| Previous | Image 1 of 2 | Next |
 |
| Waves begin eating away at recently dumped sand at Middle beach as the tide rises yesterday afternoon. Gerard O'Brien. |
Dunedin is on "high alert" as its beaches take another
hammering from king tides and winter storms that have caused
the worst damage since emergency powers were invoked in 2007.
"Reno mattresses", made of steel mesh cages filled with
rocks, part of interim protection work done in the past two
years, have been exposed by the sea at Middle beach, and an
extensive amount of sand has been lost from the Lawyers Head
end of the beach.
Freshly dumped sand was being washed away from Middle beach
as the tide rose yesterday afternoon.
Dunedin City Council community and recreation services
manager Mick Reece said yesterday the high tide was "the last
aria" in a drama that had been ongoing for a week.
"It's as serious as it's been since the emergency [in 2007]."
While St Clair beach was holding up well, the profile of the
beach had taken a battering further east.
"People are saying they haven't seen that much sand taken off
the Lawyers Head end in a long time."
The council had been learning about the oceanography of the
area, including that sea movement caused by White Island
could be having a significant effect, Mr Reece said.
"The guts of it is the Moana Rua Rd edge is gone, and we're
steadily losing the toe of the dune."
It was the first time the reno mattresses had been exposed
since they were put in place two years ago. They had slumped
forward, and the sea had been eroding the dune behind.
Contractors next week would build a new mattress and place it
behind and beneath the existing ones.
The battle between surf and turf has been going on since the
1800s, when families began settling what had been a swampy
marshland.
In 2007, storms caused major damage to the beaches. The
situation was exacerbated by vandalism to "sand sausages" -
geo-textile bags filled with sand and laid to protect sand
dunes - that were uncovered at the time.
Clutha dams and erosion
What's missing in this debate? Silt, sediment, sand, that's what's missing.
Where is it? If you look at the oceanographic 'geography' beyond the mouth of the Clutha River, you'll notice a vast undersea delta. Historically, about 1.46 million tonnes of sediment came down the Clutha annually. This was spread northward, fortifying the coastline, creating a system of beaches that were relatively stable.
Of course this balance was suddenly removed in 1956, when the Roxburgh dam was commissioned. The missing Clutha River deposits amount to about 75 million tonnes. Local coastal-dwellers noticed an increase in erosion soon after the dam was finished, so this is not a new 'climate change' issue.
Climate change is perhaps making it worse. Of course, Contact Energy will not admit any liability on this issue. It could be said that they, and many others, have their heads in the sand.
But the cost of defending beaches and coastal property and infrastructure is serious, and this issue will not be effectively addressed until the Clutha River deposits are allowed to replenish the coastline again. A good start would be to de-commission the aging Roxburgh dam, with its problematic bed profile and flood issues.
De-silting the Roxburgh Gorge would provide years of buffering deposits along the Dunedin coastline.