Determination of Large Wood Accumulation in a Steep Forested Torrent Using Laser Scanning

In order to estimate large wood (LW) recruitment potential in torrential watersheds, and to describe the interactions of wood and torrential sediments in a cascading forested torrential channel, we performed a field survey of the lower reach of the Kuzlovec Torrent in the Gradascica watershed, using a terrestrial laser scanner (TLS). The point cloud consisted of several million points. The points were coloured with images made with digital camera to support visual interpretation of the captured objects. Data were used to create digital terrain model (DTM) (0.05 m grid cell) of the torrential channel area to be used for modelling of torrential flows. Furthermore, the point cloud was used for recognition of fallen trees and dead wood (574 LW pieces were recognised), and the results were compared with manual dead wood survey. Approximately 20 % of the LW was not recognised using TLS if compared to the manual survey. The amount of LW was estimated to be ~0.1 m3 per running meter of the torrential channel, and dead tree volume (~30 m3/ha) was compared to the average forest stand for the area (346 m3/ha)–LW found close to the torrential channel represents ~8 % of the wooden biomass. The time consumed for the LW volume calculation stressed the need for the further automation of the procedures.