SUPPLEMENT TO THE ASHBURTON GUARDIAN.

ASHBURTON, SATURDAY, APRIL 4, 1891

THE ASHBURTON-RAKAIA IRRIGATION SCHEME.

Thb County Engisbek's Repokt and Estimates presented to the County Council at Meeting held ox April 8,1891. (Published hj tta Authority of the Ashburlon County Council.) Ashburton, 3rd April, 1891. The Chairman, Ashburton County Council,— Sib,—ln accordance with the Council's instructions to report on an Irrigation Scheme from the Upper Bakaia, I have now the honor to do so as follows:— I have carefully examined the river banks from Acton upwards to Black ford, a distance, of about 32 miles. The height of the terrace above the river bed at the former place is stoat 10 feet, and at the latter place, 700 feet. At Barrhill theheightof theterrace is 104 feet; at the chasm above Highbank, 250 feet; at Barker's Road, 360 feet; and 3 miles higher up, opposite B.S. 17336, the height is 540 feet. The surface of the plain has a gradually increasing ascent from Bakaia upwards, the rate of inclination m the vicinity of Bakaia being about 30 feet per mile, and on the downs near Mount Hutt 75 feet per mile. The river from Rakaia Gorge Bridge downwards has a fairly uniform fall of about 21 feet per mile. The cross sections of the terraces for most part are very steep, and m several places almost perpendicular. The cross section of that portion of the terrace m which a channel would have to be located would average about \\ horizonal to 1 perpendicular. The terrace slopes consist almost entirely of gravel of a loose nature, and are very much furrowed and water-worn for the most part, while, m placet, the flood-water has scooped out large chasms extending backwards several chains from the river bank. I am of opinion that for a channel,of such magnitude as would be required for the purpose, it would not be prudent to locate it m the face of this terrace. A coneret© channel with attendant earthworks, &c, would cost nearly as much aa a tunnel, whilst its maintenance m such an unstable position could not fail to be a costly item. Neither could I recommend the erection of a timber flume m such a perilous situation, and over such a length. „ The best point for inlet, I should say, would be at the rock-bound point of the river, nearly opposite Mr Aynsley's old homestead, at point A on the map, about 60 chains north of the Bakaia Gorge Bridge. Here the river would be under perfect control, and the natural flow of the river thero would not only keep up a constant supply, but would also keep the inlet clear of travelling shingle, depositing it on the opposite side of the bight. Were the inlet to be made south of the Gorge Bridge, there would be the difficulty of contending with a depth of 8 or 10 feet of travelling shingle m the river bed, and also the shingle from the adjacent terrace slope, besides the disadvantage of losing fully 20 feet of fall, and so preventing the terrace being surmounted at Highbank, as would be the case from the higher level. From the inlet at A to D, at Highbank, the distance is 10 miles, of which I should propose to tunnel the portions A.B and CD., together 8£ miles, and have an earthwork channel across the flat B.C. The tunnel to have concrete foun dations and to be lined with brick except where it passes through solid rock. The volume of supply would be 200,000 gallons a minute, and the decli vity of the channel 4 feet per mile. There would be an open cut from D to E, the depth at D being 26 feet, and at E about 6 feet* From E the main would proceed to F, near the south west corner of the Springfield property, m a channel 70 feet wide at E, lessen ing m width 5 feet per mile, with 2 feet depth of water; fall 6 feet per mile. For practical purposes the Lyndhurst road might be taken to represent the upper boundary of the district, and the sea line the lower boundary. It would comprise the areas of Nos. 1, 2, and 3 Irrigation districts, as referred to mmy report of 11th May, 1886, with an aggregate area of 225,000 acres m round numbers ; the main line of railway being about equidistant between the upper boundary and the sea. Sub-mains would be led off the main at distance* about half a mile apart, the widths and depths varying to suit the varying declivities of the ground, so as to obviate scour. In most cases the existing races w.©uld be utilised and enlarged to serve as sub-mains. . The supply is calculated, with constant flow, to cover the entire area to a depth of fully 10 inches annually, or say half of each holding (comprising all grass land and root crops) to that depth, assuming that irrigation would be practised for six months annually; or say one-fourth of each holding to a still greater depth, and that would probably be about the proportionate area which would obtain m practice. I estimate the cost as follows :■— Tunnel, including headworks 8§ miles at £32,500 £276,250 Earthwork, channel 8.C., and culverts ... 120° £277,450 MainD.E. 6000 MainE.F. . : • Sub-mains, including enlarging existing races iu.wu Fords, bridges, sluices, etc •• ••• , intSn Cost of land for mains and fencing do., also part sub-mams IU.UW 314,450 Contingencies 23,050 Total cost £337,500 Equal to 30s per acre. Interest on loan at 5 per cent ... £16,875 per annum Maintenance and supervision ... - 16,875 per annum £33,750 Equal to 3s per acre per annum. This is exclusive of field outlets and distribution, which would have to be undertaken by the owners of the land. ,..111 1 j The cert of taking the water on to the plain at still higher levels would

!bein an increasing ratio. In order to surmount the terrace two miles higher I up than the Lyndhurst Road, the point of intake from the river would have to be at Blackford, 4| miles to the north of point A, and there is no satisfactory site for headworks m that district. The tunnel would therefore be of greater length, and also of somewhat larger capacity to serve the increased area of 15,000 acres. The additional cost might be set down at £150,000, thus bring ing the total cost for the district so enlarged to a sum little short of half a million pounds. While I would not be prepared to say that the expenditure of £337,500 as set down for the irrigation of the district between Lyndhurst Road and the sea might not ultimately prove remunerative, I think it would be well to con sider the propriety of introducing a modified scheme at moderate expense which would serve an area equal to two-thirds of the district referred to m the foregoing. That scheme would consist m tapping the Rakaia River at Lavington, where the terrace is only 48 feet m height, and where the supply could be led on to the surface by a channel, 0.P., If miles long, as shewn by red line, the first 15 chains of which might be built m brick and covered m. The volume of water to be supplied would be 105,000 gallons a minute; the area served 150,000 acres approximately, and to be irrigated to a depth of 10 inches annually as before proposed. From point F at Holraeslca 5 miles above the railway, the maio would take the direction indicated by line P. R. on the map, terminating near Digby's, 8 miles above the railway. This upper boundary line would, generally speak ing, be situated from 2| to 3 miles north-west of the main line of railway. The wain P.R., 15 miles m length, would be 50 feet wide at P, diminishing m size at the rate of about 2£ feet per mile, with a declivity of 6 feet per mile. From that main sub-mains" of suitable sizes would be led off to the lower end of the district. I estimate the cost of the scheme as follows : Headworks, and main O.P. £38,500 MainP.R 5,000 Bub-mains and enlarging existing races ... ... ... ... 4000 Fords, Bridges, Sluices, etc ... 3500 Land for mains and part sub-mains and fonoing ... ... ... 5000 Contingencies ... 4000 Total cost £60,000 Equal to 8s per acre. Interest on fjoan , £3000 per annum. Maintenance and Supervision ...  4000 per annum. Total ... £7000 Say 11 \d per acre per annum, exclusive, of field outlets and distribution The foregoing estimates include considerable amounts for land and fenc ing, which under negotiation with owners, might probably be reduced con siderably. The scheme I formerly reported on for the irrigation of this area of about 150,000 acres, comprising Nos. 1 and 2 districts, was to have cost about £16,000 only, but it was subject to the following drawbacks :— First.—The full supply from the South Ashburton River would not have been available at all times, as the drought of the last two years has clearly proved. I shall refer to this deficiency more fally when bringing up my report on the Ashburton-Rangitata Irrigation Scheme. The South Ashburton River could probably be supplemented from the water-shed at Lake Heron, but meantime I must consider tbe Rakaia River as the sole source of supply. Second.—The main m the vicinity of Rakaia Township could not be con structed so cheaply now as at the date of my report, inasmuch as very con siderable river encroachment has since taken place, which would have been obviated by the protective work then proposed. Third,—The New Zealand and Australian Land Company having stipu lated for the route of main through their property being brought considerably lower down than the line proposed, the area which it would have then com manded would have been much reduced. Fourth.—The auxiliary race since led out of the Rakaia River at Acton, could b« enlarged and extended, so as to command an area of some 20,000 acres m the south-east corner of the district (a considerable part of which, however, is very undulating ground and not very suitable for irrigation), and if this auxiliary race were converted into an irrigation main, the proposed works at Lavington might be modified to a small extent. As the connection with the Rakaia river at Acton is already initiated, it would not seem advisable, under all the circumstances, to open up another connection so near as Rakaia township. It is abundantly clear that the principal item of cost consists m bringing the supply from the river to the top of the terrace, and the lower down this can be accomplished the cheaper the scheme becomes ; while it is also patent that the higher portions of the plain, having a greater rainfall, have so much the less need of irrigation ; but there is no reason why for some months every year that upper district might not, at a small cost per acre, receive a supply for partial irrigation, by the enlargement and multiplication of races from the present sources, as formerly proposed. While the inajorjty of the settlers can get an irrigation supply from the lower reaches of the river at moderate cost, they would not be likely to agree to share the liability of expensive schemes from the upper levels, where the in creased cost would be out of all proportion to the increased area to be served. Taking into account the permanency of the supply from the Rakaia river, and the comparatively moderate cost of the Lavington scheme, I am of opinion it is on the whole, the most suitable for the requirements of the district. I have the honor to be, Sir, Your most obedient servant, William Baxter, County Engineer.

Printed and published by the Ashburton Publishing Company, at their registered printing office, Burnett Street, Ashburton, SATUBDAY, April 4, 1891.