DRAINING A LAKE.

Star (Christchurch), Volume XLIV, Issue 601, 3 November 1932, Page 7

DRAINING A LAKE.

Big Ellesmere Scheme Put Forward. A MOLE AT TAUMUTU. If the latest scheme for the drainage of Lake Ellesmere were adopted, land on the fringes of the lake worth £763,500 would be permanently reclaimed. Against that must be placed an estimated expenditure of £583,175. The scheme, in brief, is to provide an outlet at Taumutu by the construction of a mole; this has been put forward by Mr F. Langbein, District Engineer, in a report submitted to the Canterbury Progress League, which, at the instigation of the Ellesmere Land Drainage Board, took the matter in hand. The principal object is the lowering of the usual level of the water in order that the lands bordering on the lake may not be periodically inundated as they are at the present time, but be available for cropping and grass continuously. Seven schemes in all were investigated by the Department, stated Mr Langbein’s report. Six were ruled out as impracticable. It was evident that in order to merit any consideration a scheme should be such that a connection between the lake and the sea was established at a point where they were reasonably close together. It was then apparent that the problem had to be tackled at Taumutu, if anywhere. Cost of the Mole. Using the experience of Timaru as the guide it was considered that if a mole was run seaward for a distance of twenty-five chains and, assuming as in the case of Timaru the accretion of shingle tapered to nothing at a point one mile south of the wall, the amount of shingle trapped was estimated to be 40,000.000 cubic yards, which would provide for all the shingle travelling along the beach for a period of about fifty-three years, provided the amount travelling was equal at Taumutu 1 and Timaru. The extension of the mole could be undertaken in the future when found necessary. The estimated cost of construction of such a mole with rock from Birdling’s Flat and run along the spit on a tramway was £IIO,OOO. It was suggested that an area of 29,450 acres should be reclaimed which would be reducible to 28,000 acres on account of roads, drains and other land required. That would prove very fertile according to the soil analysis carried out and the opinion expressed by Mr Aston, the chemist of the Agricultural Department. It was considered that £2O would be a conservative

estimate of its value when brought into production. An acreage of 28,500 at £2O an acre would represent £570,000, and by adding the betterment to the marginal lands it was estimated that the total betterment would be £781,000. The proposal would consist of the construction of a mole at the outlet to provide a permanent outlet, the construction of a stop bank, pumps to be installed to pump the water from the area proposed to be reclaimed, the provision of permanent pumping outfit to dispose of drainage water and the construction of drains and roads over the reclaimed area. The estimated cost of | the proposal would be £376,000 and the yearly maintenance cost £4OOO. Question of Economics. If the scheme were undertaken, several years would elapse after construction before the land would be of much use and quite a number of years would pass before it was fully developed. In considering the economics of the proposal it was therefore necessary to allow the interest and sinking fund to be a charge on the capital cost for a number of years and a period of five years was submitted as being reasonable. With interest and sinking fund at 6 per cent for five years the capital expenditure estimated at £376,000 j would be increased by £127,175. A review of the position then was: Estimated capital expenditure £376,000, interest and sinking* fund for first five years £127,175, and maintenance cost of £4OOO per annum capitalised at 5 per cent £BO,OO0 —a total of £583,175. To be compared with that figure was £763,500, the estimated value of the betterment of the lands affected. The estimated cost was approximately 76 per cent of the estimated betterment. There were, however, certain difficulties in connection with the scheme which should not be overlooked. There was the possibility of the mole requiring considerably more rock than estimated on account of settlement in the sea bed, and the rate of accumulation behind the mole had been assumed on the experience at Timaru and it might vary considerably. There were also the possibilities of large supplies of spring water being encountered when the reclaimed area was dewatered and of the sea water seeping through the shingle spit into the reclaimed area in considerable volume.