ARAPUNI SPILLWAY

Sun (Auckland), Volume II, Issue 320, 3 April 1928, Page 1

ARAPUNI SPILLWAY

Erosion of Rock Face SCOPE FOR UNEASY CONJECTURE pHANGES below the spillway at Arapuni suggest the possibility that steps to prevent serious erosion may be necessary. It is on the cards that, to permit an inspection by engineers, the diversion tunnel may be reopened, the lake level thus lowered, and water kept away from the spillway for a time.

the first fury of its release over the spillway, when the power supply over a large part of the province was Interrupted, and enormous quantities of material swept away, the Waikato had apparently settled down to something approaching respectability. Recently it became apparent, however, that the rock face at the waterfall below the spillway was becoming undermined, and this process culmininated in the collapse of a large rock structure, which gave the falls a spectacular sheer drop of 70ft. Undermining of the rock at this point is still going on, and the place is dangerous to the Incautious Investigator. Part of the rock which has collapsed was that on which hundreds of sightseers stood to view the cataract. To keep them away from the danger zone a fence was erected, above the falls, last week. Another new precautionary measure adopted by the Public Works Department is the erection of a concrete buttress wall on the eastern side of the spillway channel, near the fall. This is to prevent stray flood-waters from dashing into the apparently soft ground toward the old river bed. What will happen in the first flood is a matter for uneasy conjecture. CHANCES OF TROUBLE In the half mile between the spillway wall and the falls there is at present no hint of trouble. The river, once over the spillway, glides easily In the course it has carved fo>- itself, and it is only at the falls that trouble is likely. Once real trouble starts, however, its effects may be far-reach-ing. Should the rock-structure rbove the falls be unreliable in texture, then the river will eat back very rapidly, possibly cutting a new channel to the old bed, and possibly causing erosion uncomfortably near the spillway. At present, owing to the violence of the falls, it Is impossible to see exactly what is happening, and to what extent erosion has already taken place. Therefore it has been suggested that the spillway might be closed, so that an Inspection could be made. The position is complicated by the fact that the diversion tunnel, which would have to be reopened, by the raising of the enormous gates, was not built to stand the prolonged passage of water travelling under the huge pressure created by the big head of water impounded in Arapuni Lake. The lining of the tunnel is only two feet in thickness, and would not long stand the bombardment of boulders and debris in such a terrific current. This possibility was presented to the Public Works Department some time ago, when Armstron, Whitworth, the British contractors, suggested that an emergency outlet should be constructed. This would have been an expensive stand-by, however, and the proposition, for which estimates and complete data had been prepared, was turned down. Another complicating factor is ’nterference which Hora Hora would mffer through the reopening of the tunnel. Since the diversion immense quantities of sand and silt have been deposited in the Waikato, above and below the confluence between the old and new channels, and the bed of the river has actually been raised 28ft, with the result that between Arapuni and Hora Hora the stream is flowing

among trees and scrub that were formerly quite high on Its banks. Meanwhile, the Public Works Department, despite many predictions to the contrary, is making tangible progress with the excavation on the powerhouse site. The sealing of the coffer dam, which Armstrong, Whitworth's installed, has been reinforced by a system of concrete grouting. Holes were drilled in the rock dividing the site from the river, and into these were pumped tons and tons of liquid concrete, which have made the --ock much more water-tight. The site has been divided into halves, and one piece is being excavated at a time. At the main dam there is still a slight leakage, such as would be expected in so large a job, and cinders have been poured over the upstream side in the hope that slight fissures may thus be sealed. Armstrong, Whitworth’s "maintenance period” has not yet expired, and the company’s representatives are still on the job. Hast week an inspection wts made by Mr. C. J. McKenzie, assistant chief engineer to the Public Works Department, who was accompanied by Mr. Kissell, chief electrical engineer, and Mr. A. Dinnie, who is in charge at Arapuni.