GROYNES AT BLAKETOWN

Greymouth Evening Star, 9 March 1939, Page 12

GROYNES AT BLAKETOWN

; BENEFICIAL EFFECT ON BAR. In the course of a report, to the Gieymouth Harbour Board, last evening, concerning the groynes on the Blakclown beach, the Engineer (Mr, ID. (’.Milne) slated: February was remarkable for the exceptionally heavy sotith-wesierly seas which prevailed without interruption throughout. the month. Observations taken on the foreshore' at! Blaketown revealed that vast quantities of material were moving northwards along the beach.

Rapid: accretion occurred in the. vicinity of tlio groynes, where the beach u se to tho top of'the planking. About the middle of the: month, the piles in all four groynes were completely covered, and, at tho seaward end of the gioynes, tho level of the beach was between two and three feet above the tops of tho piles. The bank of sand and shingle had extended southwards towards the wreck of the Lauderdale, while the foreshore to thenorth of the groynes remained normal. A few days later, a bank was observed in the process of formation between the seaward end of the groynes, and lowwater mark. This' bank increased rapidly, in extent, and extended, not only io tho south of the-groynes, but also to the north, along a part of the foreshore, which had been previously unaffected by the travel of the littoral drift. It was apparent that the groynes had temporarily reached the limit of their capacity to arrest the northward movement of material, and that sand and shingle were passing the seaward end of the structures. The effect on the bar depth was immediately, evident. A survey made on February 23, disclosed that the depth had decreased from 27ft Gins to 22ft Gins at H.W.O.S.T. The river velocities during the month had been abnormally low, and a study of the records showed that the conditions of both sea and river were analogous to those which obtained in February, 193 G. The appearance of the foreshore in the latter period, however, was entirely different. In February. 1936, although considerable accretion occur-

red, it was on a very much smaller scale and consisted of a series of low banks with deep depressions between each, the average beach level being over 3 feet lower than it is at present. It is clear, therefore, that the accretion which took place during the past month was on a vastly greater scale than in the corresponding period of 1936. It has been definitely established by an investigation extending oveT a* number of years that, other factors being constant, the bar depth varies directly- with the volume of ac-

cretion on the beach to the south ot the harbour. In the two. periods under discussion, the other two factors —sea 1 Conditions and river velocity—wenp similar in both cases, while the greater volume of accretion which occurred during the past month is reflected in the ‘ bar depth of at JJ.W.O.S.T., compared with 16ft in February. -1936. 1 have stated in previous reports that, where heavy southwesterly seas are coincident 'with abnormally low river velocities, serious shoaling of the bar must inevitably result, and, although 1 see no reason to alter that opinion, the influence of the groynes on the bar depth during the recent severe conditions, is as remarkable as it is satisfactory. The construction of Groyne No. 10 was completed during the month. The chairman (Mr. J. Ryall)' said the’Engineer was to be complimented on solving, to a. great extent, the shoaling problem on the bar, by the erection of the groynes on the beach. Mr. J. B. Kent said it was not only what it meant to the Harbour Board, but the benefit to business people of the district in having the port kept open continuously. Mr. Perry said the Engineer's theory had now been proved by the results of the erection of the groynes, as set out in the report. In reply to a question, the Engineer said that shoaling could not be eliminated entirely, as they were dealing, not with the river, but with the Tasman Sea. and that, could not be controlled. To Mr. .1. Smeaton, the Engineer said it had always been the assumption in Gerymouth that it was the shingle brought down by the river which caused shoaling of the bar, but he had proved that this was not the case. It 'was the sea which caused the trouble. Every scheme previously propounded and tried to improve the bar had been based on. the assumption that the river was responsible. The report was approved.