This example illustrates the differences between tidal range, amplitude and level with a nonstandard choice of chart datum. The amplitudes of the main tidal constituents (in metres) at a location (X) are shown in the table below, where the charted mean water depth (h) is also given. The chart datum at X is not the level of the LAT. Classify the tidal variation at X using the tidal ratio F. Also, estimate the mean spring and neap tidal ranges and the maximum tidal range.

If local mean tide level at X is +0.95 m above chart datum, determine the maximum tidal elevation with respect to chart datum.

h(m) | M_{2} | S_2 | O_1 | K_1 | |

X | 2.5 | 1.8 | 0.75 | 0.6 | 0.3 |

Step-by-Step

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We calculate F from the amplitudes of the constituents:

F = (0.6 + 0.37)/(1.8 + 0.75) = 0.353, so tide is mixed, predominantly semidiurnal.

As the tide is predominantly semidiurnal, the tidal range is governed mainly by the semidiurnal tidal constituents.

So the mean spring tidal range = 2 × (1.8 + 0.75) = 5.10 m. Note that the mean neap tidal amplitude is given by the magnitude of the difference of the amplitudes of the two primary tidal constituents,

so the mean neap tidal range = 2 × |1.8 − 0.75| = 2.1 m.

The maximum tidal range = 2 × (1.8 + 0.75 + 0.6 + 0.3) = 6.90 m.

Thus,

The maximum tidal amplitude is 3.45 m.

The maximum tide level = 0.95 + 3.45 = +4.40 m CD.

The minimum tide level = 0.95 − 3.45 = −2.50 m CD.

And the mean water level (Z0) = +0.95 m CD.

The diagram shows the mean sea level 0.95 m above chart datum. The minimum tide level occurs at low spring tide which is 3.45 m below mean sea level and 2.5 m below 0 m CD, or −2.5 m CD.

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