Floc fraction in the waters of the Po River prodelta

Abstract

Three independent methods of estimating the proportion of suspended material packaged within flocs, termed the floc fraction (f), were employed using hydrographic and suspended sediment data, and core and tripod-based observation data collected from the waters and sediments of the Po River prodelta throughout 2001. Using a floc size versus settling velocity relationship established in this study, floc fraction estimates were derived as follows: (1) by calculation of floc concentration in in situ images versus total suspended concentration using knowledge of floc effective density via Stokes' approximation; (2) by parameterization of disaggregated inorganic grain size (DIGS) distributions of bottom sediments to infer floc fraction in suspension necessary to produce the observed flux to the seabed; and (3) by calculating a mean representative floc fraction estimate for the system derived from estimates of floc bulk density. Calculated estimates of floc fraction are near unity at the river mouth, with the bulk of floc deposition occurring by the 8m-isobath. Seaward and to the south of the river mouth, the suspension is not highly flocculated (f∼0.08). Subsequent transport and removal of deposited sediment from the prodelta, as suggested in previous study, is believed to occur within the bottom boundary layer.