Predictions of sediment trap biases in turbulent flows: A theoretical analysis based on observations from the literature

The physical variables affecting the trapping of particles in sediment collectors are grouped into a set of six dimensionless parameters, as a function of a dimensionless particle collection efficiency. Relevant laboratory calibration studies on sediment trap biases are evaluated to determine the quantitative dependence between collection efficiency and three of the parameters, trap Reynolds number, the ratio of flow speed to particle fall velocity and the ratio of trap height to mouth diameter, as well as trap geometry. We find that few of the parameters have been systematically tested in the laboratory and that trap Reynolds number-similarity for field conditions is maintained only for the slowest flow speeds and/or smallest trap diameters. However, the literature results do suggest some intriguing trends in biased trapping which also can be explained physically. The physical mechanisms are derived from a physical description of particle trapping based on observations of flow through traps, the mass balance for particles entering and leaving traps and a definition of particle collection efficiency, coupled with model development for cases where collection efficiency, as specified by the mass balance, deviates from one.The following testable hypotheses for biased trapping by unbaffied, straight-sided cylinders and noncylindrical traps result from our analysis. For fixed values of the other two parameters, collection efficiency of cylinders will decrease over some range of increasing trap Reynolds number, decrease over some range of decreasing particle fall velocity and increase over some range of increasing trap aspect ratio. Traps will be undercollectors or overcollectors depending on the physical mechanisms causing the biased collections. Predicting biased collections for noncylindrical traps is more complex but, in most cases, small-mouth, wide-body traps will be overcollectors and funnel-type traps will be undercollectors. Future laboratory studies are required to test these hypotheses and, in particular, parameter combinations representative of field conditions, where traps are deployed, must be tested

Aggregation of Fine Particles at the Sediment-Water Interface

The presence of a bottom sediment layer agitated by mechanical stirring or by resident organisma (tubificid oligochaetes) significantly increases the rate at which fine (1 µm) cohesive particles are removed from suspension in laboratory columns. Measured rates of particle removal are equivalent to deposition velocities ranging from 0.23 m per day to 0.41 m per day. These rates are an order of magnitude faster than deposition by gravitational settling or coagulation with larger particles in the water column as observed in experimental controls. It is hypothesized that the increased removal rate is the result of aggregation in a sediment layer at the bed-water interface characterized by loosely bound (fluffy), porous material hydrodynamically coupled to the water column. According to this hypothesis particle removal occurs when motion of the overlying water or organism activity causes suspended fine particles to collide with and stick to the interfacial sediment. This new hypothesis is supported by the mass and size distribution of tracer particles recovered in cores and sediment traps at a coastal site and by theoretical estimates of interfacial aggregation rates.This work was supported by EPA grant CR-81181-01-01, by the MIT Sea Grant College Program, under NOAA Grant NA86AA-D-SG089, and by a postgraduate scholarship awarded to the second author by the Natural Sciences and Engineering Research Council of Canada.https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/92JC0182

Operational issues involving use of supplementary cooling towers to meet stream temperature standards with application to the Browns Ferry Nuclear Plant

A mixed mode cooling system is one which operates in either the open, closed, or helper (once-through but with the use of the cooling towers) modes. Such systems may be particularly economical where the need for supplementary cooling to meet environmental constraints on induced water temperature changes is seasonal or dependent upon other transient factors such as stream- flow. The issues involved in the use of mixed mode systems include the design of the open cycle and closed cycle portions of the cooling system, the specification of the environmental standard to be met, and the monitoring system and associated decision rules used to determine when mode changes are necessary. These issues have been examined in the context of a case study of TVA's Browns Ferry Nuclear Plant which utilizes the large quantity of site specific data reflecting conditions both with and without plant operation. The most important findings of this study are: (1) The natural temperature differences in the Tennessee River are of the same order of magnitude (50F) as the maximum allowed induced temperature increase. (2) Predictive estimates based on local hydrological and meteorological data are capable of accounting for 40% of the observed natural variability. (3) Available algorithms for plant induced temperature increases provide estimates within 1F of observed values except during periods of strong stratification. (4) A mixed mode system experiences only 10% of the capacity losses experienced by a totally closed system, (5) The capacity loss is relatively more sensitive to the environmental standard than to changes in cooling system design. (6) About one third of the capacity loss incurred using the mixed mode system is the result of natural temperature variations. This unnecessary loss may be halved by the use of predictive estimates for natural temperature differences

Eulerian-Lagrangian analysis of pollutant transport in shallow water

A numerical method for the solution of the two-dimensional, unsteady, transport equation is formulated, and its accuracy is tested.The method uses a Eulerian-Lagrangian approach, in which the transport equation is divided into a diffusion equation (solved by a finite element method) and a convection equation (solved by the method of characteristics). This approach leads to results that are free of spurious oscillations and excessive numerical damping, even in the case where advection strongly dominates diffusion. For pure diffusion problems, optimal accuracy is approached as the time-step, At, goes to zero; conversely, for pure-convection problems, accuracy improves with increasing At; for convection-diffusion problems the At leading to optimal accuracy depends on the characteristics of the spatial discretization and on the relative importance of convection and diffusion.The method is cost-effective in modeling pollutant transport in coastal waters, as demonstrated by two prototype applications: hypothetical sludge dumping in Massachusetts Bay and the thermal discharge from Brayton Point Generating Station in Narragansett Bay. Numerical diffusion is eliminated or greatly reduced, raising the need for realistic estimation of dispersion coefficients. Costs (based on CPU time) should not exceed those of conventional Eulerian methods and, in some cases (e.g., problems involving predictions over several tidal cycles), considerable savings may even be achieved

Eddy-resolving simulation of plankton ecosystem dynamics in the California Current System

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 53 (2006): 1483-1516, doi:10.1016/j.dsr.2006.06.005.We study the dynamics of the planktonic ecosystem in the coastal upwelling zone within the California Current System using a three-dimensional, eddy-resolving circulation model coupled to an ecosystem/biogeochemistry model. The physical model is based on the Regional Oceanic Modeling System (ROMS), configured at a resolution of 15 km for a domain covering the entire U.S. West Coast, with an embedded child grid covering the central California upwelling region at a resolution of 5 km. The model is forced with monthly mean boundary conditions at the open lateral boundaries as well as at the surface. The ecological/biogeochemical model is nitrogen based, includes single classes for phytoplankton and zooplankton, and considers two detrital pools with different sinking speeds. The model also explicitly simulates a variable chlorophyll-to-carbon ratio. Comparisons of model results with either remote sensing observations (AVHRR, SeaWiFS) or in situ measurements from the CalCOFI program indicate that our model is capable of replicating many of the large-scale, time averaged features of the coastal upwelling system. An exception is the underestimation of the chlorophyll levels in the northern part of the domain, perhaps because of the lack of short-term variations in the forcing from the atmosphere. Another shortcoming is that the modeled thermocline is too diffuse, and that the upward slope of the isolines toward the coast is too small. Detailed time-series comparisons with observations from Monterey Bay reveal similar agreements and discrepancies. We attribute the good agreement between the modeled and observed ecological properties in large part to the accuracy of the physical fields. In turn, many of the discrepancies can be traced back to our use of monthly mean forcing. Analysis of the ecosystem structure and dynamics reveal that the magnitude and pattern of phytoplankton biomass in the nearshore region are determined largely by the balance of growth and zooplankton grazing, while in the offshore region, growth is balanced by mortality. The latter appears to be inconsistent with in situ observations and is a result of our consideration of only one zooplankton size class (mesozooplankton), neglecting the importance of microzooplankton grazing in the offshore region. A comparison of the allocation of nitrogen into the different pools of the ecosystem in the 3-D results with those obtained from a box model configuration of the same ecosystem model reveals that only a few components of the ecosystem reach a local steady-state, i.e. where biological sources and sinks balance each other. The balances for the majority of the components are achieved by local biological source and sink terms balancing the net physical divergence, confirming the importance of the 3-D nature of circulation and mixing in a coastal upwelling system.Most of this work has been made possible by two grants from NASA. Additional support is acknowledged from NSF’s ITR program

First-Order Contaminant Removal in the Hyporheic Zone of Streams: Physical Insights from a Simple Analytical Model

A simple analytical model is presented for the removal of stream-borne contaminants by hyporheic exchange across duned or rippled streambeds. The model assumes a steady-state balance between contaminant supply from the stream and first-order reaction in the sediment. Hyporheic exchange occurs by bed form pumping, in which water and contaminants flow into bed forms in high-pressure regions (downwelling zones) and out of bed forms in low-pressure regions (upwelling zones). Model-predicted contaminant concentrations are higher in downwelling zones than upwelling zones, reflecting the strong coupling that exists between transport and reaction in these systems. When flow-averaged, the concentration difference across upwelling and downwelling zones drives a net contaminant flux into the sediment bed proportional to the average downwelling velocity. The downwelling velocity is functionally equivalent to a mass transfer coefficient, and can be estimated from stream state variables including stream velocity, bed form geometry, and the hydraulic conductivity and porosity of the sediment. Increasing the mass transfer coefficient increases the fraction of streamwater cycling through the hyporheic zone (per unit length of stream) but also decreases the time contaminants undergo first-order reaction in the sediment. As a consequence, small changes in stream state variables can significantly alter the performance of hyporheic zone treatment systems

The effect of natural water temperature variation on the monitoring and regulation of thermal discharge impacts : the role of predictive natural temperature models

Pollution control policies have been an outgrowth of increased awareness that measures must be taken to handle the increasing amounts of wastes and by-products of human activity. A particular problem in the policies is how to address wastes that have large natural variations due to natural sources and changing environmental conditions. This is especially true for the control of thermal discharges from steam-electric generating facilities into large bodies of water also influenced by solar heating and inflows of water from natural sources.The basis for most pollution control policies in the United States is the set of regulations specifying ambient and effluent standards. Technology-based effluent standards have been increasingly used to provide a conservative basis for environmental protection. Ambient standards, based on impacts on humans or other life forms, however provide a viable regulatory approach for those effluents with costly treatment, particularly where large natural variability indicates the environment has a significant capacity to assimilate additional inputs. A major problem with ambient temperature standards indicated by two case studies of large thermal discharges, is the variability in induced and natural conditions which affect facility siting, design, and operation, and verification of compliance.The Browns Ferry Nuclear Plant is an example of a large thermal discharge into a varying river environment. The final set of ambient temperature limiting standards for the site which have values near naturally occurring conditions, required the owners of the plant to redesign the heat dissipation system. The final design included the use of supplemental cooling (open, helper, or closed mode) to provide flexible plant operation under varying river flow conditions. Problems with real-time monitoring for compliance with the standards led to a study of various methods of verification. Simulation of plant operation found that adjusting the standards higher than naturally occurring values had larger effects than various monitoring strategies utilizing spatial and temporal averaging. A one-dimensional natural change in temperature model used in conjunction with real-time monitoring reduced power losses due to natural variation by about one half, but could not account for all the short-term variations in natural temperatures caused by topographic and river flow changes and density effects.The Millstone Nuclear Power Station, located in a coastal environment is an example of a thermal discharge into an area with relatively constant long-term mixing conditions. Concerns over natural temperature variation were present throughout the site's history, although this has not affected plant operation since the ambient standards, based on biological evidence, were set to include full open-cycle operation. A natural temperature model, based on finite element circulation and dispersion models was developed as one means of addressing the natural variability issue. The model produced reasonable resolution of the horizontal temperature distribution and relative changes over a tidal cycle. The model had some limitations in those areas where solar heating significantly affects the vertical temperature distribution. If properly combined with baseline temperature monitoring, the natural temperature model provides an assessment tool for characterizing the physical environment around a thermal discharge. It also has potential in verification of compliance by combining with thermal plume monitoring and modeling efforts to define the ambient baseline conditions and the effects of natural conditions on the extent of a thermal plume.It is recommended that ambient standards continue to be used in the control of thermal discharges to take into account the natural assimilative capacity of large bodies of water. Real-time monitoring of compliance with maximum rise temperature standards should not be used in areas of high natural variability. Natural temperature models which cannot adequately predict highly variable situations should not be used to correct real-time monitoring efforts. Therefore, flexible effluent standards which adapt to large changing conditions should be used based on modeled plant effects and potential biological impacts. Natural temperature modeling (including extensive monitoring of baseline temperature conditions) in both preoperational and operational studies should be used to provide a balance of the understanding between physical and biological characteristics in complex environments

Modeling Water and Sediment Quality in the Coastal Ocean

The overall goal of the research project is to develop a model of water and sediment quality capable of forecasting environmental events occurring over small space and time scales in the coastal zone, including embayments, harbors, and shoreline regions

Modeling Water and Sediment Quality in the Coastal Ocean

The overall goal of the research project is to develop a model of water and sediment quality capable of forecasting environmental events occurring over small space and time scales in the coastal zone, including embayments, harbors, and shoreline regions