Upstream Erosion and Sediment Passage at Piano Key Weirs

Abstract

Piano key weirs (PKWs) are a weir type characterized by an effective rating curve. Accordingly, this control structure is primarily applied at dams to increase the spillway capacity. In recent years, PKWs also have been implemented in rivers combined with low-head hydropower or to regulate waterways. For the latter application, a weir type without gates may be favorable, but the issue of the passage of sediments arises. Such sediments are either deposited in the backwater or transported to the weir during intense floods. An efficient sediment passage is necessary to avoid inundations upstream of the weir and to maintain a navigable waterway. Two options arise: (1) to flush the sediments (e.g., through a gate in the weir), or (2) to carry them over the weir crest. The second option is favorable, if upstream riverbed aggradation can be avoided, because no mechanical devices (i.e., gates) are used. This study analyzed the sediment passage over a PKW driven uniquely by the flow. Systematic physical model tests were conducted to study the upstream riverbed behavior as well as the passage of sediments over the PKW. Three PKW configurations, two sediment granulometries, and six discharges were considered. Key results refer to the modified rating curve under high riverbed levels and to the upstream scour process of sediment deposits. Finally, the sediment passage capacity was linked to the equilibrium sediment transport conditions upstream of a PKW. Pragmatically formulated, this relation indicates—at least for the tested configurations—that sediments arriving at the PKW also pass over it.