DEAD TIMBER

Evening Post, Volume CVI, Issue 129, 28 November 1923, Page 9

DEAD TIMBER

DANGER TO BRIDGES

HOW DEFORESTATION REACTS

BATTERING RAMS CARRIED BY FLOODS. ' . .

(By "X.")

Marlborough, disappointed in its legitimate expectation of a railway connection with Canterbury, looks forward to a motor connection with the West Coast by means of an arterial road traversing the Wairau and the Buller Valleys, and crossing from one valley to the other via the intervening high "country, not far from Tophouse. "-■- ■' "'■■ A great deal of this arterial road has been constructed, but. a bridge is needed over the Wairau River; and the bridge over an important tributary of the Wairau, called The Branch, i» incomplete. . This Branch Bridge.hae had a chequered career. The.Branch issues from a gorge on to open country. In iha gorge its; waters are more or lesii coritrned; on the open country they diffuse over a wide and not very well defined bed. The gorge country is hard, -and costly for pile-driving, but -when a cylinder pile is put into such •■ country it is likely to stop there. In the softer open country a cheap beech pile can be driven at much lower cost, but is also liable to be scoured out in flood time, under certain conditions of construction, a longer bridge in the open country may be cheaper than a shorter bridge in a gorge. In this case, it also happens that the using of the gorge as a bridgesite would require a deviation lengthening the road by more than a mile. After weighing it up, the Public Works' Department decided to put the bridge-in the open and not in the gorge. ohis, by way of preface. THE TACTICS OF RIVERS. A feature of the bridge-site chosen was a sort of island in the river. The Branch ran principally on the Blenheim or north-eastern side of the island. It was thought that the lesser channel on the other or south-western side could be blocked with a solid bank, carrying the roadway on top. The Branch thought otherwise. It came down with mountain, reinforcements and knocked out the 1 bank. Thus the bridge was left without a. connection with the south-western bank. The strength of flow thus revealed to be possible in the south-western or lesser channel made it necessary, to put in, on that side of the island, a continuation of the pile bridge instead,of a solid barriep or bank. Latterly the flood has attacked the bridge in the other channel, and has shifted some piles at the Blenheim end. These will be renewed, possibly in ironbark. The vagaries of a wide-bedded, rapid-ly-nsmg river, and its capacity to transfer: its massed attack from point to •point, as though searching for a weak spot, are not new. But, according to the impressions of a visitor, recently in the district, what has happened in The Branch affords incidentally a striking instance of the evils of deforestation. In the upper basin' of the river was a. beech bush. Some years ago it was fired; probably' accidentally. The tim-.ber-value of this particular bush had not been- assessed (so far as the informant knows), but as beech will come into constantly greater utilisation, particularly as Australian hardwoods grow scarcer and dearer, any purposeless burning of beech is an economic loss at present value and probably a calamity if assessed at future value. The Branch Bridge, however, was concerned m the fate of the beech bush 'in 2S. r ways t*l*ll in the timber aspect. The ghastly.dead trunks of a burned bush do not regulate water-flow. All the regulating influence of the green bush, with its natural reticulation, is lost. Rain flows off quickly, with erosive effect, and floods and shingleshiftmg become intensified when the lifehas gone out of the forest. That is bridge-danger No. 1. . . OBSTRUCTIONS AND SCOUR. " Bridge-danger No. 2 is provided by thi corpses of the burned trees. As these fall the timber-carrying element in Uood-time greatly increases, and the informant gathers that this has happened m The Branch. When a flood pile! up dead timber against some of the piles of a bridge, the water-way becomes restricted, and the. strength of the flood is concentrated on scouring at the foot of some other pile or piles. Holes of 20 feet or 30 feet may thug be scooped. Ihe piles that are subjected to this massed attack give way. _ The particular instance on The Branch" is not m' itself necessary to prove this double danger of deforestation (accidental or otherwise). Other instances exist. Burning, converts the green bush from being an outpost line of defenceagainst flooding into being a battering ram that the hostile waters hurl against bridges .to break them down. And not only bridges. The dead timber element is a danger to railway banks and embankments wherever there is running water. The culvert is blocked. Water can t get away. Bank collapses. "Washout on the line." • Marlborough is entitled to an arterial road properly bridged, to Westport and to the West Coast. . The motor may yet be the means of making such a road one of the developmental highways of New Zealand Other districts possibly hay» similar legitimate claims. But hew few districts realise that needless destruction of native bush removes a i>ro. tection from their internal communications and substitutes a barrier? "