We propose an ecomorphodynamic model which conceptualizes the chief land-forming processes operating on the intertwined, long-term evolution of marsh platforms and embedded tidal networks. The rapid network incision (previously addressed by the authors) is decoupled from the geomorphological dynamics of intertidal areas, governed by sediment erosion and deposition and crucially affected by the presence of vegetation. This allows us to investigate the response of tidal morphologies to different scenarios of sediment supply, colonization by halophytes, and changing sea level. Different morphological evolutionary regimes are shown to depend on marsh ecology. Marsh accretion rates, enhanced by vegetation growth, and the related platform elevations tend to decrease with distance from the creek, measured along suitably defined flow paths. The negative feedback between surface elevation and its inorganic accretion rate is reinforced by the relation between plant productivity and soil elevation in Spartina-dominated marshes and counteracted by positive feedbacks in multispecies-vegetated marshes. When evolving under constant sea level, unvegetated and Spartina-dominated marshes asymptotically tend to mean high water level (MHWL), different from multiple vegetation species marshes, which can make the evolutionary transition to upland. Equilibrium configurations below MHWL can be reached under constant rates of sea level rise, depending on sediment supply and vegetation productivity. Our analyses on marine regressions and transgressions show that when the system is in a supply-limited regime, network retreat and expansion (associated with regressions and transgressions, respectively) tend to be cyclic. Conversely, in a transport-limited regime, network reexpansion following a regression tends to take on a new configuration, showing a hysteretic behavior. Copyright 2007 by the American Geophysical Union.

Landscape evolution in tidal embayments: modeling the interplay of erosion, sedimentation, and vegetation dynamics

Abstract The foreland basin of the southern European Alps is characterized by large fan-shaped alluvial systems fed by the main montane valleys and these depositional systems present an extent of 300–3000 km2, with a length of 30–70 km. Most of them are megafans, characterized by evident longitudinal differentiation in which steep piedmont sector consists of amalgamated gravels, while the distal portion has a gradient

Alluvial Fans and Megafans Along the Southern Side of the Alps

Meandering channels extensively dissect fluvial and tidal landscapes, critically controlling their morphodynamic evolution and sedimentary architecture. In spite of an apparently striking dissimilarity of the governing processes, planform dimensions of tidal and fluvial meanders consistently scale to local channel width, and previous studies were unable to identify quantitative planimetric differences between these landforms. Here we use satellite imagery, measurements of meandering patterns, and different statistical analyses applied to about 10,000 tidal and fluvial meanders worldwide to objectively disclose fingerprints of the different physical processes they are shaped by. We find that fluvial and tidal meanders can be distinguished on the exclusive basis of their remotely-sensed planforms. Moreover, we show that tidal meanders are less morphologically complex and display more spatially homogeneous characteristics compared to fluvial meanders. Based on existing theoretical, numerical, and field studies, we suggest that our empirical observations can be explained by the more regular processes carving tidal meanders, as well as by the higher lithological homogeneity of the substrates they typically cut through. Allowing one to effectively infer processes from landforms, a fundamental inverse problem in geomorphology, our results have relevant implications for the conservation and restoration of tidal environments, as well as from planetary exploration perspectives.

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Remotely-sensed planform morphologies reveal fluvial and tidal nature of meandering channels.

Salt marshes are among the most common morphological features found in tidal landscapes and provide ecosystem services of primary ecological and economic importance. However, the continued rise in relative sea level and increasing anthropogenic pressures threaten the sustainability of these environments. The alarmingly high rates of salt marsh loss observed worldwide, mainly dictated by the lateral erosion of their margins, call for new insights into the mutual feedbacks among physical, biological, and morphological processes that take place at the critical interface between salt marshes and the adjoining tidal flats. We combined field measurements, remote sensing data, and numerical modeling to investigate the interplays between wind waves and the morphology, ecology, and planform evolution of salt marsh margins in the Venice Lagoon of Italy. Our results confirm the existence of a positive linear relationship between incoming wave power density and rates of salt marsh lateral retreat. In addition, we show that lateral erosion significantly decreases when halophytic vegetation colonizes the marsh margins, and that different erosion rates in vegetated margins are associated with different halophytes. High marsh cliffs and smooth shorelines are expected along rapidly eroding margins, whereas erosion rates are reduced in gently sloped, irregular edges facing shallow tidal flats that are typically exposed to low wind-energy conditions. By highlighting the relationships between the dynamics and functional forms of salt marsh margins, our results represent a critical step to address issues related to conservation and restoration of salt marsh ecosystems, especially in the face of changing environmental forcings.

Control of wind-wave power on morphological shape of salt marsh margins

Three‐dimensional flow structures and morphodynamic evolution of microtidal meandering channels

https://www.research.unipd.it/bitstream/11577/3309552/1/Cosma%20et%20al_Manuscript.pdf

Point-bar brink and channel thalweg trajectories depicting interaction between vertical and lateral shifts of microtidal channels in the Venice Lagoon (Italy)

Piracy-controlled geometry of tide-dominated point bars: Combined evidence from ancient sedimentary successions and modern channel networks

Reading tidal processes where their signature is cryptic: The Maastrichtian meandering channel deposits of the Tremp Formation (Southern Pyrenees, Spain)

Low rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point‐bar geometries and internal facies arrangements that differ substantially from widely accepted models of point‐bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three‐dimensional forward stratigraphic model, the ‘Point‐Bar Sedimentary Architecture Numerical Deduction’ (PB‐SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB‐SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best‐fit modelling approach has been used to obtain multiple simulations for each case‐study, each of which fits the observations derived from the analysis of time‐series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point‐bar brink and channel‐thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point‐bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three‐dimensional point‐bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero‐vertical shifts of meandering channels can determine the geometry of related sedimentary bodies.

/pdf/an-integrated-approach-to-determine-three-dimensional-1l7eahmhpq.pdf

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon

Intertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine-grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend(i.e. the upper part of the point bar). This occurs because the lower point-bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three-dimensional perspective and overlook the along-bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper-intertidal domain of the Mont-Saint-Michel Bay (France), and it analyses the sedimentology of a 3D time-framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time-framed accretionary package, data from thirteen lidar topographic surveys were used to create the point-bar synthetic stratigraphy. Data shows that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point-bar-apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend-apex zone is ascribed to the development of low-energy conditions due to flow and bed configuration. High accretion rate of the point-bar-apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point-bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal-point-bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.

Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data

Marta Cosma

Papers

Point-bar brink and channel thalweg trajectories depicting interaction between vertical and lateral shifts of microtidal channels in the Venice Lagoon (Italy)

Piracy-controlled geometry of tide-dominated point bars: Combined evidence from ancient sedimentary successions and modern channel networks

Reading tidal processes where their signature is cryptic: The Maastrichtian meandering channel deposits of the Tremp Formation (Southern Pyrenees, Spain)

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon

Sedimentology of a hypertidal point bar (Mont‐Saint‐Michel Bay, north‐western France) revealed by combining lidar time‐series and sedimentary core data