OneFortyOne Forest Management

May 2022

A study into the performance of sediment control practices utilised in forest harvesting and earthworks.

A long-term monitoring programme has been established and is operating within paired (comparative) catchments in the OneFortyOne New Zealand (OFO) forest estate. The aim is to study the performance of sediment control practices utilised in forest harvesting and earthworks. The paired catchments are adjoining and of similar size, area, geology and topography, and planted in Pinus radiata of similar age.


OneFortyOne (formerly Nelson Forests Ltd) has seven forests in Nelson, Tasman and Marlborough, comprising 50 forest blocks. In 2016, OneFortyOne hosted a two-day workshop to discuss sedimentation risks and areas for potential improvement in forest management practices.


Participants represented a wide group of interested parties, including Crown Research Institutes, universities, councils, the Department of Conservation, Fish & Game, forest industry practitioners and contractors. The workshop led to a call for a comparison of sediment entering streams (clays in suspension) under current forest practices, against further sediment reduction approaches.


OneFortyOne reviewed its Environmental Management System (EMS) extensively after the workshop and in 2018 updated its EMS, with more focus on control practices to reduce sediment into streams from disturbed ground (through earthworks and harvesting), as well as using harvest slash to disperse sediment erosion entering water bodies. The company did this update partly in anticipation of a new National Environmental Standard for Plantation Forestry. 


Some New Zealand soils are highly erodible while others are more solid. Those solid soils can still move but not necessarily as much as some soils did in major storms like Cyclone Gita, Tologa Bay and Bola. This long-term trial is working in Moutere gravels, which is a relatively stable soil type. OneFortyOne and its partners will compare a harvested area with a non-harvested area – and compare standard, everyday sediment erosion control, like a sediment trap, with something completely different, like a large sediment retention pond.


The seven-year study has two phases. Jo Field, Environmental Planner for OFO explains that the first phase (“treatment catchment 1”) will measure sediment exported from current harvest and earthwork operations. The second phase (“treatment catchment 2”) will have a sediment retention pond to decrease or trap flood flows and allow maximum sediment build-up and removal.  Both phases will be compared  to a control catchment with no harvesting/earthwork operations for the course of the study period.


All harvesting and earthworks in both treatment catchments will follow the standards in OFO’s EMS. Concurrent with forest operations, there will be monitoring assessments of in-stream health to see if there is an ecological impact, and - if noticeable - determine the recovery period.


Most sediment will be ‘delivered’ during storms, so the focus of the project is on collecting good-quality, storm-related suspended sediment data from the pre-harvest to the post-harvest period. The project will quantify how much sediment can be prevented from leaving a harvesting site using sediment control practices. It will also study the aquatic ecological impact of reducing sediment generation and the costs of implementing new standards and practices. For example, if the results show that the current practice of using slash is an effective treatment there could be a reduction in cost by using slash rather than persisting with practices that require off-site resources. Conversely, if the results indicate that the ecological impacts are too disruptive, then the work will provide a basis for more investment in sediment control for ecological benefits. 


Cawthron Freshwater Scientist Karen Shearer is undertaking ecological assessments in waterways in and around the Donald Creek forestry block near Tadmor in the Tasman district, to explore the effectiveness of forestry sediment control measures in reducing sedimentation in streams and rivers during harvesting. This includes measuring water quality, the amount of fine sediment on and in the streambeds, and collecting stream habitat, algae cover, invertebrate, and fish data. This involves benthic and sediment sampling, and fish and other native species identification and quantification. 


The Donald Creek project is in partnership with the Cawthron Institute, Envirolink, and Manaaki Whenua Landcare Research scientists. It is funded 50/50 by the Ministry for Primary Industries and OneFortyOne.


The results of this research will be made available to the forest industry, scientific community, tertiary institutions, and regulatory authorities. 


In smaller sediment traps, the sediment can be removed to an area where it won’t enter a waterway. Eventually that area should be able to grow grass, and thereby reduce the ecological impact in waterways to a minimum. It’s too soon to know what will be done with the sediment from the retention pond. At the end of the project it may be returned into engineered-wetland.