Architects impression of the final seawall and Esplanade redevelopment looking south toward St Clair Hot Salt Water Pool.
The St Clair Sea-wall
The calm, settled weather of Dunedin's autumn has made Maurice Davis grind his teeth. Each passing day brings southerly storms closer, and with them the likelihood that reconstruction of the St Clair sea-wall will begin smack in the middle of the annual tempest. Jo Register backgrounds the massive $5 million project.
Architects impression of the final seawall and Esplanade redevelopment looking south toward St Clair Hot Salt Water Pool.
As Managing Director of Duffill Watts Davis Ltd, and Dunedin City Council's engineering consultant for the St Clair sea-wall construction project, Maurice Davis FIPENZ has been frustrated by a five-month delay in obtaining the necessary resource consents. "We could have been working during this good weather. It would have been great." They were five more months tacked onto years of increasingly urgent need to upgrade the dilapidated wall. The first sea-wall – "Smith's wall" – was built in 1866; the present one was constructed in part in 1912/13, and extended between 1933 and 1938. There have been several inundations, as the different structures for taming the tide have crumbled, allowing seawater to flood several blocks inland into what is now a densely built residential area.
COP Outcome development and evaluation
Significant deterioration was apparent by the 1980s, and reports commissioned by the Council in 1985, 1995 and 1999 all recommended remedial works, ranging from significant repair to complete replacement to prevent flooding, and improve coastal protection and public access to the beach. Meanwhile, fill behind the wall washed away causing the St Clair Esplanade road to slump visibly several times, stairs leading down to the beach collapsed completely, and the beach and adjoining dune were being scoured away by wave energy reflected off the old flat barrier. At the southern end of the beach, the sea-wall surrounding the recently renovated St Clair Hot Salt Water Pool was also providing less and less adequate protection from the sea.
"When the wall was first built there wasn't a lot of knowledge about good sea-wall practice," says Mr Davis. Remedial work has been carried out after each problem has emerged, but without the benefit of the technology and understanding that are now available.
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Development
The Seawall and Esplanade looking south toward St Clair Hot Salt Water Pool.
A study commissioned from Duffill Watts & King in 1999 established that the main Esplanade sea-wall was "at the end of its practical life",and replacement was a matter of growing urgency – the risk of failure in storm conditions in the next 10 years was estimated at 60-75, and outside "acceptable limits".
"We have to try and meet the challenges that the sea is going to throw at us in future," says Mr Davis (Conceptual statement). He explains that the proposal is to build a new sea-wall immediately to the seaward side of the existing one, and admits to some unknowns, such as the condition of the tiebacks anchoring the existing wall and of the services behind them. Information available at the moment is sketchy. The threat of a big storm during construction is the principal reason they propose to make use of the existing sea-wall in this way – "The prospect of a storm without that protection does not bear thinking about."
The project will include a 50-metre extension of the sea-wall along the dunes. As the wall's flat surface does nothing to dissipate the energy of the waves, the new wall will be formed with sheet piles profiled to reduce the sand-scouring effect of the tide. The concrete piles have been designed and trialled by Duffill Watts Davis especially for this project.
With the assistance of Derek Todd, a Christchurch consultant dune specialist, several hundred metres of dune re-contouring at the end of the wall has been designed. Without some kind of transition between the wall and the dune, a scouring or "end-wall effect" typically occurs. The transition will be effected using "geotextiles", sausage-shaped tubes filled with sand and buried in the dune. Whenever sand is washed away the base of the dune will be protected by the geotextiles, and will therefore be able to build itself up naturally much faster. Combined with wind-fencing and plantings of suitable vegetation, this will offer the dune a far greater chance of withstanding the tide.
"Getting machinery and equipment on to the beach to do all this will be tricky for the contractors, as will getting it off again when storms hit," Mr Davis adds, but he believes they will get some warning since most storms travel up the coast from the south.
Safety Issues
Artist impression of the St Clair seawall and Esplanade redevelopment looking north.
The weather and the sea are not the only challenges in this project. "Maintaining public health and safety is a significant concern," says Dunedin City Architect Robert Tongue, "especially given that we are required to make sure access to the domestic and commercial buildings along the Esplanade and to the beach and dunes is preserved at all times."
St Clair attracts a constant stream of surfers, young families, joggers, walkers and cafe cruisers, so this will be no small task. Large holes will be dug in the road, ten metres apart, where the big anchor blocks that will hold up the new sea-wall will be buried. There will be large vehicles and machinery on the road and the beach. "The contractors will have some major health and safety issues to work through," says Mr Tongue.
The reality of what the St Clair Esplanade and beach will have to endure during construction may come as a shock to those who frequent the beach, the very people who are most passionate about its aesthetic preservation. Mr Tongue knows there will be strong reaction. "Keeping the public happy through all this will be very difficult. Needless to say, there will be continuous public consultation. But when they see the huge holes in the Esplanade, when they see the heavy machinery moving up and down the beach..." – he shakes his head. More public consultation is to take place about "extras" – if that is the right term for measures to enhance the Esplanade environment's aesthetic and amenity value. There are proposals to upgrade seating, lighting, footpaths, toilets, pedestrian areas and beach access, and to provide facilities such as parking and a playground.
"You can't make an omelette without breaking a few eggs," says Mr Davis, but he too indicates that this will be a particularly difficult site for contractors to safeguard against public risk.
Construction
The construction sequence and methodology, and the problem of maintaining public access to the beach and to the Esplanade Road throughout the project all have yet to be worked through. Mr Tongue says that contractors will be asked to put forward their preferred methodology.
They are likely to have different equipment and different approaches to various aspects of the work, including health and safety issues. They will also have to bear in mind the quite severe deadlines the DCC is placing on the project. It aims for completion one year from the beginning of construction (currently expected to start in August 2003); and furthermore, the Council also wants access to the St Clair Hot Salt Water Pool completed before it opens for the season in late October.
So far, the most controversial part of the planned reconstruction has been the proposed wave-calming devices to be built off the pool. The resource consent panel rejected the initial suggestion of building a 20-metre-long rock groyne out from the headland near the pool, to retain sand on the beach. St Clair has an internationally recognised surf break, and board riders strongly objected to the groyne, saying it would block a channel they and others used frequently, and increase the danger of people being washed onto the rocks. The panel did, however, accept a plan to attach large wave-baffling rocks around the edge of the pool. Local newspapers quoted concerned board riders who do not believe the rocks will withstand the regular five-metre seas without loosening and wreaking havoc on the pool; but, as no appeal was lodged against the final consent, only time will tell who has judged the proposal most accurately.
In the meantime, from August onward, if you hear of a storm heaving up the seas around Dunedin, spare a thought for the contractors scurrying to prevent their assets becoming a new reef off St Clair.