EFDC_Explorer provides two approaches for representing tidal forcing boundaries, the simple harmonic forcing boundary and the astronomical tidal forcing.

Simple Harmonic Forcing

With the simple harmonic forcing boundary, harmonic constants are specified and EFDC computes the forcing time series using a simple harmonic formulation. This time series then can be combined with a normal pressure time series for different correction purposes, such as storm surge simulation or datum correction.

This forcing boundary type can be selected in the Domain | Boundary Conditions tab and clicking "E" (edit) button for Harmonics which displays the form shown in Figure 1. The user can specify the forcing of an arbitrary number of combinations for harmonic components from 234 predefined tidal constituents. This is done by LMC to select the Global Tidal Constituents and then pressing the arrow button on the form to populate the Harmonic Constituents to the right.

These constituents may be sorted either by angular speed (frequency) (degrees/hours) or period (days) with the radial button in the Sorted by frame.

In the Harmonic Coefficient Forcing Type frame the user has three options; constant, linear variation and quadratic variation. It is generally recommended to use a constant forcing. With the other two options the user must specify additional one or two amplitude and phase for each tidal constituent, respectively. EFDC will then vary the final (i.e. applied) phase and amplitude for each tidal constituent using the specified method and values.

Figure 1  Domain Tab: Simple Harmonic Forcing.

Note that when specifying an open boundary, it is possible to specify harmonic tide as an independent boundary condition or both a harmonic series and a pressure series. In the latter case the harmonic series is added to the pressure series for the final pressure to be used by EFDC in the solution. That is how the user could adjust for a datum shift (i.e. the pressure series would just be a constant offset).

Astronomical Tidal Forcing

EFDC_Explorer provides a Tide Generator Tool for generating a pressure time series computed as astronomical tidal forcing.
The basic formula for the oscillation of water level due to astronomical tides is based on the combination of the tidal constituents as follows:

In which

η = tide level (m)

z₀ = mean sea level above the datum (m)

f_i = nodal coefficient to adjust amplitude for the i^th component

H_i = amplitude of the i^th component (m)

ω_i = speed or angular velocity of the  i^th component (radians per second)

t = time (s)

(V₀ + u)_i = astronomical arguments of the i^th component

g_i = phase lag of the i^th component from Greenwich longitude

It should be noted that the units used in EE can be changed so that they are convenient for the users.

The Tide Generator Tool (EFDC Input Tides) provides the ability to generate pressure time series from astronomical tidal forcing. This is accessed via the “E” button for the Pressure series in the Domain tab | Boundary Conditions. In the Data Series: Water Surface form the user should click the Generate from Harmonics button. This will display the form shown in Figure 2. The user should complete the form in a manner similar to that described for simple harmonic forcing above. The user must specify the number of constituents, and import the data file for the constituent parameters (name, amplitude, phase) if available.

Each series can have its own name, and the user may apply a datum shift (z_o) in meters or time shift in hours to the series. A row may be deleted by selecting Shift and RMC. An example of the data structure is provided in Appendix B.

Figure 2 Domain Tab: Astronomical Tidal Forcing.