Horizontal Falls

Posted by efloow on September 2, 2015 in Uncategorized | 4 Comments

The Horizontal Falls are created when the massive tides of the Dampier Peninsula are held up by two narrow gaps in the McLarty Ranges. The tides here are the biggest in Australia and the second biggest in the world (only beaten by Novo Scotia in Canada). The water level can rise or fall as much as eleven meters every six hours.

As the tides go in and out the water cannot get through the gaps in the ranges quickly enough and a height difference occurs on each side of the gaps, hence the “falls”. For the narrow gap, this height difference can be close to six meters. It is a pretty impressive site, especially when you realise that there are millions of litres flowing across the 40m deep gaps every second at up to 15 knots!

To get there you take a 30 minute seaplane to Talbot Bay on the north coast above Derby. We chose an overnight option, where you land and stay on a permanent house boat moored in the bay (2nd September). The phenomenon is amazing, the scenery is spectacular, and to accommodation, staff and food are all excellent.

They have made it into an adrenaline rush sort of trip. There are high speed (900hp) boats that go through the falls doing the water equivalent of a burnout and jumps over the falls, the seaplane, feeding and swimming with sharks and ubiquitous helicopter ride. It all sounds a like the wrong way to treat such an amazing place, but it actually works really well, and it was very enjoyable, especially for a six, nearly seven year old boy and his father!

The adventure started when Alex got to sit in the copilot seat of the Cessna Caravan seaplane and after the usual bumpy small plane climb to 6000 feet, much to the horror of the other passengers, Alex was handed the controls. The pilot was pretty amazed at Alex’s understanding and ability (all those hours of X-plane and trips to Sydney airport runway had some value). The grin on Alex’s face was worth the cost of the whole trip! From there it never stopped; feeding and swimming with the 20 or so Tawny sharks and then the speedboat trip through and around the falls and Cyclone Bay (an amazingly sheltered bay that the pearler a used t opponent their fleets from the cyclones)

The next morning Alex and I continued the adventure with a dawn helicopter ride over the falls and another run in the speedboat. This second run was over an even bigger tide… I suspect that they though we were desensitised by now. It was a good thing that Holly decided to stay on the house boat. The final adrenaline rush was a bumpy flight back to Derby.

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Some facts on tides in and around the Dampier Penninsula (it took a while to work this out):

• Normal high tides are caused by a bulge in the water covering the surface of the earth. High tides occur on both sides of the earth at the same time. The water on the side closest to the moon bulges out because it is being sucked by the gravitational force of the moon. The water on the side furthest away from the moon also bulges out, because it is being pushed by forces including the centrifugal force of the earth.
• Bigger tide occur when the suns gravitational force is added to that of the moon which corresponds to the full and new moon.
• King and Neap tides occur when the moon is closer to earth and the suns effect is additive.
• The tides at King Sound are much larger than the normal high tides caused by the action of the moon, because they are also influenced by the shape of the local coast. King Sound, where Derby is situated, is an extension of the wide, shallow north-west continental shelf. When the water comes in from deeper areas, it is constricted and squeezed by the shallow continental shelf. Because the volume of water is being constrained in this shallow region, the currents increase to try and move the water onto the shelf. The tide is therefore amplified as it moves shoreward. Tides at Broome are very high, at 9m. But the tides are even higher in King Sound, because this area is shaped like a huge shallow channel. Here, the water is constrained on both sides and forced up the Sound, causing these massive tides and creating the King of all Australian tides at Derby.
• The tidal massive variation is also due to the amphidromic system!  This is the name given to the interference on water flow from ocean currents being altered by land masses, deep ocean basins and the rotational effect of the earth (Coriolis effect).  The best analogy to this is to think of the water going down the plug hole (the water on the outside moves faster than that on the inside).  Imagine then putting a coin near the plug hole as the water swirls around… the water will rise and back up around the coin / obstruction (a higher tide).  Novoscotia and Derby with their high tides represent the places on earth where this effect is maximal