HYDRO-ELECTRIC SCHEMES

Nelson Evening Mail, Volume LIV, 12 July 1921, Page 5

HYDRO-ELECTRIC SCHEMES

NELSON RIVERS EXAMINED

REPORT TO THE CITY COUNCIL

THE WAIROA PROPOSAL

No. 1. Some months ago the then City Council asked Messrs Toogood, Jones, and Holmes, hydro-electric engineers, to report on the possible hydro-elec-tric -schemes in the Nelson district the report, as already stated, was presented to the City Council on I| riday evening last. The following is tho first portion of the report;— His Worship the Mayor, Citv of Nelson. Sir ‘ REPORT ON HYDRO-ELECTRIC SCHEME The various rivers in your district have been examined with the, view of reporting on the best proposal for hydro-electric purposes.

SPECIAL FEATURES OF THE DISTRICT The outstanding special feature of the Nelson District, from the point of views of hydro electric development, is the intensity of floods, the low run off in times of minimum flow, and the absence of marked falls or rapids in the river.

MINIMUM FLOW The minimum flow regulates within limits the horse-power to be developed on a moderate capital expenditure. The average minimum flow from the rivers taking their source from the ranges in the western side of the Weilington Province is over one cubic foot of water per second for each square mile of country in tho watershed. From a test made by the uity Engineer on a branch of Roding we must conclude that the minimum flow for the Wairoa River is about .6 of a cubic foot of water per second for each square mile drained. ’ . , For the same head and water-shed a similar river in the Wairarapa District would give twice the power during times of drought, or rivers m your district having equal water-shed require twice the head to give the same power. , This being so we have directed our attention to the question of averaging i\p tho water conditions by using a stand-by power plant. The stand-oy plant will be referred to in more detail later in this report. We have no direct evidence of the minimum flow of any other river an your district, hut our experience leads us to venture the opinion, that the ■ water-sheds surrounding Nelson are affected, in proportion to their altitude, by the same conditions as apply to the watershed of the Roding River, and that in considering anyone of them, tho allowance made tor the Roding River will apply within fairly close limits to them all.

FLOODS There is ample evidence that the Wairoa, and other rivers, are subject to higher floods than the average, and t m considering a hydro-electric development this factor must he taken into account. It would require much detailed field work to accurately determine the flood discharge as indicated by existing flood marks, hut at the piesent juncture such ivork is not Avarranted, as, Ave have sufficient data of similar country to forecast fairly accurately, the expected pood discharge, and Ave have taken 25 per cent in excess of the average in order to provide for the safety of works liable to damage. / . . It is a sine qua non that the actual power house machinery must be entirely protected from the highest possible floods, and it is therefore desirable that the power house should be built out of any gorge if at all possible.

ABSENCE OF FALLS Ab there are no natural falls or rapids any hydra electric development will require either a darq { a long race, or a tunnel in order to give the head required. , , The question of a long race can Be eliminated in all cases but one, because unless there is a largo stoiage area at. the end of the race, or an adequate .supply of water to allow about two-thifds of the available sup ply to run to waste during Lmes ot low load, a race is not practicable A loner race or tunnel may also have the disadvantage of extending too far into the water-shed, and so limiting available supply. „, There is a possible chance of harnessing the Motueka River by the aid of a race at a point between Kohatu and Mararewa. . COQ f t Kohatu Railway Station is 623 feet above sea level and Mararewa 023 feet and Tapawera 418 feet. Thus in a distance of about five miles the fall is 141 feet or '2B feet per mile. With a water race it may be possible to get an effective head of 24 feet per mile. Wo have vet to go over the ground to ascertain if a race scheme is possible. It may be feasible to bring water down so that a power house can be placed on Stanley Brook and thus he kept clear .'of floods in the Motueka River i‘ , In the meantime we have worked up, all other possible schemes on the basis of a dam. , . , , BTn The main essentials for a high dam are rock foundations, a fairly narrow gorge, and sufficient Spillway room to pass floods safely. n . Ilt . In the Wairoa River at or about the point shewn on the plan, all these features are to hand more or less as required, and the power house can be built out of the Gorge and free from floods. The Gorge is rather wide to give a cheap, dam, hut it is stu nar row enough to employ tjie most economical section, namely, an arched type of dam. The rock, from/external appearances is reliable, but this point mil have to be tested to a much greater extent before any signs can be made. Owing to the ab-1 sence of borings, wo have had to as-j sumo that the rock will befmmd in tb© river bed at a depth of 15 feet below water level. The assumption may bo, near enougn for the present, hut before a loan proposal can be put to vour the actual depth will have to be tested with accuracy. ... jThe only other locality in your district where the conditions approach.

the requirements is in the gorge (upper) of the Motueka River. la this gorge, however, after a dam has been built, little or no extra spillway can bo provided. Some difficulty would be experienced in the securing of a suitable power house if the dam were constructed in thd most suitable place. We have discussed the general requirements in a wide manner as an introduction to this report, and We now propose to briefly compare the rivers inspected. For this purposb and to enable you to see the position at a glance we have tabulated the conditions as far as possible.

COMPARATIVE TABLES Referring to Table No. 1 and using the figures given by Mr Littlejohn for the Roding as a basis, and making due allowance for difference in altitude, tho calculated minimum flow of all the important rivers in your district is given. The calculated flood discharges are also given, and these are based on 25 per cent greater flood discharge thaji the average New Zealand conditions. Referring to table No. 2 we shew the head required in these river's to' ■ velop 1000 h.p. with, and without storage. . Where storage has been considered the load factor allowed is 33 per. cent, and this assumes that if the total consumption of power were pontihuous, the demand would be satisfied in 8 hours, or in other words the water has to be. stored for 16 hours. This is quite a reasonable allowance .for your city. ■,, Tho length of spillway required* to discharge the calculated floods is also given, and this length is bas&d on a depth of lO’O” running over ,the top of the spillway. tjie aid of these two tables it can he seen that the Wairoa, River is the only scheme using a 1 dam that has been ( brought under our notice, that is suitable* for for the purposes of your city. ; (To be continued).