HYDRO-ELECTRIC SCHEMES

Nelson Evening Mail, Volume LIV, 13 July 1921, Page 7

HYDRO-ELECTRIC SCHEMES

No. 2.

' av.wi: u*kk:a i!ivi;i: To develop l(JpO*h.p. thin »*• vor would require 31 feet of head, a storage area of 42 acres, juu! a spillway width, ot 1-lo Oft. The (him silo shewn to us would require a gravity section some 660 feet in length. The total flood disci large would hare to take place along the whole length of the dam. No suitablepower house site was easily available, and in order ro keep out of difficulties at the power house 1 and provide against silting of storage area a higher dam than 'shown would lie necessary.

MOTUEKA ItIVKi; ABOVE THE JUNCTION WITH THE WANOAFEKA

There are no dmn sites which could he. considered satisfactory a'bove the junction, of the Manga peka. While strong outcrops of rbek can lie found on one side of the river, and also in the river we siw no place where good rook earned right through. The actual head required would have to be much increased owing to the absence of good power house sites.

MOTUEKA RIVER BELOW THE JUNCTION WITH THE WANGAPEKA

There may lie a better chance of dealing with the Motueka River below the junction with the Wangapeka, and a close examination may disclose a reasonable- dam site. Even il a good dam site is found, it will he difficult to find a good location for the power house, as floods will rise 10 to 12 feet, and the power house will have to be kept out of the way, it may mean that the low head of 11 feet for 1000 h.p: cannot actually he used. Furthermore a low dam would soon lose its storage capacity, and we believe it will bo found that storage is not practicable ,and that the height of dam actually required will not be less than 30 foot above water level. In the Motueka River a dam of this height is a considerable undertaking, and unless a particularly favourable spot, can be found, it would not he possible with your resources.

MOTUEKA RIVER AT THE GOUGE (UPPER)

To produce 1000 h.p. a dam of 87 feet in dieight is required. While the gorge is too narrow to allow sufficient, spillway for Hoods it is too wide for cheap dam construction. The dam would have to be situated in the near vicinity of tho month of the gorge in. order to place the power house, tree of floods.

MOTUEKA RIVER BY RACE As stated previously, we think it desirable id investigate the possibility of a. race out of the river between Kohatu and Mararowa and the results will bo added as an appendix to this report. . .. LENGTH OF 'IHANSMISSION LINE In table No. 2wo have given the approximate length of transmission line required to bring tho energy to the City of Nelson. This has an important bearing on the economic value of any scheme. It may be found 'by actual' survey that the distance is greater or less, but the lengths given are quite near enough for present purposes, Tbs cost of transmission line will be approximately £SOO to £6OO per mile according to (the difficulty of transport , and with the voltage used. Fhr the purpose of comparison we will take £550 per mile a® the average' of the longer lines, and £SOO per mile from Brightwater to Nelson, where the conditions are easy. The cost ot transmission line is tabulated below: Approx. Approx. River. Length. Cost.

Wairoav 1 12 miles £6,000 AVangapeka 30 miles £16,o0() Molneka 30 miles., £16,500 Motucka Gorge 36 miles £19,800

Tim does not give? the full difference in cost between the ■ various transmission lines, as the cost of maintenance will be greater for the longer lengths-and rougher country.

NELSON’S DEMAND FOR, ELECTRICITY The ’population' «ofe the City of Nelson has heongiven to us ns 9000 inhabitants. There, is already installed a municipal gas plant which is selling coal gas at moderate rates to about 2400 consumers. We are informed that at present there is -'installed ,m various industries, a connected . load of about 1200 b.p./including, gas and steam powder. ; The electrical requirements of elson appear t,6 he’ln the /vicinity of 1000 h.p., and this would serve a ponnected load of about 3500 h.p. to .4000 h.p.

PROPOSED WATROA SCHEME Existing Right.s— rights and purchase of land affect the Wairoa scheme to a greater, extent than any other. If a dannis constructed in the Wairofi Corg| considerable area of settled land will liavo to be acquired and two bridges built and roads altered. It is- impossible to closely estimate the cost of this in detail, but at least ,90 acres of ' freehold are involved. The lengths of road Ijo be altered is about l-l miles. If a sum of £IB,OOO is allowed it should be greater than the amount required to settle these claims arid pay expenses. Minimum Flow.—While we are of the opinion, that on some occasions the flow of the stream will reach 100 cnsocß, at the point wo propose to dam it, the present installation should bo based on a minimum flow of 120 cusecs, bccanso over a number of years it will be found that the river will not often go below that. The turbine and generators proposed would be 2 units, each 700 h.p. To meet the case of ! tjh© flow of the river going below 120 cusecs a standby plant would have to bo installed. It is almost a necessity in every large hydraulic scheme to instal a stand-by for occasional use,, and wo can quote tho case, of Christchurch Tramway Board's steam stand-by plant, and tho proposed new steam plant for Wellington which will act as a stand-by plant to Mangahoa. In North America where the rivers are liable to varying flow steam stand-by plants are universal Tho stand-by plant required would he 250 h'.p. to meet the caise when the flow of the river fell to 100 cusecs. As, Jiowevor, 250 h.p. may not b© sufficient to keep essential industries going when a shut-down took place, the size of the sitand-by may nave to he increased, and we it should have a capacity of 500 h.p. in 2 units each 250 h.p. For the first a 250-h.v. stand-by plant will bo ample. This station: should be located in Nelson alongside fhe water front, where coal may ho handled as cheaply as possible, and ample water is available for condensing purposes. Dam.—The dam w© propose would he of the arch type built to 43 feet

above the existing water, level ah the dam site. 'We have assumed that good rock exists 15 feet: below venter' level at this point, hut it requires borings to tes: the ioundiitions. \\ s have decided upon tiie daifi height so as to bring the crest to a level whore ample spillway can he■ secured. if the dam is made higher it will involve costly gravity type flank dam wide!) will no. he economical at this jiumbure. Storage.—Tin l storage may he the dam with A feet draw off will be sulficieiit. for 33 per cent load factor. Silting.—-We have chocked up tho rate that silting will take , place on the figures supplied by Mr Littlejohn. Pomidatiion of the surface iu country similar to the Wairoa watershed is usually allowed at the rate, ot one cubic yard, per acre oi watershed, per vear. On this basis the rate oi denudation of the watershed will lie 107.000 cubic vards per year. A; least 10 per cent of it will pass away as mud, leaving 06,300 cubic, yards somewhere iu :be watershed area. Of this amount a considerable portion will he held by the stilled waters caused by the dam and will pile up in the upper reaelms until it assumes its original gi anient. Enr the purposes of this calculation we will take it that 70.000 cubic vards .sill affect the scheme. ().n comparing those figures with tho figures supplied by Mr Littlejohn for t'm town water supply dam. we find them close enough to he relied U *The cubic -mace below 3 feet- allowed for storage is 2.100,000 cubic yards and 30 years would elapse' Worm silting would affecj the scheme h\ limuinu the storage. In the com so of about 40 rears the dam may have to he raised, and for this purpose we have allowed for suitable foundations.

ESTIMATE

It is proposed to install two unite n{ 700 h.p. and lonvr- space lor a third. Wc estimate the cost- ni- this uork as follows: g Dnm and Intake 40,000 Tunnel and Pipe-line < Adr Power House .etc. . 4,006 Turbines, ete. 10,000 Electrical Machinery and Transmission Lino 18,00(1 ■ £BB,OOO Land Purchase, roads, etc. 'IB,OOO , r £106,000 10 o/o*usnal allowances 10,000 £116,000 Ist year’s capital charges at. 7h o/o £125,316 Allow £126.000 ... £OO per h.p. (To he continued). ,