Engineers Discuss River Problems In Canterbury

Press, Volume XC, Issue 27498, 4 November 1954, Page 12

Engineers Discuss River Problems In Canterbury

“The problem of Canterbury’s rivers —control and irrigation’” was discussed by two speakers in addresses to the Canterbury branch of the Royal Society of New Zealand last evening. Mr H. M. Reid, engineer for the North Canterbury Catchmpnt Board, dealt with river control, and Mr F. D. Grant, of the Ministry of Works, with irrigation. A film on the Waimakariri river, produced by the National Film Unit, was screened.

Mr Reid said gravel-bearing. rivers of the nature of the Canterbury ones constituted a special problem. When a gravel-bearing river reached the state of the Waimakariri, with a gradually flattening grade towards the sea and a heavy bed load to be transported, there was difficulty in containing flood waters within the channel and arresting bank erosion. “If we can slow down the run-off of flood water from the upper catchments of rivers, it must have a most beneficial effect on the lower reaches, and I consider every £ spent to achieve this object will pay handsome dividends in the future/’ he said. Outufilng the problems of control, Mr Reid said that before any control plan could be designed, accurate records were required of the amount and duration of rainfall over the whole catchment area. There was a need for many more rain gauges, systematically located in the Canterbury area. Once the maximum rainfall had been

ascertained, the next problem was to assess its rate of run-off from the hills and its effect on the river itself, said Mr Reid. In many cases, it was. necessary to study each individual catchment area over the whole catchment to arrive at a fair total discharge. Where possible, actual measurements of river flows were taken by current meter. “Every endeavour is made to obtain discharges at all stages of flooding, so that when combined with the known rainfall, accurate forecasts can be made for future flood levels,” he said. Problem of Bed Load

“A big problem is bed load, which is continually replenished by erosion in the headwaters and from local bank erosion,” Mr Reid said. “Once the supply of gravel exceeds the carrying capacity of the river, aggradation must take place, with its attendant vices of meandering, with more bank erosion, and the continual raising of bed levels. “The erosion problem is directly caused by excessive run-off, which m turn is the result, in most cases, of man-made circumstances,” said Mr Reid. “These circumstances in the upper catchments are the secret of good river control, and it is most unfortunate that this position was not recognised in the past.”

Discussing the control measures that it .was possible to take, Mr Reid said it was desirable to restrict the gravel rivers to a maximum width by training works, thus encouraging them to flow in straighter channels with increased velocities which would cause a

certain amount of bed scour and increase their load-carrying capacity. That would ensure gravel deposits being carried to the sea. The training works could take the form of groynes in the first instance, to be followed by the stabilising of the banks to prevent further erosion, Mr Reid said. To provide adequate protection along the banks. of a new channel, plantations of quick-growing trees should be' established near the banks, with trees of a more permanent or slow-growing nature further back. In areas, where the river bed had risen, the only quick method of protecting adjoining lands was by the erection of stop-banks or levees.

Alternative Control Plans Mr Reid said that, when the safety limit was endangered by rising bed levels, an authority coula either raise the stop-banks or lower the river bed. The first alternative was unsatisfactory for, as long as aggradation continued, bank raising must be also. The other alternative was difficult, but not impossible, and, once degradation began, a river would continue to improve itself if the bed load was reduced and future erosion held to a minimum. 9

Dams had been successfully used for control works, but were extremely expensive and hardly warranted unless other types of control were useless he said. In few cases was it possible to provide a dual-purpose dam—for flood control and power generation—as one required the dam to be empty and the other required it to be full. A great deal of investigation was required’befor dam building could be made. In order further to slow down the discharge in the lower reaches of a river, it is sometimes possible to direct \ p “ r i 10 ?. ?!• the flood waters into special holding or detention areas, where the waters are allowed to pond ?f d 3 re ., h £. W back tU the P eak oi the flood has passed ” said Mr Reid. It is also possible t 6 divert some of the flood waters into other channels trt special spillways, but these possibilities require extra i lands for flooding. In some cases, the lands win benefit by allowing the flood waters to lie on them and deposit silts, so bringing many thousands of acres of poor, light land into higher production.” , Referring to finance, Mr Reid said it was often the main obstacle to works. It had.become easier as the importance of control had become more obvious to the public. TMe Government now provided quite generous subsidies f° r approved river works and capital expenditure was assured within reason.

Irrigation Prospects Discussing .irrigation. Mr Grant said that production throughout Canterbury during the last 50 years had not Increased comparably with that of the higher ramfall districts of New Zealand. Because -at lack of moisture, farmers had not been able to exploit the techniques of improved grassland farming to the same extent as farmers in higher-rainfall areas. A greater incr£ts<Lin p P /? ductlon could be made wl tJ? fusl USe of irrigation water. , Mr Grant described irrigation projects m various Canterbury areas, and showed a series of slides.' He said it was necessary to obtain better productlon s J!? x^ with the wonderful possibilities of “our irrigable plains.” New Mayor of Sava.—Suva has an K}and-b°r n Mayor as the result of a ballot of city councillors. He is Mr V ey ?’ V on of Mr c - c ' of 1 j rs Bay - He came to Fiji in 1937. and was elected to the City SttTA n Q W M& r aT Fij e i.^uv e a, O Nov t 3 EUrOPean traders iri