Vegetation causes channel erosion in a tidal landscape

Abstract

Vegetation is traditionally regarded to reduce the erosion of channels in both fl uvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, fl at substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the fl ow, leading to fl ow concentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our fi ndings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, fl ow, and landform have an important control on landscape evolution.

Does vegetation prevent wave erosion of salt marsh edges?

“…A priori, it is critical to distinguish between lateral (marsh edge) and medial (marsh interior) erosion, because the interaction between waves and the substrate is very different in each case (direct impact force in the case of the former, and attenuated orbital wave currents at the bed in the latter). Existing evidence for the medial erosion process suggests that above-ground portions of a plant attenuate orbital wave currents within relatively dense vegetated salt marshes, but it has also been shown in steady-flow conditions that these aboveground portions can result in surface scouring in adjacent locations that are less dense (29,30). Moreover, it has been hypothesized that these above-ground portions can actually trigger the formation of the vertical cliff-like edge at the land-water interface (31,32), as a result of relative differences in landward versus seaward flow velocities and wave stresses on either side of the edge (33).…”

Signatures of sea level changes on tidal geomorphology: Experiments on network incision and retreat

“…[13] Most models, both conceptual and numerical, describing the morphodynamic evolution of tidal channels cutting through vegetated or unvegetated platforms [e.g., Redfield, 1965;Allen, 1997;Hood, 2006;Kirwan and Murray, 2007;Temmerman et al, 2007;Hughes et al, 2009], account for the possible supply of sediments (e.g., from rivers or from the sea) whereas our experimental apparatus can reproduce only purely erosive settings. Some studies suggest tidalchannel development to be the result of depositional rather then erosional processes [e.g., Redfield, 1965;Hood, 2006], whereas others consider erosion as the dominant process [e.g., Fagherazzi and Sun, 2004;Perillo et al, 2005;D'Alpaos et al, 2005D'Alpaos et al, , 2007bVlaswinkel and Cantelli, 2011].…”

“…Despite their importance in landscape evolution, tidal networks have received less attention when compared to their fluvial counterparts [e.g., Howard et al, 1994] particularly in terms of the chief processes governing their initiation and evolution, and their response to variations in external forcings [e.g., Rigon et al, 1994;Rinaldo et al, 1995]. Recently, several mathematical models have been developed to describe the morphogenesis and development of tidal networks [e.g., Fagherazzi and Sun, 2004;D'Alpaos et al, 2005;Marciano et al, 2005;Kirwan and Murray, 2007;Temmerman et al, 2007]. These models deepen our understanding of tidal network growth, otherwise analyzed solely on the basis of field observations and related conceptual models [e.g., Redfield, 1965;Allen, 1997;Perillo et al, 2005;Hood, 2006;D'Alpaos et al, 2007b;Hughes et al, 2009].…”

Relationship of Salt Marsh Vegetation Zonation to Spatial Patterns in Soil Moisture, Salinity, and Topography

“…First, enhanced sediment deposition within, and enhanced erosion around, Spartina clusters has been reported at low marsh elevations (Temmerman andothers 2007, van Hulzen andothers 2007). This mechanism may also explain the great density of microtributaries in the Spartina-dominated zone (Figure 2) at our high-elevation marsh site.…”