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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.

A highly efficient finite element model has been developed for the numerical prediction of depth average circulation within small scale embayments which are often characterized by irregular boundaries and bottom topography., Traditional finite element models use time-stepping and have been plagued with requirements for high eddy viscosity coefficients and small time steps necessary to insure numerical stability, making application to small bays infeasible. These problems are overcome by operating in the frequency domain, an intrinsically more natural solution procedure for a highly periodic process such as tidal forcing. In order to handle non-linearities, an iterative scheme which updates non-linearities as right hand side force loadings must be implemented., Pioneering efforts with the harmonic approach have had shortcomings in either not modeling all physically relevant terms and/or in not gearing towards application to small scale regions. Small embayments are often quite shallow and have rapidly varying depth, making the nonlinear terms in the governing hydrodynamic equations much more significant. This requires that more frequencies be used in order to resolve the tide and account for the greater nonlinear coupling due to bottom friction, convective acceleration and finite amplitude effects. In order to make the process of handling this wide range of frequencies manageable, a hybrid frequency-time domain approach is applied. The iterative scheme revolves around a highly efficient linear core code which can handle a wide range of frequencies. Furthermore, instead of Fourier expanding the nonlinear terms, an efficient least squares error minimization algorithm is used for the discrete spectral analysis of the iteratively updated psuedo-force time history generated by the nonlinearities., With this highly efficient scheme it is now possible to efficiently study both short period and long term residual circulation within small scale embayments., Sponsored by Northeast Utilities Service Company and New England Power Service Company under the MIT Energy Laboratory Electric Utitility Program and by The Sea Grant Office of NOAA, U.S. Department of Commerce.

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.

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.

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.