February 25th 2013
Horseshoe foreshore under threat - but what can we do?
Over the last 22 years sea levels around Magnetic Island have risen by about 6.38cm. And, at Horseshoe Bay, where a very popular foreshore park is being chewed away by successive storms, and where council recently decided not to replace storm damaged public toilets in the same, prone, position, the reality of sea level rise is beginning to shape real infrastructure decisions. Some locals are calling for walled protection. But is this a viable solution?
After posting the story: Council thinks twice after storm erosion (Click here) Magnetic Times received some interesting responses on our facebook page. As there have been calls in the past for rock walls and, now, timber walls to protect foreshores we have repeated the facebook comments between reader, Maria Julia and Magnetic Times over the issue. This is followed by a response from Magnetic Island Councillor and Deputy Mayor, Vern Veitch.
Maria Julia: We should be lobbying council to put a timber wall structure the likes that Picnic Bay has otherwise we are not going to have a Horseshoe Bay foreshore, where will all the tourists sit,what will happen to the business
And the markets. Every time we get a severe northerly the foreshore is eaten away, they bring in more earthmoving equipment put some sand back and they think they've cured the problem. The toilet block is prime example at least Vern isn't flogging a dead horse. Council be responsible with our ratepayers money and construct a wall that will last for the next 100 years.
Magnetic Times: Hi Maria, I think you would get a very interesting answer from Vern re the wall idea. He has a serious science background and has spoken at length in the past about how walls eventually exacerbate these sorts of problems.
Maria Julia: Vern has alot of strong opinions about things, especially if the council have to pay, than you see his science degree and statistics . However if we could get someone other than the council to pay for the headworks of the structure I'm sure it would be a different answer. Walls would not exacerbate this erosion problem, we are not trying to divert water flow from one spot to another as is seen in Nelly Bay, we require to alleviate the erosion that's evident and the potential storm surge damage that is occuring more and more with every strong northerly . Vern needs to see what is really happening time and again before saying that walls exacerbate problems. How long has the wall barrier been up at Picnic, that foreshore looks pretty damn good. We are all to complacent about Horseshoe Bay . We need to take action. I don't even own property along the Pacific Drive foreshore but if I did, I'd be very worried !!!
Following is an abridged response from Cr Veitch after we relayed the discussion to him for comment.
"As far as building a rock wall is concerned, there are 3 issues.
"One is certainly the cost which is in the order of $20K to 30k per linear metre of length on the mainland where resources such as trucks and quarries are readily available and dependent on how deep and high the wall needs to be.
"At a minimum, about 270 metres of wall would be needed if we only did the section from the boat ramp to the eastern end of the park and a guess at the estimated cost would be in the order of about $8m for a wall of that length and some landscaping on the mainland. Importing sand to put in front of the wall would be an additional cost.
"The second is where to get the rock from. Short of turning a headland into a quarry, and repeating the Nelly Bay situation, I suspect it would have to be carted from the mainland adding (perhaps doubling) to the costs above.
"The third issue however is that rock walls bounce waves whereas a sloped sand beach shapes itself to the waves it has been exposed to and encourages them to break and lose their energy. This is why there is rarely and beach that is usable directly in front of a rock wall as the reflected wave carries the sand further offshore. This science is well established and can be confirmed in almost any reference written on the subject. If you google “rock seawall” images you will note that very few have any usable beach.
"The other important factor in rock wall is where they start and finish. At Horseshoe, whilst it would require a detailed study by coastal engineers, there is no hard structure at the eastern end such as rocks to tie the wall to so it does not create erosion at the end of the wall.
"Before we can do any work of a structural nature, we would need a Shoreline Erosion Management Plan (SEMP) and for an area of that size, the minimum cost of a consultant to develop the plan is likely to be in excess of $100K and this would then need state and perhaps federal approval to implement. Note that we did not get approval for a rock wall at Rowes Bay and now have something more temporary that has so far cost about $6m for a shorter length than would be required at Horseshoe.
"As you can see, easy to say, difficult to do and perhaps unlikely to get approval anyway from higher levels of governments as it is considered to be a natural process.
"Just as an aside, one engineer outside council suggested we blow up a nearby headland and build an outer rock wall and turn the bay into a giant marina. Not something I could support."
Note: Great Barrier Reef Marine Park Authority website notes: "Records of sea levels at Cape Ferguson near Townsville show an average increase of 2.9mm every year between 1991 and 2006." Our figure of 6.38cm was extrapolated from this data. (Ed.)
Story and photo: George Hirst