Modeling Tidal Flow In The Great Bay Estuary,New Hampshire, Using A Depth AveragedFlooding-dewatering Model With Application ToThe Bed Load Transport Of Coarse Sediments (original) (raw)
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Modeling bed-load transport of coarse sediments in the Great Bay Estuary, New Hampshire
Estuarine, Coastal and Shelf Science, 2003
Current, sea level and bed-load transport are investigated in the Lower Piscataqua River section of the Great Bay Estuary, New Hampshire, USAda well-mixed and geometrically complex system with low freshwater input, having main channel tidal currents ranging between 0.5 and 2 m s ÿ1. Current and sea level forced by the M 2 M 4 M 6 tides at the estuarine mouth are simulated by a vertically averaged, non-linear, time-stepping finite element model. The hydrodynamic model uses a fixed boundary computational domain and accounts for floodingedrying of tidal flats by making use of a groundwater component. Inertia terms are neglected in comparison with pressure gradient and bottom friction terms, which is consistent with the observed principal dynamic balance for this section of the system. The accuracy of hydrodynamic predictions in the study area is demonstrated by comparison with four tidal elevation stations and two cross-section averaged current measurements. Simulated current is then used to model bed-load transport in the vicinity of a rapidly growing shoal located in the main channel of the lower system. Consisting of coarse sand and gravel, the shoal must be dredged every five to eight years. Two approaches are takendan Eulerian parametric method in which nodal bed-load flux vectors are averaged over the tidal cycle and a Lagrangian particle tracking approach in which a finite number of sediment particles are released and tracked. Both methods yield pathways and accumulations in agreement with the observed shoal formation and the long-term rate of sediment accumulation in the shoal area.
Journal of Coastal Research, 2012
Acquisu ion of extensive current measurements in tide-dominated environments in order to predict sediment transport is difficult and expensive. In addition. instantaneous transport rates predicted from different sediment transport equations often disagree by several orders of'magnitude. Therefore, a numerical model was applied to a shallow inlet-basin system in order 1.0 make long-term predictions of currents and bed shear needed to integrate instantaneous sediment transport predictions and provide realistic estimates of net transport puuorns. After callbr-at ion of the model, good aWt.. .. ernent between observed and predicted tides and currents was obtained. Five empirical sediment transport equations were then coupled to the numerical mode) in order to compare the range of predicted sediment transport rates within the studv area. The transport rates predicted from the five different equations integrated over two spring -neap cycles result in largely flood-directed net sediment transport rates that are within one order of magnitude. The predicted net sediment transport pattern agrees well with observed conditions showing convergence of Iran sport in shoaling-areas and divcr gence in erosional areas.
Numerical simulation of tide-induced transport of heterogeneous sediments in the English Channel
Continental Shelf Research, 2010
The three-dimensional numerical model COHERENS (COupled Hydrodynamical-Ecolo -gical model for REgioNal and Shelf seas) has been adapted to compute the rates of transport as bedload and suspended load of heterogeneous bottom sediments induced by the dominant M 2 tide in the English Channel. A pre-processing of an extensive surficial sediments dataset has been performed to determine the seabed composition (grain-size distribution or presence of rocks) at the computational grid nodes. Maximum bedload and suspended load transport rates over the tidal cycle, as well as the contributions of the 10 different sedimentary classes to the mean transports are computed. Highest sediment transport rates occur in fine sediments areas located in the surroundings of high shear stresses areas. Medium sand (d 4 = 350 µm) is found to be predominant in bedload, while suspension load implies mainly silts (d 1 = 25 µm) in the inner shoreface and both fine and medium sands (d 3 = 150 µm, d 4 = 350 µm) in the outer shoreface. The offshore residual bedload transport pathways are orientated westerly in the western part of the Channel and easterly in the eastern part defining a "parting" zone which runs from the Isle of Wight to the Cotentin Peninsula. An offshore "bedload convergence" occurs in the southwest of the Dover Strait; a narrow transport pathway bypassing it along the French coastline. These features reproduce those predicted by and provide higher resolution features like inshore headland-induced gyres, particularly along the English coastline. The new predicted general pattern of residual suspended load transport is very similar to the bedload pattern. Differences arise in the central "divergence" zone which exhibits a "Y" shape with two branches ending on both sides of the Isle of Wight, in the Baie de Seine characterized by a central "convergence" and along the English coastline studded with many headland-induced recirculations.
Simulation of the Great Bay Estuarine System: Tides with tidal flats wetting and drying
1] Realistic lunar tides of the Great Bay Estuarine System have been simulated using a fixed boundary finite element numerical model as described by . It is a two-dimensional, nonlinear, time-stepping model with a groundwater component coupled to a kinematic force balance to facilitate the realistic drainage and filling of elements during ebb and flood, respectively. The numerical model reproduces the observed M 2 tides as described by , and it successfully captures qualitatively correct residual currents and transports, realistic massconserving flooding and dewatering of the tidal flats, and current asymmetry between flood and ebb. The simulation results were sensitive to local bathymetry and the implemented friction law. The accuracy of the model is demonstrated by comparison with the 1975 Great Bay study in terms of tidal elevations at 14 tidal stations and 4 cross-sectionally averaged current measurements in the estuary. Quantitatively, the model results show good agreement with observations, displaying correlation coefficients of !0.96 in surface elevation and !0.95 in averaged current, with average RMS errors of 0.12 m and 0.26 m s À1 , respectively. In addition, tidal flat hydrodynamics, characteristic distributions of residual current, sediment bed load transport, and influence of topography on the overall circulation in the region are also discussed.
A mathematical model study of the flushing characteristics of a shallow tidal bay
The Paper describes the development and application of a mathematical model to compare the hydraulic features and flushing characteristics of Holes Bay in Dorset, for the present boundary configuration and for two proposed new outlines of the bay. The time dependent non-linear equations of mass, momentum and advective-diffusion were solved numerically using a finite difference scheme, with the effects of the earth's rotation, bed friction, a surface wind stress and a simple turbulence model being included in the momentum equations.
Tidal velocity asymmetries and bedload transport in shallow embayments
Estuarine Coastal and Shelf Science, 1990
Tidally forced circulation can cause a net near-bed transport of sediment when the tidal velocity is asymmetric about a zero mean (flood or ebb dominant) and the transport rate is nonlinearly related to velocity. The relationship between elevation and velocity is elucidated here to enable one to determine from tide gauge data and sediment transport relations whether tidal asymmetry may cause net sediment transport. Tidal elevation and tidal velocity are related through the equations of motion of the fluid. If the estuary is shallow, the change in cross-sectional area of the channel with the tide is significant with respect to total area: the equations become nonlinear and an exact solution does not exist. A relationship between elevation and velocity in a nonlinear system is derived through the continuity equation and shown to be significantly different than the linear relation. Finite difference numerical solutions of the one dimensional, shallow water nonlinear equations are compared to the continuity relation and are in good agreement. The relationship between elevation asymmetry and ratio of flood-to-ebb bedload transport is calculated for both the linear relation between elevation and velocity and the nonlinear relation. Results show that the ratio of flood-to-ebb bedload transport as calculated from the nonlinear relation between elevation and velocity is similar to the flood-to-ebb ratio calculated from the linear relation.
Depth‐Averaged 2‐D Model of Tidal Flow in Estuaries
2004
A depth-averaged 2-D numerical model for unsteady tidal flow in estuaries is established using the finite volume method on non-staggered, curvilinear grid. The 2-D shallow water equations are solved by the SIMPLEC algorithm with the Rhie and Chow's momentum interpolation technique. The convection terms are discretized by one of the hybrid upwind/central difference scheme, exponential difference scheme, QUICK scheme and HLPA scheme. The algebraic equations are solved using the strongly implicit procedure (SIP). The model is capable of handling the drying and wetting problem due to the variation of water surface elevation. The model has been tested in Tokyo Bay and San Francisco Bay. The tests show that the present model is very stable and efficient. The simulated water elevation and flow velocity are in good agreement with the measured data.
Hydraulic modelling of tidal circulation and flushing in coastal basins
The Paper highlights the increasing concern of planners and designers for the hydroenvironmental problems relating to tidal circulation and flushing in small coastal basins, harbours, and marinas, and the use of physical and mathematical models as design tools to address such problems. Details are given of techniques frequently adopted in using both physical and mathematical models to quantify tidal flow patterns and water exchange characteristics of harbours and marinas. Emphasis is placed on comparative studies where alternative basin geometries and/or bathymetries are proposed. Advantages and disadvantages of both modelling techniques are considered. An example application of each approach is presented. The main purpose of the two studies was to investigate effects of basin geometries on the tidal flow and flushing features for two specific sites-one in the USA, the other in the UK. Results of both studies are reported, together with an interpretation of the data and a summary of the findings. Notatioo C , initial spatial average tracer concentration for volume considered C, spatial average tracer concentration for same volume after n tides E average per cycle exchange coeflicient n number of tides of simulation R average per cycle retention coeflicient T P R tidal prism ratio
A two-dimensional analytical solution of groundwater response to tidal loading in an estuary
Water Resources Research, 1997
A two-dimensional transient groundwater flow equation for a confined nonleaky aquifer is solved analytically with an estuary tidal-loading boundary condition. The amplitude and phase of the tide varies with position and time, respectively. These variations, called the "damping coefficient," for amplitude, and "separation constant," for phase along the coastline, are considered in the analysis. The solution presented is more advanced than the traditional one-dimensional analytical solution with a one-dimensional tidal-loading boundary condition. The damping coefficient and separation constant in the application study are obtained from the harmonic analysis of the observed tidal level in Apalachicola Bay, Florida. The analytical solution is compared to a two-dimensional finite difference solution. A numerical simulation illustrates the response of piezometric head to tidal loading. The piezometric head and phase from both solutions match closely. The analytical and numerical solutions show that a piezometric head along both sides of the estuary can be predicted given a tidal elevation at the entry of an estuary. The fluctuation of piezometric head in response to tidal loading diminishes rapidly inland and diminishes slowly along the coastline of an estuary. There are also obvious phase shifts associated with spatial variations as the wave propagates. SUN: TWO-DIMENSIONAL ANALYTICAL SOLUTION Tidal speed, a • -0.2618 hour -• Separation constant, b • 1.67E-06 m -• Phase shift, c • 0.0 Semidiurnal Tide Amplitude at the entry of the bay, A 2 0.35 m Damping coefficient, m 2 2.32E,05 m -• Tidal speed, a 2 -0.5236 hour -1 Separation constant, b2 6.89E-05 m-• Phase shift, c2 7.0 SUN: TWO-DIMENSIONAL ANALYTICAL SOLUTION 1431 M2 -Amplitude ,=,, 2.o 20.1 f9.2 ß 6 • 9. ß 9040 , '•••"