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Sea-wall work almost done | Otago Daily Times Online News : Otago, South Island, New Zealand & International News

Work to secure and stabilise the damaged sections of sea wall and the Esplanade at St Clair should be completed by this weekend.

Since the installation of new sheet-piling at the bottom of the sea wall was completed last week, contractors have been filling gaps behind the wall to protect it from the thumping of waves.
A gap between the sea wall and the original sea wall behind it has been filled with fine gravel, and holes that appeared in the Esplanade have been filled with coarser gravel.
The holes were created when the wall was undermined by high tides and heavy seas that removed more than a metre of sand from the beach at the foot of the wall and created gaps through which fill from behind was sucked out under the wall.
Council roading maintenance engineer Peter Stand-ring said he expected the filling work to be completed by this weekend.
The sheet-piling and fill would protect the wall from further damage, but the area round the St Clair Surf Life Saving Club's ramp would remain closed to the public for a significant amount of time yet.
Contractors would next go over the damaged area looking for any less obvious damage that had not been picked up yet.
The next major stage of work would be looking at addressing the sand retention on the beach in the long term.
Council staff had narrowed down the 11 expressions of interest received from consultants across the country for that work, and were in discussions with three companies.
It was hoped a decision on a consultant would be made by the middle of next week.

Impending high tide concern at St Clair | Otago Daily Times Online News : Otago, South Island, New Zealand & International News


Sinkholes at the Esplanade at St Clair will be filled with gravel as concern builds about high tides expected later this week.
A perigee tide is expected, building up from today, and the king tide is expected on Friday.
The tides are similar to those that three weeks ago stripped St Clair Beach of sand, exposing the sea wall's toe and undermining the Esplanade.
It was ''fair to say'' that because the fill had gone from behind the sea wall, council staff and engineers were ''a bit nervous'' as to how the wall would stand up to the pounding waves the high tide would bring, council roading maintenance engineer Peter Standring said yesterday.
It had been decided to fill the small gap between the sea wall and the old sea wall behind it with a fine pea gravel, and then fill the sinkholes with bigger AP40 gravel, to brace the walls and support the wall's anchors.
The use of gravel meant if there was any need in the future to drill down into the holes, that could still be done.
On the beach, part of the St Clair Surf Life Saving club's beach access ramp was removed at the weekend so contractors could plug the final gap under the wall.
Mr Standring said the ramp was basically dangling in its place and was too dangerous to work under.
The concrete ramp was old and unsafe and it was decided it would be replaced with a new ramp as part of the work on fixing the sea wall, he said.
Contractors who had gone to the southwest end of the beach to move big rocks back on the rock wall there, found the bottom section of a set of stairs had come adrift, and the stairs had been removed.
The steps have been blocked off for safety reasons.
Mr Standring said the future of those steps was already ''under a question mark'' as earlier engineering advice had been they were at risk of ongoing damage from wave action.

Sun, surf and not much sand at all | Otago Daily Times Online News : Otago, South Island, New Zealand & International News


As the cost of plugging the gap under the St Clair sea wall spirals past $300,000, Debbie Porteous' thoughts turn to sand.
When the new sea wall was built in front of the old one at St Clair in 2004, a lot of people said something also had to be built to protect it.
After years of increasing beach scouring, the eventual exposure of its toe and inevitable sink-holes in the footpath above in the late 1990s, there were many warnings from experts.
The warnings were that wave pressure and sand erosion at St Clair Beach needed somehow to be mitigated or the wall, and the beach, would take a beating.
They were accompanied by high level of concern from the community.
Every person agreed that when waves reflected off the wall they scoured the beach.
As the beach became lower the waves could reach the wall more easily, smacking into it harder and scouring more and more sand from its front.
Contractors work on the last section of sheet piling going in in front of the sea wall to stop it being undermined.
Contractors work on the last section of sheet piling going in in front of the sea wall to stop it being undermined.
Sand moved to and fro, but it was clear it was not returning at the rate it was leaving.
Canterbury University engineering dean Prof Alex Sutherland warned that without mitigation, the vertical sea wall would continue to lower the beach level.
The Dunedin City Council's consultant engineers said a breakwater was needed to break waves offshore and create a reservoir of sand at the beach.
A Otago Regional Council hearings panel - despite turning down consent to build a breakwater - suggested the city council investigate options to mitigate waves hitting the wall and beach scouring.
''... and a longer-term management strategy to address these effects as far as is reasonably practicable.''
Members of the public welcomed a new wall, but said it should be only the first step in protecting the area.
It was the appearance of sinkholes in the Esplanade - not unlike those that have appeared there now - and the exposure of the original wall's foundations in the late 1990s that prompted the council to start seeking advice.
Councillors were soon advised the wall was unstable, and could collapse in the next big storm.
A new wall was built in front of it in 2004.
At the time council staff were clear further protection also had to accompany the wall.
They suggested that long-term, the installation of a $2.5 million artificial offshore reef, made of sand-filled sausages, to reduce the power of the waves could solve the sand erosion problems.
But they also said it would be several years before the council would be able to consider such a project.
A great citywide debate about what to do about the wall followed.
Engineer Maurice Davis, then of Duffill Watts and Davis, who prepared the initial beach report for the council, told a public meeting at the time a wall was not ideal at all. But because one had been built there at a time when no-one knew what effect it would have, and houses had long since been built behind it, the city was stuck with it.
A new wall would have to be built in the same place, as close to the old wall as possible.
Its negative effects could be alleviated by encouraging the deposit of sand on the beach and limiting the sand's longshore drift, he said.
As well as the artificial reef idea, people at the meeting also discussed other long-term options, including building a permeable or solid breakwater out from the headland and building sand-trap fences along the beach.
Duffill Watts and Davis eventually recommended the council build a 20m breakwater out from the St Clair salt water pool.
It is understood its plans were reviewed by other experts, who recommended the breakwater extend further than that, but when the council eventually applied for consent to build the sea wall, a 20m breakwater was included.
It was not a popular idea.
There was a strongly negative reaction from surfers, and a regional council panel declined to give resource consent for it based on the health and safety risks it posed to surfers.
The panel was also concerned the breakwater would project into a known channel used by surfers and affect wave breaks and currents in an area that was so important to surfing.
''We consider that the value that a readily accessible good surfing beach offers Dunedin must be preserved. The rock groyne would be inconsistent with the classification of St Clair beach as a coastal recreation area,'' its decision said.
In 2005, a report for council prepared by Duffill Watts and King on damage to a sea ramp on the new wall raised the issue of the breakwater again.
It said the wave conditions of the previous six months and the ''dramatic degrading'' of the beach provided a sound argument for the council to reconsider the project.
The beach profile was ''seriously sucked down'' and there was little sand protecting the base of the wall and ramp, director Barry Chamberlain told councillors.
It was left to staff and a council consultant to consider the groyne suggestion.
It seemed to fizzle out.
In 2007, the council commenced a substantive investigation into stabilising the city's beaches, following storm damage to dunes at Middle Beach.
Submissions reflected concern from the community about the sea wall's effect on erosion at St Clair Beach, and support for an artificial reef to solve the problem again came through strongly.
At some point, the council suggested taking sand from Tomahawk to replenish St Clair and Middle Beaches, a plan abandoned after Tomahawk locals did not take kindly to it.
From the investigations came the long-term Ocean Beach Domain Management Plan.
Adopted last year, it says the area in front of the sea wall is managed by a separate city council process and proposes no action for the area other than maintaining the sea wall.
The nature of that ''separate process'' is unclear, although parks and recreation manager Mick Reece said it was related to the consent, which says the wall shall be maintained in good repair throughout the 35-year term of the consent.
Chairman of the hearings committee Cr Colin Weatherall said St Clair Beach was out of the project's scope.
''It was always going to be the difficult quantum, but we never envisaged what's happened there now.''
Last week, he asked council staff for a report as soon as possible.
If everyone said the sand migration issues needed to be addressed, why has it not been done?It seems the reasons are complex.
The answer would require a close reading of dozens of reports, studies and assessments on the matter done by council staff and experts over the past 15 years.
There have been changes in councils and council staff.
The man responsible for the sea wall now is transportation operations manager Graeme Hamilton, who says he inherited the sea wall from the council's parks and recreation department 18 months ago.
Parks and recreation inherited it from the city architect/city planning department, which dealt with it when the wall was replaced.
Mr Hamilton's focus is on fixing the immediate problem: stopping the Esplanade from slumping any more.
He has not thought too much about sand yet.
''If the wall was built on rock, there wouldn't be an issue with sand.''
The long-term options for sand retention would be determined and reported back to the council for a decision, he said.
At this stage, it seems councillors will not receive that report until September, as it will not be ready for the next committee round in July.
Cr Paul Hudson, who was on the hearings committee which dealt with submissions on the 2008 sand investigations, said the whole area was part of those discussions and councillors were well briefed on issues along the coast, including St Clair, as part of that.
It is not that nothing has happened to try to divert the sea's energy at St Clair; work such as dumping stones at the foot of the wall had taken place from time to time, he said.
It seems likely the council may now have to seriously consider doing something about the sand situation.
And that will come back to money, of course, of which there is not much to splash around.
The 2004 sea wall and Esplanade was originally to cost $3 million and ended up costing about $7 million.
The cost of another major project is unlikely to be something any council, especially one in a tight spot, will welcome.
Any options involving more rocks or structures on the beach or in the water are also unlikely to be popular.
But perhaps people will have to accept it could well be a choice between some effects on the amenity - visual or otherwise - and no beach at all.

St Clair sea wall: Some food for thought | Otago Daily Times Online News : Otago, South Island, New Zealand & International News



Steve Moynihan, of Moynihan Coastal Consulting Ltd, provides some food for thought about the St Clair sea wall.
Quite a lot has been said about the St Clair sea wall.
I have joined the party late, mainly because I have had to carry out some calculations and prepare a drawing.
I am a coastal engineer working from home in Omakau (not because of expected sea-level rise), and I have been involved in new beach design.
Research on many beaches has shown soft (sand/shingle) beaches will adopt an equilibrium shape in plan when adjacent to a headland such as the seawater pool area at St Clair.
Equilibrium is a state where there is no more erosion because the shoreline is everywhere in line with the incoming waves. A beach that has not reached an equilibrium shape will continue to erode until it does so. This is the case at the St Clair corner, which explains why there have been problems there since first occupation in the late 1800s.
The picture published here titled ''Erosion'' has a red line showing where the back of the beach wants to be. In trying to reach this line, the beach may encounter hard rock or a seawall which constrains the back of the beach to the line of the wall.
However, a sea wall does not stop erosion and the seabed in front of the wall will have been continuing to lower until it reaches a depth at the location of the wall, as if the back of the beach did in fact lie along the red line.
The wall would have no problems if it had a strong base that went below the eroded depth, but this would not leave much of a beach in front. Of course the whole situation is very dynamic and sand will come and go, but the overall effect would be that of a poor beach.
A change could be made by pushing out the headland into the sea by way of a breakwater.
The picture labelled ''Beach'' has a yellow line showing where the edge of the beach would lie with a 150m breakwater in place. The result would be a beach of up to 50m width, with the sea wall sheltered by permanent sand.
My inexpert drawing ''Breakwater-Beach'' shows what this would look like. There would be room for activity like beach volleyball. Studies would be required to show no harm to the surf break. More beach could be obtained with a longer breakwater.
The downside is cost. A strong breakwater would be required - costing $8 million to $10 million, depending on whether sand would have to be imported, on final design, studies, and consent costs.
Food for thought anyway.