THE WAIPORI DAM.

Otago Witness, Issue 3830, 9 August 1927, Page 16

THE WAIPORI DAM.

NEW LOCALITY SUGGESTED.

REPORT BY MR FURKERT.

It will be remembered that when Mr J. G. Alexander, city engineer, was asked to report on the possibility of raising the dam at the entrance to the Waipori River gorge from 40ft to 60ft, with the ultimate -intention to increase the height to 110ft—he declined to accept any responsibilityJn the matter. All that he would sanction •was that the present dam be raised to 43ft. His investigations had led him to consider that a sound foundation could not be secured for a dam of l"Oft in height, unless at a very large expense consequent upon the necessity of extensive excavation and concrete work. He considered that it would be cheaper to build an entirely new dam lower down from the opening to the gorge, where a foundation could be secured which would be safe to carry a height of 110 ft. As it was realised that, whatever decision the City Council might adopt on the question of raising the present dam, the approval of the Public Works Department would be required, it was decided to obtain a report from Mr F. W. Furkert, engineer-in-chief of the Public Works Department, and thus let the council know how far it could go.

This report practically suggests to the council that it should abandon the present dam and build a new dam lower down the Waipori Gorge. The report reads:— Replying to yours of June 18 concerning the Waipori hydro-electric development, and more particularly the dam known as Mahinerangi, I am of opinion that the corporation would be wise to adopt the city engineer’s recommendation that the permanent dam to the full height should not be constructed on the top of the small portion already constructed at the entrance to the gorge, but that it should be built some 300yds downstream. It is possible that some slight variations in the actual siting of the works may be made which will be referred to later, but what is known as the No. 3 site-is in my opinion the cue to be adopted.

With reference to the question whether the dam should be a maximum height of 100 ft or 110 ft, this is one on which there is room for considerable argument, and while it may not be possible to show absolutely by figures that the dam >uld be built to 110 ft—not to 100 ft —yet after considering all the factors in the case, I am of the opinion that the higher level should be adopted. At the same time, this is not to be taken as an indication that I consider that the data on which Mr Alexander based his recommendation are inaccurate. Figures concerning river flow and rainfall cannot be established with extreme accuracy, and ■ there is

always a good deal of individual judgment to be taken into account. Also, when Mr Alexander made his investigati ns he was not aware of the fact that a considerable amount of water now being diverted from the Waipori watershed will in all probability not be so diverted before many years have passed. The fact also that a considerable amount of the storage of the dam, at whichever height, must be lost as a result of the dam being used to some extent for flood relief justifies a higher level than purely power considerations might dictate.

Coming to the second lot of questions asked by you, I am of opinion that: (1) The proposal to install a remote controlled power station to utilise the discharge from the new dam is a sound one. (2) The substitution of larger units for the smaller ones in the main station is also on sound lines, and also the proposal to put the tunnel under pressure from the weir and to eliminate the pres .t reinforced concrete conduit to the old surge chamber is a proper one. (3) The two-power plants below the main station so far as one may judge from the very meagre data so far available will be valuable and justifiable adjuncts, but their construction is a long way in the future, and the matter requires to be further looked into in view of the fact that long before they are likely to be required the Waipori system will be interconnected with the . Government stations to the north (either now in existence or to be constructed), and in all probability

with the Southland scheme to the south. The construction of these lower stations will of' course help to justify the higher dam. In view of your station being a high head development and the next Government station at Waitaki being low head, it might be better for both parties for you to take a block of power continuously from Waitaki, and to use your own plant for the more irregular part of your load, under which arrangement your station could operate more machinery with less water on account of the smaller load factor. Coming now to the probable cost of the dam, I went through, the figures prepared in the city engineer’s office, and consider that they are safe figures for estimating, and that in all probability the work will be done for somewhat less per unit than

the figures given by the city engineer. The question of day work versus contract, though not mentioned in your memo., was discussed both with the chair- I man of the Electric Light and Power 1 Board, and also with the city engineer and tho city electrical engineer. As a general rule the more straightforward a I job is the more likely it is to be done cheaper by a contractor, and the more uncertain it is the more the employment of day work is indicated. However, it ap-

pears to me that under the circumstances existing at Waipori the better way would be to divide the work up and do as much by small contracts as possible, reserving to the corporation’s own staff the difficult and uncertain works, such as excavating the foundations, setting of valves, intricate reinforced concrete work, coffer dams, etc. The carting of the cement might weil be done on a tonnage basis by contract, the landing of shingle on the job at per cubic yard, the placing of the concrete at per cubic yard, either with appara tus first supplied and erected by the corporation, or, alternatively, found by the contractor, and after the work is well organised the boxing might well be done -at so much a hundred superficial feet, though this is not an item on which very great saving could be made whichever wav it is done. If such a method as indicated here is carried out, it is very necessary that the control of the sub-contractors as well as the day workers should .be in the hands of the resident engineer on the works, who would take the necessary steps to co-ordi-nate all the operations and prevent conful sion or interference.

With reference to the actual siting of the works on the No. 3 site, I am of opinion that the type of dam requires further consideration. ” The city engineer’s figures plainly show that a dam of the Ambursen type would be cheaper than a gravity dam. The site is unsuitable for a simple arch dam, owing to there being no sound rock to act as abutments to the horizontal arch up to the full height. However, the rock at the bottom is excellent and higher up abutments to the central arch could be made by what are known as gravity section ends to the dam. I discussed this I matter with the city engineer when in I Dunedin, and I understand that he is going into this type of dam. A number of determinations have been made in this office for various combinations and curves prepared showing the variations in the quantities of concrete; (1) by keeping the central angle < .istant and varying the height and radius; (2) by keeping the height and radius constant and varying the central angle; and (a) by keeping the central angle and height con slant and varying the radius. From this it appears that by the combination of No 4 or 5, a dam with a height of 110 ft can be constructed with approximately 25,000 [ cubic yards of concrete. The actual quantity, however, is not so definite as the relative advantages of that particular combination ; in other words, the values given in these curves while correct relatively to one another may be all too great or all too small according as the rock when excavated requires to be taken out either higher or lower than the assumptions made in this comparison. The basis of the designs, however, was where the arch abutted on to natural rock, it should bear on unimpeachable hard blue rock, but the 1 gravity portion which forms the abutment in the upper parts of the arch might itself be founded on rock of poorer quality piovided a cut-off wall is carried down to the full depth necessary to prevent any percolation underneath. With more detailed knowledge of the actual position or rock able to act as a satisfactory foundation, the site may perhaps be slightly varied with advantage. To show the method of treatment, I forward herewith a rough print of the sketches made here, but as these were only done in pencil they naturally do not come out very well in a print, but they will be quite sufficient for the city engineer to follow our general idea, which is not put forward as by any means final, but as an indication that saving may be possible by the adoption of a type other tlian the cellular. Concerning the spillway, it is probable that the best type for this would be syphonic, which" might be made to commence discharge at any predetermined height, and by means of suitable air valves could be prevented from functioning if it was at any time desired to hold the water above any predetermined level temporarily It could also serve to draw down the water to anv desired level within such limits as are likely to be required without the necessity for opening any sluice-gates. The only pipes through the dam would thus be "such as are necessary to let out the amount of water required for actual power generation and losses en route to the power house (if any).