1642 research outputs found
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Hydro-morphodynamics modelling for the mobilization assessment of UXOs and DMMs
No full textA waves, currents and sediment model is implemented in openTELEMAC to simulate morphodynamic changes on the East Coast of Florida, USA. This model produces temporal and spatially varying data for an external software to simulate the exposure and mobilization of UXO and DMM.
The model retrieves its boundary conditions from global climate (ECMWF ERA-5 reanalysis) and ocean currents (HYCOM GoM regional analysis) models, thus providing a link between mesoscale dynamics and local conditions. Comparison between the model boundary forcings, model results and field measurements during the passage of Hurricane Matthew (October 2016) indicate that the model can simulate the hydrodynamics of the area. Comparisons with available LiDAR data also indicate that the model agrees with erosion and deposition patterns observed along the beach. Lastly, the model was run using conditions for an entire year, which produced a suitably long dataset for undertaking UXO and DMM mobilization studies
Local overshoot and wind effects on wave overtopping at vertical coastal structures
No full textWind effects on wave overtopping over a fully impermeable vertical sea wall were studied numerically using the open-source computational fluid dynamics library OpenFoam. A pressure gradient correction term was incorporated in the momentum equations. In recent studies, it was found that, in the absence of wind, an increase in wave steepness results in a reduction of wave overtopping. This is related to the instability of the standing wave formed at the front of a vertical structure. Such instability was noticed in the range of steepness 0.285–0.443 from previous physical experiments for a regular wave interacting with a vertical structure. The existence of this regime was confirmed in the current study. It was also found that the stability of the standing wave determines the shape and volume of the overshooting jet, which has a strong effect on wave overtopping. When the wind is relatively weak (e.g. a wind speed of 10 m/s) it is unable to alter the overshooting jet very much, meaning a weak wind effect on wave overtopping. When the wind is strong (e.g. a wind speed of 30 m/s) it completely deforms the overshooting jet resulting in overtopping discharge almost three times that without wind
A new semi-unsteady sand transport formula in GAIA
No full textThe numerical morphodynamic model GAIA predicts bed changes due to hydrodynamics by means of empirical sand transport formulae. The physical transport processes in the near shore, particularly the wave related processes, which are important during storms, are still poorly represented by the majority of the available transport formulae. Wave related processes become important in shallow waters, where currents are relatively small and waves undergo shape changes. As a consequence of the lack of wave related processes in the sand transport formulae, sand bar formation and migration cannot be modelled yet in TELEMAC. To address this, wave related transport processes were implemented in GAIA using the semi-unsteady sand transport formula of [11], known as the SANTOSS formula. The novelty of this formula, compared to other existing sand transport formulae in GAIA, is that SANTOSS calculates the sand transport rate accounting for wave non-linearity without the need of a phase-resolving wave model. The formula, implemented in a fully coupled 2D model (TELEMAC2D-TOMAWAC-GAIA), was tested against in situ measurements, showing good agreement with the measurements
Estuarine flocculation – a review of the key contributing factors
No full textMost estuaries around the world are dominated by combinations cohesive sediments (generally referred to as mud) and mud:sand mixtures (Manning et al., 2010), the transport and fate of which plays a major role in most estuarine management and many marine engineering projects. Mud typically is composed of mineral grains that originate from both fluvial and marine sources, together with biological matter - both living and in various stages of decomposition (Deng, 2022). It is the combination of these features that makes estuarine mud sticky in nature (e.g. Chassagne et al., 2009; Parsons et al., 2016; Ye et al., 2021), and for this reason these sediment types are referred to generically as cohesive sediments (Whitehouse et al., 2000; Mehta, 2023). In contrast to purely non-cohesive sandy sediments, muddy sediments can flocculate into larger aggregates called flocs (Manning et al., 2017; Spencer et al., 2021, 2022), and this poses a serious complication to modellers of estuarine sediment dynamics. Consequently, understanding the mud processes has been a subject of intensive research effort (e.g. Dyer, 1986; Dronkers and Van Leussen, 1988; Healy et al., 2002) and there has been much progress been made in the past few decades in advancing our understanding of cohesive sediments in aquatic environments, especially estuarial regions. Dyer (1989) proposed key areas that would require research focus on cohesive sediments. It is the aim of this chapter to provide a state-of-the-art review of many of the key scientific advances in research assessing the processes and behaviour of muddy sediments, and the research progress made during the last 20–30-year period. This presentation provides an overview of the following topics: estuarine sediment composition, suspended sediments, flocculation dynamics, cohesion, particle iteraction, mixed sediments, hindered settling, and floc structure. The presentation includes succinct background science and theory on each component topic, together with respective highlights from key research contributions, and also drawing on relevant case studies and real-world examples
Sediment suspension, flocculation and settling over bio-physical cohesive substrates in saline water
No full textBiologically-active mud-sand mixtures, commonly known as cohesive sediments, are ubiquitous in estuaries and continental shelves. Due to flocculation, cohesive sediments transport as flocs and their impacts to ecosystem and earth system are distinctly different from those of non-cohesive sediments. Typical floc samples generated from kaolinite clay, fine sand and xanthan gum (a natural proxy of extracellular polymeric substances (EPS)) were collected and analysed from a series of controlled laboratory flume experiments. Not only floc characteristics but also their interactions with flow turbulence in the boundary layer and bed deposition are presented. The results indicate that flocculation occurs pervasively and rapidly in all the cohesive experimental runs from within 0.5 hour of the start of the experiment. Pure mineral cohesion without xantham gum lead to higher suspended sediment concentration (SSCs) in the water column than that of mixed bio-physical cohesion. Bed elevations and turbulence kinetic energy (TKE) near the bed show more fluctuations in bio-physical cases with low qantities of xanthan gum. Moreover, in the low bio-physical cohesive condition, the flocs aggregation show a dominance in flocculation from 0.5 to 6 hours and then breakage increasing between 6 to 9 hours. With the addition of xanthan gum and the associated higher bio-physical cohesion, flocculation can be impeded by lower sediment entrainment and suspension from the substrates. The number of flocs, Macroflocs size, settling velocity and mass settling flux (MSF) have reductions compare with those of low bio-physical case. Finally, a conceptual schematic illustrating the processes of bio-physical cohesive sediment suspension, flocculation, settling, and deposition has been presented to emphasise the importance of taking flocculation and settling into consideration in future morphodynamics, sedimentation studies and predictive models
A novel 3D volumetric method for directly quantifying porosity and pore space morphology in flocculated suspended sediments
No full textFlocculated suspended sediments (flocs) are found in a variety of environments globally, and their transport and behavior bear substantial importance to several industries including fisheries, aquaculture, and shipping. Additionally, the modelling of their behavior is important for estuarine and coastal flood prediction and defence, and the process of flocculation occurs in other unrelated industries such as paper and chemical production. Floc porosity is conventionally assessed using inferential indirect or proxy data approaches. These methods underestimate floc porosity % by c. 30% and cannot measure the micro-scale complexity of these pore spaces and networks, rendering inputs to models sub-optimal. This study introduces a novel 3D porosity and pore space quantification protocol, that produces directly quantified porosity % and pore space data.
• 3D floc data from micro-CT scanning is segmented volumetrically
• This segmented volume is quantified to extract porosity and several pore space parameters from the floc structur
Seabed Proximity Effect on Free Spanning Pipelines: the FIST JIP
No full textNatural free spans occur in offshore pipelines when its stiffness prevents it from following the contours of the seabed., typically either due to the seabed contour or an erosion process after the pipe is laid. Free spans may be subjected to vortex induced vibrations that can lead to rapid accumulation of fatigue damage. Most offshore pipelines free spans occur in the proximity of the seabed. The influence of the seabed proximity on the hydrodynamic behaviour of the system is dictated by the span gap. When the seabed proximity significantly affects the flow around the pipeline in free span, the span may be considered a small gap span. When assessing VIV in small gap spans, current practice is to either nearly ignore the effect from proximity or to ignore VIV based on a critical gap approach. Previous research on crossflow VIV showed that both approaches are limited.
Thus, a joint industry project to study proximity effects in free spans in scour trenches, the FIST JIP, was formed. FIST JIP performed extensive test program focused on in-line VIV near seabed, with and without scoured trenches, to document the proximity effects. This paper presents the experimental work from FIST JIP and a suggestion to incorporate its results to better account for seabed proximity
A storm driven turbidity maximum in a microtidal estuary
No full textMany macro- and mesotidal estuaries are characterized by Turbidity Maxima Zones (TMZs), regions with suspended solid concentrations that are much higher than those found throughout the rest of the estuary. Such regions are located near the upriver limit of salt intrusion and their position and extent are modulated and driven by tidal oscillations, especially in estuaries where tidal forcing is large. Hence, pronounced TMZs are not typically expected in micro-tidal estuaries. Field experiments were carried out in the microtidal estuary of the Misa River (northeast coast of Italy) with the aim to analyze riverine-coastal ocean interactions during different climatic conditions, freshwater discharge and tidal forcing. The goal was also that of identifying factors and episodic conditions that could lead to the evolution of ephemeral TMZs in this microtidal estuarine system. Observational results, combined to a flocculation model suite, describe the hydrodynamics, morphological bed evolution, water chemistry and floc dynamics within the estuary during wintertime quiescent and stormy periods. Pronounced TMZs with different location and extent were observed during two storms with different intensities, when enhanced freshwater discharge, wave action and tidal oscillation generated significant stratification of the lower estuarine water column. Higher turbidity values were observed throughout the TMZ during the smaller/weaker storm, while stronger surface mixing during the stronger storm led to greater dispersion of the (re-)suspended particulate load throughout the upper water column, providing a less pronounced TMZ along the bed of the lower estuary. Observations in the Misa River, potentially valid for other microtidal estuaries, show that: 1) episodic storm conditions that significantly increase freshwater discharge can lead to the evolution of an ephemeral TMZ that is modulated, but not controlled, by tidal oscillations and surface mixing conditions; 2) ephemeral TMZ localization, intensity, and extent during episodic storm events is a function of storm intensity; 3) moderately enhanced freshwater flow during an episodic storm event promotes a high degree of stratification, allowing for the formation of large flocs with great settling rates, leading to a pronounced TMZ forming downriver of the landward limit of seawater intrusion; whereas higher freshwater flows during stronger storm events lead to less stratification, greater bottom turbulence and potential TMZ suppression near the riverbed, with shear conditions promoting smaller flocs with lower settling and a greater potential for suspended particulate export from the lower estuary to coastal waters
A laboratory study on the behavior of estuarine sediment flocculation as function of salinity, EPS and living algae
No full textThe interactions between organic and inorganic particles in the context of flocculation is an on-going topic of research. Most current researches do not distinguish between the effects of EPS (produced by microorganisms) and living microorganisms (like algae). In this study, the effect of salinity, EPS and living algae on sediment flocculation are investigated separately. Several types of measurements were performed, which can be divided into the following categories: sediment at different salinities, sediment in the presence of EPS at different salinities, sediment in the presence of living algae at a given salinity. Results show that increasing salinity enhances slightly sediment flocculation. In the presence of EPS there was hardly any flocculation in demi-water, but the flocculation was significant in saline water. The living algae cells were shown to flocculate with themselves and form large flocs. These algae flocs can bind to sediment particles to form larger flocs, both in demi-water and sea water. Size-wise algae-sediment flocs were largest, EPS-sediment flocs came second, and salt-sediment flocs were smallest
GAIA - a unified framework for sediment transport and bed evolution in rivers, coastal seas and transitional waters in the TELEMAC-MASCARET modelling system
No full textIn rivers, coastal seas and transitional waters, sediment transport processes involve a variety of interacting factors that dynamically vary over time and space. The flow dynamics within these highly heterogeneous natural systems influences the spatial patterns of erosion and deposition of the bed sediments, which in turn shapes and conditions the bottom morphology. By taking advantage of the established modelling framework in both two- and three dimensions for unstructured meshes proposed within the Telemac-Mascaret system, the new module Gaia provides a code structure for solving sediment transport and morphological evolution problems. By a clear treatment of sedimentary processes that happen in the water column, in the bed structure, and at the water–bed interface, Gaia efficiently manages the spatial and temporal variability of sediment size classes, properties and transport modes for two- and three dimensions. In addition, this module can easily be expanded and customised to particular requirements by modifying user-friendly, easy-to-read, and well-documented FORTRAN-90 subroutines