Evaluating the potential for sediment delivery at forest road-stream crossings in New Zealand

Abstract

Forest road-stream crossings can represent a significant pathway for sediment delivery to streams. Careful planning of road location, stream-crossing design and implementation of best management practices (BMPs) for water quality protection are necessary to provide access for the expansion in forest harvest volumes in New Zealand while meeting the goals of the Resource Management Act 1991. However few studies in this country have examined the relationship between current BMPs and sediment delivery potential at forest road-stream crossings. A field survey of 39 corporate haul road-stream crossings covering six regions in New Zealand was conducted to characterise the potential for sediment delivery to the stream. Mean length, slope and road camber of the crossing approaches was 40 m, 7.2% and 4.4%, respectively. Median cover on the running surface component of the approaches was 75% and estimates of potential erosion using the Universal Soil Loss Equation modified for forest land (USLE-forest) were relatively low (range = 0.01 to 8.73 tonnes/ha/yr; median = 0.7 tonnes/ha/yr) in comparison to previous forest road erosion studies in New Zealand (range = trace amounts to 150 tonnes/ha/ yr). Bare soil area within 15 m of the crossing ranged from 2.3 to 421 m2, with a median value of 82 m2. Soil erodibility decreases with time through the processes of surface armouring and vegetation re-establishment on cut slopes, fill slopes and water table drains. Collectively, these findings show that aggregate surfacing and an avoidance of long, steep road gradients can reduce sediment delivery potential at road-stream crossings in the long term and minimise soil disturbance during construction in the short term.