Evaluation of wood degradation for timber check dams using time domain reflectometry water content measurements

Wood is currently widely used in low environmental impact transverse structures for stream management and restoration. During the service life of these structures, chemical, physical, and biological processes can naturally degrade the wood fibres with a consequent increase of porosity. This typically causes wood moisture to increase with decay; however, conventional wood water content measurement instruments are calibrated for wood that has not been degraded. In this study, we analysed larch wood (Larix decidua Mill.) logs at different degradation levels by surveying their water content with the time domain reflectometry (TDR) technique. TDR has been extensively used to measure water content in soil; however, it has rarely been applied to wood materials. A laboratory analysis was first performed to calibrate the TDR measurements with those obtained with the gravimetric method. Three field campaigns were conducted in three different check dams built between 1950 and 2003 and located in the NW Italian Alps. The proposed TDR calibration curve allowed for the determination of the volumetric wood water content with satisfactory accuracy. This curve showed two distinct regression slopes: the first slope was related to volumetric water contents between 0.0 and 0.16 m3 m−3, whilst the second slope was related to water contents above 0.16 m3 m−3. The TDR in situ measurements showed water content values ranging from 0.08 to 0.54 m3 m−3 for the old timber structures and from 0.11 to 0.45 m3 m−3 for the newer structures. This agrees with the conservation state of the timber, which was influenced by the position of the logs in the structure during the life of the check dams. Finally, we found an operative way to relate the wood water content to the wood degradation level.