![Sample our Earth Sciences journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5930/TOC-Subject-Sample-2021-Earth-Sciences.jpg"})
Abstract
Coarse and fine sediment production from surface erosion was measured at three sites on roads formed in weathered schist in Queen Charlotte Forest, eastern Marlborough Sounds, over periods ranging from 2 to 3 years. Two 25 × 4 m run‐off plots on freshly graded sections of forest road (gradient 5°) at 460 m elevation yielded annual averages of 0.48 and 0.46 kg m−2, respectively, for coarse sediment production from the road surface and drainage ditch. The cutbank contribution was negligible. Two 5 × 2 m plots established on fresh sidecast fill (gradient 38°) below a newly constructed road yielded an average of 7 kg m−2 yr−1. Sidecast fill with partial vegetation cover (average 50%) contributed c. 3 kg m−2. Fine sediment production measured on a section of road in strongly weathered schist at 70 m elevation was estimated at 2.3 kg m−2 yr−1. Extrapolation of the results to the 39 km of forest road and the 21 km of access tracks and firebreaks suggests that about 20001 of material is being removed by surface erosion each year (621 km−2 of forest). This could increase to 7000 t or 218 t km−2 yr−1 at the time of harvesting. Log landings could add a further 20%. Background rates of erosion are in the range 300–600 t km−2 yr−1. Up to 2001 of fine sediment may enter local embayments annually which could cause suspended sediment concentration in sea water to rise temporarily to 1000 mg l−1.
