Particle fluxes into permeable sediments: Comparison of mechanisms mediating deposition

Fine particle deposition in coarser sediment beds plays an important role in biogeochemical cycling in sediments by (1) supplying organic matter, attached microorganisms, and nutrients and (2) mediating interfacial fluxes through changes in permeability. In some cases, particles could be associated with contaminants or particles could clog pores in the sediment resulting in ecological impacts through changes in porewater chemistry (e.g., dissolved oxygen profiles). We compare two mechanisms for fine particle deposition to permeable sediments. First, for flat beds, particle delivery by sediment grain-scale flows coupled with particle retention within the sediment could result in deposition in excess of the gravitational settling. Second, pressure-driven flows across ripples may deliver additional particles directly into the sediment, although sediment transport can counteract deposition. Flume experiments were used to measure deposition rates over a range of particle and sediment sizes for both flat and rippled beds. Flat bed deposition rates were greater than the particle settling velocities and comparable to the deposition rates to rippled beds under similar flow conditions. An empirical expression based on parameters for flow, sediment, and particle conditions predicts where each mechanism dominates the deposition rate. This result illustrates the potential importance of grain-scale processes for the deposition of fine particles to permeable sediments regardless of bed roughness. Predictions of particle fate, penetration into the sediment, and degradation following the deposition still require a description of bed topography in addition to sediment properties.