The sea — both a friend and an enemy to Kaikoura

Press, 17 August 1978, Page 17

The sea — both a friend and an enemy to Kaikoura

Late in June, the business section of Kaikoura township came within a metre of being flooded by the heaviest seas many people had ever seen. But while the town narrowly weathered the north-easterly storm, the beach was disappearing into the sea and the shingle dune between the sea and Lyall Creek was going with it. One week later the sea had returned much of the shingle which it had scoured from the beach. But the message had been brought home — in stronger terms than most would have preferred — just how vulnerable to violent weather Kaikoura has become. Since then, the Marlborough Catchment Board has bulldozed more shingle on to the beach to build up the dune between the creek and the sea. But the board’s chief engineer, Mr P. A. Thomson, does not see this as the end of the job. “The situation is serious enough to cause a great deal of concern,” he says. “The business area is in danger to an extent, but the sea would have to do more than it has up to now. Yes, it’s vulnerable; there’s no doubt about that.”

On the bright side, Mr Thomson says that if there is nothing worse than the June storm, then “We’re pretty safe.” The board’s biggest problem is deciding whether the changing weather patterns and severe erosion are long-term factors. One man who believes they are is Dr R. M. Kirk, of the University of Canterbury’s geography department. He has spent about 10

years studying the beach behaviour and the evolution of the shoreline. He says that the Kaikoura beaches have developed in much the same way as other east coast beaches over the last 5000 to 7000 years — with one complicating factor: the Kaikouras. The mountains have been jacked up 2m by a series of earth movements over the last 1000 years. Every time the mountains have been propped up there has been erosion of the sea floor and shingle has been swept up in ridges. “It is sufficiently long since they last popped up to exhaust the available

supply of gravel,” says Dr Kirk. There are more immediate causes, which come to mind. Mr Thomson points to the topping of the beach by the Kaikoura County Council about four years 'ago. A car park was formed at the time, but the topping caused the waves to break back on to the beach. Nor has the board’s own work in the area all been to the good of the beach,

he says. Some of the creek reconstruction work about 1972 upset the natural pattern of sea movements. Work on Lyall Creek has involved blasting away some of the creek’s rock bed to allow it to dig a deeper channel when it floods — rather than spreading over the township. In considering solutions to the problem of erosion, Mr Thomson says that the board wants to avoid interfering with coastal patterns. “We need a permeable beach that allows the water to soak back,” he says. “There is a solid layer at some depth below the beach, a sort of natur-

al feature, and this looks as though it will protect the beach. “We could run into all sorts of trouble if we upset the natural situation.” If the board built a rock groyne, or a series of them, the wind might blow on one side and the sea eat away the other. Mr Thomson has a lot of respect for a sea which some people suggest inevitably takes back whatever is taken from it. He sees the bulldozing of shingle back to form an artificial dune as temporary, but effective. In the long-term, however, the board would have to look at “something bigger.” An off-shore breakwater might be the answer, but Mr Thomson says it would be very costly. A groyne would be the “last desperate resort.” Any attempt to armour the beach would require a close watch on other parts of the coast to see that it had no adverse effects. One has only to look at the constant maintenance of the coast north and south of Kaikoura that has occurred since the main highway and trunk line were constructed to realise the great problems that exist. The Ministry of Works

and Development has been committed to a programme of protecting the seawall wherever the sea has eaten away at- it: an on-going process that has lasted 35 years and has no clear end in sight. Dr Kirk believes — and he has told the residents as much — that a large rugged seawall could solve the problem, but at the expense of the beach. “You don’t usually have a beach with a seawall,” he says.

The beach must be re-nourished, whether by trucking in more shingle, or by finding out where the sea has deposited what it took in June. “There may be a fair chance of getting much of it back” he says. The erosion, Dr Kirk believes, will continue until the land is jacked up again, or an artificial replacement, like a seawall,

is constructed to gain tne same effect. The Kaikoura coast is generally subject to strong southerly winds. The town is protected from these by the bay, but it is left vulnerable to north-easterly seas which hit usually around mid-winter. But whatever the longterm solution, Mr Thomson says that the board is watching the picture pretty closely, and is prepared to move in in an emergency.

Bv

MIKE HANNAH