Abstract:

The erosion-deposition processes of estuarine bars are the basis of estuarine geomorphology and channel evolution. Most studies revealed the annual or seasonal change in deposition-erosion of estuarine bars based on chart data and sedimentary dating. However, understanding of erosion-deposition changes during the flood-ebb tide and spring-neap tide cycles is insufficient. This study conducted high-resolution in-situ observations of hydrodynamics and bedform changes on the tidal flat front of northern Chongming Dongtan, using bottom-mounted tripods over 15 flood–ebb tide cycles and one spring–neap tide cycle. The study found increases and decreases in velocity in four erosion-deposition phases over one flood-ebb cycle: flood tide erosion, flood tide deposition, ebb tide erosion, and ebb tide deposition. In addition, the flood flow originating from offshore shoals caused a high suspended sediment peak (SSP), which settled rapidly and formed the major silting layer after flood slack. The ebb flow caused another SSP and formed the major scouring layer due to long-term scouring by high velocity flow during the rapid ebb flow stage. From neap tide to spring tide, the bed exhibited a scouring trend due to the generally low suspended sediment concentration (SSC) of flood flow from offshore shoals. However, from spring tide to neap tide, the bottom showed a depositional trend due to a high and increasing SSC of flood flow from shoals. The above findings provide a reference for further understanding of estuarine erosion-deposition and evolution processes.

Key words: mouth bar, Chongming Dongtan, flood-ebb tide cycle, spring-neap tide cycle, sediment dynamics