Documentation of forest and other woody land along trajectories of non-destructive rockfall V2 2018-04 Frank Perzl, Andreas Kofler, Elisabeth Lauss, Hanna Krismer, Monika Rössel, Christian Scheidl and Karl Kleemayr 1 Introduction In this guideline, an approach to consider forest structure and characteristics along a rockfall trajectory guideline is presented to determine key parameters, which help to evaluate the protective role of forests stands against rock-fall events. Therefore, this work should be seen as extension to “Guidelines for collecting data about past rockfall events” (ASP 462 – RockTheAlps; WP1 – Activity A. T1.2) proposed by Žabota & Kobal (2017). In this concept, the approach and the survey setup for a comprehensive rockfall hazard event documentation, focusing on woody vegetation, are illustrated. The main forest parameters are explained and in a “best practice example”, the general workflow and some exceptional cases are discussed. 2 General concept The spatial distribution of (woody) land cover units like forest stands with relative uniformity and the internal structure of woody vegetation of these units of homogeneity (U) is documented along rockfall trajectories (real rockfall paths) from the farthest block deposit (FBD) identifiable to the starting point of the rockfall hazard (the center of the scar – CS). Methods and work steps: 1) Preliminary orthophoto-mapping of the rockfall trajectory. 2) Preliminary segmentation of the rockfall trajectory into units of homogeneity (not stocked areas and stands of woody vegetation) by means of aerial image interpretation. 3) Preliminary determination of sample plot positions (plot centers) within the unity of homogeneity along the trajectory. 4) On-site inspection of the trajectory uphill: correction of the direction of the trajectory and of the U-borders, classification and description of U, log sampling along the segments, correction of sample point positions and plot sampling within Us. 1 2.1 Parameters TORRID explicit Depending on the classification of land cover unit the amount of collected data varies. Clearing areas and non-forested areas are registerd by their length and position. For the forested sections, the required parameters for the TORRID toolbox (explicit) are determined. The necessary parameters for TORRID explicit cannot all be determined directly in the field, but can be calculated using the following measurable parameters, which are collected following the work steps of the general concept stated above. The key parameters of TORRID explicit are: Basal area [m²/ha]: The angle count sampling method (ACS) (Bitterlich, 1984) and, in case of young wood and shrubs below callipering limit of ACS, tree diameters at breast height (DBH) from stem surveys on fixed area plots are used to calculate the basal area. Stand density [N/ha]: The method is set up for determination of the number of wood plants per ha regardless of plant size. Species composition: Stand composition can be specified as proportions of basal area or canopy cover on base of the proposed survey. Ratio high forest/shrub forest: The method enables the calculation of the percentage of different types of land cover of the total length of the rockfall path. Top height [m]: Different indications of the height of the woody vegetation like the top height may be calculated on base of the surveys. 3 Pre-investigation First step of pre-investigation is the collection of all information about the rockfall hazard from initial hazard reports like photos and witness statements if available. These reports will help to identify the rockfall sources and paths. Registration of the hazard and of all suitable reports in the hazard database prior to terrestrial survey enables clear allocation to data sources by identity numbers. Next step is mapping of the rockfall trajectory or determination of the most probable direction from FBD to CS on a current aerial orthophoto (mapping scale ~ 1:2,000) with contour lines. If available, relief images from high resolution digital elevation models should also be used. Relief images provide the identification of the downhill direction, terrain features and possible rockfall sources. The rockfall trajectory is drawn as a polyline. The edges of the polyline are recommended to not be smaller than about 20 m in length on a mapping scale of 1:2,000. Information about vegetation and other surface cover provided by the orthophoto and relief imagery is used for segmentation of the trajectory into units of homogeneity of slope and surface cover with special consideration of the density and structure of woody vegetation. On-site views from points on the opposite slope may be particularly suitable for mapping and segmentation of trajectories on base of topographic maps, optical and relief imagery. 2 Pre-investigation of trajectory may be done in office or on-site or both, depending on the situation. Especially prior information provided by hazard reports and remote sensing sources are key factors. In some situations, above all in case of rockfall deposits under dense forest cover, mapping and segmentation of the trajectory as well as positioning of sample plot centers is a consecutive task of on-site investigation of hillslope. Figure 1: Mapping and segmentation of the rockfall trajectory. Figure 1 is a sketch of mapping and segmenting of the rockfall trajectory taking into account the spatial distribution of units of woody vegetation. The units 2 and 3 portray forested areas, whereas the units 1 and 4-6 represent non-forested areas, which are registered solely by length and GPS position. A more detailed description of the trajectory recording, as well as a detailed description of examples considering special forest conditions can be found in chapter 5. We recommend a minimum length of a unit of 20 m in downslope direction (regarding canopy cover on woody land, CC). Planning of on-site surveys requires an assessment of the number of samplings. The number of sampling plots in each unit is a crucial question regarding validity and explanatory power of the data. The number of plot samples required to give a defined degree of accuracy is difficult to be predicted in advance. Therefore, we suggest to define a fixed number of samplings per length of unit instead of statistical calculation of the required number of samples on base of assumptions. The respective sampling plot set up is illustrated in chapter 4.2. 3 We suggest 1 plot per 60 m length of the segment (LS) of the trajectory within a unit, at least one plot per unit. The ratio R is rounded mathematically (R ≈ LS/60). The distance of samples along the segment of the unit is simply DS = LS / (R + 1). The general concept for measuring the structure of woody land (forest stands) is a plot survey along the trajectory within units of homogeneity. We suggest a combination of angle count sampling (Bitterlich, 1984) of the "tree layer" and several small fixed area plots in order to survey shrubs and regeneration. 4 On-site inspection of trajectory 4.1 Description of the course of the trajectory and of units The description of the course of the trajectory may follow the structure of template C. EVENTID: Identifier of the hazard event record in the database. PATHID: Identifier of the rockfall path (trajectory) in the database (or sequential number). EPSG-Code: Code of the coordinate reference system used. Description of trajectory and units starts at FBD with GPS measurement of position (Figure 1). Inspection of the situation of the surrounding and uphill area may show appropriate conditions for a sampling at FBD. No positional shift is required. The next point of the course will be the uphill border of unit 1 (Figure 1). At FBD the first sampling is positioned. The FBD is the starting point of the first segment of the traverse. Hence, attributes of the first row of the table are: PATHID: Identifier of the rockfall path. UNITID: Consecutive number of the unit of homogeneity (Figure 1). PLOTID: In case of a sample plot, consecutive number or identifier of the sample plot. No: Number of the segment of the traverse (Figure 1). GPS-X (Easting): Coordinate (Easting) of the starting point of the first segment (GPS). GPS-Y (Easting): Coordinate (Northing) of the starting point of the first segment (GPS). Altitude: Altitude of the point (GPS or barometric measurement). Ideally, a team of 2 groups will conduct the recordings, considering time management and work flow. One part of the staff (group 1) stays at FBD for sampling. The other part (group 2) may explore the terrain uphill to define and mark further sample points. Group 2 measures: Azimuth: The Azimuth of the next segment of the course to the borderline of unit 2 (e.g. L1.1, Figure 1). Inclination: The Inclination of the segment. 4 Distance: The planar length of the segment. Logs: Additionally, group 2 counts the number of woody logs with a diameter ≥ 20 cm, which intersect the segment of the trajectory on the way to the next point. This enables the calculation of a log density. 4.2 Plot sampling of woody vegetation Figure 2 shows the design of tree and stand structure measurements. Sampling method may be: 1) Angle count sampling (ACS) of woody vegetation (DBH ≥ 12 cm) in combination with four crosswise arranged fixed area subplots (4 m x 5 m) for survey of bushes, coppices and regeneration (h ≥ 1.3 cm). 2) Alternatively: Survey of woody vegetation (DBH ≥ 12 cm) on fixed area plots (20 x 20 m or 15 x 15 m) in combination with crosswise-arranged subplots for plants (h < 1.3 m). 3) Optionally: survey of woody regeneration ("single-stem trees", 0.1 m ≤ h < 1.3 m) on half of the area of the subplots. As general sampling procedure, we propose method 1 (ACS in combination with fixed area subplots).