Accretion increases with period of inundation and tidal flats are inundated for longer than saltmarshes. However, vegetation increases sedimentation capture.
See Burningham, H. (2008). Contrasting geomorphic response to structural control: The Loughros estuaries, northwest Ireland. Geomorphology, 97(3), 300-320. This is a classic are that displays both tidal flats and salt marsh along the Bellanagoal River. Also references therein.