ASHLEY RIVER PROTECTION

Press, Volume LXXIV, Issue 22343, 5 March 1938, Page 15

ASHLEY RIVER PROTECTION

+ — COMPLETION IN ABOUT THREE MONTHS TOTAL COST OF MO LIE THAN £200,000 WORK BEGUN FIVE \EARS AGO Another three months' work will probably bring about the completion of the Ashley'river protective scheme, begun nearly five years ago and estimated to cost considerably more than £200,000 by the time it is finished. The river will then be confined to its bed by a series of stopbanks and groynes stretching for 13 miles from Fernside to the mouth at Waikuku. With a catchment area of 440 square miles on the eastern slopes of the Puketeraki ranges, the Ashley river is liable to heavy flooding when there is a downpour of rain in this district which includes Mount Thomas and Mount Grey and the Lees Valley. Tins has endangered life and property in a large area of country south of the river, and also on low-lying land to the north nearer the coast. Although it is neither as big nor as important as the protective works built in the bed of the Waimakanri river, the scheme affords protection to many districts. As was seen at the time of the exceptional flood early in 1930. Ashley, Sefton, and Saltwater Creek, on the north bank towards the mouth, and Fernside, Rangiora, Soutnbrook. Coldstream, Waikuku, and Woodond, on the southern side of the river, are in the danger zone with the river not under control. A big factor in the flooding has been the restriction of the flow near the mouth, where a short bridge and a heavy growth of willows caused considerable obstruction. This "bottleneck" effect has been overcome by the construction of a longer reinforced concrete bridge on the Main North road and the clearing of the riverbed of much of its natural growth. It was in 1926 that Mr F. C. Hay, a consulting engineer and an authority on river control, was invited by the Ashley River Trust to prepare a scheme to protect surrounding country from floods in the river. He drew plans for protective measures to cope with a flood of 44.000 cusecs, and in general this plan has been followed. There have been minor modifications, I chiefly the strengthen in? of the works ! afler the phenomenal liood two years i ago. Unemployed Relief Measure • ! The trust suggested that the Gov- | eminent should carry out the scheme ! as an unemployment relief measure, I and it was begun as such under the I control of the Public Works Depart- ! ment. After some trouble with the J workers at the beginning of 193 G, the scheme was declared a standard Pub- ! lie Works Department job, and the I men are now employed full time under the co-operative contract system. Good progress has been made since the start in May, 1933, and the scheme is expected to be finished within the fiveyear period. After the flood in February, 1930. when there was a flow of 110,000 cusecs. it was realised that Mr Hay' 3 provision for a flood of 44,000 cusecs was not sufficient. His scheme was designed to withstand a 44,000-cusec flood with two feet of free-board, cor- ' responding to 70,000 cusecs at the point , where the banks would be overtopped. To ensure the safety of the scheme in a flood even as great as that in 1936, all banks on the upper stretches of the river, built before 1936, have been raised a foot and the groynes from three to four feet. The channel has been widened nearer the mouth so that the flow of flood waters would not be impeded, and it is considered that in its finished state the scheme will give complete protection. Banks and groynes were damaged in several places by the flood and repairs took some time. When the scheme is completed, the protective works will be handed over to the trust by the department. Work still to be done is the extension of stop-banks to meet the sandhills at the coast, the clearing of the central control channel, and the removal of willows near the mouth, the raising of some of the banks built in the Fernside district, when the scheme was begun and further protective planting. From Fernside to the mouth, stopbanks have been built along both sides of the river and these are protected by groynes running out into the riverbed at an angle to the banks and pointing upstream. Banks and groynes are raised with spoil taken from the land side of the banks, the groynes being constructed on the solid principle with stone gabion heads (stones enclosed in a wire mesh at the point). The groynes give protection to the banks by holding some dead water against the banks in time of- flood and preventing any scour by the more swiftly moving waters in the stream. Were the groynes pointed downstream, the river would be drawn to the banks, probably causing scour where the groynes join the *h juries Hugre Quantity of Spoil Twenty-two groynes, varying in length from two chains to more than 40, have been constructed on both sides of the river, between Fernside and Coldstream, being placed to give most protection where there is a bend in the river. There are 21 miles of stopbanks in the whole scheme and, up to the end of January, 660,000 cubic yards of spoil had been used in the building of these and the groynes. By the time the work is complete, about 750,000 yards will have been used. The banks vary in height to six feet and are four feet wide on top, with two-to-one side slopes. Groynes have a width on top of eight feet. Allowance is made for shrinkage in consolidation. As many as 500 men have been engaged on the work at one time. About 280 are working at present, some from Christchurch and others from the Rangiora district. The number varies from time to time, particularly during the harvest, when the workers are given the opportunity to take seasonal work. In the earlier stages, the men were accommodated in camps at the Rangiora bridge and at the Main North road bridge, but they now mostly travel to and from their homes each day. The spoil is shifted by barrow and small truck lines. Since its introduction, the co-operative contract system has proved very satisfactory, the work proceeding harmoniously. The men make wages well in excess of the basic rate and at the same time the department receives a greater output from each man. The average amount of spoil handled by a man is from eight to nine cubic yards daily, varying according to local conditions such as the distance to be travelled, the height of the bank, and j the nature of the material. ! Clearing willows from the lower reaches and the removal of all obstructions in the main control channel, the main work still to be done, : is in progress. Nine traction engines are being used «to haul the willow , trees out bodily bv the roots. While the trees growing haphazardly in the riveri bed are being pulled out. the stopbanks are being protected by the systematic planting of willow cuttings on i the river side, between the banks and the control channel. About 220 acres i have been planted in this way and more planting will be done in the autumn. : Automatic flood-gates at' the mouth of the Waikuku and Taranaki creeks ; have yet to be provided and there are ■ also several smaller gates required | elsewhere. The big ones will cost

about £IOOO each. As an extension of the general scheme, stop-banks have been built at Stoney Creek, Sefton, to control the creek and prevent it flooding and depositing shingle over adjoining properties. The engineer in charge of the scheme is Mr R. Mclntyre. \vh.o has had considerable experience'iti irrigation and river-control work.