The Model and its Limitations

The Model and its Limitations

The numerical model used to generate the current diagrams was originally developed in a test exercise looking at the tsunami generated in the Halifax Harbour explosion of 1917[7, 3, 4]. At the time this report was written, an introduction to the model was available at the internet site http://www.mar.dfo-mpo.gc.ca/science/ocean/home.html by following the links Research --> Modelling --> Coastal Embayments Circulation and Dispersion --> Halifax Harbour. The tidal currents have been subject to only limited verification. Where they have been compared to current meter data they agree to within the uncertainty of the observations (10 - 20%). The currents seaward of MacNab's Island have inaccuracies due to the approximations made in specifying the tide at the outer limit of the model and these should be treated as unreliable. Data vs model comparisons in this area show that the model is producing across-channel currents stronger than observed.

For those technically inclined, the model is a linear harmonic finite element model with grid resolution from 20 m to 1 km with an average node separation of 170 m. In practical terms this means that the model will not be accurate for currents that change at scales approaching the local resolution. This will be a problem near poorly resolved solid docks and areas with small bottom features and rapidly changing depths.

The model only uses one component of the tides - the M2 Lunar component with period 12.42 hours. The relative magnitudes of the other constituents can be estimated from the amplitudes from an analysis of the Halifax Harbour tide gauge data (Table 2). The currents for the semi-diurnal components, S2 and N2, will have magnitudes approximately proportional to the ratio of their amplitude to the M2 amplitude. The diurnal current components, K1 and O1, would scale as one half their ratio to the M2 amplitude. These components of the currents are approximately considered by taking into account the tidal range (see section 4). Other nontidal contributions to the Harbour currents, which at times have magnitudes greater than the tides, are described in section 3 below.

Constituent	Amplitude (m)
M2	0.628
S2	0.138
N2	0.138
K1	0.100
O1	0.048

Table 1: The amplitude of the major tidal constituents from an analysis of 1973 data from the Halifax Harbour tide gauge. The components additional to M2 are approximately accounted for by scaling the vectors as described on section 4.