Effect of blocked driftwood on the hydraulic performance of a gated standard weir

Abstract

Driftwood belongs to riverine ecosystems and is stored and transported in every natural stream. Hydraulic structures built in streams typically alter the flow characteristics and might consequently get in conflict with driftwood. The latter can get trapped at limited cross-sections and then hinders the water to pass. The upstream water level subsequently raises, since the flow needs more energy to pass the obstacle. This might lead to inundations of upstream zones or to overtopping of dams. Both are not acceptable. The herein presented study addresses this issue, focusing on a standard weir with piers (to hold gates or flaps). Such a configuration is frequently used to regulate the flow on dam spillways. We have conducted systematic model tests supplying large driftwood volumes, varying the discharge and the bay width. The reduced discharge coefficient under driftwood impact was derived, allowing to determine the related reservoir level rise. Furthermore, three technical installations, denoted as countermeasures, were tested to avoid the observed discharge capacity limitation of a jammed weir. These measures included (i) overhanging piers (protruding into the reservoir), (ii) driftwood racks installed upstream of the weir, as well as (iii) the removal of the piers generating "wide" bays. The tests indicated that, under the herein tested conditions, all measures were highly efficient. The discharge coefficient remained typically at almost the free weir flow capacity (>90%), even under a high driftwood occurrence.