The St Clair Sea-wall
Development
(Click to enlarge)
The Seawall and Esplanade looking south toward St Clair Hot Salt Water Pool.
(click to enlarge)
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.
