An SPH study on viscoplastic surges overriding mobile beds: The many regimes of entrainment

Flow-type landslides entrain mobile bed material, but the processes involved are diverse and require systematic study. We perform direct numerical simulations using the open-source SPH package DualSPHysics with a regularized Herschel–Bulkley rheology. We then compare model output with physical test data, and hence investigate the effects of varying the bed yield stress $_{,b}$ and bed depth ℎ$_b$, interpreting the results using a newly-identified set of dimensionless numbers. Results reveal diverse interaction regimes between surges and mobile beds, including ‘‘rigid bed’’, ‘‘lubrication’’, ‘‘shallow ploughing’’, ‘‘surfing’’, ‘‘plunging’’, and ‘‘deep ploughing’’. Shallow, borderline-stable beds ‘‘lubricate’’ the surge: once destabilized, these beds cause strong acceleration of the combined flow front. Deeper borderline-stable beds allow the surge material to ‘‘plunge’’ downward, massively displacing bed material upward and downstream. For stabler beds, ‘‘ploughing’’ and ‘‘surfing’’ are associated with intermediate and high values of $_{,b}$, respectively. In both cases, beds retard the surge, with mobile dams forming for ‘‘ploughing’’ regimes. Across all regimes identified, the influence of $_{,b}$ is non-monotonic, with intermediate values decelerating the combined flow fronts the most. Furthermore, the different interaction regimes exhibit unique velocity profiles. We develop phase diagrams based on three dimensionless numbers, demarcating these regimes

Mass exchange between geophysical flows and beds: Idealised computational modelling using a Herschel-Bulkley rheology

A key mechanism by which geophysical flows evolve is mass exchange with the underlying bed, either by entraining material from the bed, or by depositing material. Although it is known that some consequences of these mass exchange processes include changes in the volume, momentum and local rheology of the flow, the circumstances under which specific changes occur are not well-established. Given the enormous number of competing mechanisms present in geophysical flows, it is not surprising that the state of the art for modelling entrainment is essentially still empirical. In this study, we implement a Herschel-Bulkley (non-Newtonian) rheology into an existing open-source Smoothed Particle Hydrodynamics solver (DualSPHysics). This rheology can reasonably represent clay-rich flows, typical of those observed in the French Prealps. We hence undertake a highly-idealised, quantitative investigation of entrainment mechanisms for flows overriding non-fixed beds. For the beds, we vary the yield stress and the depth. Preliminary results reveal a rich variety of behaviours that can be obtained for different bed properties, including both acceleration and deceleration of the flow material. These mechanisms are reminiscent (but not identical) of observations from other studies where geo-materials were used

High-resolution topography: tools and analysis of the life and death of salt marshes

Salt marshes are grassy platforms that develop on sheltered coasts with high sediment supply. They may be found on sub-tropical shores where they often coexist with mangrove swamps, or in temperate climates where they might front brackish and fresh wetlands. These landscapes filter pollutants, protect coastlines against storm surges, and sequester carbon at high rates, making salt marshes some of the most valuable ecosystems on Earth. However, their survival is jeopardised by imbalance between formative and destructive processes: salt marshes rely heavily on external sources of sediment, and the poor sediment supply may prevent them from recovering from wave-driven erosion or from matching accelerating sea level rise. The sustained existence of a salt marsh ecosystem depends strongly on its topographic evolution. Hence, quantifying marsh platform topography is vital to improve coastal management, and the current development of high-resolution topographic data acquisition techniques presents geomorphologists with important opportunities to achieve this objective. This thesis addresses the need for topographic analysis tools specific to the morphology of salt marshes and explores a selection of potential uses for these tools. First, I propose a novel, unsupervised method to reproducibly isolate salt marsh scarps and platforms from a Digital Terrain Model (DTM). This method takes the form of a multiple routing algorithms grouped under a single programme referred to as the Topographic Identification of Platforms (TIP). Field observations and numerical models show that salt marshes mature into subhorizontal platforms delineated by subvertical scarps. Based on this premise, the programme identifies scarps as lines of local maxima on a slope raster, then fills the DEM from the scarps upward, thus isolating mature marsh platform objects. I then test the TIP method using lidar-derived DTMs from six salt marshes in England with varying tidal ranges and geometries, for which topographic platforms were manually isolated from tidal flats. Agreement between manual and unsupervised classification exceeds 90 $\%$ for resolutions up to 3m. I also find that our method allows for the accurate detection of local block failures as small as 3 times the DTM resolution. Ultimately, I show that unsupervised classification of marsh platforms from high-resolution topography is possible and sufficient to monitor and analyse topographic evolution over time. The relevance of such monitoring is however dependant on the frequency and time-span of data acquisition, a point which I discuss further in the conclusive chapter. Second, I use the TIP method to extract the distribution of elevations of multiple marsh platforms in the United Kingdom and the United States. I compare marsh elevations relative to current sea level and run simple 0-dimensional settling simulations in order to explore constraints on suspended sediment concentration and particle size. These experiments set a basis for comparison with observed accretion rates from field sources, as lidar-derived accretion rates are found to be inaccurate. I find that the marsh platforms examined occupy a narrow range of elevations in the upper tidal frame, situated between Mean High Tide and the Observed Highest High Tide. At these elevations, accretion models using sinusoidal tidal forcing do not allow these platforms to be inundated nor experience deposition. However, when forced with year-long tidal records, I find not inconsiderable deposition rates that follow hyperbolic contour lines when expressed as a function of sediment concentration and median grain size. I find that the deposition of coarse, concentrated sediment is necessary for platforms in the upper tidal frame to immediately match sea level rise, suggesting a strong dependance on infrequent high-deposition events for short-term accretion. This is particularly true for marshes that are very high in the tidal frame, making accretion increasingly storm-driven as marsh platforms gain elevation. Finally, I reflect on the capacity of marshes to regenerate after erosion events within a context of changing sediment supply conditions and how this may affect the long-term, dynamic equilibrium of marsh platforms. Finally, I add a module to the TIP method to determine the topographic signature of retreat and progradation on the edges of salt marsh platforms in mega-tidal Moricambe Bay (UK) in 2009, 2013 and 2017. I first describe the TIP method, and from the outlines it determines I generate transverse topographic profiles of the marsh edge 10m long and 20m apart. Profiles are grouped into categories depending on whether they experienced erosion or accretion in the 2009-2013 or 2013-2017 periods respectively, and I find that profiles belonging to the same retreat or progradation event have distinctly similar morphologies, regardless of the event magnitude. Progradation profiles have a shallow scarp and low relief that decreases with event magnitude, facilitating more progradation. Conversely, steep-scarped, high-relief retreat profiles that dip away from levees as retreat reveals older platforms. Furthermore, vertical accretion of the marsh edge is found to be primarily controlled by elevation in the study site, suggesting an even distribution of deposition that would allow bay infilling were it not limited by the migration of creeks. The scope of this research within future research on marsh margins is further discussed in the conclusive chapter

Toward coherent space–time mapping of seagrass cover from satellite data: an example of a Mediterranean lagoon

Seagrass meadows are a highly productive and economically important shallow coastal habitat. Their sensitivity to natural and anthropogenic disturbances, combined with their importance for local biodiversity, carbon stocks, and sediment dynamics, motivate a frequent monitoring of their distribution. However, generating time series of seagrass cover from field observations is costly, and mapping methods based on remote sensing require restrictive conditions on seabed visibility, limiting the frequency of observations. In this contribution, we examine the effect of accounting for environmental factors, such as the bathymetry and median grain size (D50) of the substrate as well as the coordinates of known seagrass patches, on the performance of a random forest (RF) classifier used to determine seagrass cover. Using 148 Landsat images of the Venice Lagoon (Italy) between 1999 and 2020, we trained an RF classifier with only spectral features from Landsat images and seagrass surveys from 2002 and 2017. Then, by adding the features above and applying a time-based correction to predictions, we created multiple RF models with different feature combinations. We tested the quality of the resulting seagrass cover predictions from each model against field surveys, showing that bathymetry, D50, and coordinates of known patches exert an influence that is dependent on the training Landsat image and seagrass survey chosen. In models trained on a survey from 2017, where using only spectral features causes predictions to overestimate seagrass surface area, no significant change in model performance was observed. Conversely, in models trained on a survey from 2002, the addition of the out-of-image features and particularly coordinates of known vegetated patches greatly improves the predictive capacity of the model, while still allowing the detection of seagrass beds absent in the reference field survey. Applying a time-based correction eliminates small temporal variations in predictions, improving predictions that performed well before correction. We conclude that accounting for the coordinates of known seagrass patches, together with applying a time-based correction, has the most potential to produce reliable frequent predictions of seagrass cover. While this case study alone is insufficient to explain how geographic location information influences the classification process, we suggest that it is linked to the inherent spatial auto-correlation of seagrass meadow distribution. In the interest of improving remote-sensing classification and particularly to develop our capacity to map vegetation across time, we identify this phenomenon as warranting further research.</p

Detecting the Morphology of Prograding and Retreating Marsh Margins—Example of a Mega-Tidal Bay

Retreat and progradation make the edges of salt marsh platforms their most active features. If we have a single topographic snapshot of a marsh, is it possible to tell if some areas have retreated or prograded recently or if they are likely to do so in the future? We explore these questions by characterising marsh edge topography in mega-tidal Moricambe Bay (UK) in 2009, 2013 and 2017. We first map outlines of marsh platform edges based on lidar data and from these we generate transverse topographic profiles of the marsh edge 10 m long and 20 m apart. By associating profiles with individual retreat or progradation events, we find that they produce distinct profiles when grouped by change event, regardless of event magnitude. Progradation profiles have a shallow scarp and low relief that decreases with event magnitude, facilitating more progradation. Conversely, steep-scarped, high-relief retreat profiles dip landward as retreat reveals older platforms. Furthermore, vertical accretion of the marsh edge is controlled by elevation rather than its lateral motion, suggesting an even distribution of deposition that would allow bay infilling were it not limited by the migration of creeks. While we demonstrate that marsh edges can be quantified with currently available DTMs, oblique observations are crucial to fully describe scarps and better inform their sensitivity to wave and current erosion

Sediment accumulation in embayments controlled by bathymetric slope and wave energy: Implications for beach formation and persistence

High energy, rocky coastlines often feature sandy beaches within headland‐bound embayments. Not all such embayments have beaches however, and beaches in embayments can be removed by storms and may subsequently reform. What dictates the presence or absence of an embayed beach and its resilience to storms? In this paper, we explore the effect of offshore slope and wind conditions on nearshore sediment transport within idealised embayments to give insight into nearshore sediment supplies. We use numerical simulations to show that sand can accumulate near shore if the offshore slope is &gt;0.025 m/m, but only under persistent calm conditions. Our modelling also suggests that if sediment in an embayment with an offshore gradient steeper than 0.025 m/m is removed during a period of persistent stormy conditions, it will be unlikely to return in sub‐decadal timescales. In contrast, sediment located in embayments with shallower gradients can reform swiftly in both calm and stormy conditions. Our findings have wide implications for contemporary coastal engineering in the face of future global climate change, but also for Quaternary environmental reconstruction. Our simple method to predict beach stability based on slope can be used to interpret differing responses of embayments to periods of changing coastal storminess such as the medieval climate anomaly‐little ice age (MCA‐LIA) transition

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

Anomalous versus slowed-down Brownian diffusion in the ligand-binding equilibrium

Measurements of protein motion in living cells and membranes consistently report transient anomalous diffusion (subdiffusion) which converges back to a Brownian motion with reduced diffusion coefficient at long times, after the anomalous diffusion regime. Therefore, slowed-down Brownian motion could be considered the macroscopic limit of transient anomalous diffusion. On the other hand, membranes are also heterogeneous media in which Brownian motion may be locally slowed-down due to variations in lipid composition. Here, we investigate whether both situations lead to a similar behavior for the reversible ligand-binding reaction in 2d. We compare the (long-time) equilibrium properties obtained with transient anomalous diffusion due to obstacle hindrance or power-law distributed residence times (continuous-time random walks) to those obtained with space-dependent slowed-down Brownian motion. Using theoretical arguments and Monte-Carlo simulations, we show that those three scenarios have distinctive effects on the apparent affinity of the reaction. While continuous-time random walks decrease the apparent affinity of the reaction, locally slowed-down Brownian motion and local hinderance by obstacles both improve it. However, only in the case of slowed-down Brownian motion, the affinity is maximal when the slowdown is restricted to a subregion of the available space. Hence, even at long times (equilibrium), these processes are different and exhibit irreconcilable behaviors when the area fraction of reduced mobility changes.Comment: Biophysical Journal (2013

Academic careers in Computer Science: Continuance and transience of lifetime co-authorships

International audienceScholarly publications reify fruitful collaborations between co-authors. A branch of research in the Science Studies focuses on analyzing the co-authorship networks of established scientists. Such studies tell us about how their collaborations developed through their careers. This paper updates previous work by reporting a transversal and a longitudinal studies spanning the lifelong careers of a cohort of researchers from the DBLP bibliographic database. We mined 3,860 researchers' publication records to study the evolution patterns of their co-authorships. Two features of co-authors were considered: 1) their expertise, and 2) the history of their partnerships with the sampled researchers. Our findings reveal the ephemeral nature of most collaborations: 70% of the new co-authors were only one-shot partners since they did not appear to collaborate on any further publications. Overall, researchers consistently extended their co-authorships 1) by steadily enrolling beginning researchers (i.e., people who had never published before), and 2) by increasingly working with confirmed researchers with whom they already collaborated