Tides are the periodic rise and fall of sea level resulting from the gravitational interaction and motion of the Sun, Moon and Earth acting on ocean waters. Tidal patterns are modified by other influences such as the shape and depth of oceans and the weather. The combination of factors is complex and causes local tides to vary greatly from one site to another.
Derby has Australia’s highest tides and one of the highest in the world.
The Bay of Fundy, Nova Scotia, Canada, has 15m tides. Other high tides occur at Bristol (UK) 14.6m, Mont Saint Michel (France) 12.3m, Puerto Gallegos (Argentina) 13.2m and Bhaunagan (India) 12.2m.
High tides in Western Australia are Derby 11.8m, Yampi Sound 10.9m, Broome 9.8m and Wyndham 8.4m.
Storm conditions can cause higher tide levels than predicted. Tides can be predicted with a high degree of accuracy from analysis of long term tide records. The tide predictions provide the time and height of high and low water for a particular day at a particular place. Spring tides occur every 14 days at the new and full moons. Neap tides occur every 14 days at the half moon.
Tide charts are available from the Derby Visitor Centre.
Online Tide Prediction data is available from the Department for Planning and Infrastructure (DPI) website.
Highest Tides for 2013
|Tuesday January 15
|Thursday February 14
|Friday March 29
|Sunday April 28
How Tides Are Measured
First Mean sea level has to be determined then the tidal datum point is established. This datum is usually selected on the basis that the water level will seldom fall below it.
Mean sea level at places all around Australia was determined by observations at 30 ports from 1 January 1966 to 31 December 1968. Derbys datum was established by correlating a relatively short period of observation of tidal movements at Derby with the same period of tides at a standard port - in this case Port Hedland.
Derbys tides were first measured by visual observations from 3 September 1958 to 11 October 1958 at the old wooden jetty. The last datum adjustment was in October 1998.
The Chart Datum found on navigational maps is the lowest predictable level to which the tide is likely to fall. It is the approximate lowest low water. All soundings on a navigational chart are referenced to Chart Datum. 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 information is important for safe navigation in ports and coastal waters.
Predicted Tide Charts for Derby are available at the Derby Visitor Centre.
(Information courtesy of Transport WA Coastal Management Branch Tides and Waves Section).