Dams are civil structures indispensable to modern civilization, as they make available water for irrigation and electricity production, among others. In parallel, they can be a threat if not properly designed and operated. A particular risk that can potentially lead to dam failure is the blocking of the spillway inlet with driftwood or debris.
The present study investigated, on the base of physical modelling, this blocking and the related consequences. More precisely, considerable quantities of driftwood were supplied upstream of a gated ogee weir and the subsequent reservoir level rise was measured. A particular focus was laid on extreme events in terms of driftwood occurrence and discharges. It was found that a gated ogee being blocked with driftwood performs with a reduced discharge coefficient, as long as no countermeasures are taken. These include pier overhang, absence of piers and racks. The performances of these countermeasures were studied and criteria developed to limit the perturbing effect of driftwood.