Abstract
Piano key (PK) weirs are used in a variety of flow control structure applications, including spillway crests and open channel diversion structures. However, to the best of authors’ knowledge, structure-specific design guidance for scour mitigation is still needed. To fill this gap of knowledge, a systematic experimental campaign was conducted by testing different configurations of horizontal aprons with a cutoff wall. Protection structures were located at the toe of the PK weir. Namely, experiments were performed at large-scale to assess the effect of three apron lengths on downstream scour hole geometry under different hydraulic conditions. It was observed that a horizontal apron deflects the plunging jets originating from the PK weir, thus significantly reducing scour. Experimental evidence allowed corroboration that significant scour depth reduction occurs for an apron length 1.5 times the weir height, with longer aprons found to provide marginal benefits. Finally, also provided herein are tools to estimate the main scour characteristics and help practitioners in optimizing apron design.
Highlights
When considering current design standards, sustainability principles, and observed trends in flooding, the control of local scour represents a challenging task
A dissipative process may occur on the apron due to the formation of a submerged hydraulic jump
Limited published guidance is available for designing a minimal apron and cutoff wall lengths to protect or mitigate local scour at the toe of a piano key (PK) weir
Summary
When considering current design standards, sustainability principles, and observed trends in flooding, the control of local scour represents a challenging task. Often, localized scour phenomena are caused by falling nappe or oblique plunging jets originating from weirs. Depending on jet characteristics (e.g., submergence, geometry, and inclination), scour morphology at equilibrium could be either two- or three-dimensional (considering the vertical and streamwise directions with the third dimension being transverse) [4,5,6,7,8,9,10]. E.g., [11,12,13,14,15,16,17] can represent a valid help for practitioners with upstream and downstream erosion at PK weirs discussed in [18], none of them provide guidance for PK weirs’ scour-induced processes and selection of a horizontal apron and cuttoff wall for scour protection
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