EM 1110-2-1100 (Part II)
30 Apr 02
(f) Before presenting example applications of models, it is stressed that all models are not equal with
respect to computational speed and accuracy. Depending on how the governing equations are written and
solved and how and what boundary conditions are defined and used to drive the models, any two finite-
element or finite-difference models can produce different results. Therefore, for some applications, one
model may produce accurate results while another formulation will be unstable and produce totally unrealistic
simulations. For this reason, any application of a numerical model should begin with a calibration and
verification procedure in which the model is run to reproduce the hydrodynamic flow field corresponding to
a specific time period during which prototype data (i.e., surface elevations and currents) have been collected.
Additional data comparisons may include hydrodynamically driven parameters such as temperatures and
salinities.
(g) In the calibration phase, model parameters such as the friction factor distribution or depth resolution
are adjusted to optimize the comparison of model-generated data to measured prototype data. Comparisons
are generally made to surface elevations and velocities, but may include reproduction of temperature and
salinity. In the verification procedure, the model is used to simulate an alternate time period containing
additional prototype data. Model coefficients are not adjusted in the verification phase. An acceptable
calibration and verification demonstrates that the model is capable of simulating the total flow regime over
the entire computational domain. This ability is basic and necessary to the use of numerical models to
quantify the flow-field response to proposed or existing changes in flow-field boundaries.
(h) If data are available or can be collected, the calibration and verification procedures represent a
minimum criterion required of a model before it can be applied to any specific problem. If these two steps
are not performed in a satisfactory manner, model results can only be considered qualitative. Situations do
exist where prototype data are not available and cannot be obtained within the time and cost limits of the
budget. In these cases, models can be used to generate qualitative trends; however, unverified model results
should not be the basis for quantitative decision making unless some data are available, even at a minimum
level, to demonstrate that the model is producing realistic results. This aspect of modeling may be vital to
a particular project because decisions based on unverified model results can be legally challenged and shown
to be invalid.
(2) Example - tidal circulation modeling.
(a) The New York Bight project (Scheffner et al. 1993) is an excellent example of tidal circulation
modeling. The goal of the study was to develop a hydrodynamic simulation tool that could be used to address
questions concerning how certain modifications to the New York Bight may affect the local or global
hydrodynamics of the system and how the computed flow fields affected the transport of certain water quality
parameters. The model was therefore required to be capable of simulating the flow-field hydrodynamics,
temperature, and salinity distribution over a large computational domain.
(b) The model selected for this study was the CH3D (Curvilinear Hydrodynamics in Three (3)
Dimensions) (Johnson et al. 1991) model, a three-dimensional, finite difference formulation model with
boundary-fitted coordinates. The modeled area represents the region extending offshore from Cape May, NJ,
and Nantucket Shoals to beyond the continental shelf, including Long Island Sound, the Hudson and
East Rivers, and New York Harbor. Depths vary from less than 10 m to more than 2,000 m. The
geographical boundaries are shown in Figure II-5-32a. The computational grid used to represent this area
of interest is shown in Figure II-5-32b. The grid contains 76 cells in the alongshore direction and 45 cells
in the cross-shore direction. There are 2,641 active computational cells in the horizontal and 10 in the
vertical.
II-5-56
Water Levels and Long Waves
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