Tatsusaburo Isaji

A Model of the Tidally Induced Residual Circulation in the Gulf of Maine and Georges Bank

Abstract A three-dimensional nonlinear numerical hydrodynamic model using Legendre polynomials to represent the vertical structure of the horizontal currents has been used to study the tidally induced residual flows in the Gulf of Maine–Georges Bank study region using a 6.25 km square grid. Tidal elevations in terms of the M2 phase and amplitude along the open boundaries are specified using Schwiderskis deep ocean tidal model. The model predicts strong clockwise circulation gyres around Georges Bank and Nantucket Shoals with a weak gyre around Browns Bank. Strong inflow to the Gulf of Maine is predicted near the Southwestern tip of Nova Scotia. These results are in good agreement with recent model predictions of Greenberg.

Tidal exchange between a coastal lagoon and offshore waters

A field program was performed to determine the exchange of water between Ninigret Pond and Block Island Sound (BIS) at tidal and subtidal frequencies. Time series on sea level variations in BIS and Ninigret Pond, spatially integrated velocities across the breachway connecting the two (obtained by a GEK) and wind speed and direction were collected from April 21–June 3, 1980. A hybrid hydrodynamic model incorporating a simplified one-dimensional approximation for the breachway channel systems and a two-dimensional vertically-averaged finite element solution on triangular grids for the pond proper was used to model the ponds’ response to ocean forcing. Model predictions were in good agreement with the field data. Both indicated a factor of 5.5 reduction in the semidiurnal tidal amplitude, and a high water shift of 2.5 hr, relative to BIS. Model and field data also show that as the frequency of sea level forcing becomes lower the pond sea level response is in phase and of the same magnitude as that in BIS.

A Simplified Model for Assessing the Impact of Breachway Modifications on Coastal Pond Circulation and Flushing Dynamics

A simplified hydrodynamics and salt balance model system has been developed for a tidal inlet-costal pond system to predict the consequences of inlet modifications on the inlet flow rate, pond tidal range, and the pond salinity. The hydrodynamics portion of the model solves the momentum and continuity equations for the inlet where the pond behavior is incorporated in the inlet continuity relationship. The salt balance is determined using a tidal prism approach where complete mixing and steady state conditions are assumed. For a single inlet bay system and assuming no inertial acceleration term or fresh-water inflow, an approximate solution of the governing equations is presented. Extension of the model system to a multiple inlet-bay system, following the computational algorithm developed by Seelig et al, is presented and applied to the Charlestown Pond complex located on the southern Rhode Island coast.

A Numerical Model of the M2 and K1 Tide in the Northwestern Gulf of Alaska

Abstract A two-dimensional vertically averaged hydrodynamic model in spherical coordinates with 0.2° latitude, 0.35° longitude resolution was employed to predict the M2 and K1 tidal elevations and currents in the northwestern Gulf of Alaska; 50°N, 145°–168°W to the southern Alaskan coast. Boundary conditions for the model were derived from Schwiderskis global ocean tidal model and observations. Model predictions for the amplitude and phase of the sea elevations are generally within 5 cm and 2° of the observation for the M2 constituent and 7 cm and 5° for the K1. The M2 standing wave pattern in shelikof Strait and the tidal responance in Cook Inlet observed in earlier studies are confirmed by the numerical simulations.

Oil spill fishery impact assessment model: Sensitivity to spill location and timing

Abstract An oil spill fishery impact assessment model system has been applied to the Georges Bank-Gulf of Maine region to assess the sensitivity of probable impact on several key fisheries to spill location and timing. Simulations of the impact on the fishery of tanker spills (20 million gallons released over 5 days), at two separate locations for each season of the year, and blowout spills (68 million gallons released over 30 days) at one location, with monthly releases and at six other locations with seasonal spills have been studied. Atlantic cod has been employed as the principal fish species throughout the simulations. Impacts on Atlantic herring and haddock have also been investigated for selected cases. All spill sites are located on Georges Bank with the majority in the general region of OCS leasing activity. The results of these simulations suggest a complex interaction among spill location and timing, the spatial and temporal distribution of spawning, the population dynamics of the species under study, and the hydrodynamics of the area. For the species studied, spills occurring during the winter and spring have the largest impact with cod being the most heavily impacted followed by haddock and herring. In all cases, the maximum cumulative loss to the fishery of a one time spill event never exceeded 25% of the annual catch with the exact value depending on the number of ichthyoplankton impacted by the spill and the compensatory dynamics of the population.

Transport and Fate of Sediment Suspended from Jetting Operations for Undersea Cable Burial

Cable laying on the ocean bottom, particularly in coastal areas, is increasingly used to avoid the routing problems found on land in densely developed coastal zones. One technique used for sand or relatively soft sediments is the use of a jet plow system. During the jetting process a fraction of the sediment is introduced into the water column which may be transported by currents away from the site before settling back down to the sea floor. A computer simulation model, SSFATE, was developed and used to simulate the resulting sediment water column concentration and resulting deposition from jet plow operations, Applications of the model are presented for cable burial operations in the northeast United States for typically non-cohesive sediments

Application of a Sediment Transport Model (BFSED) to Assess the Impact from Bridge Reconstruction in the Barrington River, RI

The study demonstrates a complete modeling system for sediment transport computations. The system contains an implementation of the BFSED model within a map-based, graphical user interface including a sophisticated, boundary -fitted hydrodynamics model (BFHYDRO), tools for collecting and manipulating input data and analyzing and animating model results. The hydrodynamic model predicts that changes in the tidal current velocity as a result of the proposed bridge causeway configuration are limited to the area near the Route 114 Bridge. Opening the existing west causeway results in a decrease in current speed across the bridge opening, but an increase in current speed over an area adjacent to the section removed from the west causeway. The area of increased current5 speed is a potential new source of sediment erosion and transport up and down the river. The results of the coupled BFSED-hydrodynamic model predict a marked decrease in the speed of maximum flood currents across the bridge opening after construction of the replacement bridge. The hydrodynamic model also predicts an increase in tidal current speed in areas adjacent to the causeway sections to be removed, potentially changing the dominant process in these areas from net deposition to net erosion and causing a predicted redistribution of the eroded sediment to other parts of the estuary. The BFSED model was used to predict the maximum depth of erosion expected to occur in the areas of increased flow, the volume of sediment eroded from these areas, and the spatial distribution of the resulting sediment deposits.

AN OIL SPILL MODEL SYSTEM FOR ARCTIC WATERS

ABSTRACT An Arctic coastal sea model system, consisting of submodels for weather, hydrodynamics, waves, suspended sediment, ice cover, oil spill trajectory and fates, and ecosystem dynamics is presented. The structure and operation of each sub-model and linkages among the various model components are briefly described. The weather, hydrodynamic, ice, and oil spill fates components of the model system are applied to predict hydrodynamics and oil spill trajectories from selected release points in the Bering Sea. Trajectory data are analyzed to describe the percent of trajectories affecting land and the direction, length, and duration distribution of trajectories. A detailed sensitivity study is performed to determine the role of the various mean current components and wind drift effects in describing spill trajectories. Model predictions highlight the importance of accurately representing the environmental data used as input to the model.

Stepwise-Continuous-Variable-Rectangular Grid

This paper describes a newly developed horizontal gridding method, the stepwise-continuous-variable-rectangular grid (SCVR). The gridding approach is based on simple rectangular gridding and has some similarity to a standard finite difference nesting structure. Unlike typical nested grids, which allow only a limited level of refinement, one-way propagation, and in some cases require separate model executions; the SCVR gridding strategy permits unlimited refinement, consistent integration, and a single model execution. The SCVR strategy will be demonstrated by application to a barotropic long wave propagation model. The advantage of this approach is that large areas of widely differing spatial scales can be addressed within one consistent model application. Grids constructed by the SCVR are still structured, so that arbitrary locations can be easily located to corresponding computational cells. This mapping facility is particularly advantageous when outputs of the hydrodynamics model propagate to subsequent application programs (e.g. Lagrangian particle transport model) that use another grid or grid structure.