FOREST ROAD EROSION IN THE GRANITE TERRAIN OF SOUTHWEST NELSON, NEW ZEALAND

Past and present sediment production rates from surface erosion on roads in erodible granite terrain of two adjacent forests in southwest Nelson are presented. Yields were monitored for 20 to 32 months at five plots on representative sections of forest roads. Data were collected between 1980 and 1983 from 18 plots on sidecast. Erosivity indices were closely related to sediment production during individual storms. Average annual yields ranged from 1.6 kg m⁻² to 11 kg m⁻² for the road surface-drainage ditch combination and from 1 kg m⁻² to 5 kg m⁻² for sidecast. These figures are extrapolated, with the aid of a detailed road survey, to the 209-km road network on the 12,500 ha of granite terrain in both forests to estimate total sediment supply from surface erosion. Current yields are estimated at 4700 t yr⁻¹ (37 t km⁻² of forest), and at the time of peak road construction, 11,000 t yr⁻¹ (88 t km⁻¹). After upgrading and expansion of the road network at harvesting, sediment yields are predicted to rise to somewhere between 20,000 t yr⁻¹ and 40,000 t yr⁻¹ (160 t km⁻² to 320 t km⁻²). Background erosion rates for nearby river catchments are estimated at about 500 t km⁻² yr⁻¹.