Processes controlling the deposition of fine particles at sediment water interface.

The prediction of particle deposition rate is critical to gauging the amount of organic matter processing in permeable sediment beds. Recent studies have provided insight into the mechanisms controlling fluid flow and particle filtration at the sediment water interface. This study focused on revealing the processes mediating deposition for flat and rippled sediment beds. Flume observations of deposition were made over a wide range of flow, particle, and bed conditions. The coupling of diffusive transport across the interface with filtration of particles in the sediment bed results in significant enhancement of deposition, relative to gravitational settling alone. Diffusion of fluid and associated solutes or particles could play an essential role in the processing of organic matter in permeable sediments. Fluid flow predictions by the model, such as slip at the sediment water interface, correlation of flux and flow, and penetration into the sediment, appear to be supported by flume experiments. For most treatments, the same fluid flow and particle transport processes controlled deposition to both flat and rippled beds. These processes occur at the grain scale and mediate total deposition over the larger scales in the flume and, presumably, in natural settings. Sediment motion limits deposition due to disruption of interfacial flows and filtration at the interface. Assessing the effects of ripples and bedload improves the predictive capability of deposition models based on laboratory data. These results provide further evidence of the importance of permeable sediment environments as organic matter reservoirs.