Spatial-temporal variations of bare flats in the Qinjiang River estuary, Maowei Sea

Estuarine tidal flats generally consist of vegetated wetlands in upper-middle intertidal zones and fringed bare flats in middle-lower intertidal zones. Spatial-temporal variations of bare flats directly regulate geomorphic erosions-accretions and vegetation successions of the whole tidal flats. Based on multi-satellite remote sensing imageries from Landsat 5/8 TM/OLI and Sentinel-2 MSI collections, random forest classification algorithm and Digital Shoreline Analysis System were used to examine the variations and characteristics of bare intertidal flats in the Qinjiang River estuary (QRE), Maowei Sea (MWS) of Beibu Gulf in 1986—2021. Results indicate that, (1) the overall area of bare flats in the QRE has shown obvious losses during the past four decades, which can be divided into four stages: maintained equilibrium of erosion-accumulation between 1986—2002, accumulated rapidly between 2002—2007, declined sharply between 2007—2014 and slowly deposited between 2014—2021. (2) Tidal flat accretions distributed mainly in the secondary Shajing outlet (SJO) and its inner channel of the QRE along the edges of tidal channels, while erosions occurred in the secondary SJO, Shachong outlet (SCO) and inner tidal channels. (3) Dredging projects in the MWS were the direct driven force that triggered the losses of bare intertidal flats in the QRE from 2008 to 2014, and the expansions of intertidal vegetation and weakened hydrodynamics followed by extensive oyster cultivations after 2015 caused seaward progradation of the bare flats. In addition, the modern sea-level rise has not significantly affected the dynamics of bare flats, and the losses were partly due to the reduced riverine suspended sediment load into the MWS. This study provided theoretical and technical supports for the sustainable and efficient utilizations of estuarine tidal flats.