Effect of driftwood on hydraulic head of Piano Key weirs

Driftwood is often transported by a river course during flood events and can be problematic where it accumulates at hydraulic structures. Near obstacles, driftwood can block and thus reduces the open flow area for water passage, resulting in a lowered discharge capacity combined with increasing flow depths in the upstream region. The blocking of driftwood at weirs and spillway inlets is of particular interest, as it may affect the safety of such structures by drastically reducing the maximum spilling capacity. The high discharge efficiency of Piano Key weirs (PKWs), relative to linear weirs, may influence driftwood collection. The flow depth over a PKW for a given discharge will be less than would be required by linear structures, thus providing less flow momentum per unit weir length for flushing driftwood. If the catchment is forested, the potential effects of driftwood on the PKW head-discharge relation should be considered as part of the hydraulic design process. This paper describes systematic model tests considering the effect of driftwood on the PKW discharge efficiency. Trunks and rootstocks were supplied upstream of different laboratory-scaled PKW configurations. The test series closely approximated real-world conditions by investigating the nature of driftwood accumulations on the PKW discharge efficiency.