Sediment production from forest roads in Queen Charlotte Forest and potential impact on marine water quality, Marlborough Sounds, New Zealand

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 .

Smothering of estuarine sandflats by terrigenous clay: the role of wind-wave disturbance and bioturbation in site-dependent macrofaunal recovery

“…Apart from catchment urbanisation, soil erosion in New Zealand is exacerbated by young volcanogenic soils, steep land, deforestation and high annual rainfall. Catchment erosion can occur both through the continuous process of surface erosion, but also through larger slips or landslides that contribute large quantities of sub-surface soils to aquatic systems (Fahey & Coker 1992, Konar & Roberts 1996. Our own observations and anecdotal evidence from around New Zealand suggests that such landslide events do occur in connection with storm events, producing massive slugs of fine terrigenous sediments entering estuaries.…”

Terrestrial deposits on intertidal sandflats: sediment characteristics as indicators of habitat suitability for recolonising macrofauna

“…Megahan & Kidd 1972, Doeg & Koehn 1990, Fahey & Coker 1992, Davies & Nelson 1993, Grayson et al 1993). Fine sediments which are removed from the land in this way may find their way to coastal areas within hours/ minutes (Fahey & Coker 1992), particularly in association with flood events (Nichols 1977, Wheatcroft et al 1997, Wheatcroft 2000. On mixing with seawater, fine terrestrial sediments flocculate and can settle onto the seafloor; their accumulation on the seafloor can adversely affect the resident benthic communities (Johnston et al 1981, Fahey & Coker 1992.…”

“…Fine sediments which are removed from the land in this way may find their way to coastal areas within hours/ minutes (Fahey & Coker 1992), particularly in association with flood events (Nichols 1977, Wheatcroft et al 1997, Wheatcroft 2000. On mixing with seawater, fine terrestrial sediments flocculate and can settle onto the seafloor; their accumulation on the seafloor can adversely affect the resident benthic communities (Johnston et al 1981, Fahey & Coker 1992. In storm-associated sedimentation, terrestrial sediment may be washed into a catchment stream and transported downstream to the estuary, after which it may be deposited directly onto the sandflat, or remain in suspension for a time prior to deposition.…”

Forest road erosion in New Zealand: overview

“…Most of the road erosion research has been carried out in this and the Marlborough Sounds region. In parts of the Marlborough Sounds, erosion of forest roads below 200 m altitude was a concern due to the presence of strongly weathered schist, and a perceived threat to marine mussel farming, high water quality, recreation and scenic values (Fahey and Coker, 1992). The establishment of small experimental basins from 1964 to 1975 (New Zealand's participation in UNESCO's International Hydrological Decade) later provided opportunities to examine the erosional effect of logging tracks (skid trails) and landings on stream water quality (O'Loughlin, 1979;O'Loughlin et al, 1980, Pearce andHodgkiss, 1987;Hicks and Harmsworth, 1989).…”