Flume experiments of log incipient motion in rivers

In natural rivers the transport phenomena of floating debris involve a large number of problems that are relevant to both environmental and technical aspects. A large part of the research developed has been motivated by linkages between fish habitat and geomorphological processes and forms influenced by large woody debris, but drifts reduce also the capacity of bridge openings, contribute to scour around piers and abutements, and increase lateral forces on bridges. Large woody debris (LWD) is defined as a log having at least 10 cm mid-point diameter, being 2 m in length while the term CWD (coarse woody debris) usually refers to smaller pieces. Key pieces are individual logs with rootwads that are less likely to move than other wood pieces during a bankfull flow. They play a relevant role in snags amassing because key pieces constitute the first step of the woody accumulation process. To identify hydraulic thresholds for movement and transport of key pieces for different log geometries we developed a series of experiments in a flume model 0.30 m wide and 5.0 m long with about 10% slope transversal to the main flow direction. The log, with and without rootwads, is assumed fallen on the sloping bank and the hydraulic threshold is analyzed as a function of log geometry and hydrodynamic action. The key pieces are simulated by wood cylinder ended with an octagon (to take into account the rootwad) of different lengths and thicknesses. Besides the acquisition of water level and discharge as usual, the video recording of the experiments permits the identification of the log position at the threshold of motion into the flume. A conceptual model, based on the 3-D stationary equilibrium of gravity, buoyancy, friction and hydrodynamic forces acting on a rigid body, was developed to allow the interpretation of scale flume model experiments. From the preliminary analysis of laboratory experiments it seems that the condition of incipient motion is mainly controlled by the ratio between the rootwad size and the log length.