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Tides around South Africa

Tidal Stages:

• Sea level rises over several hours, covering the intertidal zone; flood tide.
• The water rises to its highest level, reaching high tide.
• Sea level falls over several hours, revealing the intertidal zone; ebb tide.
• The water stops falling, reaching low tide.

Southern Africa experiences semi-diurnal tides:

• Two high tides and two low tides each day (~24 hr. period).
• The two high waters are generally not the same height and the same is true for the low waters.

Image: The lunar gravity differential field at the Earth's surface is known as the tide-generating force. This is the primary mechanism that drives tidal action and explains two equipotential tidal bulges, accounting for two daily high waters and two daily low waters.

Tides are probably the marine phenomenon studied for the longest period, since its measurement only requires the ability to measure distance (height).

In terms of the theory behind tides, Isaac Newton was the first to formulate concepts of gravitational attraction, which forms a key component of tidal theory (1687).

More details available at http://www.satides.co.za/about.html.

For tide predictions at your city, go to http://www.koolasun.co.za/weather/tide-tables/cape-town-tides.html.

A graphic view of the tides for Durban for a week is shown in http://www.tide-forecast.com/locations/Durban-SouthAfrica/tides/latest.

• The main tide off South Africa is the semidiurnal principal lunar tide (referred to as M2).
• The second largest semidiurnal tide is the principal solar tide, S2 (47% as important as the M2).
• The main diurnal tide is the principal solar tide, K1 (58% of the M2).
• The second largest diurnal tide is the diurnal principal lunar tide O1 (41% of M2).
• The largest tides occur where the dimensions of a bay causes resonance with the tide. The Bay of Fundy (Nova Scotia, Canada) has a natural periodicity of about 12 hours, and this causes the tides to reach almost 15 m during spring tide.
• An animation of the tides (in relation to the sun and the moon) can be found at http://www.sfgate.com/getoutside/1996/jun/tides.html.
• Examples of mechanical tide predicting instruments from the previous century can be found at http://co-ops.nos.noaa.gov/predma3.html.
• Nowadays, tides are not predicted by machines, but by computers What do you think tidal information is used for?
• The largest tides are found in the Bay of Fundy, where freight vessels are allowed to settle on the bottom during low tide.