The Wanganui Herald (Published Daily.) Friday, June 11, 1920. SEA EROSION.

Wanganui Herald, Volume LIII, Issue 160673, 11 June 1920, Page 4

The Wanganui Herald (Published Daily.) Friday, June 11, 1920. SEA EROSION.

On several previous occasions mention has been made in these columns ot the serious erosion of the sea which is taking place at Dunedin's iavourite watering-place and to the similarity of conditions prevailing there and at the South Spit, Wanganui. The encroachment of the sea at St. Clair is causing the authorities grave anxiety. Various suggestions have been thrown out and several experiments have been tried, but up to the present no measure of success has attended the efforts to curb the sea’s power. Mr A. Slinger, a well-known Dunedin, engineer, now comes forward and suggests that groynes are the only solution of the erosion problem. His observations are of particular interest to Wanganui readers, for the reason that the conditions existing at St. Clair are identical with the sea's operations on the South Spit, and the suggestions he throws out are similar to what Mr Napier Bell ever twenty years ago recommended the Wanganui Harbour Board to adopt. Mr Slinger discards the idea of breakwaters out to sea or of a sea wall along the foreshore. He pins his faith to groynes. He says that a judicious use of groynes of proper design and carefully maintained, should stem the present invasion of the sea and gradually coax back the sand and restore the beach. When the uselessness ot the present groynes was pointed out, he replied that they were the mere skeletons of groynes, a row of piles' with a wailing bolted to them. He advocates the type which he has seen doing good service at various places on the British coast—two or more rows of horizontal wailings bolted to the piles, and vertical sheeting affixed to the wailings. The ordinary paling fence illustrates the type of structure.

FACTS TO BE OBSERVED

In giving reasons for his preference Mr Slinger emphasised certain facta which those who have studied conditions at the Wanganui entrance must have noted—that wherever there is an opening between rocky promontories and the waves have scope to spend their force, they will warp or build up beaches, and the wind will blow the sand into dunes behind the beaches. The natural tendency of the sea is to build up. He points out that chiefly through human agency the waves have been prevented in places from spending their force naturally, and that interference with wave action often causes trouble. Mr Slinger entirely opposes any form of sea wall, for two reasons: Firstly, a rigid defence there would be the undoing of the natural defence in the shape of warped-up sand (as present developments are bitterly proving): and. secondly, there could be no rigid defence there, as a solid foundation for it does not exist. Sheet piling, for example, whether driven in vertically or at a very sloping angle, would inevitably fall into the sea. It would merely be an invitation to the ocean to scour away the sand and underlying mud from the seaward side of its base with the recoil of each wave for a fresh spring. Then would follow collapse and—more money thrown into the sea. The proposal to build a sea wall is the direct antithesis of Mr Slinger’s axiom of assisting Nature to build up the defences of the flat as they formerly existed. The same objections apply to more temporary (ind less expensive ex'pedients. such as putting an apron of loose stones and boulders at the base of the sandhills. They tend to accentuate the scour in the interstices, and, resting on no foundation, would soon sink out of sight, and really make matters worse instead of better. All this is very interesting in view of the fact that sooner or later the Wanganui Harbour Board will have to find a solution for a similar problem to which the Dunedin authorities arCt direction attention. It may be added that Mr Slinger was recently consulted by the Wairoa Harbour Board, and his report was referred to three well-known harbour engineers —Messrs W. Ferguson (formerly of Wellington), Cyrus Williams (Lyttelton), and J. Blair Mason (formerly of Dunedin). Their joint report has recently been given. It rceommends precisely what Mr Slinger had advised.