Arapuni Power Station
Arapuni Power Station is the seventh and penultimate hydroelectric power station on the Waikato River, in the North Island of New Zealand. It is owned and operated by Mercury Energy. It also the oldest currently generating, the first government-built, and the largest capacity single hydroelectric power station on the Waikato River.
Construction finally began in 1924, but was hampered by repeated heavy rain and floods. The station, with three turbines and provisions for a fourth, was commissioned in mid-1929. Shortly afterwards, Arapuni was closed for two years while a water seepage problem was investigated. Two Swedish experts, Professor Hornell and Mr Werner, supported by New Zealand geological experts, were engaged and various remedial measures were recommended and carried out, including an impervious lining to the headrace. With the addition of a fourth turbine, the station was recommissioned in May 1932. Increasing demand for electricity led to the powerhouse being doubled in size, and provision made for four more units. Turbines 5 and 6 were commissioned in 1938. The last two turbines were commissioned in 1946. In 1990 a repair, refurbishment and upgrade project was completed, the first stage of which involved construction of a diversion channel. In 2000 it was determined that the historic seepage problem had worsened, interim repairs were carried out while a more permanent solution was devised, and implemented in 2005–07. Work was carried out on four of Arapuni's turbines to increase capacity from 24.7 MW (33,100 hp) to 26.7 MW (35,800 hp) each and to improve their peak efficiency.[1] [2]
1929 Abstract or a paper submitted by by Frederick William Adolph Handman for discussion before the Institution of Civil Engineers on April 23rd, 1929.[3]
1930 Extracts from an article in Engineering 1930/11/28:-
'After the completion of the main dam, the flow
of the river was passed down the head-race and
over the spillway. The power house is ultimately
to consist of eight units of 25,000 h.p. each. The
initial installation of three units was completed
only about two years after the dam; a fourth
was in course of being put in last summer. The
bulk of the ultimate flow to be utilised was therefore
passed over the spillway, and, in time, resulted in
unexpectedly serious erosion at the falls, converting
them into two steps, the upper one of which moved
back some 200 ft. up stream. On this occurrence,
it was decided to reduce the flow over the spillway
by excavating two more penstocks at the station
site, and fitting the discharges with Glenfield and Kennedy jet dispersers.
On the night of June 7 last, a serious crack, reported
to have been accompanied by a tremor, appeared, at
the intake, in the island formed between the headrace and spillway channel and the river bed proper.
This crack extended from the junction of the spillway and intake structures in a down-stream direction towards the Waikato bed. It was 2 in. to 2 1/2 in.
wide, and, after disappearing, could again be clearly
traced over a very considerable distance. At the
same time, water also made its way directly across
the island site and discharged from the side of the
gorge. The step was at once taken of lowering the
level of water in the head-race, by permitting it to
escape through the diversion tunnel at the dam,
the result being, of course, that the station was put
out of action. The station itself suffered some
damage from the movement. On the head-race
being drained, the crack appears to have been
traced for a considerable distance upstream, as
indicated very approximately by the dotted line
on Fig. 1. Within a few hours of the lowering of
the level in the head-race the crack began to close,
and the whole mass of country ultimately returned
to nearly its original position.
'The Government immediately commenced investigations by their own geological and other experts, and decided to call in an expert from Europe to report on the scheme from the civil engineering point of view. Their- selection fell on Professor P. G. Hornell, of Vattenbyggnadsbyran, consulting engineers, of Stockholm. Professor Hornell was accompanied by Mr. P. W. Werner of the same firm, whose London representative is Mr. B. Ilellstrom. ... Professor Hornell rejects the theory that the crack was caused by hydrostatic pressure resulting from the high level in the head-race, owing to the length of time (two years) during which the race had been in use without previous sign of trouble. The cause is rather traceable, in his opinion, to the presence of water in the head-race and capillary action in the rock. The rock has been shown to swell as much as 1/2000th per unit length, and although it must have been nearly saturated before water was let into the head-race, considerable stresses may have been produced between strata of unequal saturation. .... rovided remedial measures are carried out, Professor Hornell considers that the plant can be reconditioned satisfactorily, and the whole scheme completed at an economical figure. In order to prevent water soaking through into the rock, it is recommended that the head-race be lined, the cracks which have formed also being grouted, and the deeper strata drained by low-level tunnels at right angles to the axis of the river and head-race. .... The report of Professor Hornell is in distinct contrast with that of the Government’s own geological experts, who proposed buttressing the whole block of country affected by the damage, this is something like a mile in length, and from 300 ft. to 500 ft. broad, with a gorge in the neighbourhood of 200 ft. deep on one side, the magnitude of the proposition will be evident. After putting forward their proposal, the geologists seem to have left more specific planning, in connection with the working out of their idea, to the engineering profession, but Professor Hornell would appear to have found a rather more logical and certainly more economical way of dealing with the situation.'