A coupled geomorphic and ecological model of tidal marsh evolution

Abstract

The evolution of tidal marsh platforms and interwoven channel networks cannot be addressed without treating the two-way interactions that link biological and physical processes. We have developed a 3D model of tidal marsh accretion and channel network development that couples physical sediment transport processes with vegetation biomass productivity. Tidal flow tends to cause erosion, whereas vegetation biomass, a function of bed surface depth below high tide, influences the rate of sediment deposition and slope-driven transport processes such as creek bank slumping. With a steady, moderate rise in sea level, the model builds a marsh platform and channel network with accretion rates everywhere equal to the rate of sea-level rise, meaning water depths and biological productivity remain temporally constant. An increase in the rate of sea-level rise, or a reduction in sediment supply, causes marsh-surface depths, biomass productivity, and deposition rates to increase while simultaneously causing the channel network to expand. Vegetation on the marsh platform can promote a metastable equilibrium where the platform maintains elevation relative to a rapidly rising sea level, although disturbance to vegetation could cause irreversible loss of marsh habitat. accretion ͉ erosion ͉ sea level ͉ vegetation ͉ wetland S ubsidence, erosion, sea-level rise, and anthropogenic changes to sediment delivery rates are affecting coastal marshes worldwide. In some regions these influences are converting significant portions of marshland to open water (1, 2). The fate of intertidal salt marshes is of societal importance and scientific interest; marshes provide highly productive habitat and serve as nursery grounds for a large number of commercially important fin and shellfish (3, 4). Additionally, marshes offer great value as buffers of coastal storms in cities such as New Orleans, which is separated from the Gulf of Mexico by marshland (5, 6).A variety of vertical accretion models have been used to address the response of tidal marshes to environmental change, including accelerated sea-level rise and reduced sediment supply (7,8). In these models, bed elevation of the marsh platform is adjusted according to a deposition rate that is proportional to water depth at high tide, a proxy for duration and frequency of inundation. In such models, an increase in the rate of sea-level rise is accompanied by an increase in water depth until the increasing deposition rate becomes equal to the sea-level rise rate. With the exception of recent work by Morris et al. (9), these models neglect the role of vegetation, despite Redfield's (10) hypothesis that vegetation and physical processes influence morphodynamics equally strongly in the intertidal zone. Vegetation traps inorganic sediment and provides a source of organic sediment. Based on field measurements, Morris et al. (9) argue that biomass density, and therefore deposition rates, increase with water depth up to some optimal depth. The role of biomass density in enhancing deposition rates in th...

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

“…[7] The laboratory experiments presented herein support the description of network evolution and dynamics addressed by conceptual [e.g., Allen, 1997] and numerical models [D'Alpaos et al, 2007a;Kirwan and Murray, 2007], and deepen our understanding of the effects of cyclic changes triggered by RMSL variations. The latter in fact, cannot be observed directly in nature owing to the long timescales involved, and have only been addressed so far on the basis of numerical experiments [D'Alpaos et al, 2007a].…”

“…[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].…”

“…These studies showed that laboratory experiments can successfully model tidal network dynamics. Although tidal network response to RMSL changes has been addressed by conceptual [e.g., Allen, 1997] and numerical models [D'Alpaos et al, 2007a;Kirwan and Murray, 2007], the effects of cyclic variations in RMSL on the characteristics and structure of tidal networks remain poorly understood, and the possibility of detecting signatures of past conditions imprinted on the landscape justifies analyses of the type proposed herein.…”

“…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].…”

Understanding and predicting wave erosion of marsh edges

“…Much attention has been devoted to the understanding and description of the processes which lead to observed equilibria in the vertical direction, or lack thereof, producing a rather comprehensive understanding of the controlling biological and physical processes [Allen, 1990;Morris et al, 2002;D'Alpaos et al, 2007;Kirwan and Murray, 2007;Marani et al, 2007;Mudd et al, 2009;Marani et al, 2010]. On the contrary, "lateral" evolution mechanisms have received comparatively much less attention, even though marsh degradation associated with edge erosion is arguably the chief mechanism by which marshes in coastal areas worldwide are being lost [Schwimmer, 2001;Gedan et al, 2009;van de Koppel et al, 2005;Mariotti and Fagherazzi, 2010].…”