Falling aprons at circular piers under currents

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Traditional guidelines on rock protection at circular piers predominantly focus on preventing shear failure (by choosing a sufficiently large rock size), winnowing failure (by designing an appropriate filter) and edge failure (by selecting a sufficient extent). In particular areas (e.g. in an eroding river channel or in an area with large-scale bed forms), the rock protection may also face a degradation (lowering) of the bed. As a result, the edges will be undermined and stones will roll down and cover the newly developed slope (a falling apron). This process is not well understood and theoretically-based design guidelines for falling aprons are not available, only empirical relationships for bank revetments. Indicative laboratory experiments were conducted in order to derive guidelines to account for bed degradation in the design of rock protection at circular piers under currents. This paper summarizes the experimental set-up, monitoring techniques, test program and results of the conducted experiments. All tested rock protection layouts consisted of a single stone grading with a sufficient stone size and layer thickness to prevent shear failure and winnowing failure. In total, 7 live-bed tests were conducted with varying current velocities, bed degradation levels and rock layouts. During the tests, the bed protection level near the pier was monitored with video cameras. Before and after each test, the bathymetry was recorded by stereo-photography. The analysis focussed on the successive failure stages, the redistribution of stones over the slope and the evolution in time. The results showed that, as the undermining progressed, the stones at the edges of the protection were redistributed through a combination of rolling, sliding and sinking. Finally, a protective mound was formed, with stones covering the slopes on all sides. The observed stone layer thickness on the slopes gradually reduced towards the outside. A design rule was derived to account for bed degradation by quantifying the stone volume required for a falling apron.