Quantify woody debris / slash content along the coastal, and inland catchment areas, characterizing it by woody debris source or type, volume per hectare, and the total volume accumulated.

Sampling Methodology Review

Line Intercept Sampling Plots

Due to the nature of the debris, the Line Intercept Sampling (LIS) technique will be adapted from that used for cutover waste assessment as per NZQA Unit Standard 6956 – Carry Out Waste Assessment in Cutover Forest.  Since its original description for merchantable cutover residue assessment in Kaingaroa Forest by Warren and Olsen in 1964, the line intercept method (LIS) has been extensively used for measuring the quantity of wood lying on or near the ground. This continued to develop and be tested as a simple and practical methodology (Bailey 1970, Wagner and Wilson 1976, Hall 1986, Bell et al 1996).  Proof its mathematical basis (De Vries 1973) and a review of the practical aspects of LIS to overcome bias and improve precision (Wagner, 1982, Pickford and Hazard 1978, Bell et al. 1996), meaning it has been adapted worldwide for woody debris and woody fuel loading sampling.  Therefore the use of LIS for woody residue assessment is simple and well-proven.

An equilateral triangle sample plot should be used to remove the bias of piece orientation as it is accumulated on the beach from the waves or alongside a river.   Each side of the triangle will be 10m, with each plot being a total length of 30m (horizontal length), with a total sample length of 30m per sample point with 10m on each side as per Figure 1.  Where the slope of the transect is greater than 5 degrees, a slope-adjusted distance should be applied to each side of the triangle by simply adding a small distance on each side.

Each woody debris piece would be measured for the diameter at which point it intercepts the line, even if it intercepts more than once.   To avoid measuring smaller twig / small branch material a cut-off diameter of less than 7cm will be used, whereby all material greater than or equal to 7cm will be recorded and measured.  Comments should be made, combined with photos collected of each to characterise the small debris.   There is no minimum length restriction recorded piece.

To further address the bias in debris accumulation orientation it is proposed that each equilateral triangle sample has a random orientation to the predominate river flow or beach front based on random bearing between 0 and 90 (Figure 2).

Measurement of Pieces

Diameter is to be measured to the nearest cm in line with the woody debris orientation as shown in Figure 3.

Classification of Debris

Each intercept diameter measured will be classified into the following.

N Native Species
CH Conifer (Pine/Douglas) Plantation Harvest Residue
evidence of flush cuts / slovens / processor damage / branches cut off
CT Confer (Pine/Douglas) Plantation Tree Residue
evidence of full tree slippage of preharvest standing trees / full tree lengths / root plates visible / branches attached.
CN Conifer (Pine/Douglas) Non-Plantation
old man’s pine, riverbank large old open grown pine.
CO Conifer (Pine/Douglas) Other, unsure of classes above.
PT Posts / Timber
PW Poplar or Willow

Calculating Woody Debris Volume and Summary

Volume will be calculated using the following LIS formula.

Volume of debris:  Volume m3/ ha = (π2/8*L)*sum(d2)


V= volume per unit area (m3/ha)
d= piece diameter at intersection (cm)
L= length of the sample line (m)

Resulting dataset will yield volume per ha by woody debris type.  This yield of woody debris can then be applied to the mapped area of woody debris.    Depth of transects will be recorded and maybe used to add bulking factors for depth of woody debris.

Locating LIS Sampling Plots and Intensity

Plots should be randomly located within pre-identified mapped areas.  Due to the nature of the material in isolated piles or long narrow beach accumulation the plot placement can be adapted from one of two approaches.

  1. Predefined Plot Locations

The preferred method, where random plots using geo-spatial plot sample tools (e.g GeoMaster Assessment Planner) identifies plots which will be navigated to using a GPS.  Plots will be placed using a random sampling approach (best suited for narrow width of the sample areas).

  1. Onsite Random Systematic Grid Plot Locations

Select a systematic interval between plots e.g., every 25m or 50m along the baseline (beachfront or river flow).   Where possible sketch / draw on a map of the accumulated debris area.  This will include taking photos using a mobile device, and/or drone imagery, video or mapped orthophoto to aid in later mapping of area in hectares of woody debris.   This is to ensure during analysis the data can be qualified by total area of the debris pile and the sampling intensity applied.

Methodology Workflow in Practice

  1. Determine the baseline (beach, river flow)
  2. Locate plot using GPS or random systematic grid, mark with stick paint mark – POINT A.
  3. Record the GPS Location of the plot.
  4. Determine plot orientation from base line with a random number between 0 and 90 (select from Table 1).
  5. Using a 30m tape or survey rope, hold the tape at 0m and 30m at Point A. Lay out the first transect of 10m from the start point to POINT B.  Then completed the triangle by pulling the remaining tape out to 20m POINT C.
  6. Record wood debris diameters where piece at intercept is =>7cm and classify each piece by type.
  7. At each corner check slope of transect and where greater than 5 degrees add an additional length to transect as per Table 2, recording any additional wood debris.
  8. Record as depth of the pile per transect 10m length which was not able to be assessed (if you measured all the pieces then depth will be 0m). Record to nearest 0.5m
  9. Continue to measure wood debris each side of the triangle.
  10. Take photos of each side of the transect, and any other items of interest for future reference.

Mapping Woody Debris

Using aerial imagery along the length of the coastline to be assessed, map polygon areas of debris.  Locate plots as per random plot methodology across the mapped area.  Provide a GPX, Shapefile and PDF Map.

Satellite or Aerial Photography

A range of imagery sources may be used, whereby initially use of Planet Labs 3-4m resolution satellite imagery can be used as shown in Figure 4 and Figure 5.

Aerial photography or higher resolution satellite imagery (example Figure 6) can also be used depending on the area and availability to help improve the mapping quality.

Drone Mapping

Where areas are likely not to have other imagery sources or the debris is quickly changing (being chipped, remove from site or continuing to wash away), it will be useful to capture the site with drone mapping.   Automated detection of woody debris could also be applied to the scale of imagery as shown in Figure 7

Flight Specifications: typically, an 80-120m above ground level mapping grid flight, with a 70% overlapping swath should result in an effect map in most circumstances for an orthomosaic image of woody debris.  This makes collecting time efficient and depending on the resolution of the camera will result in a <5cm pixel over the area.


Appendix A – Random Bearings 0 to 90 and Slope Adjustment Table for 10m

Appendix B – Plot Form Paper Format


Bell, G. et al., 1996. Accuracy of the line intersect method of post-logging sampling under orientation bias. Forest Ecology and Management, 84(1-3), pp.23–28.

Competenz Unit Standard 6956 – Level 4 – Carry out waste assessment in cutover forest.

Hall, P., 1998. Logging Residue at Landings, (May), pp.1996–1998.

Howard. J, Ward. F 1972 Measurement of Logging Residue – Alterative applications of the Line Intersect Method. USDA Forest Service Research Note PNW-183

Wagner, C.E. Van, 1968. The line intersect method in forest fuel sampling. For. Sci., 14: 20-26. De Vries, P.G., 1986. Sampling Theory

Wagner, C.E. Van, 1982. Practical aspects of the line intersect method.

Warren, W.G., 1989. Line intersect sampling: a historical perspective. In: State-of-the-art Methodology of Forest Inventory: A Symposium Proceedings. USDA Forest Service, Pacific Northwest Research Station, Portland, OR, pp. 33–38.

Warren.W.G. and Olsen,P.F.,1964: A line intersect technique for assessing logging waste. Forest Science, vol.10 pp.267-276

Further detail on this can be found in a review of the line intercept approach.




Pan Pac Forests, Red Axe Intelligence, Aratu Forests, HB Forestry Group, Kees Weytmans