FLOODWATERS OF THE ORARI

Timaru Herald, Volume CLVIII, Issue 23387, 19 December 1945, Page 4

FLOODWATERS OF THE ORARI

Diversion Scheme To Rangitata Catchment Board Hears Report Advocating the thorough investigation of a scheme to divert the flood-waters of the Orari River to the Rangitata River, a deputation of Geraldine ratepayers waited on the South Canterbury Catchment Board at Timaru yesterday. After hearing: their case, the Board decided to keep the subject in mind. Mr R. E. Mcllwrick said that the settlers from the sea to the hills were concerned with the danger from the flood-waters of the Orari. The deputation hoped that the Board would realise the state of mind of the settlers, said Mr Mcllwrick, and would consider the scheme they suggested. If the scheme proved to be impracticable, he felt that a thprough investigation of the causes of the trouble and the proposal would produce something worth while. It was not feasible to interfere with the normal flow of the Orari, said Mr C. J. Kerr, but any attempt to draw off the flood-waters of the river would be welcomed. He suggested a dam or a weir in the river and a channel to take the flood-waters to the Rangitata. With some of this excess water the waten-races in the lower area could be substantially augmented as well as the Raukapuka-Downs scheme, which was in a poor state. Tire rates from these two services would go a little way to defray the cost of the scheme. All that was asked was a thorough investigation by the Board’s engineers.

That the proposed scheme over four and a-half miles would present less difficulties than one over 20 miles was mentioned by Major-General W. E. Barker, the third member of the deputation. Having seen other flood-water control schemes working satisfactorily in other parts of the world, he said that he could not see insurmountable difficulties in the present proposal. The Orari river was once well below the level of the plain below the railway line. It was now filled with shingle. Unless the drift of shingle was controlled at the head of the river, it could not be stopped. The proposed scheme might be a vast one, but it would be simple in application. The schemes that the Board had in mind for the Orari would involve a terrific cost and should not be carried out without a further thorough investigation. Major-General Barker said that he did not think that anyone had ever seen the Rangitata overflow, and the only danger, if there was one, was at the Island. The Orari and Rangitata did not flood at the same time. He asked, on behalf of the Farmers’ Federation, that a further investigation be made before a milestone of debt and maintenance was placed around the necks of the people as the result of the costly scheme that the Board had in mind.

Thanking the deputation for bringing their problems to the Board in the way they had, the chairman (Dr. P. R. Woodhouse) said that such cooperation was welcomed from those who were unfortunate enough to be the Board’s ratepayers. MajorGeneral Barker had mentioned the accumulation of shingle in the Orari, and Dr. Woodhouse invited assistance from the landholders to control erosion in the hills by tree-planting. The Board had had the problem of the Orari in mind for some time, concluded Dr. Woodhouse, and the chief engineer (Mi- G. R. Milward) had considered the scheme proposed by the delegation after he had read the report in the newspapers of the discussion at the meeting of the Federated Farmers at Geraldine.

Flood Discharge The Orari river, with a watershed of 141,400 acres and an estimated runoff of a quarter of an inch an hour, could discharge a maximum of 36,000 cusecs at the month of the Orari Gorge, explamed Mr Milward. The Rangitata River had a watershed of 440,300 acres and could discharge 110,000 cusecs. Consequently, assuming that half the Orari flood were diverted to the Rangitata, the maximum flood in the Rangitata would be increased by 18 per cent. As the other half of the Orari flood would be carried in the existing riverbed, the remodelling of the stopbank system and clearing of the willows, etc., would still have to be done, although on a less elaborate scale. “The Rangitata River flows at a much lower level than the Orari,” said Mr Milward. “This means that when the flood-water was gravitated to the Rangitata it would have to be discharged over a spillway with a drop of about 100 feet. It would appear that the alignment of the diversion would follow approximately along the 800 ft contour, and regarding the size of channel necessary the following hydraulic facts are given: “If the risk of scouring is to be avoided the mean velocity would have to be less than five feet a second. “In order to discharge half the maximum flood, the half being 18,000 cusecs, the cross section of the channel would be approximately 4000 square feet. In other words, if the average depth of floodwater in the diversion channel were designed to be six feet, the width of the channel would have to be 666 feet or 10 chains. “Whatever form of design was adopted, it is evident that earthmoving to the extent of 2,000,000 to 3,000,000 cubic yards would be involved. “It is at the intake of the diversion channel that the greatest difficulties would be encountered,” continued Mr Milward. “The Orari is known to carry large quantities of shingle when in flood and this shingle would either have to be trapped by a dam or passed down the old course. In any case it should not be permitted to block the diversion channel. “The riverbed and banks appear most uninviting for the siting of control weirs or spillways near the 800 ft contour where the width between the natural terraces each side of the river bed is at least half a mile. Further up the gorge, on the 900 ft contour, it may be possible to build a secure dam; but from this level it would be impossible to gravitate the diverted water in an earth channel without steep gradients and the consequent scouring of banks and bed. Estimate of Costs “An estimate for annual maintenance is impossible, unless a scheme is designed, but certain items are apparent,” said the engineer. “The accumulation of shingle at the intake would have to be maintained. These two items alone would involve considerable annual expenditure. “In order to arrive at an estimate for the project, many months of surveying, plotting and calculations would be necessary; but it is clear that many hundreds of thousands of pounds would be involved. It is no exaggeration to state that the total cost could exceed £1,000,000,” concluded Mr Milward. “The capital value of the land to be included in the Orari-Waihi-Temuka scheme is only a little more than £1,000,000 and the area about 60,000 acres. Even allowing for the protection of public services, roads, railways, etc., the proposal would be still much too costly and uneconomic, and I cannot recommend the Board to give it any support.” After the deputation withdrew, Mr W. H. Hall said that the engineer’s report satisfied him. There were other more important works in the Board’s district to claim the attention of the engineers than the making of another survey of the Orari and Rangitata Rivers He did not favour anything being done immediately. Mr M S. Turton: The cost of the scheme proposed by the deputation would be prohibitive; even if it were only half as great it would mean that the innri would have to lie rated at £9 an acre. “The proposed scheme is one that appeals to the layman, but after hearing the engineer’s report I am convinced that the scheme is not'practicable.” said Mr L. V. Talbot. “It would be wise to investigate the scheme further to satisfy those advancing it ” Mr C. V. Kirke: We have every confidence in our engineer. It would be appropriate if we left our engineer to make any further Investigation that he mav think necessary. On the motion of Mr Turton, it was decided to keep the subject in mind. “Even if this diversion scheme were

carried out, our present scheme would also have to be put into effect,” said Mr J. R. A. Hawcridge.