WORDS & PICTURES: Scott Rawstorne from Global Paddler.
Anyone who has spent time at the seaside or beside an estuary will have noticed that the water level is not constant. It rises and falls on a regular basis. Particularly perceptive people will also have noticed that water is pushed into estuaries as the sea level rises and it is drained out of estuaries as it falls. The overarching term generally used to describe all of these processes is tides.
An excellent detailed explanation of the causes and effects of tides can be found on the National Oceanic and Atmospheric Administration (NOAA) website. There is a lot to take in if you want to completely understand the processes involved but for the purposes of planning paddling adventures on tidal waters in Australia, I have broken it down into the few things I think you might want or need to know.
Tides are driven by gravitational forces exerted on the world’s oceans by the Moon and the Sun, and centrifugal forces applied to the world’s oceans by the revolving Earth-Moon partnership and the Earth-Sun relationship. Gravitational forces draw the ocean towards the Moon and towards the Sun. Centrifugal forces send it to the opposite side of the Earth from the Moon and the Sun. Broadly speaking, the net effect of this is that water levels rise in those places that are on the same side or directly opposite side of the Earth from the Moon and the Sun, and they fall everywhere else. This sounds chaotic but the Moon is much closer than the Sun, so its forces are much stronger. The Sun’s forces generally only serve to enhance or diminish the effect of the Moon. As the Earth rotates on its axis 360 degrees every 24 hours, two bulges of water, which could be regarded as waves, move horizontally around the planet in synchronization with the position of the Moon. As a general rule, this creates two high tides and two low tides per day, usually just under 6 hours apart.
There are a small number of places in the world where the difference between high and low tide is virtually non-existent for one or two days a month. One of those is Adelaide in South Australia where the phenomenon is known as a dodge tide.
The height of the tide varies depending on the distance of the Earth from the Moon and the Sun, and the extent to which the forces exerted by the Sun enhance those of the Moon. The highest tides occur at New Moon when the Sun and the Moon are on the same side of the Earth as each other and Full Moon when they are on opposite sides of the Earth. These are called spring tides. The lowest tides occur when the Sun and the Moon are at right angles to each other. These are called neap tides.
The difference between high tide and low tide is known as the tidal range. A higher high tide will usually be accompanied by a lower low tide and therefore a greater tidal range. Greater tidal ranges result in faster tidal flows through channels and estuaries because there is more water moving in-between high and low tide. It is important to note that the speed of the tidal flow is not constant. It is slowest near high tide and low tide, and quickest at the mid-point between the two.
The speed of the tidal flow can also be affected by the shape of the land, above and below the water. In general, it will be quicker in narrow and shallow sections. Tide times for estuaries are always later inland than at the coast.
If you are planning on paddling in a tidal area, make sure to check the predicted time and height of the tides, and consider what effect tides might have on your chosen venue. Too much water may leave a fantastic beach submerged. Not enough could mean a portage is required or worse still, your day is cut short. In addition, the direction and speed of the tidal flow could either impede or assist your journey.
Tide predictions are published in a number of places. The Bureau of Meteorology, WillyWeather, and Weatherzone websites are all great sources of tidal information. There are also applications (“apps”) for smart phones. If you don’t have access to the internet, check in the local newspapers, watch the weather report on the television news, or pick up a tide chart from your friendly neighbourhood fishing shop or outdoor store.