Reduction of deltaic channel mobility by tidal action under rising relative sea level

As Holocene river deltas continue to experience sea-level rise, sediment carried by distributary channels counteracts delta-plain drowning. Many deltas worldwide are subject to tidal action, which strongly affects the morphology of distributary channels and could also influence their mobility. Here we show, through physical laboratory experiments, that distributary-channel mobility can be dramatically reduced in systems affected by tides in comparison to an identical system with no tides, and that the mobility of distributary channels decreases as the ratio of tidal to fluvial energy increases. This effect occurs even if new accommodation space is created by rising relative sea level. By analyzing synthetic stratigraphy derived from both digital elevation data and time-lapse photography, we show also that the reduction of channel mobility in tidal deltas increases channel stacking and connectivity in the stratigraphic record

Tidal Channel Patterns: Field Investigations, Numerical Modelling and Laboratory Experiments

Tidal meandering channels are ubiquitous features of tidal landscapes and play a fundamental role on the eco-morphodynamic evolution of these environments. However, only a handful of papers provide details on tidal meander planimetric shape, morphometric characteristics and morphodynamic evolution, and the internal achitecture of tidal meanders has not been explored in detail. Moreover, the morphodynamic evolution of tidal meanders and the related sedimentary products have often been interpreted on the basis of the well developed models and theories existing for their fluvial counterparts, despite a number of differences were a priori identifiable. Toward the goal of improving current understanding of the morphodynamic evolution of tidal meanders, five main issues have been investigated in the present work: i) rates of migration and evolutionary dynamics of tidal meanders; ii) assessment and quantification of differences and analogies existing between the planform features of tidal and fluvial meanders; iii) variations of tidal meander hydrodynamics in response to different tidal phases, and the role that these variations exert on tidal meander sedimentary products; iv) role played by bidirectional flows, tidal asymmetries and lateral tributaries; v) assessment of influence of tide amplitude, basin slope and initial shoreline configuration on tidal channel network ontogeny and evolution via laboratory experiment. A multidisciplinary approach has been adopted, with different methodologies encompassing remote sensing techniques, field observations, numerical modelling and physical-laboratory experiments. Activities have been carried out in parallel with sedimentological studies, in order to provide a comprehensive framework. The main results from this work highlighted that: I) once conveniently scaled with channel width, tidal meander migration rates are very similar to those displayed by fluvial meanders, thus challenging the paradigm of tidal meanders as a stable landscape features; II) differences and analogies between tidal and fluvial meander planforms can be addressed in a quantitative way, and different metrics exist thta allow one to successfully quantify these differences; III) strong asymmetries exist between different tidal phases, exerting a crucial role on the depositional patterns of tidal meanders; IV) under certain conditions, lateral tributaries can strongly influence the evolution of bends modifying local mechanisms of flow and sediment distribution; V) tidal channel network features evolve differently in response to different tidal ranges, basin slopes and relative sea level changes, whereas the number of breaches along the initial shoreline seems to have little effect on the evolution of the network itself

Experimental delta evolution in tidal environments: Morphologic response to relative sea\u2010level rise and net deposition

Tide-influenced deltas are among the largest depositional features on Earth and are ecologically and economically im-portant as they support large populations. However, the continued rise in relative sea level threatens the sustainability of these land-scapes and calls for new insights on their morphological response. While field studies of ancient deposits allow for insight into deltaevolution during times of eustatic adjustment, tide-influenced deltas are notoriously hard to identify in the rock record. We present asuite of physical experiments aimed at investigating the morphological response of tide-influenced deltas subject to relative sea-levelrise. We show that increasing relative tidal energy changes the response of the delta because tides effectively act to remove fluviallydeposited sediment from the delta topset. This leads to enhanced transgression, which we quantify via a new methodology for com-paring shoreline transgression rates based on the concept of a\u2018transgression anomaly\u2019relative to a simple reference case. We alsoshow that stronger tidal forcing can create composite deltas where distinct land-forming processes dominate different areas of thedelta plain, shaping characteristic morphological features. The net effect of tidal action is to enhance seaward transfer of bedloadsediment, resulting in greater shoreline transgression compared to identical, yet purely fluvial, deltaic systems that exhibit static oreven regressive shorelines

Field migration rates of tidal meanders recapitulate fluvial morphodynamics

The majority of tidal channels display marked meandering features. Despite their importance in oil-reservoir formation and tidal landscape morphology, questions remain on whether tidalmeander dynamics could be understood in terms of fluvial processes and theory. Key differences suggest otherwise, like the periodic reversal of landscape-forming tidal flows and the widely accepted empirical notion that tidal meanders are stable landscape features, in stark contrast with their migrating fluvial counterparts. On the contrary, here we show that, once properly normalized, observed migration rates of tidal and fluvial meanders are remarkably similar. Key to normalization is the role of tidal channel width that responds to the strong spatial gradients of landscape-forming flow rates and tidal prisms. We find that migration dynamics of tidal meanders agree with nonlinear theories for river meander evolution. Our results challenge the conventional view of tidal channels as stable landscape features and suggest that meandering tidal channels recapitulate many fluvial counterparts owing to large gradients of tidal prisms across meander wavelengths

Morpho-sedimentary evolution of a microtidal meandering channel driven by 130-years of natural and anthropogenic modifications of the Venice Lagoon (Italy)

Abstract Tidal channels form the pathways for tidal currents to propagate and distribute clastic sediments and nutrients, thus providing a primary control on tidal-landscape ecomorphodynamics. Most tidal channels in both estuarine and lagoonal environments have a tendency to meander, yet very few studies exist that investigate the full spectrum of processes controlling tidal meander morpho-sedimentary evolution. The Venice Lagoon (Italy) offers a unique opportunity to shed light on this topic, because a long record of morphological and sedimentary data is available, which allows one to relate tidal channel evolution to the hydrodynamic and morphological changes undergone by the lagoon. In particular, during the last 130 years, feedback between rising relative sea levels and anthropogenic interventions have caused severe modifications of the Lagoon hydro- and morpho-dynamics. Here we investigate how these modifications fed back into the morpho-sedimentary evolution of a meandering tidal channel located in the northern Lagoon. Combining extensive datasets of aerial photographs, topographic and bathymetric surveys, geophysical investigations, sedimentary core analysis, and numerical modeling, we show that enhanced local tidal ranges and water discharges determine adjustments of channel cross-sectional geometries proportional to increasing tidal prisms, while changes in local tidal asymmetries caused modifications of the local sediment transport regime, resulting in the development of bar-pool patterns according to the dominant tidal phase. Such bar-pool patterns eventually determine channel migration through a bar-push mechanism controlled by a fluvial-like, quasi-linear relationship between local channel curvature and lateral migration rates. Critical differences in sediment transport regime are however highlighted between fluvial and tidal meanders, the latter being potentially characterized by high concentrations of suspended sediment during periods of slack waters when wind-driven sediment transport processes are not negligible. This could hamper the formation of high-relief bedforms, with profound implications for the sedimentology of tidal point-bar deposits

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

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

Long‐Term Monitoring of Coupled Vegetation and Elevation Changes in Response to Sea Level Rise in a Microtidal Salt Marsh

Tight interplays between physical and biotic processes in tidal salt marshes lead to self-organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long-term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation-vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea-level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site-specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species-specifically to changes in environmental forcings by modifying species-preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation-species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution

Seaward expansion of salt marshes maintains morphological self-similarity of tidal channel networks

International audienceTidal channel networks (TCNs) dissect ecologically and economically valuable salt marsh ecosystems. These networks evolve in response to complex interactions between hydrological, sedimentological, and ecological processes that act in tidal landscapes. Thus, improving current knowledge of the evolution of salt-marsh TCNs is critical to providing a better understanding of bio-morphodynamic processes in coastal environments. Existing studies of coastal TCNs have typically focussed on marshes with either laterally stable or eroding edges, and suggested that TCN morphology evolves primarily through the progressive landward erosion of channel tips, that is, via channel headward growth. In this study, we analyze for the first time the morphological evolution of TCNs found within salt marshes that are characterized by active lateral expansion along their seaward edges and anthropogenically-fixed landward boundaries. We use remote-sensing and numerical-modeling analyses to show that marsh seaward expansion effectively limits headward channel growth and prompts the evolution of TCNs that maintain self-similar morphological structures. In particular, we demonstrate that the overall TCN length increases proportionally to the rate at which marshes expand laterally and that these morphological changes do not significantly alter the drainage properties of the coupled marsh-TCN system. Such behavior is not observed in marshes that are not expanding laterally. Our results allow for elucidating the mechanisms of TCN formation and evolution in tidal wetlands, and are therefore critical to improving our current understanding of coastal-landscape ecomorphodynamics, as well as to developing sustainable strategies for the conservation and restoration of these environments

Tidal Channel Patterns: Field Investigations, Numerical Modelling and Laboratory Experiments

Tidal meandering channels are ubiquitous features of tidal landscapes and play a fundamental role on the eco-morphodynamic evolution of these environments. However, only a handful of papers provide details on tidal meander planimetric shape, morphometric characteristics and morphodynamic evolution, and the internal achitecture of tidal meanders has not been explored in detail. Moreover, the morphodynamic evolution of tidal meanders and the related sedimentary products have often been interpreted on the basis of the well developed models and theories existing for their fluvial counterparts, despite a number of differences were a priori identifiable. Toward the goal of improving current understanding of the morphodynamic evolution of tidal meanders, five main issues have been investigated in the present work: i) rates of migration and evolutionary dynamics of tidal meanders; ii) assessment and quantification of differences and analogies existing between the planform features of tidal and fluvial meanders; iii) variations of tidal meander hydrodynamics in response to different tidal phases, and the role that these variations exert on tidal meander sedimentary products; iv) role played by bidirectional flows, tidal asymmetries and lateral tributaries; v) assessment of influence of tide amplitude, basin slope and initial shoreline configuration on tidal channel network ontogeny and evolution via laboratory experiment. A multidisciplinary approach has been adopted, with different methodologies encompassing remote sensing techniques, field observations, numerical modelling and physical-laboratory experiments. Activities have been carried out in parallel with sedimentological studies, in order to provide a comprehensive framework. The main results from this work highlighted that: I) once conveniently scaled with channel width, tidal meander migration rates are very similar to those displayed by fluvial meanders, thus challenging the paradigm of tidal meanders as a stable landscape features; II) differences and analogies between tidal and fluvial meander planforms can be addressed in a quantitative way, and different metrics exist thta allow one to successfully quantify these differences; III) strong asymmetries exist between different tidal phases, exerting a crucial role on the depositional patterns of tidal meanders; IV) under certain conditions, lateral tributaries can strongly influence the evolution of bends modifying local mechanisms of flow and sediment distribution; V) tidal channel network features evolve differently in response to different tidal ranges, basin slopes and relative sea level changes, whereas the number of breaches along the initial shoreline seems to have little effect on the evolution of the network itself.Le reti di canali meandriformi costituiscono una delle principali componenti dei sistemi mareali, e giocano un ruolo di fondamentale importanza nell’evoluzione eco-morfodinamica di questi ambienti. Tuttavia, solo un numero limitato di studi scientifici ne ha analizzato le configurazioni planimetriche, le caratteristiche morfometriche e l’evoluzione morfodinamica. Inoltre, l’evoluzione morfodinamica e i prodotti sedimentari dei meandri a marea sono spesso stati interpretati sulla base di teorie e modelli sviluppati per i loro omologhi fluviali, nonostante numerose differenze tra le due tipologie siano identificabili a priori. Nell’intento di comprendere più approfonditamente l’evoluzione morfodinamica dei meandri a marea, nel presente lavoro sono stati studiati 5 differenti argomenti: i) tassi di migrazione e dinamiche evolutiove dei meandri a marea; ii) stima e quantificazione delle differenze planimetriche esistenti tra meandri fluviali e tidali; iii) variazioni dell’idrodinamica dei meandri a marea in risposta all’alternanza delle fasi mareali, e influenza di queste variazioni sui prodotti sedimentari propri dei meandri a marea; iv) ruolo della bidirezionalità del flusso, delle asimmetrie mareali e dei tributari laterali; v) stima dell’influenza dell’ampiezza di marea, delle pendenze topografiche del bacino tidale e della configurazione iniziale della linea di costa sulla nascita ed evoluzione morfologica delle reti di canali a marea. Nelle suddette analisi é stato utilizzato un approccio di tipo multidisciplinare, combinando metodologie quali remote-sensing, osservazioni in situ, modellazione numerica ed esperimenti su modelli fisici. Le attività sono state condotte in parallelo con studi sedimentologici, così da fornire un quadro che fosse il più esaustivo possibile. I principali risultati ottenuti evidenziano che: I) se convenientemente normnalizzati con la larghezza del canale, i tassi di migrazione dei meandri a marea sono molto simili a quelli dei loro corrispettivi fluviali, inficiando così il paradigma che vede i meandri tidali come un’entità morfologica essenzialmente stabile; II) le differenze tra meandri tidali e fluvali non sono solo qualitative, e diverse sono le metriche che possono essere utilizzate per quantificare queste differenze; III) le asimmetrie tra le diverse fasi di marea sono significative, e influenzano i patterns deposizionali in modo determinante; IV) gli affluenti laterali possono influenzare fortemente l’evoluzione dei meandri, modificando i meccanismi locali di distrubuzione dei flussi e dei sedimenti; V) le reti di canali a marea evolvono in modo diverso in risposta a differenti ampiezze di marea, pendenze del bacino tidale e cambiamenti del livello relativo del medio mare, mentre la configurazione iniziale della linea di costa non sembra avere effetti significativi sull’evoluzione della rete stessa