coastal engineering

Direct Measurement of Sediment Response to Waves with Smart Sediment Grains

A comprehensive characterization of small-scale fluid-sediment interactions will improve understanding of large-scale ocean engineering phenomena, resulting in more accurate wave forecasting and improved ocean circulation models. The critical shear stress is typically used to determine the initiation of sediment motion in coastal applications. However, this shear stress criterion was primarily developed for steady flows and has been inconclusive in some wave environments where sediment motion may be induced by horizontal pressure gradients. Evidence suggests that the incipient motion formulation should account for the combined effects of the horizontal pressure gradients and bed shear stresses. Other researchers have made one-dimensional and two-dimensional measurements of near-bed velocities. However, previously available technologies could not resolve the three-dimensional velocities at the bed or directly measure sediment motion. Accurate investigation of the hydrodynamic forces that initiate coastal sediment transport requires high resolution measurements at the fluid-sediment interface. We have made previously unavailable high spatial and temporal resolution laboratory measurements at the sediment bed with state-of-the-art instruments. Additionally, we have performed one of the first direct measurements of sediment motion in response to waves with newly-developed electronic grains. The MEMs sensors are 2.5 x 1.5 x 1.4 cm and measure three-dimensional accelerations, store the data onboard, and transmit them wirelessly after retrieval. The Smart Sediment Grains (SSGs) were developed by embedding MEMs sensors in gravel-sized Delrin plastic spheres. These spheres allow uninhibited movement in any direction, similar to a smooth sand grain. The SSGs are the first freely moving electronic grains that measure sediment dynamics which previous technologies could not, giving insight into the underlying wave forces driving sediment transport. The SSGs enhance our ability to measure the motion, transport, and settling of sediments in the nearshore by capturing translation and rotation of the sediment. This will improve our predictive capabilities of sediment transport phenomena such as beach erosion and seabed evolution in response to wave forces; as well as improve parameterizations of the bottom friction for ocean circulation and wave energy dissipation models. The SSGs have been successfully deployed in small and field-scale wave flumes to measure the response of coarse gravel sediments to wave forcing. High resolution profiling Acoustic Doppler Velocimeters and a Particle Image Velocimetry system, comprising a laser and four high speed cameras, measured the three-dimensional fluid velocities at the bed. These measurements provide resolution high enough to fully examine the small-scale fluid forces exerted on each individual sediment grain. The SSGs accurately captured the sediment response to the waves at the onset of sediment transport. Additionally, broader incipient motion experiments were conducted with a variety of sediment grain diameters and densities for comparison. The results suggest evidence of pressure gradient influenced incipient motion; in contrast with the more commonly used threshold for sediment motion based on the bed shear stress. Calculated values of the Sleath parameter, used to quantify the effects of the pressure gradients, were comparable with field observations of pressure gradient induced sediment transport. The data also suggest that vortex shedding could be a factor in triggering sediment transport. We have directly measured incipient motion in waves by resolving the near-bed fluid velocities and collecting direct measurements of sediment motion with state-of-the-art instruments. The data are being used to validate theoretical and numerical models of the wave bottom boundary layer and bottom friction estimates. These results will be synthesized to propose a comprehensive incipient motion criterion comprising the effects of the shear stress and the pressure gradients, also taking into account a variety of flow and sediment characteristics. The current configuration of the SSGs helps to identify the characteristics of incipient motion and determine orientation. These mobile nodes make a significant step towards resolving the Lagrangian dynamics of individual coarse gravel-sized particles within the mobile bed layer in the nearshore. On a larger scale, they will reduce the effects of beach erosion by improving beach nourishment design. With technological advancements, these SSGs can be minimized and made field-deployable with enclosures configured to other applications to provide transformative measurements in geotechnical engineering, hydrology, oceanography and human health monitoring.
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