How-to: forest roughness length?

For wind engineers, it is a well-known problem; but a new report by Sogachev et al. has a unique take on forest roughness. The report describes a way to parametrise forests and utilise it for roughness-length and canopy-drag flow models.

Forest parametrisation

Only three parameters are needed to describe the vertical structure of the forest: tree height, plant area index, and height of maximal plant density. The parameters can be derived from LiDAR scans or, as the report shows, estimated from traditional forestry relationships.

Forest roughness length

The forest parametrisation is directly applicable in CFD models that apply full descriptions of the canopy drag. However, the report gives a novel approach to translating the parametrisation to roughness length using a 1D CFD solver, for use in any micro-scale flow model.

The approach is automatic and objective and demonstrates the importance of including the vertical foliage distribution when estimating the forest roughness length. The picture shows the horizontal distribution of the roughness length for the ?sterild site (Sogachev et al. 2017). Download the report here.

Reference

Sogachev, Andrey, Dalibor Cavar, Mark C. Kelly, and Andreas Bechmann. 2017. Effective Roughness and Displacement Height over Forested Areas, via Reduced-Dimension CFD. DTU Wind Energy E 0161. https://orbit.dtu.dk/files/141969986/Effective_roughness_over_forested_areas_for_CFD.pdf.