GROYNE EXPERIMENT

Evening Post, Volume CVI, Issue 30, 4 August 1923, Page 13

GROYNE EXPERIMENT

PLAN TO FEND THE FLOOD

Climatic conditions and the mountainous character of the country have made the ■ task of the railway-builders a hard one.

The average annual rainfall at Otira' is -195 inches, or over 16 feet.' In winter, Otira has known a snowfall as heavy as 5 feet. Avalanche and mountain torrent threaten a railway from each side.

Fed by melting snows or heavy rains, the mountain rivers quickly become roaring torrents. Where such a river hugs the steep hillside, the railway-builder must push the river away, so as to leave a space wide enough for the rails to lie clear of the avalanches and slips that thunder down the steeps, and also clear of the water. Being between tho devil and the deep sea, the railway-builder must have clear space on either hand. It may be explained here that th« Otira railway tunnel unites the valleys of the Bealey River (east) and tho Rolleston Eiver (west). The Bealey ia a tribu-

tary of the Waimakariri. The Rolleston is a tributary of the Otira, which in turn runs into the Teremakau.

At the confluence of the Rolleston and the Otira, the latter charges on to the line of the railway almost at right angles. In flood, the ordinary rock-faced wall of a railway bank could never stand such an impact. The Public Works Department therefore experimented with a system of groynes quite new to railway works in this country. Long groynes were built out from the railway bank squarely to the current, or " pointing slightly upstream. At the end of each groyne huge blocks of concrete, weigh-

ing each from 60 to over 100 tons, were built in. The designer anticipated the powerful scour of the flood waters at the end of the groyne, and his idea was that, as the river scooped out a hole, the heavy concrete block would sink into it instead of being carried away. Thus the end of the groyne would only go deeper through the rage of the flbod, and would provide a still firmer 'foundation on which the groyne's end could be built up again by the building-in of new concrete blocks on top of the sunken ones. It is claimed t.hnt tin's plan has entirely succeeded in protecting U*e railway") and

that the photograph, published herewith illustrates its operation.

A glance at the photograph shows that the end of one of the groynes has collapsed. A first impression is that the groyne is a failure. At the time the photograph was taken the end of one groyne had certainly sunk, but only as allowed for in the plan; and the contention is that the sinkage- (which has been known to amount to 25 feet) improves the resistance of the groyne by giving a better protection and foundation; and that the cost of building up a certain proportion of. groynes affected by scour is a rea-

sonable economic insurance for a railway so exposed to elemental forces. While the cost of groynes is considerable it is offset by the fact that the cost of facing the railway bank i s eliminated The photograph shows that the bank is unfaced—that it is simply made up by dumping the material excavated from the tunnel „AS the flood-fending groynes carry the war into the enemy's country, the bunk can be left without further defence.

llus riparian-protective work—lho purpose ol whK-b is nvalauchc-avoirlanco a» well "as nvov-reewtance—i« as interesting

as the tunnel itself. In fact, the extratunnel features of this Alpine railway crossing are altogether remarkable. The tunnel itself reveals only some of the obstacles. On each side of it immense difficulties were met. In a short length of nine miles on the Canterbury side of the tunnel, between Avoca and Otarama, there are three high steel viaducts, one of which carries the rails 236 feet above the floor of the gorge, and no less than seventeen short tunnels, the longest of which is about 2000 feet. HARNESSING DESTRUCTIVE FORCE. It is well worth anyone's while' to climb the. hills to a look-out point above the Otira. portal of the big tunnel. Looking down the valley he will see the KoUeston (crossed by an impressive railway bridge) flowing beneath his feet for a short distance to its point of confluence with the Otira, which, as mentioned above, charges into tho Rolleston at right angles, aud, therefore, at right angles to the line of railway following the Rolleston's bank. From* this point downward the railway-builder has to deal with_ the powerful rivers now allied against him. Even the united body of water, in its normal flow, looks nothing dreadful, but the character of tho wida floor of the river-bed is a sufficient indication, to anyone who knows the Southern Alpine rivers, of the enormous destructive force that can be generated. But that force is not always destructive; in fact, in one place the engineers have carefully utilised it. After putting in a system of groynes that has driven the river from tha left bank—the railway bank—to the opposite side of the wide valley, they have then turned it back again, by means of further groyne work, so that it once more hits the left bank near the confluence point of a mountain torrent called Goat- Creek. Reason: Goat Creek, which the railway has bridged, piles up at its confluence a talus—a huge heap of boulders and debris carried by the Goat Creek torrent down the steep mountain side. I Ihese huge stones, having found a mora level lodgment on the flat bed of -ths parent river, require more than tho water-power of Goat Creek to continue their progress down river. Therefore the engineers, by' their groynes on the right bank, turn the Otira River tack and cause it to charge into the foot of the Coat Creek talus. In this case the Otira. acts-as a special traffic constable, and the great stones of the talus niovo on. If allowed to' "accumulate, they would rise to a, level threatening tho superstructure ot- the railway bridge over Goat Creek. • .

Thus is carried on the war of man with Nature—the essence of which war is sometimes to repel, sometimes to harness, those elemental forces which Nature ]>rovides for the use of him who can understand and control them. At Goat Creek the destructive energies of Nature- are used constructively.

The Otira scheme of electrification is planned on the basis of 1000 tons gross of freight per day from west to cast, and 700 tons per day from east to west, carried in . four trains each way, and, in addition, one cis-coach passenger train each way py day.