Experimental study on entrainment of large wood in lowland rivers

Wood transport in rivers, typically occurring during flood events, represents a relevant hazard for its potential to create obstruction at bridges and narrow cross-sections. Therefore, the understanding and prediction of entrainment and transport dynamics of woody material of different shapes, density and dimensions is of great interest for river managers. The paper presents results from laboratory experiments carried out to assess the entrainment conditions of large wood in lowland rivers, i.e. with negligible longitudinal slopes, relatively smooth bed and low flow Froude numbers. The tests were performed in a straight flume with fixed bed and smooth side walls for several flow conditions. Entrainment was studied for circular and square logs having different initial orientation. Integrating the results with other data available in the literature allowed to derive a semi-empirical entrainment threshold based on a force balance. This threshold is based on simple parameters, such as normal flow characteristics, log size and density as well as median diameter of bed sediments.