Margin lateral erosion is arguably the main mechanism leading to marsh loss in estuaries and lagoons worldwide. Our understanding of the mechanisms controlling marsh edge erosion is currently quite ...limited and current predictive models rely on empirical laws with limited general applicability. We propose here a simple theoretical treatment of the problem based on dimensional analysis. The identification of the variables controlling the problem and the application of Buckingham's theorem show, purely on dimensional grounds, that the rate of edge erosion and the incident wave power density are linearly related. The predictive ability of the derived relationship is then evaluated, positively, using new long‐term observations from the Venice lagoon (Italy) and by re‐interpreting data available in previous literature.
Key Points
Marsh edge erosion rate is a linear function of wave power density
Marsh edge erosion rate is a function of marsh cliff height
Traditional, power‐law, formulas are inconsistent with theory and observations
We have developed an analytical model of salt marsh evolution that captures the dynamic response of marshes to perturbations in suspended sediment concentrations, plant productivity, and the rate of ...relative sea level rise (RSLR). Sediment‐rich and highly productive marshes will approach a new equilibrium state in response to a step change in the rate of RSLR faster than sediment‐poor or less productive marshes. Microtidal marshes will respond more quickly to a step change in the rate of RSLR than mesotidal or macrotidal marshes. Marshes are more resilient to a decrease rather than to an increase in the rate of RSLR, and they are more resilient to a decrease rather than to an increase in sediment availability. Moreover, macrotidal marshes are more resilient to changes in the rate of RSLR than their microtidal counterparts. Finally, we find that a marsh's ability to record sea level fluctuations in its stratigraphy is fundamentally related to a timescale we call TFT, or filling timescale, which is equal to the tidal amplitude divided by the maximum possible accretion rate on the marsh (a function of plant productivity, sediment properties, and availability). Marshes with a short‐filling timescale (i.e., marshes with rapid sedimentation or small tidal amplitudes) are best suited to recording high‐frequency fluctuations in RSLR, but our model suggests it is unlikely that marshes will be able to record fluctuations occurring over timescales that are shorter than decadal.
Key Points
Our analytical model describes marsh response to changes in environmental forcing
The model allows instant assessment of key dynamical behavior of salt marshes
Marshes are more resilient to decreasing (rather than to increasing) SLR rates
On funneling of tidal channels Lanzoni, S.; D'Alpaos, A.
Journal of geophysical research. Earth surface,
03/2015, Letnik:
120, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Tidal channels dissect the tidal landscape and exert a crucial control on the morphodynamic evolution of these landscapes. Improving our understanding of channel equilibrium morphology is therefore ...an important issue for both theoretical and practical reasons. We analyze the case of a tidal channel dissecting a relatively short, unvegetated tidal flat characterized by microtidal conditions and a negligible external sediment supply. The three‐dimensional equilibrium configuration of the channel is determined on the basis of a hydrodynamic model, describing the cross‐sectional distribution of the longitudinal bed shear stresses, coupled with a morphodynamic model retaining the description of the main physical processes shaping the channel and the adjacent intertidal platform. Both channel bed and width are allowed to adapt to the flow field so that an equilibrium altimetric and planimetric configuration is eventually obtained, when erosion becomes negligibly small, and asymptotically constant elevations are reached everywhere within the domain. Model results reproduce several observed channel characteristics of geomorphic relevance, such as the relationship between channel cross‐sectional area and the flowing tidal prism, the scaling of the width‐to‐depth ratio with channel width, and the longitudinal distributions of bed elevations and channel widths. In analogy with empirical evidence from estuaries, tidal channel funneling is usually assumed to be described by an exponential trend. Our theoretical analyses, modeling results, and observational evidence suggest that a linear relationship also provides a good approximation to describe longitudinal variations in channel width for short tidal channels. Longitudinal bed profiles characterized by a strong planform funneling tend to attain an upward concavity, whereas a low degree of convergence implies an almost linear profile. Finally, the model allows one to analyze the influence of environmental conditions (sediment characteristics, basin size, tidal amplitude, etc.) on the geomorphological features of tidal channels (equilibrium cross‐sectional area and bottom profile, width‐to‐depth ratios, and planform shape). Wider and deeper channels develop as the width of the domain increases, as the tidal amplitude increases, or as the mean platform elevation decreases. Conversely, narrower and shallower channels result from an increase in the critical shear stress for erosion or a decrease in the flow conductance. We thus believe that this model provides a useful tool for quantitative analyses of long‐term morphodynamics of tidal landscapes.
Key Points
A morphodynamic model for the 3‐D equilibrium morphology of tidal channels
A linear landward decrease in channel width can describe channel funneling
Model provides a useful tool for quantitative long‐term tidal morphodynamics
Accretion rate in salt marshes is governed by inorganic soil deposition and soil organic matter (SOM) accumulation. Existing (limited) observations and modeling results suggest that SOM amounts, ...biomass production, and decomposition processes should vary widely and systematically at the marsh scale. However, we lack observations aimed at understanding how SOM production is modulated spatially within a marsh, and at elucidating the relative importance of the controlling processes. The little existing data suggest that competing effects between biomass production and decomposition processes determine an approximately spatially constant contribution of SOM to total accretion. Here we investigate this idea using concurrent observations of SOM and decomposition rates from marshes in North Carolina. Our results indicate that systematic spatial variations in SOM are small, possibly as a result of an at least partial compensation of opposing trends in biomass production and decomposed organic matter. Our analyses show that deeper soil layers are, on average, characterized by lower decomposition rates and higher stabilization factors than shallower layers, likely because of differences in the persistence of water‐logged conditions. Overall, decomposition processes are sufficiently rapid that the labile material in the fresh biomass is completely decomposed before it can be sufficiently buried and stabilized. Our findings point to the importance of the fraction of initially refractory material and of stabilization processes in determining the final distribution of SOM within the soil column.
Key Points
Soil organic matter decomposition rate and decomposable fraction tend to decrease with depth below the marsh surface
Observed decomposition rates indicate that labile fraction is fully decomposed before it is buried sufficiently deep to become stable
The fraction of refractory organic matter and the stabilization of labile material are main determinants of soil organic matter content
In tidal environments, channel networks act as essential drainage pathways. Although the complex interactions between environmental factors have been studied extensively, the effects of the initial ...bathymetry on tidal network ontogeny are poorly understood. In this contribution, we used a numerical model to mimic a schematic tidal basin subjected to tidal forcing. The effects of the initial bathymetry and vegetation growth are analyzed by changing the features of randomly generated bed perturbation and the intertidal platform slope. Different perturbation densities mildly affect the growth of tidal networks, which, at equilibrium, share similar values in terms of channel length, tidal prism, and cross‐sectional area. The complexity and structure of channel networks are more sensitive to variations in the perturbation distribution. Increasing the initial bathymetry slope can shorten channels and reduce the tidal prism and drainage efficiency. Vegetation growth is found to invariably promote channel lengthening and narrowing, increasing the complexity and drainage efficiency of the system. An asymmetrical tidal forcing generally leads to longer channels and smaller unchanneled lengths. Under ebb‐dominant conditions, channels get deeper, and the increased channel length ensures a higher drainage efficiency. The insights of our study provide a deeper understanding of the environmental factors controlling the equilibrium morphology of tidal channel systems and their overall resilience. Further implications concern the restoration and management of coastal areas through the informed use of topographic changes and planting arrangements. Finally, accounting for the uncertainties associated with initial conditions is relevant when modeling other earth systems and comparing them with real systems.
Plain Language Summary
Tidal channels form networks connecting the sea to the inner land and act as essential pathways in coastal landscapes to exchange water, sediments, and nutrients. The channel network morphology and its efficiency in draining the basin exhibit different sensitivities to environmental factors. In this study, the evolution of a tidal network is simulated using a modeling framework to analyze the effects of small topographic features, vegetation, and tidal conditions on tidal channel characteristics. Small topographic features can significantly enhance the complexity of the channel network and increase the number of branches. The presence of vegetation generally promotes channel expansion and improves the drainage efficiency. However, the effect of vegetation on channel drainage efficiency may become weaker when the basin is sloping to the sea. The drainage efficiency can be further enhanced under flood‐ or ebb‐dominant conditions. Furthermore, under ebb‐dominant conditions, the tidal channel system will dissect the tidal basin more efficiently, resulting in a higher drainage efficiency than in flood‐dominant conditions. These findings are relevant for assessing the resilience of tidal channel systems and could be useful for the design of restoration projects and the management of coastal areas.
Key Points
Randomly distributed bed perturbations may increase channel drainage efficiency by changing the channel network structure
Vegetation promotes channel elongation and drainage efficiency, while a sloping basin has opposite effects due to reduced tidal prism
Ebb‐dominant conditions produce a rearrangement of total channel length and mean unchanneled length, ensuring a higher drainage efficiency
Tidal salt marshes are widespread along the World's coasts, and are ecologically and economically important as they provide several valuable ecosystem services. In particular, their significant ...primary production, coupled with sustained vertical accretion rates, enables marshes to sequester and store large amounts of organic carbon and makes them one of the most carbon‐rich ecosystems on Earth. Organic carbon accumulation results from the balance between inputs, that is, organic matter produced by local plants or imported, and outputs through decomposition and erosion. Additionally, organic matter deposition actively contributes to marsh vertical accretion, thus critically affecting the resilience of marsh ecosystems to rising relative sea levels. A better understanding of organic‐matter dynamics in salt marshes is key to address salt‐marsh conservation issues and to elucidate marsh importance within the global carbon cycle. Toward this goal, we empirically derived rates of organic matter decomposition by burying 712 commercially available tea bags at different marshes in the microtidal Venice Lagoon (Italy), and by analyzing them following the Tea Bag Index protocol. We find values of the decomposition rate (k) and stabilization factor (S) equal to 0.012 ± 0.003 days−1 and 0.15 ± 0.063, respectively. Water temperature critically affects organic matter decomposition, enhancing decomposition rates by 8% per °C on average. We argue that, at least in the short term, the amount of undecomposed organic matter that actively contributes to carbon sequestration and marsh vertical accretion strongly depends on the initial organic matter quality, which is a function of marsh and vegetation characteristics.
Plain Language Summary
Salt marshes are important coastal environments regularly flooded by the tide and dominated by herbaceous plants, providing several valuable ecosystem services. They are, however, threatened by the effects of climate changes and human interferences. As organic matter accumulated in salt‐marsh soil importantly contribute to surface elevation necessary for marshes to keep up with sea level rise and to store atmospheric carbon, this project aims to improve our understanding of decomposition processes affecting organic matter preservation and their controls in salt‐marsh environment. Toward this goal, following the so‐called Tea Bag Index protocol, we buried 712 commercially available tea bags in salt‐marsh soils of the Venice Lagoon (Italy) measuring the reduction of their organic content due to decomposition processes after 3 months. Our results confirm that salt marshes are among biomes with the slowest decomposition rates. However, we observed a loss of about two‐thirds of the initial labile organic mass after 90 days and that initial litter quality, depending on litter and vegetation characteristics, exerts a primary control on the amount of preserved organic matter contributing to carbon sequestration and marsh accretion.
Key Points
Decomposition rates in Venice marshes display a mean value of 0.012 ± 0.003 days−1, confirming them among biomes with the slowest decomposition rates
We find that a one degree increase in temperature leads to a 8% increase in decomposition rates
Litter quality exerts a primary control on the amount of preserved organic matter contributing to carbon sequestration and marsh accretion
We have monitored and analyzed, through remote sensing and ancillary field surveys, the rapid (O(1) year) development of a tidal network within a newly established artificial salt marsh in the Venice ...Lagoon. After the construction of the salt marsh, a network of volunteer creeks established themselves away from an artificially constructed main channel (with mean and maximum annual headward-growth rates of 11 m/yr and 18 m/yr, respectively). The rapid formation of this system of tidal creeks provides a unique opportunity to test the reliability of a model of tidal network initiation and development, previously proposed by the authors. The restored marsh presents the characteristics of a controlled environment analogous to a large-scale field laboratory, as it allows comparison of the morphologic features of real and simulated network structures under the reasonable assumption of neglecting accretion and deposition processes over the timescales of observation. Our results compare favorably with observational evidence, showing that the model proves reasonably capable of reproducing the main features of the actual channel-network patterns. The model reproduces statistical network characteristics of eco-morphodynamic and hydrodynamic relevance and captures the dominant modes of the network-incision process.
The formation and evolution of tidal networks have been described through various theories which mostly assume that tidal network development results from erosional processes, therefore emphasizing ...the chief role of external forcing triggering channel net erosion such as tidal currents. In contrast, in the present contribution we explore the influence of sediment supply in governing tidal channel initiation and further elaboration using an ecogeomorphic modeling framework. This deliberate choice of environmental conditions allows for the investigation of tidal network growth and development in different sedimentary contexts and provides evidences for the occurrence of both erosional and depositional channel‐forming processes. Results show that these two mechanisms in reality coexist but act at different time scales: channel initiation stems from erosional processes, while channel elaboration mostly results from depositional processes. Furthermore, analyses suggest that tidal network ontogeny is accelerated as the marsh accretional activity increases, revealing the high magnitude and prevalence of the depositional processes in governing the morphodynamic evolution of the tidal network. On a second stage, we analyze the role of different initial topographic configurations in driving the development of tidal networks. Results point out an increase in network complexity over highly perturbed initial topographic surfaces, highlighting the legacy of initial conditions on channel morphological properties. Lastly, the consideration that landscape evolution depends significantly on the parameterization of the vegetation biomass distribution suggests that the claim to use uncalibrated models for vegetation dynamics is still questionable when studying real cases.
Key Points
Accelerated tidal network ontogeny with increasing sediment supply
Ecogeomorphic feedbacks enhanced for high biomass density at low marsh elevations
Inheritance of tidal channel features from the initial tidal basin morphology
The morphological evolution of shallow tidal systems strongly depends on gradients in transport that control sediment erosion and deposition. A spatially refined quantitative description of suspended ...sediment patterns and dynamics is therefore a key requirement to address issues connected with dynamical trends, responses, and conservation of these systems. Here we use a combination of numerical models of sediment transport dynamics, high temporal resolution point observations, and high spatial resolution remote sensing data to overcome the intrinsic limitations of traditional monitoring approaches and to establish the robustness of numerical models in reproducing space‐time suspended sediment concentration (SSC) patterns. The comparison of SSC distributions in the Venice Lagoon (Italy) computed with a numerical model with SSC retrievals from remote sensing data allows us to define the ability of the model to properly describe spatial patterns and gradients in the SSC fields. The use of point observations similarly allows us to constrain the model temporally, thus leading to a complete space‐time evaluation of model abilities. Our results highlight the fundamental control exerted on sediment transport intensity and patterns by the sheltering effect associated with artificial and natural intertidal landforms. Furthermore, we show how the stabilizing effect of benthic vegetation is a main control of sediment dynamics at the system scale, confirming a notion previously established in the laboratory or at small field scales.
Key Points
We fuse remote sensing and field data to test circulation and transport models
Natural and artificial structures affect sediment dynamics in tidal environments
Stabilizing effect of benthic vegetation is a main control of sediment dynamics
In this paper, we examine variations in climate characteristics near the area of Cortina d'Ampezzo (Dolomites, Eastern Italian Alps), with particular reference to the possible implications for ...debris-flow occurrence. The study area is prone to debris-flow release in response to summer high-intensity short-duration rainfalls and, therefore, it is of the utmost importance to investigate the potential increase in debris-flow triggering rainfall events. The critical rainfall threshold is agreed to be a crucial triggering factor for debris-flows. Data from a monitoring system, placed in a catchment near Cortina (Acquabona), show that debris-flows were triggered by rainfalls with peak rainfall intensities ranging from 4.9 to 17.4 mm/10 min. The analyses of meteorological data, collected from 1921 to 1994 at several stations in the study area, show a negative trend of annual rainfall, a considerable variation in the monthly rainfall distribution, and an increase in the temperature range, possibly related to global climate changes. Moreover, high-intensity and short-duration rainfall events, derived from data collected from 1990 and 2008, show an increase in exceptional rainfall events. The results obtained in a peak-over-threshold framework, applied to the rainfall data measured at the Faloria rain gauge station from 1990 to 2008, clearly show that the interarrival time of over-threshold events computed for different threshold values decreased in the last decade. This suggests that local climatic changes might produce an increase in the frequency of rainfall events, potentially triggering debris flows in the study area.
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