Assessing large wood and wood-jams volume by TLS surveys

Wood in rivers can affect the hydrological regime, sediment transport and the ecological status of the water course. On the other hand, it may also increase the risk of flooding and clogging the runoff sections. The importance to know the amount of wood in rivers, has encouraged the employment of new technologies in order to obtain information on wood material faster and more precisely than through the traditional method based on manually field surveys. An alternative method to define the volume of large wood (LW) and wood jams (WJ) is proposed in this research, using the Terrestrial Laser Scanner (TLS) and the derived high-resolution Digital Elevation Model (DEM). The study was carried out along an area of about 7 ha of the gravel-bed Piave river (North-East of Italy). Wood volumes extracted from the DEM is compared to those obtained from the field activities. Results shows the potentiality of this device to provide accurate values. The volume of WJ, derived from the DEM, is slightly overestimated than the result obtained from the traditional methodology (+5.6%, corresponding to +15.7 m3 ha-1). The calculation of LW volume appears more uncertain, showing a consistent underestimation (-80.7% corresponding to -67.41 m3 ha-1). This is probably due by the difficulty to detect in detail the smaller logs by the TLS device. However, the integration of DEM with other data provided by specific techniques, able to analyse more in detail the LW dimensions, might allow the improvement of such methodology.