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Forestry Engineering Research

22 November 2023

Forest roads and associated crossings can represent major sources of sediment delivery to streams, which can negatively impact water quality and aquatic habitat. We evaluate the best management practices for water control and surface cover at forest road-stream crossings in New Zealand. Learn more out about Forestry Engineering Research.


Forest roads and associated crossings can represent major sources of sediment delivery to streams, which can negatively impact water quality and aquatic habitat.  The importance of controlling sediment delivery from earthworks, including road and stream crossing construction, is underscored in technical documents, such as the Environmental Code of Practice for Plantation Forestry and the Forest Road Engineering Manual.  Further, New Zealand councils are required to adopt more stringent water quality rules in 2015 as per the National Policy Statement (NPS) for Freshwater Management.  The new NPS for Freshwater requires a detailed knowledge of sediment loading from major sources, such as forest roads at stream crossings.  However, the sediment-reduction efficacy of specific stream crossing BMPs has not been quantified for a wide range of road types and site conditions (e.g. climate, soil erodibility, slope, traffic).  Improved understanding of BMP costs and benefits (i.e. reductions in sediment loading) are critical to facilitate the adoption and proper implementation of recommended BMPs by the forest industry and to comply with the requirements of the NPS for Freshwater Management.

Consequently, a field survey of forest roads at stream crossings is being conducted to evaluate Best Management Practices (BMPs) related to water control, surface cover, and the crossing structure itself.  From March through June 2015, 29 crossing evaluations have been completed in the Southland, Gisborne, and Canterbury regions for a range of road classes (spur, secondary, and arterial) and road use conditions (e.g., closed road, active use, or new construction).  Field measurements included the drainage length of the stream-crossing approach (i.e., the distance from the stream to the nearest water control structure), as well as estimates of road camber and surface cover associated with the major road components (e.g. table drains, cut and fill batters) within 15 meters of the crossing.  The Universal Soil Loss Equation as modified for forest land (USLE-Forest) was used to estimate annual surface erosion rates associated with the road formation component of the stream-crossing approach.  Data collection related to the crossing structure included crossing type, stream bankfull width, and culvert material, diameter, and depth of fill (if applicable).  A questionnaire was completed that evaluates crossing installation and alignment, the use of gravel to control erosion near the stream, approach length and slope, passage of streamflow and fish, and fill stabilization with vegetation or rock cover.  Culvert sizing will be evaluated in accordance with upstream catchment area and streamflow characteristics.

This research will improve understanding about implementations of BMPs at forest road-stream crossings in New Zealand, as well as their effectiveness in reducing sediment delivery and avoiding the restriction of streamflow and fish passage.  Identification of potential water quality problem areas, such as inadequate water control and surface cover near the stream, can be used to guide future research regarding low-cost and practical BMPs for road-stream crossings.  Examples could include the addition of water control structures (e.g., cross-drain culverts or turnouts) to reduce the stream-crossing approach length or the use of slash to control erosion on fill batters near the stream.


Workshops on Cable Rigging Configurations

Dr Hunter Harrill has been disseminating the findings from his Ph.D. research to forest managers and logging contractors through a series of workshops focusing on cable rigging configurations. From March through June 2015, Dr. Harrill has lead five workshops around the country, including Gisborne, Mosgiel, and Rotorua. Learning outcomes from the workshops include:

  • Improving fundamental knowledge of tension and deflection relationships
  • Recognising situations that lead to high and/or shock loads in the rigging
  • Appreciating advantages and disadvantages of common rigging configurations
  • Learning about new/alternative wire ropes and the effect of bending/sheave size on forces
  • Understanding how payload analysis software works and the benefits of advanced planning
  • Measuring deflection in the field
  • And, most importantly, through interaction with other workshop participants, appreciating common issues affecting rigging configurations


UC Logging Fuel Use Research

Having worked for a timber harvesting and wood products processing company back in Kenya, Paul Oyier is pursuing a Master of Forestry Science degree on logging fuel use, having been awarded a New Zealand Development Scholarship. He notes that determining energy consumption in forestry operations is fairly complex and requires accurate data collected through real time studies or informed surveys. This data can be used to model the relationships between energy inputs (e.g. fuel) and productivity outputs (e.g. volume of timber harvested).

Through a survey of logging contractors, Paul has collected data on fuel consumption and production by machines and common harvest systems in New Zealand to determine the average fuel consumption per unit volume of wood. The purpose of the study is to determine differences in fuel consumption among commonly used harvesting systems and machines and to compare the survey results with published fuel consumption data. Paul notes that fuel is a major cost in logging, accounting for up to 25% of the total cost of running a logging business, and therefore is a significant determining factor of the logging business profitability. A better understanding of the factors affecting fuel consumption is important for decision making during harvesting planning and logistics. However, he notes that very little is known about average fuel consumption per unit volume of timber harvested owing to a lack of published fuel use information. He further notes that logging activities such as felling, primary transportation (extraction), processing, and loading are conducted by different logging crews under dissimilar forest settings, each with distinct stand and terrain variables. Furthermore, logging machinery executing these primary functions vary with type, make, and power ratings.

Understanding of machine variability and the influence of harvest site factors on the quantity of fuel used to harvest a given volume of wood is of economic importance to all the stakeholders in the logging industry. More importantly, New Zealand is currently shifting towards fully mechanised steep terrain logging with more research aimed at integrating mechanisation and increased efficiency with crew safety on the slopes. Therefore, focussing on being fuel efficient through mechanisation, now and in the future, can make a difference in overall logging productivity, operational efficiency, and economic viability for logging contractors, landowners and forest management companies.


Previous Research and Projects


Forest Roads Research (3 parts)

Survey of Forest Road Engineering Practice
The purpose of this research is to assess the current state of road engineering practice in the New Zealand forest industry. The outcome will assist in the identification of opportunities to improve forest road engineering practices. Preliminary findings from this survey have identified the need to better research aggregate gradation and pavement thickness design for unsealed forest roads.


Aggregate Gradation for Unsealed Roads
This research involves sampling and analysing aggregates used by New Zealand forest companies to construct forest roads. The aim is to determine the gradation of current materials and to compare this to theoretical gradation curves and existing gradation specifications used by forest engineers throughout the world. The research will help development, testing and evaluation of an improved aggregate gradation suitable for New Zealand’s environmental and operating conditions.


Unsealed Pavement Thickness Design for Forest Roads

Current unsealed pavement thickness design methods used by the New Zealand forest industry generally rely on experience and trial and error. This research will evaluate and test a range of unsealed pavement thickness algorithms to determine whether the resulting pavement designs can improve the reliability and cost-effectiveness of pavement construction for forest roads.

Simon Fairbrother


Manual and Mechanised Felling Studies

A series of studies are being carried out to test the productivity implications of felling with a chainsaw on steep terrain, as well as looking at tree size limits for mechanised felling.

Jacob Saathof


Radio-controlled chokers for Cable Yarders

Radio-controlled chokers can improve the productivity and safety of cable yarding operations by avoiding the need of the poleman (also called a chaser) to unhook a turn of wood as it reaches the landing. However the radio-controlled chokers are expensive, and can add to the workload of the choker-setters in the cut-over because of the extra weight. Together with Dr. Karl Stampfer from Austria, we are carrying out to trials to test the Fortronic chokers in NZ conditions. In addition to studying productivity improvement, we are also using heart-rate monitors to see if there is any additional strain on the choker-setters.

Rien Visser


High-resolution Aerial Photography for Tree Level Forest  Management

Heather Wakelin completed undertook this Masters projec. High resolution aerial imagery is not only getting better, but is also becoming increasing affordable for use of managing forest stands. It can be used for simple tasks such as tree counting, but also survey wind throw, planting mortality rates and perhaps have advanced applications in planning. The future may allow us to manage a forest at the individual tree level, instead of at the stand scale.

Heather Wakelin


Methodologies for Setting Logging Rates

How to set a logging rate (what a contractor is paid to fell, extract, process and load out logs). We surveyed a number of forestry companies in NZ and the USA to determine what current methodologies are being used to determine logging rates. The findings show that the approach fits into three broad categorises, whereby the pros and cons of each approach to setting logging rates is discussed.


Analysing Shiftwork Patterns at the KPP

Kim Rose, a Forestry honours student, evaluated 6 months of detailed data from the Kaingaroa Processing Plant to see if fatigue affected productivity or value recovery. Very detailed information from the plant was obtained every 30 minutes that included overall throughput, tree dimensional information as well as the percentage of higher value logs cut. Kim analysed this information relative to the time into shift (12 hour shifts were being worked), as well as looking for differences between day and night time shifts.

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