Sergio Fagherazzi

Coastal eutrophication as a driver of salt marsh loss

Salt marshes are highly productive coastal wetlands that provide important ecosystem services such as storm protection for coastal cities, nutrient removal and carbon sequestration. Despite protective measures, however, worldwide losses of these ecosystems have accelerated in recent decades. Here we present data from a nine-year whole-ecosystem nutrient-enrichment experiment. Our study demonstrates that nutrient enrichment, a global problem for coastal ecosystems, can be a driver of salt marsh loss. We show that nutrient levels commonly associated with coastal eutrophication increased above-ground leaf biomass, decreased the dense, below-ground biomass of bank-stabilizing roots, and increased microbial decomposition of organic matter. Alterations in these key ecosystem properties reduced geomorphic stability, resulting in creek-bank collapse with significant areas of creek-bank marsh converted to unvegetated mud. This pattern of marsh loss parallels observations for anthropogenically nutrient-enriched marshes worldwide, with creek-edge and bay-edge marsh evolving into mudflats and wider creeks. Our work suggests that current nutrient loading rates to many coastal ecosystems have overwhelmed the capacity of marshes to remove nitrogen without deleterious effects. Projected increases in nitrogen flux to the coast, related to increased fertilizer use required to feed an expanding human population, may rapidly result in a coastal landscape with less marsh, which would reduce the capacity of coastal regions to provide important ecological and economic services.

Numerical models of salt marsh evolution: ecological, geomorphic, and climatic factors

Salt marshes are delicate landforms at the boundary between the sea and land. These ecosystems support a diverse biota that modifies the erosive characteristics of the substrate and mediates sediment transport processes. Here we present a broad overview of recent numerical models that quantify the formation and evolution of salt marshes under different physical and ecological drivers. In particular, we focus on the coupling between geomorphological and ecological processes and on how these feedbacks are included in predictive models of landform evolution. We describe in detail models that simulate fluxes of water, organic matter, and sediments in salt marshes. The interplay between biological and morphological processes often produces a distinct scarp between salt marshes and tidal flats. Numerical models can capture the dynamics of this boundary and the progradation or regression of the marsh in time. Tidal channels are also key features of the marsh landscape, flooding and draining the marsh platform and providing a source of sediments and nutrients to the marsh ecosystem. In recent years, several numerical models have been developed to describe the morphogenesis and long-term dynamics of salt marsh channels. Finally, salt marshes are highly sensitive to the effects of long-term climatic change. We therefore discuss in detail how numerical models have been used to determine salt marsh survival under different scenarios of sea level rise.

Tidal networks 3. Landscape-forming discharges and studies in empirical geomorphic relationships

In this final part of our study [Fagherazzi et al., this issue; Rinaldo et al., this issue] we propose a simple model for predicting the local peak ebb and flood discharges throughout a tidal network and use this model to investigate scaling relationships between channel morphology and discharge in the Venice Lagoon. The model assumes that the peak flows are driven by spring (astronomical) tidal fluctuations (rather than precipitation-induced runoff or seiche, sea surge, or storm-induced tidal currents) and exploits the procedure presented by Fagherazzi et al. [this issue] for delineating a time-invariant drainage area to any channel cross section. The discharge is estimated using the Fagherazzi et al. model to predict water surface topography, and hence flow directions throughout the channel network and across unchanneled regions, and the assumption of flow continuity. Water surface elevation adjustment, not assumed to be instantaneous throughout the network, is defined by a suitable solution of the flow equations where significant morphological information is used and is reduced to depending on just one parameter, the Chezy resistance coefficient. For the Venice Lagoon, peak discharges are well predicted by our model. We also document well-defined power law relationships between channel width and peak discharge, watershed area, and flow, whereas curved, nonscaling relationships were found for channel cross-sectional area as a function of peak discharge. Hence our model supports the use of a power law dependency of peak discharge with drainage area in the Venice Lagoon and provides a simple means to explore aspects of morphodynamic adjustments in tidal systems.

Tidal network ontogeny: Channel initiation and early development

[1] The long-term morphological evolution of tidal landforms in response to physical and ecological forcings is a subject of great theoretical and practical importance. Toward the goal of a comprehensive theoretical framework suitable for large-scale, long-term applications, we set up a mathematical model of tidal channel network initiation and early development, which is assumed to act on timescales considerably shorter than those of other landscape-forming ecomorphodynamical processes of tidal systems. A hydrodynamic model capable of describing the key landforming features in small tidal embayments is coupled with a morphodynamic model which retains the description of the main physical processes responsible for tidal channel initiation and network ontogeny. The overall model is designed for the further direct inclusion of the chief ecomorphological mechanisms, e.g., related to vegetation dynamics. We assume that water surface elevation gradients provide key elements for the description of the processes that drive incision, in particular the exceedance of a stability (or maintenance) shear stress. The model describes tidal network initiation and its progressive headward extension within tidal flats through the carving of incised cross sections, where the local shear stress exceeds a predefined, possibly site-dependent threshold value. The model proves capable of providing complex network structures and of reproducing several observed characteristics of geomorphic relevance. In particular, the synthetic networks generated through the model meet distinctive network statistics as, among others, unchanneled length and area probability distributions. Copyright 2005 by the American Geophysical Union.

Critical width of tidal flats triggers marsh collapse in the absence of sea-level rise

High rates of wave-induced erosion along salt marsh boundaries challenge the idea that marsh survival is dictated by the competition between vertical sediment accretion and relative sea-level rise. Because waves pounding marshes are often locally generated in enclosed basins, the depth and width of surrounding tidal flats have a pivoting control on marsh erosion. Here, we show the existence of a threshold width for tidal flats bordering salt marshes. Once this threshold is exceeded, irreversible marsh erosion takes place even in the absence of sea-level rise. This catastrophic collapse occurs because of the positive feedbacks among tidal flat widening by wave-induced marsh erosion, tidal flat deepening driven by wave bed shear stress, and local wind wave generation. The threshold width is determined by analyzing the 50-y evolution of 54 marsh basins along the US Atlantic Coast. The presence of a critical basin width is predicted by a dynamic model that accounts for both horizontal marsh migration and vertical adjustment of marshes and tidal flats. Variability in sediment supply, rather than in relative sea-level rise or wind regime, explains the different critical width, and hence erosion vulnerability, found at different sites. We conclude that sediment starvation of coastlines produced by river dredging and damming is a major anthropogenic driver of marsh loss at the study sites and generates effects at least comparable to the accelerating sea-level rise due to global warming.

On the shape and widening of salt marsh creeks

We have developed a model that simulates aspects of initial channel formation in a youthful salt marsh environment. The model mimics the evolution of the cross section of a channel by coupling calculations of bottom shear stresses caused by tidal motions with erosion, taking into account the deposition of cohesive sediments. The simulations characterize flow in a reference cross section that includes an incipient channel zone and a marsh surface zone, with assigned water surface level and initial bottom elevation. This model mimics key characteristics of salt marshes where discharges due to tidal motion repeat in time with approximately the same magnitude and water surface level. Significant reductions in the tidal prism due to increasing bottom elevation above mean sea level, however, are not treated. Rather, the model is suitable for youthful salt marshes where relatively large water depths are maintained. Prolonged deposition reduces the area available for flow and thereby changes the shear stress distribution at the bottom, leading locally to erosion and alteration of the channel cross section. The simulations suggest that two mechanisms contribute to the longitudinal widening exhibited by salt marsh channels, which typically is disproportionately greater than that exhibited by river channels. The short duration of the maximum discharge (spring tide) and corresponding erosion rates, when compared with deposition rates, prevent the channel from reaching a deep, narrow equilibrium configuration. Furthermore, autoconsolidation of cohesive sediments, often occurring in salt marsh environments, leads to a downward increase in the resistance of the sediment to erosion. As scour occurs locally, the flow encounters more resistant sediment layers; so rather than deepening the channel over a narrow zone, flow and bottom stresses become more uniformly distributed leading to a wider channel than would otherwise occur in the absence of autoconsolidation. Based on flow and sediment properties estimated for the Venice Lagoon, Italy, simulations are consistent with observations of salt marsh creeks at this location.

Stability of creeping soil and implications for hillslope evolution

The geomorphic behavior of a soil-mantled hillslope undergoing diffusive creep involves a coupling between changes in land surface elevation, soil transport rates, soil production, and soil thickness. A linear stability analysis suggests that the coupled response of the soil mantle to small perturbations in soil thickness or surface topography is influenced by two factors. The diffusive-like behavior of soil creep has a stabilizing effect wherein perturbations in land surface elevation are damped. The relation between the soil production rate and soil thickness may be either stabilizing or destabilizing. A monotonically decreasing production rate with soil thickness reinforces the stabilizing effect of diffusive land surface smoothing. An increasing production rate with soil thickness has a destabilizing effect wherein perturbations in soil thickness or the soil-bedrock interface are amplified, despite the presence of diffusive land surface smoothing. This coupled behavior is insensitive to the transport relation, whether the soil flux is proportional to the land surface gradient or to the product of the soil thickness and land surface gradient. The latter type of relation, nonetheless, could lead to a more complex hillslope form than might otherwise be expected for purely diffusive transport. Moreover, the response to periodic (sinusoidal) variations in the rate of stream downcutting at the lower hillslope boundary involves upslope propagation of coupled (damped) waveforms in the land surface and the soil-bedrock interface. The distance of upslope propagation goes with the square root of the product of the transport diffusion-like coefficient and the period of the downcutting rate. The upper part of the hillslope is therefore insensitive to relatively high-frequency variations in stream downcutting, so together with a stable behavior of the coupled soil-mantle-bedrock system, this part of the hillslope may exhibit a tendency toward uniform lowering, while the lower part behaves transiently. Conversely, in the presence of low-frequency variations in stream downcutting, hillslope morphology and soil thickness variations are more likely to reflect unsteady conditions over the entirety of the hillslope.

Geomorphic structure of tidal hydrodynamics in salt marsh creeks

[1] This paper develops a geomorphological theory of tidal basin response (tidal instantaneous geomorphologic elementary response, or TIGER) to describe specific characteristics of tidal channel hydrodynamics. On the basis of the instantaneous unit hydrograph approach, this framework relates the hydrodynamics of tidal watersheds to the geomorphic structure of salt marshes and, specifically, to the distance traveled by water particles within the channel network and on the marsh surface. The possibility of determining the water fluxes from observations of geomorphic features is an appealing approach to the study of tidally driven flow rates. Our formulation paves the way to the application of recent results on the geomorphic structure of salt marshes and tidal networks to the determination of marsh creek hydrology. A case study shows how the asymmetry in the stage-velocity relation and the existence of velocity surges typical of the tidal hydrographs can be explained as an effect of the delay in the propagation of the tidal signal within the marsh area.

A linear relationship between wave power and erosion determines salt-marsh resilience to violent storms and hurricanes

Significance In recent years, there has been a flurry of restoration projects aimed at mitigating the impact of coastal storms using salt marshes and vegetated surfaces (called “living shorelines”). Based on a large dataset of salt marsh erosion and wave measurements collected all around the world, we find that erosion rates of marsh boundaries and incident wave energy collapse into a unique linear relationship. Our result clearly shows that long-term salt marsh deterioration is dictated by average wave conditions, and it is, therefore, predictable. Violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. This result is of high value for coastal restoration projects and the use of living shorelines to mitigate storms effect. Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. It is still unclear how resistant salt marshes are to extreme storms and whether they can survive multiple events without collapsing. Based on a large dataset of salt marsh lateral erosion rates collected around the world, here, we determine the general response of salt marsh boundaries to wave action under normal and extreme weather conditions. As wave energy increases, salt marsh response to wind waves remains linear, and there is not a critical threshold in wave energy above which salt marsh erosion drastically accelerates. We apply our general formulation for salt marsh erosion to historical wave climates at eight salt marsh locations affected by hurricanes in the United States. Based on the analysis of two decades of data, we find that violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. In contrast, moderate storms with a return period of 2.5 mo are those causing the most salt marsh deterioration. Therefore, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans.

Interactions between barrier islands and backbarrier marshes affect island system response to sea level rise: Insights from a coupled model

Interactions between backbarrier marshes and barrier islands will likely play an important role in determining how low-lying coastal systems respond to sea level rise and changes in storminess in the future. To assess the role of couplings between marshes and barrier islands under changing conditions, we develop and apply a coupled barrier island-marsh model (GEOMBEST+) to assess the impact of overwash deposition on backbarrier marsh morphology and of marsh morphology on rates of island migration. Our model results suggest that backbarrier marsh width is in a constant state of change until either the backbarrier basin becomes completely filled or backbarrier marsh deposits have completely eroded away. Results also suggest that overwash deposition is an important source of sediment, which allows existing narrow marshes to be maintained in a long-lasting alternate state (~500 m wide in the Virginia Barrier Islands) within a range of conditions under which they would otherwise disappear. The existence of a narrow marsh state is supported by observations of backbarrier marshes along the eastern shore of Virginia. Additional results suggest that marshes reduce accommodation in the backbarrier bay, which, in turn, decreases island migration rate. As climate change results in sea level rise, and the increased potential for intense hurricanes resulting in overwash, it is likely that these couplings will become increasingly important in determining future system behavior.