Engineer Discusses Alternative Plans

Northern Advocate, 29 June 1944, Page 4

Engineer Discusses Alternative Plans

Willi his 11-page report in one hand and a 4ft wooden pointer in the other, to demonstrate a map and sketch plans on the boardroom wall, Mr Mandeno outlined to members of the hoard his proposals for generation of power at Lake Omapere.

Members, each of whom held a copy of the report, circulated prior to the meeting, asked many questions. The consulting engineed explained that it would be necessary to raise the lake level a maximum of 6ft and possibly lower it a maximum of 4ft making a variation of 10ft between highest and lowest levels.

Meet Requirements For Years In reply to one question, he said the present consumption of the two reticulated counties. Bay of Islands and Mangonui, might be assumed to be 10,000.000 units annually, Hokianga and Whangaroa might be expected at a guess to consume another 3,000,000 when' reticulated to the same degree. The Utakura scheme which he recommended would provide about 1G,500,000 units, working at capacity. Upper Utakura Scheme Outlined

Mr Mandeno’s report made it clear that various alternatives had been considered, but practical discussion could be limited to two plans which he outlined in detail as to estimated cost and construction-use of the lake water into the Waihou Valley on the northern side or into the Utakura Valley on the western side. He recommended the latter, which involved the construction of two power stations.

“In the case of the Utakura scheme," Mr Mandeno stated in his report, "it is proposed to dam the Utakura Stream I i present lake outlet stream) 20 chains above the Te Rore-a-tiki Falls by means of an earth dam 50ft in height, thus forming a small lake 75 acres in extent, which would communicate by means of a canal with Lake Omapere, maintaining the same water level. A short canal in earth would convey the water from this lake to the forebay and thence the water would oe conveyed in a steel penstock 3500 ft in length to the powerhouse near the confluence of the Utakura and Waibarakeke Streams in Mr Imm s' property. Here, two turbine-driven, generating sets would be installed. This station would absorb 410 ft of head. This would drown the Te Ahi Parera and Korotanki Falls.

Plans For Lower Utakura "From this point the Utakura Stream falls at a fairly rapid rate for several miles, and it is possible to develop a further 200 ft of fall by conveying the water discharged from the first powerhouse in a water-race 100 chains long to a second station near Mr Harrison's homestead. Although both stations will be so fully protected that they can run unattended, it is proposed that they would be controlled by two operators resident at the upper station."

Total capacity of the turbines to be installed would be 7750 h.p., capable of generating 5000 kilowatts. Total Cost Nearly £300,000

The upper Utakura scheme, Mr Mandeno estimated, would cost probably £128,670, while the lower scheme would cost another £68.540. These figures, he stressed, were based on pres-ent-day costs and were considered conservative. Quotations had been secured from reputable contractors and, in approximate form, from Australian and English manufacturers of plant. Annual maintenance of such automatically controlled schemes is remarkably low. With capital charges, based at 5 per cent on the initial construction costs, total annual cost of the two schemes would amount to £12.976.

Units At Half Present Cost Quoting the average price per unit of bulk power sold throughout the North Island by the Public Works Department in 1943 at 0.323 d, Mr Mandeno stated that the two Utakura stations, if operated to capacity, could produc epower at 0.19 d per unit. The upper scheme, if developed solely, would provide power at 0.22 d per

unit. Taking into consideration the cost of transmission of bulk power from Auckland to Kaitaia, Public

Works Department power delivered to the north cost 0.415 d per unit This comparison was carried a stop further in a statement that the actual cost to the St a- of delivering the equivalent outran of Lake Omapere into the Bay board’s district would, he estimated, be £28.500 per annum, against £12.978 by local generation. Comparison of Power Production

An interesting comparison of units of

power, obtainable from the Utakura combined schemes, against the amount at present being purchased from the P.W.D. and generated at the small Kerikcri auxiliary station was given in the report. For the year ending March 31, 1944, ihe Bay of Islands Power Board purchased 9.258,1112 units of bulk power from the P.W.D.. and generated 721,(142 units at Kerikcri, a total of 9,980,454. This figure had increased by 600,000 units on the previous year and by 3,300,000 units on the year 1941-42. Now, I lie upper Utakura station alone would be capable of generating 9.(150.000 units annuallv, while between 0,500.000 and 7,000,000* units could be relied upon from the lower project, making a grand total of about 16,500,000 units from Lake Omapere. With the forthcoming reticulation of all Hokianga and Whangaroa counties of the Farthest North portion of Mangonui County, besides numerous small areas at present not reticulated in the Bay of Islands and Manger, ui Counties, it was anticipated that the total Omapere output would be used within a brief period of years. Watersheds and Rainfall “Lake Omapere," Mr. Mandeno staled.

“is approximately 3100 acres in extent, and the watershed at present draining into it is approximately acre::, including the lake surlaee itself. Other watersheds, amounting to 1020 acres, can readily be diverted into the lake. Still further watersheds could be brought in at some future date, if warranted. “By analysing rainfall records at Oknihau and Kaikohc over the 14 years between 1020 and 1039, it is found that an average flow of 32.5 cubic feet per second could have been obtained throughout that period. This included an abnormal spell of four dry years 11930, 1931, 1932, and 1933). which were unique in the 35 years tor which figures were scrutinised. The average annual rainfall over the lake was 47.(5 inches tor each of the four years, as against an average of 00.7 inches for each of the 14 years, 1920-39.” Yet. that flow of 32.5 cubic feet per second could have been maintained throughour that dry spell. Mr. Mandeno declared. By diverting' additional watershds. now flowing away from the lake, an average flow of 45 cubic feet per second could be gained. The storage required, he stated, could be readily . obtained, and involved Hooding of only a small area of farmland, the great proportion of which was of little value. Under much of this land lay a great depth of peat, which is subject to subsidence.

Alternative Waihou Scheme The Waihou Valley scheme, while examined by survey and cost estimate in full detail, was not recommended by Mr. Mandeno, and was not adopted by the board. Its greatest handicaps were the need for nearly 1000 tons of welded steel pipes and for more than 2400 feet of tunnelling; also the fact that loss of power through the steel penstock would be much greater than in the alternative scheme. Twc Tunnels. Dam. Long Penstock

In the Waihou scheme, the water would be conveyed from an intake canal at the north-west corner of tire lake through a tunnel 29 1 i chains in length, under the Okaihau railway station into the bed of the Waiharakekc stream. This stream would be dammed to a height of 40 feet by means of a short earth clam jusl below tite railway station. Water would then pass upstream for approximately 25 chains, thence by means of a tunned eight, chains long, under tire main highway and under the railway, into a lorebay. Thence it would enter a welded steel' penstock 7000 feet in length, which would carry it to the powerhouse, where it would drive two pel ton wheels. The powerhouse would be located near Mr. MacKenzie’s homestead. The total head available would be 000 feet. The water would be spilled into the. Waihou (or Whakanekcnekei river, emptying into the Hokianga Hat hour at Rangiahua. In the Utakura scheme, the water would be spilled into the lower Utakura river, which comes cut into the Hokianga near Horeke. Cost of Waihou Scheme Outlined Estimated cost of the Waihou Valley development would be approximately £190,450. The penstock would have a carrying capacity of 100 cubic feet per second, against 125 cubic feet per second for the upper Utakura scheme and considerably more for the lower Utakura scheme, which would take in the flow j from several intermediary streams. Wai- j hou. he estimated, would be capable of j generating 12.500.000 units of power per j annum. ;