An investigation of bed armoring process and its implications for channel bed stability

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

The influence and interaction of sediment grain size distributions and channel flow hydraulics over the formation of armor layers in gravel bed rivers has yet to be fully defined. Imbrications and microclusters develop as part of the armor layer and contribute to the overall increased stability of armored beds. Recent research on microclusters has focused on how the turbulent flow field is affected by their presence, which has not been fully elucidated. This research will connect the processes of bed armoring, microcluster formation, and bed stability and provide the quantification necessary to manage flow releases so that the downstream channel surface does not erode and potentially harm the downstream ecosystem. Flume experiments will create armored beds testing 4 sediment mixtures against 4 flow rates. Microcluster density and bed topography will be measured from laser profile scans of the bed surface. Acoustic Doppler Velocimeters (ADVs) will record point velocities (u,v,w) and fluctuations (u?,v?,w?) over the armored surface and around microclusters, from which turbulence descriptors, such as Turbulent Kinetic Energy (TKE), are calculated. Flow data will be analyzed using the Double Averaged Navier-Stokes (DANS) approach and quadrant analysis, which together with the detailed bed topography will provide insight into the momentum transfers occurring and the coherent flow structures present in the flow field at specific locations over the armored bed. Using ADV measurements, it will be possible to differentiate the boundary stresses as due to microcluster, topographic surface, viscous stress, or stress from the interaction of multiple clusters. At the conclusion of the proposed research, the effect of bed substrate grain size distributions and flow rate during armor layer formation will be quantified and linked to the creation of structure in the armor layer topography and the stability of the channel bed. An understanding of the controlling variables over armor layer formation and bed stability will be gained. This research will provide a means of predicting the flow rate that will break the downstream armored bed layer as a function of the substrate in the channel and the existing bed microtopography, both of which can be measured in the field.