Analysis of Wood Density to Improve Understanding of Wood Buoyancy in Rivers
Résumé
Drift wood (or in-stream large wood, LW) plays an important role in river ecosystems by influencing hydrodynamics and morphology. The final goal of this work is to improve our understanding of wood buoyancy in rivers through the assessment of wood density. We analyse wood pieces retained in the Genissiat dam, French Rhone, and a set of freshly cut riparian trees from the Ain River. Different protocols were set to measure density and buoyancy of these two series of wood samples and to test the effects of drying and wetting, species and wood decay stages. Living and dead trees show average wood density ranging from 590 to 1,080 kg m-3 and from 350 to 910 kg m-3 respectively. Differences in water content clearly affect buoyancy, which ranges from 0.36 (36 % emerged; Abies) to 0.18 (Acer and Fraxinus) as initial values, and increases up to 0.48 (Abies) for dry samples and decreases up to 0 (100 % submerged; Acer) for wet wood. We observed a significant negative linear correlation between wood density and buoyancy. The results from this work will help to understand the evolution of buoyancy through time and estimate local conditions of entrainment and transport.