Flow, turbulence and morphodynamics of wood structures in rivers: challenges due to shape, porosity, position

Wood structures used in river restoration, such as installed log-jams, root boles or entire trees, are challenging for hydraulic engineers. Their impact on flow, turbulence and morphology is difficult to predict quantitatively and even qualitatively for some configurations. Wood structures inherit distinct shape, porosity, position and orientation relative to the flow and analogies to ‘standard’ bluff bodies from literature are not easily transferable. A generalising hydraulic classification scheme and studies towards a standardisation of flow and turbulence properties are thus still lacking. Even more so, if morphodynamics and the associated flow adaptations are of concern.

As a starting point, flow and morphologic changes due to installed tree structures were investigated at the TU Wien hydraulics lab for clear-water conditions in a 2.5 m wide flume. Flow depth was H = 0.25 m at a Froude number of Fr = 0.25 and a flow-Reynolds number of Re = 7.7 x 10⁴. The structures were composed of a circular root-plate (D = 0.4 m diameter) with a cylindrical stem attached (0.12 m diameter, 3.0 m long). Root porosity was realised by cutting out sectors of the root-plate and additionally by attaching a porous filter mat. The orientation of the tree was kept streamlined with the flow and two vertical positions of the structure were tested, with the stem afloat near the surface (positively buoyant) and with the stem deposited on the bed (negatively buoyant).

The floating installation induced fast-response tunnel-scour underneath and slow-response shear layer scour at both sides downstream of the root-plate. The drowned installation induced initial horseshoe vortex scour until the lower edge of the root-plate was reached and tunnel-scour reshaped the scour hole at its final stage. The vertical and lateral extent of the scour hole controlled flow divergence underneath the root and into the near-wake, causing pronounced upwelling downstream. Upwelling fluid further diverted the shear layers laterally outward and increased the wake width. With increasing root-plate porosity, scour depth, upwelling strength and lateral shear-layer divergence decreased. Root-plate porosity reduces the global velocity gradient between wake and ambient flow as well as the strength of downward directed flow into and upward directed flow out of the scour hole into the wake.