Sources of Sedimentation

In:
2017 Forest Growers Conference
Report No:
AC2017-19
Report Date:
October 18, 2017
Author(s):
Dr Kris Brown: University of Canterbury

Erosion sources and sediment pathways to streams associated with forest harvesting activities

Conference presenter: Dr Kris Brown, University of Canterbury School of Forestry

The rapid increase in harvest volumes projected over the next decade gives rise to concern about the risk these operations pose to water quality. Many of the forests to be harvested are first-rotation, on steep, erodible soils. New roads will have to be constructed: current estimates indicate some 1,400 to 2,000 km of forest roads will need to be built annually until 2025.

The aims of this research were to:

  1. establish how much of a contribution forest roads make to watercourse sedimentation
  2. develop some best practice guidelines to minimise the risk of ‘breakthroughs’, where sediment from forest roads and trails reaches streams via concentrated overland flow.

Researchers visited 23 recently harvested forests, looking for ‘breakthrough channels’ via which water and sediment were reaching waterways. On average they found 3.4 breakthroughs per kilometre of stream, or one for every 6.5 hectares of harvest area. These frequencies compare closely with harvesting activities studied in the USA.

Some other key findings were:

  • ground-based logging resulted in nearly twice as many breakthroughs as cable logging
  • 73% of breakthroughs were associated with concentrated runoff from roads, trails, stream crossings, and machine tracks on hills
  • road surface type (e.g. aggregate-surfaced truck road, bladed skid trail, or overland skid trail) and the spacing between water control structures were important factors for understanding hydrologic connectivity at road-stream crossings
  • steep skid-trail approaches with poor water control and surface cover can result in potential erosion rates exceeding 100 tonnes/ha/yr.

Guidelines for harvest planners

The research concluded that, for most operations, minor adjustments will result in significant gains. Careful planning of road location and gradient is the biggest factor, while other guidelines include:

  • plan to minimise tracking and stream crossings
  • locate roads and stream crossings to avoid steep grades (reduces earthworks and makes water control easier)
  • maintain a buffer between disturbed soil and streams (could be simply a slash barrier)
  • install road drainage structures to control water in small amounts and reduce runoff velocity
  • position road drainage structures to avoid direct and indirect discharges to streams
  • inspect roads and skid trails periodically post-harvest to ensure drainage structures are continuing to function properly.

Presentation below.

Document:
19 Sources of Sedimentation_K Brown (922.42 Kb)
Top