£431,000 Plan Advanced To Improve Selwyn River

Press, Volume XC, Issue 27397, 9 July 1954, Page 7

£431,000 Plan Advanced To Improve Selwyn River

A £431,869 scheme for the improvement of the Selwyn river by the installation of protective works was explained to representatives of five counties concerned by the North Canterbury Catchment Board at Dunsandel yesterday. Discussions will now be held by each local body before a second meeting is called with the board. If this meeting decides to approve the scheme, further surveys will be made, and the surrounding farmlands will .be classified to assess the degree of benefit from the scheme, so that a rating scheme can be devised.

In a voluminous report to yesterday’s meeting, the chief engineer to the board (Mr H. Murray Reid) explained the engineering works recommended to alleviate flooding and bank erosion on the Selwyn river and its three principal tributaries—the Hawkins, the Waireka, and the Hororata rivers. His scheme, said Mr Reid, was designed to prevent, as far as economically possible, the flooding of agricultural land, farms and lines of communication; to reduce erosion of river banks: and to encourage the rivers to flow in more regular courses.

Between 1947 and 1952, a total of £66,800 was spent on a scheme of improvement to the Selwyn, said Mr Reid. Stop banks were then constructed from the mouth to within 20 chains of the Ellesmere bridge. But after the flood of April, 1951, which cost nearly £25,900 of the bill for the scheme, the board decided to suspend further work until surveys had been taken over the whole catchment area. Further Surveys Required Up to the date of bis report, said Mr Reid, surveys had been made along 42 miles of the Selwyn river; over 13j miles of the Waireka, from its junction with the Selwyn; along 26 miles of the Hawkins; and along 14| miles of the Hororata. Aerial surveys had also been taken.

“If the scheme is proceeded with, further surveys will be required to fix the line and height of stop banks more exactly,” he said, “and further smaller

works beyond the liriiits of existing river surveys may be found necessary.” Before any channel design could be started, it was necessary to determine a reasonable figure of flood discharge, said Mr Reid. Few figures were available, and the assessment had to be based mainly on rainfall data and judgment. A minimum of 30,000 cusecs was agreed upon, and this figure was adopted as the design discharge, with

two feet of freeboard on stop banks at this rate. Proposed Measures Eleven proposals were made by Mr Reid in his explanation of the works necessary. They -were:— (a) Stopbanking all floodable lands. (b) The prevention of overflows down old channels or from low portions of river banks. « (c) The provision of sufficient clear

water-way to pass the design figure of 30,000 cusecs in the lower reaches. (d) The erection of stopbanks in “safe” places; on • existing terraces where they suit the line, and not necessarily along the actual design channel in the.river bed. (e) The river bed (design channel width only) to be kept clear of all trees, stumps and accumulations of flood debris. (f) The construction of protection works to prevent and heal bank erosion. (g) The construction of training works, groynes, and pilot cuts in the rivers to lead flood waters into the correct channel and abate bank erosion. (h) The planting and fencing of plantations along the design channel and in “safe” spots, to consolidate the alignments and provide materials for future protective works. Many miles of the rivers have no trees growing at the present time. (i) The acquisition of all land in the riverbed to facilitate control. - (j) The fencing of lands acquired, and stopj>anks where necessary. (k) The provision of an adequate annual sum to cover the cost of maintenance and flood damage reserve. “This is a most important item,” Mr Reid commented.

Finance and Rating* Estimated costs for the Selwyn river sections of the ’scheme totalled £295,747, said Mr Reid. For the Waireka river sections, the estimate was £40,333; for the Hawkins river, £47,600; for the. Hororata river, £900; for gully control work, £7500; for other river works, £10,000; and for compensation, £5OOO. These items made a total of £407,080, to which was added £12,210 for construction maintenance over the first two years, and £12,579 for engineering services. Rates to be levied over the areas concerned would vary with the amount of benefit obtained from the scheme, said Mr Reid. At present, accurate figures could not be given;, an experienced classifier would be required for that. But as a rough guide Mr Reid had taken out figures which he considered to be fair average rates for the whole scheme.

Mr Reid said the present capita] value of the five counties concerned in the scheme (Tawera, Selwyn, Malvern, Ellesmere and Springs) amounted to £1,860,000, but it was possible, when all valuations were brought up to date, that the total would be nearer £2,750,000. Subsidy Expected On the estimated cost of the scheme, it was assumed that a subsidy of £5 for £2 would be available, Mr Reid said. The board’s share of the total was £123,392, and assuming provision for flood damage at £2500 and for maintenance at £5OOO annually, the average rate on a loan of £123,400 for a period of 20 years would be 1.172 d for repayment, .645 d for maintenance and .322 d for flood damage—a total of 2.139 d in the £.

Expenditure on maintenance and flood damage was unpredictable, said Mr Reid, and it was suggested that surplus should be allowed to accumulate, and when a sufficiently large amount was obtained, the rate should be reduced or removed until it was required again. The chairman of the catchment board (Mr R. M. D. Johnson), commenting on the possible increase in the total capital value of the counties concerned, said the increase would have the effect of lowering the rate in the £. The board was interested only in the amount qf money obtained by rates.