The Effect of Large Wood Accumulations With Rootwads on Local Geomorphic Changes

Large wood (LW) can be transported along a river during floods, increasing flood-associated hazards, particularly when it accumulates at river-spanning infrastructures such as bridges and weirs. While most flume studies have explored LW movement with simple wooden elements (dowels), only a few studies have used elements with more complex LW geometries, such as rootwads, under unsteady flow conditions. Quantitative assessment of interactions amongst more complex wood elements and river flow has rarely been attempted the effect of this additional complication has even been ignored, in both field and laboratory studies. In this study, flume experiments were conducted to assess the effect of rootwads on local scour and deposition in a flume with a mobile gravel-bed. The experiment was conducted under unsteady flow conditions, with a constricted segment of the reach, recreating conditions to wood accumulations and blockage. Results revealed that LW with rootwads tends to generate more stable accumulations than LW without rootwads, leading to the formation of more porous loosely packed accumulations. In this initial set of flume experiments, the patterns of scour were quite variable, but on average, the porous and stable LW accumulations with rootwads showed more spatially extensive disturbance of the bed. LW accumulations without rootwads led to the development of scour pits that reached the bottom of the flume more quickly than in the LW accumulations without rootwads. The mean accumulated bedload volumes were of similar magnitude overall, however, highlighting the many contingencies in the chain of processes between dam formation and resultant bed scour.