Keywords: 
• boundary layer with small flow depths

Abstract: An extremely shallow water environment is defined as an environment where the water depth is much smaller than the thickness of a fully developed benthic boundary layer. The intertidal flat and tidal creeks represent examples of such environments. Observations from the Jiangsu coast show that, towards the end of an ebb tide phase, the bed surface and tidal creeks are subjected to water flows with a low speed (0.1 m/s in magnitude) and a very small water depth (1 cm in magnitude). As a result, flattopped ripples and plane bed are formed on the lower parts of the intertidal zone, whilst smallscale secondary creeks develop at the bottom of major creeks over the upper part of the intertidal zone. In these cases, the flow structure within the boundary appears to be maintained, with the u100 value derived being applicable to the calculation of sediment transport rates. Tidal surges occurring over the middle part of the intertidal flat are another type of behavior for the small flow depth boundary layer, representing destruction to the Von Kárm ánPrandtl flow structure; the height of the tidal surge is equivalent to the critical water depth associated with the system collapse, which can be expressed quantitatively as Hb=4 z0. Thus, the processes of the small flow depth boundary layer play a unique role in influencing the sedimentological and morphological patterns of intertidal flats. 