Landscape-scale flow patterns over a vegetated tidal marsh and an unvegetated tidal flat: Implications for the landform properties of the intertidal floodplain

• We investigate the landscape-scale effect of vegetation through field measurements.
• On vegetated tidal marshes flow was concentrated in the channels.
• On unvegetated tidal flats homogeneous sheet flow occurred.
• Differences in flow patterns precipitate differences in landforms.
• Levee-basin topographies observed in the marsh are absent on the unvegetated flat.
• Microtopography as an indirect effect of vegetation on marsh flooding and drainage.

Vegetation is increasingly recognized as an important control on flow and landform patterns in many landscape types. Field studies on the landscape-scale effect of vegetation in fluvial and tidal floodplains are relatively scarce while insights are especially based on flume and numerical models. Large-scale flow patterns and landforms were measured on a vegetated tidal marsh and unvegetated tidal flat, in particular dynamic changes in two-dimensional water surface slopes, flow velocities and directions on an extensive network of locations. It was found that during flooding and drainage of the vegetated tidal marsh, the flow was concentrated in and routed through bare tidal channels, whereas on the unvegetated tidal flat more homogeneous sheet flow occurred. On the marsh the peak flow velocities were lower on the vegetated platform (< 0.1 m/s) compared to their adjacent tidal channels (0.3− 1 m/s), while on the bare tidal flat the channel and platform flow velocities were not significantly different from each other (0.1 − 0.4 m/s). These differences in flow patterns have important implications for the landform differences between vegetated and bare tidal floodplains: (1) for tidal channels with channel widths smaller than 10 m, width-to-depth ratios (= β) were smaller for the tidal marsh channels (β ~ 3) than for the tidal flat channels (β ~ 9) (due to flow concentration towards the channels in the marsh); and (2) the vegetated marsh platform exhibits a clear levee-basin topography (due to vegetation-induced platform flooding from the channels), while there is no levee-basin topography on the bare flat (due to sheet flow). This study not only emphasizes previous findings on the direct effect of vegetation, acting as frictional constraints on flow in tidal marshes, it also suggests that the indirect effect of vegetation, namely its induced micro-topography, influences the flooding and drainage behavior of marsh systems potentially altering feedbacks caused by direct vegetation effects.