Monday, 15 June 2009

From the Kayak #2 - River interference and its effects

Picture from here.
Please click to enlarge.

From the kayak

We have had some wild weather on the coast this year which caused me to reflect on the movement of sand. Beaches come, beaches go, and for the board riders the shifting sands can mean the difference between fantastic waves and nothing. The sand moves up the coast from Bass Strait, finally spilling back off the continental shelf near Hervey Bay in Queensland. Sometimes it is piled up onto beaches and sometimes it gets dragged offshore, decreasing our beaches to narrow strips above high tide.

This is natural. This is the way it has always been. River mouths moved. Sometimes they were enormous, deep channels, sometimes almost blocked up. This didn’t suit us so around the middle of the last century we started to build walls to control the rivers. We needed them to stay where we wanted them, and to remain deep at all times so that we could get boats in and out. Huge blocks, thousands of them each weighing many tonnes, were placed on all our rivers where they met the sea.

It seemed a good plan at the time – another great demonstration of man’s mastery of nature. Let’s take the Tweed River on the border of NSW and Queensland and see what happened. The walls were constructed but after a time, what fishermen called the bar, started to build up just outside the ends of the walls. This deposit of sand made it dangerous to take a boat in and out of the river. So a dredge was used to deepen the channel.

Around the same time the walls were lengthened, but no matter how many times this happened, the sand seemed to keep coming back just outside the walls. Something else was happening too. Large amounts of sand were building up to the south of the river. The shoreline moved out hundreds of metres. This did not seem like a bad thing.

But wait, if sand built up there, where did it come from and where was it going? The inexorable movement north was being held back. Much smaller amounts were available to replenish Gold Coast beaches. Change was significant. The Kirra Caravan Park simple disappeared into the sea.

Dredging continued to keep the river mouth deep enough to allow boats in and out and a bold plan was conceived. The sand flow north would be facilitated by giant pumps that would pump sand from the south side, under the river, and deposit it north of the river. A huge jetty and pumping system was constructed and the sand started to appear again north of the river. Not everyone was happy. The beached changed. The sheltered bay that was a favourite of the oldies at Rainbow Beach disappeared, to become a “super bank” for the board riders. The waves at Kirra were so far away that you needed a cut lunch to get there.

Some say the contractor doing the pumping was being paid on the amount of electricity the pumps consumed so it was in their interests to pump as much sand as they could. This started to cause problems on the southern side and at least one oceanographer thinks we need another wall to protect that area.

Let’s think about all of this. We have spent vast sums to improve the navigation of our rivers. Lismore, Grafton, Kempsey, Taree, just to name a few, are all towns that were at the preferred location to bring a ship up the river. To get there they had to negotiate the bar, to get in from the sea and back out again. The towns are evidence that this was achieved. But there is no way that ships like that could get to those towns now.

So how clever were our engineers with their walls? Not very I would say. They spent vast sums for nil net benefit and probably made things worse. Sure, dredging would have been ongoing and costly, but nothing like what has been spent.

We built the walls, decided that they didn’t work, and added sand pumps at two entrances on the Gold Coast. At what environmental cost? The electricity used to run the pumps is all generated by burning coal. We tried to control nature and it didn’t work. We made thinks worse and we now keep chasing our tails trying ever more expensive options to fix what we have done. It would seem to me that the best thing we could do is to take all the walls out, but I can’t see that happening. Rather than admit that we might have got it wrong, we will continue looking to engineers to come up with something “better”.

Steve Posselt

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