Shark nets

WHY NETS?

Sharks eat humans occasionally, and there is intense debate as to how to minimise or eradicate this horror. Whether the actual risk is high or not, the fact that counts is that as shark attack rates increase, fewer and fewer people feel safe to swim in all sorts or waters. There is a biologically fundamental fear of deep water, just as there is a fear of the dark, and both of these fears have been developed over millions of years to protect against predators or foreign tribes.

When it’s dark at night at least you can still hear, whereas in the oceans, none of our senses are of any use at all, and so the feeling of vulnerability will be magnified if you know that sharks are not far away. And if you are trying to rationalise away this natural feeling of vulnerability with some facts and figures that compare lightning strikes, or car accident rates, or domestic violence figures, to shark attacks, then you are using the rational left side of the brain to tell the intuitive right side that there’s nothing at all to worry about. Good luck with that.

When both fears combine, then even the toughest surfers will admit to the anxiety levels rising after the sun sets. As it get’s darker and darker, and the waves get harder to judge, then bit by bit the other surfers go in, until there is just one surfer out the back and it’s quite dark. Now the awareness of water depth, the ever increasing darkness and the mere concept of being attacked becomes far too much and that surfer will want the next wave in even if it is a bad one!

So the fears are biologically inherent, and very hard to undo, and when there have been many shark attacks over recent years, and reports of 1000 great whites along 1000 kilometres of the east coast of Australia, then it would in fact be negligent on the part of authorities to not do more to make swimming and surfing safer on one hand, and to quite simply make people ‘feel’ safer on the other hand, however you might view this politically.

If ever mankind should be feeling a deep connection with our oceans it is now, and the stronger we feel a bond with our oceans the more we will want to protect them.

THE USUAL NET METHODS

Common netting systems that are used to enclose swimming areas, in lakes, harbours and rivers, are not viable for the open ocean beaches.

Swimming-enclosure

There are usually two approaches. Net material can be fixed to poles which have to be driven into the sea bed, or nets are held down by weights or chain at the bottom of the net, holding it down at the seabed, with a string of floats or a floating boom on the upper edge, which keeps the top of the net at the water surface.

Swimming-enclosure-Nielsen

There are problems with either approach. Using poles is hugely expensive since each pole has to resist the full range of the oceans’ forces. Each pole is expensive in its’ own right, since it has to be rigid and very strong, then it has to be located accurately, and then driven into the seabed, sometimes many meters. Once the line of poles is in place then the net material has to be fixed to each, this is a difficult process since wave action will create unwanted movements by the craft that is being used to instal the net material.

Floating nets are far less expensive to make and deploy, but are ineffective when larger waves are involved. Although tidal movements can be accounted for by using enough net material to allow for the highest of tides, (which merely goes slack at lower tides) when waves are involved this approach has its limits. When the wind is onshore and therefore from the same or similar direction as the swell, then any slack built into the system will be taken up as the buoyant upper edge gets pushed towards shore by the wind. Once it reaches it’s limit then the buoyant upper edge will quite simply be forced under water as each wave passes. This then creates the potential for unwanted sea creatures entering the enclosure.

THE BUOYANT POLE NET SYSTEM

This alternate method does not place the floating elements flat on the water surface which leads to problems, but vertically within the water column. The hollow buoyant poles (made from 900mm PVC water pipe) are held partially below and partially above the water surface. Each pole is attached to a sand filled geo-textile bag with more mass pulling down than the buoyancy force of the poles pulling up. This creates tension in the system and keeps the pole and the netting upright.

The lower netting edge is held firm against the sea bed by chain, with the poles and netting extending several meters above mean water level. On average a ratio of 2 to one would be used for the proportions of the pole under water to above water.

As waves meet the nets system, it will tilt and move with the swell until the majority of the wave energy has passed through the net material, at which point it will then move back to vertical and right itself. As the net system tilts shorewards with the wave movement, the upper extended part remains above the water surface at all times. The height above mean water level will be determined according to the standard seasonal maximum swell size for that wave zone. When rare but much larger storm swells are predicted then the entire system can be submerged to the seabed by releasing valves in each pole wirelessly from shore. Refilling the poles with air can then take place when the swell has abated by attaching compressed air lines to sets of poles which have already been connected by flexible airlines during manufacture. The number of poles that can be refilled with air at the same time will depend on the length and diameter of the poles and the size of the compressor.

THE ACTIVE SYSTEMS

What we have designed is an active net system that makes the nets ‘visible’ to virtually all sea creatures.

These active shark nets emit pulses of energy in the four different wavelengths of sound, light, electrical and electromagnetic, so that the nets can easily be sensed and avoided. AC currents are supplied by power banks fitted to the 900mm wide top caps of the buoyant poles. The battery packs are charged by solar panels atop the poles as well as 600w wind turbines. The system is designed to lie dormant until sensors pick up the motion of either large schools of fish or sea creatures above a certain size.

The sound is emitted by a magneto that forces a small hammer to strike a plate attached to the inside of the hollow poles. These 900mm PVC water pipes are effectively huge drums partially submerged so the sound of each hammer strike will travel hundreds of metres through the water.

The light shall be emitted by hundreds of LED lights attached in strip lines to the nets and the poles.

The DC electric currents will be designed to emit pulses only along the section of the net at which the shark is moving. There is no need to activate all sections of the net system at the one moment, or all of the time. Each buoyant support pole services the electrical needs for just one section of the netting. As a result energy costs to active the four wavelengths of energy will be very low. If nothing swims by then it floats there dormant. Also if one link in the chain of poles fails, the remainder of the system still functions.

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