Ata Bilgili

Numerical modeling of tides in the Great Bay Estuarine System: dynamical balance and spring-neap residual modulation

Abstract The Great Bay Estuarine System, in New Hampshire, USA, has been the focus area for an attempt to develop a robust finite element method model for estuarine hydrodynamics. Past studies used a nonlinear, time stepping, kinematic model with limited success (Ip et al. Advances in fluid mechanics III, WIT, Southampton (2000) 569; Bilgili et al. J. Geophys. Res. – Oceans 107 (2002); Erturk et al. Estuar. Coast. Shelf Sci. 47 (1998) 119). We add dynamic physics (that is, local accelerations) for deep-water areas and keep kinematic physics (that is, without local and advective accelerations), with the inclusion of a porous medium beneath the open channel, for shallow and dewatering areas. The choice of which physics set to apply is made on an elemental depth dependent basis. The addition of the local acceleration terms for deep-water areas is seen to greatly improve accuracy in matching of tidal phasing over previous studies. Simulations involving M2/M4/M6 tidal constituents result in strong agreement to observed data from the 1975 Great Bay field program (Swift & Brown, Estuar. Coast. Shelf Sci. 17 (1983) 297), in terms of both tidal heights and cross-section averaged velocities. Comparisons with 10 tidal elevation observation stations and four cross-section averaged current transects show good agreement, displaying average normalized root mean square misfit values of 0.08 and 0.25, respectively. Study of the simulated momentum balance shows the size of the contributions from acceleration terms to be on the order of a third the size of the contributions from the pressure gradient and bottom stress terms. Although relatively small, they are observed to peak at the crucial time of tidal reversal. Application of the model for long-term simulation using an M2/N2/S2 forcing shows the ability to realistically capture the spring–neap cycle. The tidally rectified flow is generally described as a constant spatial pattern with overall amplitude modulation following the spring–neap cycle.

Particles in the coastal ocean : theory and applications

Part I. Background: 1. The coastal ocean 2. Drifters and their numerical simulation 3. Probability and statistics - a primer 4. Dispersion by random walk 5. BCs, boundary layers, sources 6. Turbulence closure Part II. Elements: 7. Meshes: interpolation, navigation, and fields 8. Particles and fields Part III. Applications: 9. Noncohesive sediment - dense particles 10. Oil - chemically active particles 11. Individual-based models - biotic particles Part IV. Appendixes.

The use of Lagrangian particle methods to investigate ocean-estuary exchange in well-mixed estuaries

A Lagrangian particle method which has been parallelized and embedded within a 2-D finite element code is used to study the transport and fate of contaminant plumes and ocean-estuary exchange processes in a well-mixed Gulf of Maine estuary. The particle method has been extended to include a random walk model in the horizontal that simulates sub-grid scale turbulent transport processes. This module has been formulated to allow for spatial variability in the diffusivity. The 2-D finite element model includes a porous medium transport module to treat the effects of wetting and drying of estuarine tidal flats. Due to the highly-complex, spatially dependent nature of tidal mixing and shear dispersion, contaminant transport is most naturally addressed through the Lagrangian methodology. Our approach involves instantaneous, massive particle releases that enable the quantification of ocean-estuary and inter-bay exchange along with the associated residence time. The results show that estuary-ocean exchange is significantly enhanced, and hence residence times reduced, by the presence of turbulent mixing, which combines with the effects of the sheared tidal currents to drive strong interbay exchange, and/or river input, which drives a mean throughflow. The particle approach helps to uncover the strong spatial dependent nature of the residence time within the estuary which has important ramifications for local water quality. The interbay exchanges are considered as a Markov process as discussed by Thompson et al. [11] and this framework is found to be useful.