18 April, 2016

Locked behind the Bar

The Clyde River enters the ocean at Batemans Bay where Malua is currently in the marina.  I have to cross the bar to get out to sea.  It is quite a benign bar as sea bar go with little swell and very few breaking waves BUT the bar is shallow.  And when I say shallow you can see from the above photos taken just after crossing the bar it was less than 1.7m over the datum of .9 which leaves Malua who draws 2.1m very little water under the keel.  A slight swell and we touch bottom.
The bar has been a limitation for the development of Batemans Bay for years especially the on-water activities.  I made my first presentation to the NSW authorities in 2003 when they where developing an estuary strategy for the Clyde.  Unfortunately I left to go sailing in the Pacific before the final draft was published and there was a big gap between cup and lip and the importance of dredging the bar was hidden in the detail of the report.
The battle continued for years with submissions to Council, the local politician and finally the issue came to a head and the Council along with Department of Lands let a tender for the dredging of the bar.  The contract as in most cases was won by the cheapest bid and a small river dredge appears at the start of winter to remove the sand from the end of the breakwall.  The sand was pumped on to the local beach but the dredging came to a halt more often than it was working either from a broken pipe or bad weather.  The operation was a farce.
Some sand was removed but actually all the dredging did was remove the lip at the end of the shallows and stirred up the sand over the width of the bar.  For a time the bar was deeper but as soon as the infamous bad weather of an east coast low appeared so did the sand and we are back to what it was before.
The issue is that the breakwall that was built some years ago was designed incorrectly and has a curve towards the sea side of its length so the river water containing the silt turns just before it reaches the sea water and the Ph changes and it drops its load.  Furthermore the speed of the flow changes as it is hit by the cross current flowing at right angles to the river mouth within the bay and the sand is deposited right at the end of the breakwall.  The solution is to extend the wall towards Snapper island some 800 m from the end.  This would improve the river flow and hold the sand in suspension and stop the lateral drift of sand on the bottom.
You may ask why have the authorities not modelled this river mouth to determine the optimum solution?  These days it can be undertaken on a computer for a few dollars.  Way back in the days of old when I was at Stellenbosch University they modelled a river mouth as a real model along with water, waves, sand and small boats.  It was a delight to watch the sand move in the bay then see the result when they placed a proposed breakwall in the bay and what would happen to the sand.  The final result was tested under many sea conditions and the results passed to the builders.  The outcome a breakwall and a safe harbour for ships and yachts.
Oh no it is too expensive to do a study on the Clyde River mouths so we will again throw good money after bad and let another tender to dredge the bar this winter.  Well, it will be too later for me to take Malua out sailing except at the full moon spring tides which is just once a month.  So Malua is now trapped behind the Clyde River bar and I watch as other set sail for distant shores at the end of the cyclone season.

No comments: