Forest parameterisation

For smaller and medium block sizes, a dense forest means a strong obstacle, as energy is constantly destroyed by repeated tree contacts and ideally the falling block is stopped.

The interaction between the falling block and the tree trunk is calculated in ROFMOD as an elastoplastic impact between two mass points.

The chapter Overview ROFMOD states that ROFMOD is a deterministic model. However, this is only correct if modelling is carried out without forest. When modelling with forest, ROFMOD calculates with some probabilistic submodels. This is necessary in order to be able to map the wide scatter of ranges, heights and fall energies observed in the field due to the direct forest effect.

The direct forest effect can alternatively be modelled in ROFMOD 5.0 with two approaches:

A.Mode "Normal":
Definition of forest density (number of trunks/ha) and distribution of trunk diameters (with two distribution parameters a, b).
For each trunk contact, a trunk diameter is randomly selected from the distribution, the specific hit distance x (parallel to the terrain profile) is calculated and then the hit is modeled.

Graphic trunk diameter distribution
For the forest modelling with mode (A) two parameters a, b have to be defined, which define the trunk diameter distribution. ROFMOD 5.0 offers the user a graphic window, which shows the resulting distribution for the defined values a, b. The maximum tree diameter is also defined. The maximum tree height is also displayed.

B.Mode "Distance":
Definition of a fixed trunk diameter, the hit distance x as well as the mean terrain slope in the relevant forest section.
After the distance x (parallel to the terrain profile) a hit is modelled.

ROFMOD considers the following factors:

•The trunk diameters involved in the reactions are randomly generated according to defined trunk diameter distributions (forest stand types) (mode A only).

•The repetition rate of the contact reactions (hit distance x) depends on the selected trunk number/area, the mean trunk diameter of the respective stand type and the mean diameter of the falling block (calculated automatically for mode A, defined by the user for mode B).

•Equally distributed, random variation of the centrality of the impact (striation to direct hit) causes different deflections and energy losses (both modes).

•The wood mass involved in the reaction is calculated according to the impact height, impact velocity and trunk diameter. This allows all effects to be simulated, from the elastic avoidance of small trunks to the breakage of large trees (both modes).

•Limitation tree height: If the fall trajectories are higher than 60 * trunk diameter over the profile, no hits are modelled. As soon as the flying height of the fall block is less than or equal to the tree height, a hit occurs immediately. This limitation of the tree height with this "degree of slenderness" is currently fixed (for both modes).

Important notes

Due to the random factors implicit in forest modelling, the forest effect can generally only be recorded with a large number of modelled falls (at least 100).

In the field it can often be observed that individual blocks can pass through short forest sections without (or with very few) tree hits. It is therefore advisable for the modelling of profiles with relatively short (< 150 to 300m) forest sections to also carry out a scenario for modelling without forest.

Modeling with mode (A) or mode (B) - to the hit distance

ROFMOD calculates the hit distance x as follows:

F = forest area (1 ha), n = number of trunks /ha,
sdm = trunk diameter, bd = block diameter

The above formula shows the reciprocal relationship between n (trunk number/ha) and x (hit distance).

It often seems easier to estimate the hit distance x, i.e. the potential distance between two tree hits, and thus model the forest according to mode (B). This has the advantage that the time-consuming and sometimes difficult determination of the number of trunks per ha is no longer necessary.

In addition, many managed forests have a very low variability with regard to the trunk diameter and can be well described with a fixed, mean trunk diameter.

When selecting mode (B), however, it is imperative to note that an estimated hit distance in the field always refers to the expected size of the fall block: A small block/stone in an identical forest section has a greater hit distance than a massive block.

Selectable random variation for the hit distance x:

ROFMOD 5.0 calculates a separate hit distance for each block when it enters the forest, based on the calculated hit distance x according to the above formula.

The hit distance is defined as a normally distributed random number with the following parameters:

Mean value = hit distance, standard deviation = k * hit distance.

The scaling of the scatter (k) was set by default to k=0.1 (=10%), but can be changed in the simulation under "Simulation/Advanced Simulation Parameters"/"Random Part of Hit Distance". If k=0 is set, the first tree hit always occurs after the same hit distance, without any random part.