RIVER DIVERSION

Evening Post, Volume CV, Issue 122, 25 May 1928, Page 10

RIVER DIVERSION

ARAPUNI SCHEME EFFECTS Of EROSION OLD FOREST REVEALED The-diversion .of-'the Waikato Kiyer at Arapuni, thereby turning a vast volume of water through loose pumice land, one ■effect rot which, inter alia, was to lay..bilre.'a.' prehistoric forest, was the subject.o-feMr.-'lI.' W. Furkert's -presidential.address this week to the inaugural .geiieVal.uneeHng of the Weilingfcon Philosophical Society. The lecture was = illustrated-, by a number of lantern-slides,'.the pictures ami spoken word,:'aided .by'diagrams, conveying in a. mostl.lucid;.in.ann.cr to .those present; an idea;.of.;>'h'at,'.wns a unique event in the; ahrials"o£--engi:necring in this part of •tiie^wq'r]d:-'; ■'•'*■'■.'';'.'' '•;•'•' ' The/genesis' of'; tic- river diversion, Mr.'FurkertVcxrMained, .was the decision . to':con'strui',t a'hydro-electric station on the Waifcatb River at the Arapuni Gorge.' The work consisted of raising the river level by a dam to such a height as: to cause it to flow again through a.channel^ through whuli it flowed possibly, thousands of. years ago. The old channel ran. roughly' parallel to the present one, but lot) feet above it at < the-'.point of bifurcation. Tor about 60 chains the old river bed shows lid appreciable fall, but from the point at the end of-the level section whjere' the spillway is now built, the channel falls'with increasing slope until, it finallyl assumes the character of a cataract, and eventually falls over a cliff, which at the time of. diversion was about 70 feet high. At the foot of this cliff there was a wide expanse of I flat country, which, was known from borings-to. consist -very..largely of sand,' with a small mixture.of pumice. Jt was known that, when, the closing of the gates at- the. dam .caused ,the water to risej sufficiently to run down the old river lied, the water '.pouring over the cliff would, soon .erode, a channel into the-Waiteti.Stream, enlarging this channel until, it.reachedtho-Waikato again. Estimates were made as to the probable size, of this new'channel, and all the vegetation;on this area, plus a reasonable allowance on each side, was chopped down and burnt. ■ ; PREHISTORIC FOREST. Calculation had led to the expectation that ,'th'c-. overflow into the ,new channel would take place on Now. Year's/Day, andthe error was inappreciable when,' the", variety of factors is considered. The lecturer detailed- what happened when the water in increasing volume began to scour out its now channel. The third'of: January was the day

on which the flow over the spillway had become practically the regular flow in the Waikato Hiver,'arid'was the day on which the maximum amount of material appears to have been swept out of the.flat below the fall. By the next day scouring and erosion had taken place to such an/extent that the buried forost' had begun to show up. By the 7th January the forest was uncovered down to the originall clay soil on which the trees had' grown;- approximately a. COft erosion-. /'These trees were foupcl to be in quite- a good state of preservation, still standing ■where some 10,000 years ago they/.flourished as a green forest of totara^and'.riniii. . EFFECTS OF EROSION. Mr. Furkert. then, dealt with the effect downstream when.the vast quantity of eroded material poured into the old Waikato'channel. The water supply of Hamilton,'. 36' miles distant from the spillway, was affected so much that it was said that if one had a bath one came out dirtier'than before. It was

estimated that. the fine pumice and other material in suspension in the water at one time.averaged over 14,000 cubic yards per day. . At Horn Horn, nearer to the site oE the diversion, the trouble from the deposit of debris at. the power station was very great, pumice accumulating on-the screens and also blocking the turbines. Silting in the river from Hamilton'to the niouth had been very slight. • WHEN THE' CLIFF FELL. The most spectacular . thing which has. occurred to date;. in the diversion of the. river has : been .'■the-collapse of the cliff which formed: the waterfall on to the AVaiteti ila.t.C This, did not take place until 24th-March;- The fall of rock which then .took', place was not observed by- any -of: the- Arapuni stuff, lint a incmb'cr of the .AVanganui Public. Works staff,, who happened to be there on a holiday, was standing on the west side of the. waterfall.at the time. He felt a tremor, like- an.earthquake, and saw the country, silbside-on the cast side: ho then, felt .another tremor, and the'west sidp subsided. After that he did not wait to see, any;more, and beat a hasty retreat, belieying. discretion to be the better, part of valour. The sides of the gorge are noiv'splaying out. Although; some' unforeseen difficulties had arisen with regard to the diversion, none, said Mr. -Furkett, had been serious, and the scheme was working in accordance with,expectations, and in another j-enr'or so, with the completion of the power house, the Arapuni hydroelectric scheme would be in operation.