Tides
Tides are the rising and falling of sea levels that result from the gravitational interaction and motion of the sun, the moon and the Earth acting on ocean waters.
Apart from astronomical factors, tidal patterns are modified by many other non astronomical influences such as the shape and depth of oceans, and the weather. The combination of factors influencing local tides is complex and varies greatly from one site to another.
Chart datum
As the level of the sea is constantly rising and falling, the depths shown on charts must have a common level from which they are measured. This level is the lowest predictable level to which the tide is likely to fall. It is known as Chart Datum.
Chart Datum is not a horizontal surface but may be considered as such over a limited local area. It is the level so low that the tide will not frequently fall below. Usually defined in terms of the approximate lowest low water level. All soundings on a navigational chart are referenced to Chart Datum.
To be able to calculate the total depth of water, you must add the depth obtained on the chart to the tide height at that time and place.
Tide cycle
Tides are often distinguished by their cyclic patterns. One tidal cycle is the sequence from one high tide to the next high tide. Tides that go through one cycle during a single rotation of the Earth are described as diurnal tides. Those that go through two cycles are described as semi-diurnal tides.
A third common category is known as mixed tides. Mixed tides cycles are a combination of the other two and are usually characterised by two unequal high waters and two unequal low waters each day. In the northern part of the State (Carnarvon northwards) the tidal cycle is semi-diurnal. In the southern part of Western Australia the tidal pattern is considered as mixed tides consisting of both semi-diurnal and diurnal tide cycles.
Tide range
The coastline of Western Australia is renowned for its diverse range of tides. The difference between the height of the tide at high water and the height at low water is called the range of the tide. The tide range on the West Australian coastline varies from approximately 12 metres at Collier Bay in the Kimberleys to about 0.7 metres in Fremantle.
Tide predictions
Although tides are influenced by both astronomical and non astronomical factors, local tides can be predicted with a high degree of accuracy from analysis of long term tide records.
