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New Zealand Journal of Forestry (1980) 25(2): 184–198
©New Zealand Institute of Forestry

Research article
The impact of forest road erosion in the Dart Valley, Nelson.

M. P. Mosley

A survey of erosion on a road system constructed in granitic terrain in the Dart Valley, Nelson, for production forest development has been carried out. Rates of sediment input into the stream system in 1978-9 averaged 255 m3/km/yr over the 25 km road system, but figures for individual roads varied widely, from 14 m3/km/yr for a 10-year-old valley bottom road to 1270 m3/kmfyr for a 1-year-old mid-slope road which had suffered a number of large culvert or fill slope failures. Total rates of erosion on the road system were three times greater, but much sediment is fed on to vegetated slopes beneath the roads, and is stored there. Total sediment input from the road system into the stream system in 1978-9 was estimated as 12 000 t/yr, in comparison with an estimate for natural sediment yield from the catchment of 9600 t/yr. Much of the sediment has been stored in headwater channels {which are, however, probably subject to periodic flushing by major storm events), deposited upon point bars above the low-flow water level in the main river, or flushed out to sea in suspension. The impact of the road-derived sediment upon the Dart River is therefore probably limited. The estimated natural sediment yield of the Wangapeka is an order of magnitude greater than that from the Dart; sediment from the Dart Valley road is therefore unlikely to have had any significant impact upon the Wangapeka, or the Motueka downstream.
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