Tides and Tidal Streams Flashcards

What causes spring tides?

Answer: The gravitational pull of the sun and moon acting in alignment (at new and full moon)

Spring tides occur when the sun and moon are in alignment (syzygy), at new moon and full moon. Their combined gravitational pull produces the highest high tides and lowest low tides.

What is the 'Rule of Twelfths' used for?

Answer: Estimating the rise or fall of tide during each hour of a roughly six-hour tidal cycle

The Rule of Twelfths estimates tidal rise/fall each hour: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12. It assumes a sinusoidal curve over approximately six hours between high and low water.

What is 'chart datum' in relation to tides?

Answer: The reference level on a chart below which depths are shown and above which tidal heights are measured

Chart datum is the reference level used on charts, typically LAT (Lowest Astronomical Tide) in UK waters. Depths shown on the chart are measured below this level, and tidal heights are measured above it.

What is the difference between 'height of tide' and 'depth of water'?

Answer: Height of tide is the vertical distance from chart datum to the water surface; depth of water is chart depth plus height of tide

Height of tide is measured from chart datum upwards to the water surface. Actual depth of water at any point = charted depth + height of tide (both measured from chart datum).

What is the approximate interval between successive high waters at a given location?

Answer: 12 hours 25 minutes

In a semidiurnal tidal pattern (as in most UK waters), high water occurs approximately every 12 hours and 25 minutes, reflecting the lunar day of about 24 hours 50 minutes.

What does 'neap tides' refer to?

Answer: Tides with the smallest range, occurring when the sun and moon are at right angles to each other (first and last quarter moon)

Neap tides have the smallest tidal range and occur at first and last quarter moon when the sun and moon are at 90 degrees to each other, partially cancelling their gravitational effects.