Intertidal flats are shallow-water environments that undergo cyclical variations in water depth, leading to a
frequent occurrence of extremely shallow water stages (ESWS; water depths<0.2 m). However, relatively little
is known about the hydrodynamic conditions and erosion–accretion processes during ESWS, because the water
depth is too shallow to measure in situ sediment dynamic processes using traditional methods. To address this
gap, based on in situ measurements with four advanced instruments, we quantified the hydrodynamic conditions,
erosion and accretion during ESWS in calm weather conditions on a highly turbid intertidal flat in Jiangsu coast,
China. Our results revealed that marked erosion and accretion occurred during ESWS at the flood and ebb tidal
stages, respectively. The resulting bed-level changes were three times greater during ESWS than relatively deep
water stages (RDWS; water depths>0.2 m), and the rate of change was an order of magnitude faster than during
RDWS. This larger and faster bed-level change occurred even though the ESWS duration only accounted for 10%
of the entire tidal cycle. This result occurred because the bed shear stress due to combined current–wave action
during ESWS, was, on average, two times higher than during RDWS at the flood stage causing more extensive
erosion. Whereas during the ebb stage, this shear stress during ESWS was only half of that during RDWS resulting
in greater accretion. The main implications of these results are that, because ESWS occur frequently (twice every
tide) and are associated with large bed shear stress and bed-level changes, these conditions are likely to play an
important role in morphological changes of intertidal flats. Our study shows that ESWS have a key influence on
intertidal flat hydrodynamics and sediment dynamics. Thus our results are the basis for an improved understanding
of the coastal morphodynamic processes on intertidal flats.