Liu, H. and Wu, J., 2017. Determining topographically controlled flows through a combined contraction and hollow in the Pearl River estuary, China.

The modern Pearl River delta, with a two-fifths area of rocky islands or hills, is estuarine accumulation dominated, where most river-borne sediments are still trapped within the estuary. However, the role of topographic flows in controlling riverine sediment dispersal and delta development is not straightforward. In this paper, the topographically controlled flows through a combined contraction and hollow were investigated to understand their roles in modern delta development. Moving shipboard surveys, bottom-mounted tripods, and water-level gauging along the Yamen Channel were undertaken in the 2012 wet season to measure mean flows, bottom boundary layer flows, tidal elevation, and water depth. During the peak tidal flows, an accelerating high-speed zone occurred on the stoss (upstream) side of the hollow. Under the unstratified conditions, the momentum balance in the hollow is roughly achieved among barotropic pressure gradient, horizontal advection, and local acceleration. The form drag induced by the hollow was estimated to be one order of magnitude larger than the skin friction generated by bed roughness. The form drag can speed the upstream current in the hollow, forming the high-speed zone on the stoss slope of the hollow. Theoretical investigations reveal that a self-adjustment mechanism of the combined hollow and contraction can calm the channel flow speed, so that more flow energy can be dissipated and more river-derived sediments can be trapped within the estuary. This recognition implies that the Pearl River delta is estuarine accumulation dominated.