Settling Velocity Observations in a Shallow Estuary: Deviations from Rouse Dynamics

Abstract:

The accuracy and reliability of cohesive sediment transport models rely on prudent parameterization the floc settling velocities, which can be estimated from field measurements using a number of different techniques. High-frequency acoustic Doppler velocimeter measurements provide one estimate by assuming a balance between vertical turbulent mixing and settling (i.e., a Rouse profile). Floc settling velocity is frequently related to floc diameter and density through Stokes’ Law, so LISST particle size measurements can also be used to infer settling velocity given an independent estimate of floc density. Finally, the settling velocity can be measured directly using in situ video-based systems such as the INSSEV-LF, or floc cam. We applied each of these approaches to estimate floc settling velocities over the course of three field deployments on the shallow shoals of South San Francisco Bay, with results showing stark disagreement among the different methods. Using a combination of traditional time series analysis and machine learning techniques, we show that variations in settling velocity estimates can be attributed to the influence of waves, horizontal advection, and water column biology. These findings help to elucidate floc settling dynamics in shallow, wave-driven flows, and can help improve settling velocity parameterizations in estuarine sediment transport models.