UNIVERSITY RESEARCH Wind Tunnel Tests Help Show Why Trees Blow Over

Press, Volume CX, Issue 32382, 22 August 1970, Page 7

UNIVERSITY RESEARCH Wind Tunnel Tests Help Show Why Trees Blow Over

CContributed by the Information Office, Univereitv of Canterbury) Inside the wind tunnel of the School of Engineering at Ham a miniature forest bends and sways as gusts similar to those of a Canterbury nor’-wester sweep through it.

The information gained in these laboratory tests and in field tests may help to prevent the gravest danger to Canterbury’s exotic forests: windthrow. In a nor’-wester before the heavy snowstorm of July, 1945, the Balmoral forest suffered severe damage and in March, 1964, 7200 acres-—nearly half—of the Eyrewell forest fell in a nor’wester.

that trees tend to fall in a gale when the pruned height is a quarter of the total height and that they will tend to break when the pruned height is a third of the tree’s height When twothirds of the tree’s height has been pruned, damage is unlikely. Also less likely to be damaged is a tree with its canopy down to ground level provided the erown size does not differ from that of pruned trees. “When we find in these tests where the centre of pressure is we can begin to think about shaping trees by controlling spacing so that they will provide the minimum of resistance,” Mr Papesch said. One paper on this research has been produced for the Forest Service and another is in preparation, but much more work is yet to be done. Mr Papesch is undertaking the research as a doctoral thesis, but the outcome may save thousands of dollars for the forest industry in New Zealand in years to come.

Some 400 million cubic feet of timber was subsequently salvaged and sold to Japan as peeler logs.

forces generated up to a maximum loading. Resistance was affected by age. The maximum resistance of a 29-year-old Pinus radiate production tree with 50 cubic feet stem volume was 87,0001 b ft. For a 39-year-old tree the figure dropped to 81,0001 b ft But a 19-year-old tree with 40 cubic foot stem volume had a resistance of 120,0001 b ft. Having measured the resistance bending moment of trees, Mr Papesch plans to find out in field tests during the coming spring where the centre of pressure exists on a tree and the associated drag. It is expected that by comparing the loading on a tree with what it can resist the conditions under which

a tree will fall will be capable of ready assessment. To undertake the loading tests Mr Papesch is setting up apparatus which will record 12 load ratings from trees simultaneously on an analogue recorder and these readings will be correlated with an anemometer set up on a tower so that the wind speed at the time is also known. Effects Of Canopy In the first series of tests the stem volume and root capability were the significant factors. In the coming tests the crown size of the trees Will be the key. Mr Papesch plans to test a theory

Between these two dates nor’-west winds gusting to 70 miles an hour or more were recorded on 16 occasions at Christchurch airport. The nor’-west fohn wind, reaching gale force as it comes off the foothills, is clearly a menace to forests. That is why the New Zealand Forest Service has made grants totalling 84500 to Mr A. J. G. Papesch, a lecturer in the department of mechanical engineering at the University of Canterbury, to study the mechanism of windthrow and to suggest methods of preventing it. When the wind tunnel, designed and built in the department, was completed in--1966, the Forest Service was experimenting with a strip planting scheme planting strips of forest every eight years at right angles to the nor’-wester. It would have taken 32 years or longer for conclusive results to come from these tests. Mr Papesch heard of the tests almost by chance and use of the wind tunnel was suggested to accelerate them. About 300 even-height “trees” made of wire and nylon and similar to a kitchen bottlebrush, except that they are of a typical Pinus radiate shape, have been used in the tunnel. Each, is wired to strain gauges and the effect is recorded as wind is circulated round the tunnel through the working section. Wind speeds erf up to 120 miles an hour may be reached in the tunnel. Test Findings These tests, which are continuing, will determine how forest roads and other features aggravate the bending moment of trees in a gale. They have already shown that right-angle corners should be avoided in forest planting and have confirmed findings from' past damage that trees on the perimeter of a forest withstand gales better than those inside. They are also providing information about the angles to Which rows of trees should be set, the characteristics of graded forests in the strip planting patterns and forest management generally. The laboratory tests are being supplemented by field work to measure the resisting forces of a tree to wind and the capability of the roots to maintain their grip. In these tests a bulldozer was used to pull over trees as load measurements were taken. “I have been able to show, though the results are not yet conclusive, that the age and stem volume of a tree correlate very well with the resisting bending moment,” said Mr Papesch of these tests. Trees 25 to 30 years old withstand bending strain better than older trees with similar stem volume. Drop With Age The root system, Mr Papesch said, was significant only after a certain strain point had been reached. Trees could withstand considerable bending provided the roots could withstand the