Erosion ideas in doubt

Press, 4 July 1985, Page 4

Erosion ideas in doubt

By

OLIVER RIDDELL

in Wellington A study on erosion in the eastern South Island high country by a Christchurch scientist, Mr lan Whitehouse, has been published by the Ministry of Works and Development. It follows on a decade of intensive investment in research by various Government agencies and, according to Mr Whitehouse, has led to a reappraisal of perspectives on and attitudes to high country erosion. Understanding of some of the causes and rates of high-country erosion had increased, he said, but knowledge was far from complete and continued research would be needed. Recent studies showed sediment yields and erosion rates in the Southern Alps naturally to be very high. They severed the link between the building up of shingle and stones down-

stream and the accelerated erosion induced by man in river headwaters, in the Waimakariri Basin. “Recent studies link this downstream aggradation with early engineering works in the riverbed intended for flood protection,” Mr Whitehouse said. Research had shown the age and long life of many screes in watercourses that had been attributed to European mismanagement of the land. Some of the man-induced soil instability in the Southern Alps was now attributed to fires lit in the Polynesian as well as European eras, he said. Some of the erosion scars that had been attributed to the depletion of vegetation by early European pastoral management did not result from that cause. “Distinction between ‘accelerated’ and ‘normal’ erosion may be difficult,” Mr

Whitehouse said. Research studies emphasised the importance of geology, stored gravel deposits, and proximity to the river channel in controlling the location and magnitude of the source of the sediment. Most erosion in the Southern Alps had been found to occur during high-intensity storms and earthquakes.

Mr Whitehouse said all this research had led to some reappraisals. For example, the slopes at 1100 metres to 1400 metres in the upper Dry Acheron Stream in the Big Ben Range, south-east of Lake Coleridge, had been described trditionally as severely eroded. But soil studies and dating of the screes indicated that the landscape had looked the same for the last 500 to 1000 years. Repeat photography of erosion sites had been used to define the extent and

nature of change over the years. Such photos might also show the dramatic effect a single event could have on erosion. Photographs of the southeast side of Foggy Peak in Canterbury’s Mount Torlesse Range, showed that a single 150-year return period storm in April, 1951, had eroded about 425,000 cubic metres from a gully and deposited it in the valley below. Mr Whitehouse said his paper had not tried to produce definitive statements on. the distinction between “natural” and “accelerated” erosion or on the relative contribution of sediment from different sorts of erosion. Implications from the research done had relevance for the pastoralist, land manager, ecologist, or soil conservator, as well as for geologists and geomorphologists.