DIVERSION OF ORARI RIVER

Press, Volume LXXXI, Issue 24753, 19 December 1945, Page 5

DIVERSION OF ORARI RIVER

ENGINEER'S REPLY TO PROPOSAL

A reply to a proposal, sponsored by the Geraldine .branch of Federated Farmers, for diverting the floodwaters of the Orari river to the Rangitata river, .was made by the engineer of the South Canterbury Catchment Board (Mr G. Milward) at a. meeting of the board, after a deputation of ratepayers of the areas affected explained the. diversion proposal. The scheme would be costly and uneconomic, said Mr Milward, and he could not recommend the board to give it any support. The Orari river, with a watershed of 141,400 acres, and estimated to run off a quarter-inch an hour, could discharge a maximum of 36,000 cusecs at the mouth of the Orari Gorge. The Rangitata river, with a watershed of 440.300 acres, could .discharge 110,000 cusecs. Assuming that half the Orari flood was diverted to the Rangitata, the maximum flood in the Rangitata would be increased by 17 per cent., and, also, as the other half of the Orari flood was to be carried in the existing river-bed, the remodelling of the stop-bank system and the clearing of the willows would still have to be done, although on a iess elaborate scale. The Rangitata river flowed at a much lower level than the Orari. This meant that when the flood water was gravitated to the Rangitata it would have to be discharged over the spillway, with a drop of about 100 feet. It would appear that- the alignment of the diversion would follow approximately along the 800 feet contour. If the risk of scouring was to be avoided the mean velocity would have to be less than five feet a second. To discharge half the maximum flood (half being 1(1,000 cusecs), the cross section of the channel would be approximately 4000 square feet, continued Mr Milward. If tjie average depth of the floodwater in the diversion channel was designed to be six feet, the width of the channel would have to be 666 feet, or 10 chains Whatever form of the design was adtfpted, it was evident,- he said, that earth moving to the extent of two to three million cubic- y.ards, would be involved. It' was at the intake of the diversion, channel, however, that the greatest difficulties would be encountered. The Orari was known to carry large, quantities of shingle when in Hood, and this shingle would either have tp 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 appeared 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 was 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 earth -. channel without steep gradients and consequent scouring of banks and bed. The estimate for annual maintenance was impossible unless a scheme was designed, but certain items were apparent; said" Mr Milward. The accumulation of shingle at the intake-would have to be dealt with, and public crossings over the diversion would have to be maintained. These two items alone would involve considerable - annual expenditure: Many months of surveying, plotting, and calculations would be necessary to arrive at an estimate for the project, he said, *nd It was clear that many hundreds of thousands of pounds would be involved. It was no exaggeration to state that the total cost could exceed £1.000.000. The capital value df land to be included in the Orarl-Waihi-Temuka scheme was only a little more than £1.000,000. and the area about 60.000 acres On the motion of Mr M. S. Turton, it was decided to thank the deputation for submitting the scheme for the diversion and to leave the matter to the engineer to decide whether or not he thought It was* necessary to make further investigations, and to act accordingly. The 'deputation comprised Messrs R. E. Mcllwrick, C. J. Kerr, and Major-Genera] R. E. Baker.