Sediment trapping of turbidity maxima in the Changjiang Estuary

Abstract

Sediment trapping in the turbidity maxima (TM) is highly variable in different estuaries or at different times in the same estuary. How related processes influence the TM behavior and its structure has not yet been completely clear. Here we present moored and shipboard observations in the 2008 flood and dry seasons in the lower reaches of the Changjiang Estuary, where the Deepwater Navigation Channel was still under construction. The excavation of the pristine mouth bar has caused the saline wedge to move upstream, resulting in new TM zones. The magnitude and extent of the TM are strongly seasonal under the influence of distinct sediment processes. The TM appears as a high concentration undercurrent in the flood season, whereas they occur as a low concentration sediment cloud over the water column in the dry season. The hyperpycnal flow can be formed by flocculation settling, and the sediment cloud can be caused primarily by resuspension. The sediment trapping in the TM occurs more often during accelerating and decelerating ebb tides, when the bed stress is small but the flow is highly accelerating and decelerating. Sediment resuspension and deposition at these tidal reversals can be induced by the growing and decaying flow spirals, respectively. Therefore, the diversity of the TM may be associated with density stratification, accelerating flows, flocculation, or generally the interaction between stratified flows and cohesive sediments.