Dams are civil structures essential to modern civilization. However, they can be a threat if not properly designed and operated. A particular risk that potentially can lead to dam failure is the blocking of the spillway inlet with driftwood or debris. This study investigated, on the basis of physical modeling, this blocking as well as the related backwater rise and discharge-capacity reduction. Considerable quantities of driftwood were supplied upstream of an ogee weir with piers, and the subsequent reservoir level rise was measured. Particular focus was placed on extreme events in terms of driftwood occurrence (volume) and discharges (design value). It was found that a gated ogee blocked with driftwood performs with a reduced discharge coefficient as long as no countermeasures are taken, such as pier overhang, the removal of piers, or the installation of a rack. The performance of these countermeasures was studied, and criteria were developed to control the perturbing effect of driftwood.

Driftwood belongs to natural rivers just like water and sediment do. A sound ecosystem requires driftwood, although it might jam at civil structures, altering the flow section and rise the backwater. Safety considerations suggest removing wood from rivers, whereas ecological experience asks for its presence. The situation might become critical if spillways clog during floods, so that their discharge capacity reduces. For narrow bays, full racks mounted upstream of the weir or overhanging piers trap the driftwood distant from the flow control section. The hydraulic capacity is then maintained, but the driftwood has to be removed after the event. We thus investigated herein with a physical model a novel partial rack, motivating the driftwood for uncongested appearance to partially pivot and pass, but ensuring a high discharge capacity under hypercongested appearance. The partial rack configuration was specified, together with the related trapping rate and discharge efficiency.

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.