methods for synthesizing scientific information. Collectively, the report lays out a strategy to create a seamless connection between all USGS activities to accelerate and make USGS science more efficient by fully integrating disciplinary expertise within a new and evolving science paradigm for a changing world in the 21st century.

Open-File Report

Documentation of the U.S. Geological Survey sea floor stress and sediment mobility database

The U.S. Geological Survey Sea Floor Stress and Sediment Mobility Database contains estimates of bottom stress and sediment mobility for the U.S. continental shelf. This U.S. Geological Survey database provides information that is needed to characterize sea floor ecosystems and evaluate areas for human use. The estimates contained in the database are designed to spatially and seasonally resolve the general characteristics of bottom stress over the U.S. continental shelf and to estimate sea floor mobility by comparing critical stress thresholds based on observed sediment texture data to the modeled stress. This report describes the methods used to make the bottom stress and mobility estimates, statistics used to characterize stress and mobility, data validation procedures, and the metadata for each dataset and provides information on how to access the database online.

Open-File Report

IOOS modeling subsystem: vision and implementation strategy

Numerical modeling is vital to achieving the U.S. IOOS® goals of predicting, understanding and adapting to change in the ocean and Great Lakes. In the next decade IOOS should cultivate a holistic approach to coastal ocean prediction, and encourage more balanced investment among the observing, modeling and information management subsystems. We believe the vision of a prediction framework driven by observations, and leveraging advanced technology and understanding of the ocean and Great Lakes, would lead to a new era for IOOS that would not only produce more powerful information, but would also capture broad community support, particularly from the general public, thus allowing IOOS to develop into the comprehensive information system that was envisioned at the outset.

Book

Priorities for IOOS ® Data Management and Communications (DMAC)

Dramatic increases in the volume of online data and rapid advances in information technology have transformed many aspects of our society. In the coastal ocean, the amount of data is also growing dramatically due to new sensor and modeling technologies. Lagging behind this deluge of ocean data, however, is an effective framework of standards, protocols, tools and culture needed to transform the way we generate knowledge and value from ocean data. The Data Management and Communications (DMAC) sub-system was envisioned to provide such an information management capability for IOOS®, promoting standards and policies to be implemented by data providers across the IOOS enterprise. DMAC needs to build upon the successes and lessons learned during development of web service standards and promote a set of end-to-end standards and procedures for the entire ocean-data life cycle, including documentation through metadata, quality control and quality assurance, effective data discovery, and stewardship through archiving. Because information technology is constantly changing, a multiyear, top-down design and implementation plan is not workable. DMAC should start by promoting a set of protocols that are functional for specific use cases, creating a modular framework in which modules can be replaced as technologies change. In addition to promoting protocols, DMAC needs to support training, flexible online documentation, support, and social networking that enable users to share code, techniques and experiences. Through this bottom-up approach, trust and understanding will foster adoption by the community. Finally, a compliance and certification process should be developed that allows IOOS to ensure that they meet the needs of customers and other stakeholders while complying with regulatory requirements related to the data. If this approach is followed, we will enable breakthroughs in ocean data–driven technology similar to those common elsewhere in our society, fulfilling the broader mission of IOOS.

Book

Workshop discusses community models for coastal sediment transport

Numerical models of coastal sediment transport are increasingly used to address problems ranging from remediation of contaminated sediments, to siting of sewage outfalls and disposal sites, to evaluating impacts of coastal development. They are also used as a test bed for sediment-transport algorithms, to provide realistic settings for biological and geochemical models, and for a variety of other research, both fundamental and applied. However, there are few full-featured, publicly available coastal sediment-transport models, and fewer still that are well tested and have been widely applied. This was the motivation for a workshop in Woods Hole, Massachusetts, on June 22–23, 2000, that explored the establishment of community models for coastal sediment-transport processes.

Eos Science News

Serving ocean model data on the cloud

The NOAA-led Integrated Ocean Observing System (IOOS) and the NSF-funded Ocean Observatories Initiative Cyberinfrastructure Project (OOI-CI) are collaborating on a prototype data delivery system for numerical model output and other gridded data using cloud computing. The strategy is to take an existing distributed system for delivering gridded data and redeploy on the cloud, making modifications to the system that allow it to harness the scalability of the cloud as well as adding functionality that the scalability affords.

Oceans Conference Record

Ocean and coastal data management

We introduce data management concepts, including what we mean by "data" and its "management," sources of data, interoperability, and data geometry. We then discuss various components of a data management system. Finally, we summarize some existing ocean and coastal data management efforts. We make specific recommendations throughout the paper. We are generally optimistic that ocean and coastal data management is an interesting and solvable challenge that will provide great benefit to society.

Conference Paper

Connections Among the Spatial and Temporal Structures in Tidal Currents, Internal Bores, and Surficial Sediment Distributions Over the Shelf off Palos Verdes, California

The topography of the Continental Shelf in the central portion of the Southern California Bight has rapid variations over relatively small spatial scales. The width of the shelf off the Palos Verdes peninsula, just northwest of Los Angeles, California, is only 1 to 3 km. About 7 km southeast of the peninsula, the shelf within San Pedro Bay widens to about 20 km. In 2000, the Los Angeles County Sanitation District began deploying a dense array of moorings in this complex region of the central Southern California Bight to monitor local circulation patterns. Moorings were deployed at 13 sites on the Palos Verdes shelf and within the northwestern portion of San Pedro Bay. At each site, a mooring supported a string of thermistors and an adjacent bottom platform housed an Acoustic Doppler Current Profiler. These instruments collected vertical profiles of current and temperature data continuously for one to two years. The variable bathymetry in the region causes rapid changes in the amplitudes and spatial structures of barotropic tidal currents, internal tidal currents, and in the associated nonlinear baroclinic currents that occur at approximate tidal frequencies. The largest barotropic tidal constituent is M2, the principal semidiurnal tide. The amplitude of this tidal current changes over fairly short along-shelf length scales. Tidal-current amplitudes are largest in the transition region between the two shelves; they increase from about 5 cm/s over the northern San Pedro shelf to nearly 10 cm/s on the southern portion of the Palos Verdes Shelf. Tidal-current amplitudes are then reduced to less than 2 cm/s over the very narrow section of the northern Palos Verdes shelf that lies just 6 km upcoast of the southern sites. Models suggest that the amplitude of the barotropic M2 tidal currents, which propagate toward the northwest primarily as a Kelvin wave, is adjusting to the short topographic length scales in the region. Semidiurnal sea-level oscillations are, as expected, independent of these topographic variations; they have a uniform amplitude and phase structure over the entire region. Because the cross-shelf angle of the seabed over most of the Palos Verdes shelf is 1 to 3 degrees, which is critical for the local generation and/or enhancement of nonlinear characteristics in semidiurnal internal tides, some internal tidal-current events have strong asymmetric current oscillations that are enhanced near the seabed. Near-bottom currents in these events are directed primarily offshore with amplitudes that exceed 30 cm/s. The spatial patterns in these energetic near-bottom currents have fairly short-length scales. They are largest over the inner shelf and in the transition region between the Palos Verdes and San Pedro shelves. This spatial pattern is similar to that found in the barotropic tidal currents. Because these baroclinic currents have an approximate tidal frequency, an asymmetric vertical structure, and a somewhat stable phase, they can produce a non-zero depth-mean flow for periods of a few months. These baroclinic currents can interact with the barotropic tidal current and cause an apparent increase (or decrease) in the estimated barotropic tidal-current amplitude. The apparent amplitude of the barotropic tidal current may change by 30 to 80 percent or more in a current record that is less than three months long. The currents and surficial sediments in this region are in dynamic equilibrium in that the spatial patterns in bottom stresses generated by near-bed currents from surface tides, internal tides, and internal bores partly control the spatial patterns in the local sediments. Coarser sediments are found in the regions with enhanced bottom stresses (that is, over the inner shelf and in the region between the Palos Verdes and San Pedro shelves). Finer sediments are found over the northwestern portion of the Palos Verdes shelf, where near-bottom currents are relatively weak. The nonlinear asymmetries in the i

Scientific Investigations Report

Role of sediment resuspension in the remobilization of particulate-phase metals from coastal sediments

The release of particulate-phase trace metals due to sediment resuspension has been investigated by combining erosion chamber experiments that apply a range of shear stresses typically encountered in coastal environments with a shear stress record simulated by a hydrodynamic model. Two sites with contrasting sediment chemistry were investigated. Sediment particles enriched in silver, copper, and lead, 4−50 times greater than the bulk surface-sediment content, were the first particles to be eroded. As the shear-stress level was increased in the chamber, the total mass eroded increased, but the enrichment of these trace metals fell, approaching the bulk-sediment content. From the temporal distribution of shear stress generated by the hydrodynamic model for a site in Boston Harbor, resuspension fluxes were estimated. The erosion threshold of this site is exceeded during spring tides, releasing the particles enriched in trace metals into the water column. Due to the higher trace metal content and the regularity of resuspension, low-energy resuspension events (up to a shear stress of 0.2 N/m 2 ) contribute up to 60% of the resuspension metal flux in an average year. The estimated annual quantity of copper and lead resuspended into the water column is higher than estimates of the total riverine flux for these metals. These results indicate that sediment resuspension is a very important mechanism for releasing metals into the water column and provide new insight into the chemical and physical processes controlling the long-term fate of trace metals in contaminated sediments.

Massachusetts

Processes influencing the transport and fate of contaminated sediments in the coastal ocean– Boston Harbor and Massachusetts Bay

Most of the major urban centers of the United States including Boston, New York, Washington, Chicago, New Orleans, Miami, Los Angeles, San Francisco, and Seattle—are on a coast (fig. 1.1). All of these cities discharge treated sewage effluent into adjacent waters. In 2000, 74 percent of the U.S. population lived within 200 kilometers (km) of the coast. Between 1980 and 2002, the population density in coastal communities increased approximately 4.5 times faster than in noncoastal areas of the U.S. (Perkins, 2004). More people generate larger volumes of wastes, increase the demands on wastewater treatment, expand the area of impervious land surfaces, and use more vehicles that contribute contaminants to street runoff. According to the National Coastal Condition Report II (U.S. Environmental Protection Agency, 2005a), on the basis of coastal habitat, water and sediment quality, benthic index, and fish tissue, the overall national coastal condition is only poor to fair and the overall coastal condition in the highly populated Northeast is poor. Scientific information helps managers to prioritize and regulate coastal-ocean uses that include recreation, commercial fishing, transportation, waste disposal, and critical habitat for marine organisms. These uses are often in conflict with each other and with environmental concerns. Developing a strategy for managing competing uses while maintaining sustainability of coastal resources requires scientific understanding of how the coastal ocean system behaves and how it responds to anthropogenic influences. This report provides a summary of a multidisciplinary research program designed to improve our understanding of the transport and fate of contaminants in Massachusetts coastal waters. Massachusetts Bay and Boston Harbor have been a focus of U.S. Geological Survey (USGS) research because they provide a diverse geographic setting for developing a scientific understanding of the geology, geochemistry, and oceanography of coastal systems in general. Scientific data from this region can also be used to inform decisions about important economic, environmental, and political issues. From the economic viewpoint, the annual value of tourism and shipping in Massachusetts and Cape Cod Bays is about $1.5 billion and $1.9 billion, respectively. Commercial and recreational fishing generates about $240 million per year in the same region (U.S. Environmental Protection Agency, 2005b). The environmental issue is the 300-year history of waste discharge from the Boston metropolitan area into the harbor. This history is punctuated by cycles of environmental degradation, public outcry, and improvements in the sewage treatment system. With each improvement, however, the continuous growth of population in greater Boston (fig. 1.2) and the resulting increase in the volume of waste exceeded the capacity of the treatment system, thereby setting the stage for a new contamination crisis. By the 1980s, the levels of contaminants in sediments of Boston Harbor were among the highest in the nation (National Oceanic and Atmospheric Administration, 1987). Fish were diseased, shellfish beds were closed, and swimming beaches were unsafe after heavy rains; in general, water quality and aesthetics were below acceptable standards. Legal and political issues have always been part of Boston Harbor’s history. The environmental conditions in the 1980s were highlighted in a 1983 legal suit brought by the city of Quincy against the Metropolitan District Commission (MDC, the state agency responsible for sewage treatment) and heads of three state agencies for discharging untreated or poorly treated sewage into the harbor (Dolin, 2004). The suit never went to trial, but through the actions of a Massachusetts Superior Court, the issue of Boston Harbor contamination remained on the political and public agenda. The judge called the harbor “unsafe, unsanitary, indecent, in violation of the law (Clean Water Act), and a danger to the health and welfare of the people” (Forman, 1984). To force the state legislature to implement a plan to improve harbor conditions, the judge threatened to place the MDC in receivership and curtail new sewage hookups for industry. Under intense lobbying by business, the legislature created the Massachusetts Water Resources Authority (MWRA) in December 1984. The independent MWRA was established to manage Boston’s waste treatment system and was given the authority to float bonds to pay for major improvements in the treatment system. In 1985, a Federal court began hearings on a suit brought by the Conservation Law Foundation, the Environmental Protection Agency (USEPA), and towns of Quincy and Winthrop against the MDC and MWRA (as heir to responsibilities of the MDC) for years of violation of the Clean Water Act. The judge ruled against the defendants and required all the parties to submit a construction plan and schedule for a new sewage treatment system. From these submissions, he developed a schedule for treatment system upgrades that would give the “citizens of this commonwealth a public assurance that Boston Harbor will be cleaned up within a defined period of time” (Dolin, 2004). The MWRA’s Boston Harbor cleanup program (Levy and Connor, 1992) has transformed the Boston sewage system. Key improvements were to (1) reduce contaminants at the industrial source; (2) remediate leaks in the sewage-collection system; (3) eliminate sewage sludge discharge to the harbor; (4) upgrade sewage treatment from primary to secondary; (5) construct a new ocean outfall 15.2 km offshore in Massachusetts Bay for discharge of treated effluent (fig. 1.3); and (6) implement improvements in the combined-sewer-overflow system. As part of the harbor cleanup program, the MWRA developed a comprehensive monitoring program (summarized in MWRA, 2004) to assess changes in the harbor and bays that specifically related to the new sewage system. Additional information about conditions and processes in the coastal system on a regional scale and over a long time period was and continues to be important in predicting and interpreting local change. Implementation of the MWRA’s program and the mission of the USGS to understand the geology of the nation’s offshore waters provided an opportunity to conduct a cooperative multidisciplinary research program. This USGS program addresses basic scientific questions as well as concerns raised by management regarding the design, implementation, and assessment of the new sewage treatment system. Already active in Boston Harbor during the late 1970s, the USGS expanded research into Massachusetts Bay with a multidisciplinary program in 1989.

Massachusetts

Surficial geology in central Narragansett Bay, Rhode Island: interpretations of sidescan sonar and multibeam bathymetry

The United States Geological Survey (USGS) is working cooperatively with the National Oceanic and Atmospheric Association (NOAA) to interpret the surficial geology in estuaries along the coast of the northeastern United States. The purpose of our present study is to determine the distributions of surficial sediments and sedimentary environments in two areas of Narragansett Bay, Rhode Island, using sidescan sonar imagery, high-resolution bathymetry, and sediment data. This study provides a framework for future studies on topics such as benthic habitats and oceanographic processes that control the transport and distribution of bottom sediments. This study mapped two separate areas totalling about 33 km² One area lies in West Passage between Plum Point, Quonset Point and Conanicut Island; the other area lies in East Passage around Dyer Island and extends south of Prudence Island.

Rhode Island

Sediment dynamics in the Adriatic Sea investigated with coupled models

Several large research programs focused on the Adriatic Sea in winter 2002-2003, making it an exciting place for sediment dynamics modelers (Figure 1). Investigations of atmospheric forcing and oceanic response (including wave generation and propagation, water-mass formation, stratification, and circulation), suspended material, bottom boundary layer dynamics, bottom sediment, and small-scale stratigraphy were performed by European and North American researchers participating in several projects. The goal of EuroSTRATAFORM researchers is to improve our ability to understand and simulate the physical processes that deliver sediment to the marine environment and generate stratigraphic signatures. Scientists involved in the Po and Apennine Sediment Transport and Accumulation (PASTA) experiment benefited from other major research programs including ACE (Adriatic Circulation Experiment), DOLCE VITA (Dynamics of Localized Currents and Eddy Variability in the Adriatic), EACE (the Croatian East Adriatic Circulation Experiment project), WISE (West Istria Experiment), and ADRICOSM (Italian nowcasting and forecasting) studies.

Adriatic Sea