Erosion sources and sediment pathways to streams associated with forest harvesting activities in New Zealand.
Streams and rivers are abundant in our plantation forests and the Forest Industry prides itself on protecting the intrinsic values they provide, including clean water and quality aquatic habitat. In ensuring they remain protected, the industry has both legal obligations through Resource Management Act and Regional Council requirements, as well as a professional obligation through Forest Accord agreements and the adoption of the NZFOA Environmental Code of Practice (ECoP). It is well known that forest harvesting, including the development of roads and infrastructure, can compromise these waterway values. With the law being effects-based, a clear focus should be on concentrated flows of runoff and sediment to stream channels (aka ‘breakthroughs’) that are linked to our harvesting practices. Improved knowledge about breakthroughs can be used to prescribe site-specific best management practices (BMPs) to protect water quality and aquatic habitat. To better understand how often breakthroughs occur and to identify and describe the most common breakthrough sources, the stream channels associated with 23 recent commercial forest harvests were surveyed. A breakthrough had to have the following characteristics to be recorded: evidence of scour from surface runoff and/or a concentrated trail (pathway) of sediment that reached the stream channel. Overall, there were 3.4 breakthroughs per kilometre of stream, or one breakthrough for every 6.5 hectares of harvest area, which is almost identical to studies from the Piedmont region of the southeastern United States where they found one breakthrough for every 6 to 8 hectares of harvest area. In this study, ground-based logging was associated with nearly twice as many breakthroughs as cable logging (i.e. 6.2 versus 3.3 breakthroughs per kilometre of stream) due to more soil disturbance from trails near streams. Seventy-three percent of breakthroughs were associated with concentrated runoff from roads, trails, stream crossings, and machine tracks on the hillslope. Road-stream crossing approaches delivered concentrated runoff to stream channels more often if they were associated with log truck roads and/or surface runoff could travel long distances from the adjacent and uphill road segment to the stream. Log truck roads are compact by design to support heavy loads, whereas skid trails often retained some of the protective functions of the forest floor (i.e. higher infiltration capacity). Longer road drainage lengths are associated with greater runoff volumes and velocities, thus increasing the potential for hydrologic connectivity with the stream. Conversely, where skid trail approaches to stream crossings did lead to breakthroughs, potential erosion rates were 44 times higher than those of log truck roads at stream crossings. This finding highlights the importance of closing skid trails properly upon harvest completion, especially at stream crossings. Installation of water bars and turnouts to control surface runoff and application of slash to skid trail approaches can be used to reduce the potential for sediment delivery at road-stream crossings. Overall, the level of protection provided to our waterways post-harvest is good. However, the majority of breakthroughs that were recorded could be directly attributed to operational activities and a more consistent adoption of BMPs, especially in the specific areas identified in this report, would reduce our impact even further. Common characteristics of breakthrough sources can include poor infiltration capacity, large quantities of exposed soil, and steep slopes. This project report provides specific examples (as observed in this study) of forestry practices used to reduce the frequency of sediment breakthroughs and their impacts on water quality and aquatic habitat, such as: Planning to locate roads away from streams, control road gradient and reduce earthworks Harvest planning to minimise tracking and stream crossings Installation of road drainage structures to control water in small amounts and reduce surface runoff velocity Positioning of road drainage structures to avoid direct or indirect discharges to stream channels Closure of temporary road-stream crossings that includes both water control and provisioning of immediate surface cover.
SubjectsField of Research::07 - Agricultural and Veterinary Sciences::0705 - Forestry Sciences::070504 - Forestry Management and Environment
- Staging