Yes / The aquatic vegetation patch plays a significant role on sediment net deposition in the vegetated channels. Particularly, the flow is decelerated at the leading edge of a patch that tends to induce vertical updraft, that is, a diverging flow region, in which vegetation greatly affects the pattern of sediment net deposition. This study focuses on the simulation of the sediment net deposition in the whole vegetation patch region through an innovative random displacement model, a Lagrange method, with probability-based boundary conditions, instead of the reflection or sorption boundary at the channel bottom. The probability model of deposition and resuspension is proposed according to the flow field characteristics in the different regions of the vegetation patch. The variation of the sediment deposition and resuspension with the turbulent kinetic energy is analyzed to illustrate the effect of the turbulence induced by vegetation, represented by the dimensionless turbulent kinetic energy (ψ), on the sediment deposition and resuspension. The sediment deposition predicted by the proposed model agrees well with the experimental measurements. Results show that the effect of vegetation on the sediment deposition and resuspension motions begins to prevail when the vegetation-induced ψ is larger than its threshold, ψ *. The threshold of ψ is predicted to be within 6.8–10 according to the simulation results in this study. As the turbulent kinetic energy increases, the deposition probability decreases continuously when ψ > ψ *. / ational Natural Science Foundation of China (NSFC). Grant Numbers: 52020105006, 11872285; UK Royal Society - International Exchanges Program. Grant Number: IES\R2\181122; Open Funding of State Key Laboratory of Water Resources and Hydropower Engineering Science (WRHES), Wuhan University. Grant Number: 2018HLG01

Ce travail de thèse s'inscrit dans la thématique de la pollution particulaire de l'air intérieur et concerne plus particulièrement les phénomènes de remise en suspension liés à l'impact, au niveau du sol, des pas d'une personne qui marche sur un plancher chargé en particules. La première partie présente un état de l'art des connaissances de la pollution particulaire. Les différents paramètres expérimentaux qui influencent la remise en suspension des particules ainsi que les coefficients utilisés pour la quantification du phénomène sont recensés. Les différentes perturbations mécaniques et aérodynamiques générées lors de l'impact d'un pied avec le sol pendant la marche d'une personne sont présentées et comparées. Nous terminons la première partie par une présentation des différents modèles de remise en suspension des particules émanant d'une surface. La seconde partie est consacrée à la présentation de la maquette qui a été mise au point au laboratoire LaSIE à La Rochelle afin d'étudier le dépôt et la remise en suspension des particules par un simulateur mécanique du pas humain depuis un sol chargé en particules. Cette étude a permis de classer les différents types de surfaces utilisées dans le bâtiment en fonction de leurs émissions en particules après l'impact mécanique d'un solide. La troisième partie du travail consiste à mesurer les vitesses de l'écoulement de l'air généré par un simulateur mécanique automatisé du pas humain en différents emplacements. Les mesures de la vitesse ont été effectuées par trois types de méthodes largement utilisées dans le domaine de la mécanique des fluides : deux méthodes de Vélocimétrie Laser (Vélocimétrie par Image des particules et Vélocimétrie Laser Doppler), mais également une méthode d’Anémométrie à Fil Chaud. Ces mesures nous ont permis de trouver la zone de forte vitesse qui correspond à la zone où nous avons une remise en suspension importante. Nous avons également étudié l'influence de l'état de la surface sur les vitesses de l'écoulement générées à proximité du sol suite au mouvement du simulateur mécanique. Ces mesures ont mis en évidence que l'influence de la rugosité de la surface sur la vitesse de l'écoulement généré par le pas est marginale. Finalement, nous avons étudié analytiquement le détachement des particules en utilisant un modèle basé sur le bilan des moments de forces. Des perspectives à la fois sur l'amélioration des deux maquettes mises en place au LaSIE et à l'ERM, ainsi que le développement d'un code numérique pour simuler le pas humain sont présentées et argumentées en conclusion. / This thesis work is part of indoor air particle pollution theme and more particularly the phenomenon of walking-induced particle resuspension. The first part presents a state of the art knowledge of particulate pollution. The different experimental parameters that influence the particles resuspension, as well as the coefficients used for the phenomenon quantification, are identified. The various mechanical and aerodynamic disturbances generated during person walking are presented and compared. We end the first part with a presentation of the different particles resuspension models. The second part is dedicated to present the experiment made on a small scale model developed at the LaSIE laboratory in La Rochelle to study particles deposition and resuspension by amechanical simulator of the human step from a particle-laden soil. This study classified the different types of surfaces used in the building according to their particulate emissions after the mechanical simulator impact. The third part of the work consists of measuring the airflow velocities generated by an automated mechanical simulator of the human footstep at different locations. The velocity measurements have been done in the Royal Military Academy at Brussels (RMA) by three types of methods widely used in the field of fluid mechanics : two methods of Laser Velocimetry (Particle Image Velocimetry and Laser Doppler Velocimetry), but also a method of Wire Anemometry. These measurements allowed us to find the high-speed zone that corresponds to the area where we have a significant resuspension. We also studied the influence the surface roughness on the flow velocities generated near the ground following the movement of the mechanical simulator. We have shown that the influence of the surface roughness on the speed of the flow generated by the pitch is marginal. Finally, we analytically studied the detachment of particles using a model based on the momentum balance. Perspectives on both the improvement of the two models set up in LaSIE and the RMA, as well as the development of a numerical code to simulate the human step, are presented and argued in conclusion.

Wave heights and periods of surface waves in the Baltic Sea have been modelled for a two-year period (1999-2000) with the wave model Hypas on an 11x11-km grid scale. There is a clear seasonal variation with higher waves during winter and lower during summer. This is mainly a reflection of the wind climate in the area where the winters are windier than the summers. The largest waves are found in the Skagerrak and over the deeper, eastern areas in the Baltic Proper. In the Baltic Sea, the surface waves influence the bottom sediment by initiating resuspension down to 80 m depths. This process is dependent not only on the waves but also on the varying grain size diameters. Fine and medium sand resuspend more often than other sediment types, and these sediments cover together about 25% of the Baltic Proper area. On average sediment is here resuspended 4-5 times per month with a duration for each event of 22 hours. The highest resuspension frequencies are found on the eastern and southern side of the Baltic Proper. During resuspension sediment grains are lifted up into the water mass and matters earlier bound in the sediment can be released. This may stimulate both production and degradation of organic matter.

A comprehensive investigation into the relationship between physical forcing and sus- pended particles in the shallow shelf region of the Western English Channel has been conducted, in order to evaluate the temporal dynamics of suspended particle populations. Measurements were taken across tidal cycles and seasons at station L4, part of the Western Channel Observatory (WCO), using the combination of a free-fall microstructure profiler and holographic imaging. Confirmation that L4 is weakly stratified is given, and that the formation of the seasonal thermocline is substantially altered by the spring-neap cy- cle. Stratification is variable and prone to periodic and partial erosion from atmospheric forcing during any point in any season. L4 undergoes moderate turbulent dissipation, principally as a result of tidal forcing. Typically, values of ε do not exceed 10−4 W kg−1 . L4 also exhibits tidal asymmetry, chiefly in response to stratification which, albeit weak, is frequently able to suppress turbulence when generated from the sea bed. The potential energy anomaly is small at L4, as expected for a weakly-stratified environment. Maxi- mum values in summer were shown to not exceed 50 J m−3 . Values of bed stress, τ0 , are rarely greater than around 0.18 N m−2 . Nonetheless, the critical erosion threshold falls below this, and is therefore smaller than that observed in similar locations around the UK. Seasonality in the amount of material resuspended from the seabed is important at L4. The presence of certain biological particles strongly influence particle size and may also determine if a given particle is lifted from the bed. Particles ≥ 200 μm are relatively rare, the site is dominated by particles smaller than this value in line with many other UK sites. Under certain conditions the theoretical maximum limit of particle size, the Kolmogorov length scale, does not hold and many examples of occasions when this threshold is exceeded are shown. This may generate important consequences in subsequent work undertaken at this site and other temperate shelf locations globally, particularly as these results indicate that maximum particle size appears to be governed less by the size of the local turbulent eddies and more by the presence of biological particles. This is another key seasonal component to particle dynamics in the Western English Channel. Phytoplankton populations are readily advected into and out of the L4 site, calling into question the current sampling strategy of the WCO to rely exclusively upon point measurements. Small increases in atmospheric forcing have the ability to rapidly disperse patches of phytoplankton, possibly to the point of cell mortality. Traditional sampling techniques for assessing zooplankton density have been shown to radically underestimate the number of animals present at L4, which will increase error estimates on current ecosystem models.

Lors des interventions humaines pendant les arrêts de tranche des installations nucléaires d’EDF, on remarque une remise en suspension de certains radionucléides sous forme d’aérosols (1 µm < dp < 10 µm). Dans le cadre d’une augmentation des interventions effectuées de façon simultanée en bâtiment réacteur, il devient important de mieux comprendre la remise en suspension due à l’activité des opérateurs pour adapter leur radioprotection. Le but des travaux de cette thèse est de quantifier la remise en suspension des particules suite à la marche des opérateurs sur un sol faiblement contaminé. Pour cela, la démarche suivie consiste à coupler un modèle de remise en suspension aéraulique avec des calculs numériques d’écoulement sous une chaussure, puis à caractériser expérimentalement certains paramètres d’entrée du modèle (diamètre de particule, forces d’adhésion, mouvement de la chaussure).Le modèle de remise en suspension Rock’n’Roll proposé par Reeks et Hall (2001) a été choisi car il décrit de manière physique ce mécanisme et est basé sur le moment des forces appliquées à une particule. Il nécessite la maîtrise de paramètres d’entrée tels que la vitesse de frottement de l’air, la distribution des forces d’adhésion et le diamètre des particules.Concernant le premier paramètre, des simulations numériques d’écoulement ont été réalisées, à l’aide du code de calcul ANSYS CFX, sous une chaussure de sécurité en mouvement (numérisée par CAO 3D) ; les cartographies des vitesses de frottement obtenues donnent des valeurs de l’ordre de 1 m.s⁻ ¹ pour une vitesse angulaire moyenne de 200 °.s⁻ ¹ .Concernant le deuxième paramètre, des mesures AFM (Atomic Force Microscope) ont été réalisées avec des particules d’alumine ainsi que des particules d’oxyde de cobalt en contact avec des surfaces en époxy représentatives de celles rencontrées dans les installations d’EDF. L’AFM permet d’obtenir la distribution des forces d’adhésion et révèle une valeur moyenne bien plus faible que ce qui peut être calculé de façon théorique en utilisant par exemple le modèle JKR proposé par Johnson et al. (1971). De plus, cette technique, tenant compte des rugosités de surface, montre que plus la taille de la particule augmente, plus la moyenne des forces d’adhésion diminue. Enfin, l’analyse des mesures AFM a permis d’obtenir une corrélation liant la distribution des forces d’adhésion au diamètre des particules, remplaçant celle de Biasi et al. (2001) initialement utilisée dans le modèle Rock’n’Roll et permettant ainsi d’adapter le modèle aux particules et aux revêtements de sol étudiés. Le couplage, effectué dans le code de calcul ANSYS CFX, entre les calculs de vitesses de frottement et le modèle de remise en suspension, a permis de déterminer des taux de remise en suspension théoriques pour le cas d’un cycle unique de marche. Ce couplage a été dans un premier temps validé par une comparaison à l’expérience pour le cas simple d’une plaque en rotation dans un volume contrôlé. En complément, des expériences à l’échelle d’un local ventilé de 30 m³ ont été réalisées en marchant sur un revêtement époxy ensemencé en particules de tailles calibrées (1,1 µm et 3,3 µm). Ces expériences ont permis de mettre en évidence les paramètres influant la remise en suspension des particules, tels que la fréquence de pas et la taille des particules. / In nuclear facilities, during normal operations in controlled areas, workers could be exposed to radioactive aerosols (1 µm < dp < 10 µm). One of the airborne contamination sources is particles that are initially seeded on the floor and could be removed by workers while they are walking. During the outage of EDF nuclear facilities, there is a resuspension of some radionuclides in aerosol form (1 µm < dp < 10 µm). Since the number of co-activity will increase in reactors buildings of EDF, it becomes important to understand particle resuspension due to the activity of the operators to reduce their radiation exposure. The purpose of this Ph.D thesis is to quantify the resuspension of particles due to the progress of operators on a contaminated soil. Thus, the approach is to combine an aerodynamic resuspension model with numerical calculations of flow under a shoe, and then to characterize experimentally some input parameters of the model (particle diameter, adhesion forces, shoes motion).The resuspension model Rock'n'Roll proposed by Reeks and Hall (2001) was chosen because it describes physically the resuspension mechanism and because it is based on the moment of forces applied to a particle. This model requires two input parameters such as friction velocity and adhesion forces distribution applied on each particle.Regarding the first argument, numerical simulations were carried on using the ANSYS CFX software applied to a safety shoe in motion (digitized by 3D CAO); the mapping of friction velocity shows values of about 1 m.s⁻ ¹ for an angular average velocity of 200 °.s⁻ ¹ . As regards the second parameter, AFM (Atomic Force Microscopy) measurements were carried out with alumina and cobalt oxide particles in contact with epoxy surfaces representative of those encountered in EDF power plants. AFM provides the distribution of adhesion forces and reveals a much lower value than what can be calculated theoretically using JKR model (Johnson et al. (1971)). Moreover, this technique, taking into account the surface roughness, shows that adhesion forces decrease while particle diameter increase. Finally, the analysis of AFM measurements gives a correlation linking the distribution of adhesion forces to the particle diameter, replacing the one given by Biasi et al. (2001) originally used in the Rock'n'Roll model and thereby adapt the model to particles and flooring studied in our case.Coupling, performed in ANSYS CFX software, between the calculations of friction velocity and model of particle resuspension, gives theoretical resuspension rate during shoe motion. This coupling was initially validated by comparison to the experience for the simple case of a rotating plate in a controlled volume. Secondly, experiments at the scale of a ventilated room of 30 m³ were performed by walking on an epoxy coating initially seeded by calibrated particle size (1,1 µm and 3,3 µm). These experiments highlight the parameters influencing the suspension of particles, such as step frequency and particle size.

<p>Nedbrytning av organiskt material är en nyckelfaktor som påverkar omvandlingar av de många grundämnen som utgör eller är associerade till just organiskt material. En stor del av nedbrytningen av akvatiskt organiskt material (OM) sker i gränsskiktet mellan sediment och vatten. Eftersom så många biogeokemiska cykler styrs av nedbrytningen av OM är det viktigt att ha kunskap om processer och påverkansfaktorer både på mikro- och makronivå. Mineraliseringshastigheten av OM är en vanligt förekommande mätparameter, men vanligtvis inkluderar mätningarna inte de naturliga processer som kan påverka nedbrytnings-hastigheterna, t.ex. fysiska krafter.</p><p>Syftet med den här studien är att studera mineraliseringshastigheten av det OM som finns naturligt i ytsediment i söt- och brackvatten när det utsätts för fysiska krafter som orsakar förändringar i redox-förhållanden, resuspension eller advektivt porvattenflöde. Fem</p><p>laborativa experiment har utförts för att belysa syftet:</p><p>Åldrat ytsediment från en sötvattens å utsattes för olika redox förhållanden där oxisk respiration, sulfatreduktion respektive metanogenes gynnades. Resultaten visade ingen skillnad i mineraliseringshastighet beroende på behandling. Detta motsäger studier utförda i marina miljöer, där anoxiska förhållanden ger en lägre mineraliseringshastighet än oxiska.</p><p>Vidare gjordes två studier på brackvattensediment där effekten av resuspension var i centrum. Den ena studien fokuserade på frekvens och varaktighet av resuspensionstiderna, den andra på olika typer av sediment. Studierna visade att väldigt korta resuspensionstider med upp till 48 timmars stillhet mellan varje resuspension ökade mineraliseringstakten med fem gånger jämfört med diffusivt utbyte, och mer än dubblerades i jämförelse med kontinuerlig resuspension eller resuspension i långa perioder. Resuspensionen under kort tid var troligen gynnande då resuspension fysiskt stör bildningen av stabila bakteriesamhällen. Mineraliseringshastigheterna i sediment som domineras av väldigt fin, fin eller medium sand visade lika hastigheter, medan grov sand visade en signifikant lägre mineraliseringshastighet. Likheterna mellan de tre första sedimenttyperna kan dock ha påverkats av tillgång på lättnedbrytbart OM då sediment och vatten hämtades in under en algblomning.</p><p>Till sist studerades effekten på mineraliseringshastigheten av advektivt porvattenflöde. Detta gjordes på åldrat sediment dels från en sötvattensbäck dels från en brackvattenstrand. Inget av de två sedimenttyperna visade någon skillnad i mineraliseringshastighet i jämförelse med diffusivt styrda system. Det är i motsats till tidigare marina studier, men är i linje med den första studien, där mineraliseringshastigheten var oberoende av redox-förhållande.</p><p>Den generella slutsatsen från den här studien är nödvändigheten att studera samma aspekter i olika typer av akvatiska system, eftersom responsen verkar vara annorlunda beroende på system, t.ex. söt- brack- och saltvatten. Faktorer som kan förklara de här skillnaderna finns ännu inte, vilket gör att småskaliga studier och modeller blir viktiga verktyg för att utreda detta.</p> / <p>Organic matter mineralisation is a key parameter that affects most other element transformations associated with organic matter. A substantial part of aquatic organic matter (OM) mineralisation takes place at the interface between sediment and water. Understanding OM mineralisation is important at both the micro and macro scales, since it drives many biogeochemical cycles. OM mineralisation rates are widely measured, but generally not all the natural factors possibly affecting the rates, such as physical forcing, are considered.</p><p>This thesis examines the mineralisation rates of indigenous OM in fresh and brackish surface sediments, subjected to different physical forces inducing changed redox conditions, resuspension, and advective pore water flow. Five experiments were performed to this end.</p><p>Aged surface sediment from a freshwater river was subjected to different redox conditions favouring oxic respiration, sulphate reduction, and methanogenesis, respectively. Results indicated no difference in mineralisation rate irrespective of treatment. This contradicts what has been found in marine environments, where anoxic mineralisation rates are lower than oxic ones.</p><p>Further, two studies of resuspension of brackish sediments were performed, one addressing the impact of the frequency and duration of the resuspension events, and the other addressing the impact of resuspension on different types of sediments. The studies found that very brief resuspension events followed by calm periods of up to 48 h increased mineralisation rates by five times compared to diffusion, and more than doubled the rate compared to continuous or long-term resuspension. The short-term events were possibly favoured because resuspension physically disturbs the arrangement of a stable bacteria community. Mineralisation rates on sediments dominated by very fine, fine, or medium-grained sand were the same, while coarse sand displayed a significantly lower rate. The similar rates of the three first sediment types could stem from access to labile OM, due to an ongoing algae bloom when the sediment and water samples were collected.</p><p>Finally, the effect of advective pore water flow on aged sediment from one fresh and one brackish sediment was studied. Neither of the sediments displayed a mineralisation rate different from those occurring in incubations in which only diffusive exchange occurred. This contradicts the findings of previous marine studies, but is in line with the first study, which did not detect different mineralisation rates irrespective of redox conditions.</p><p>The general conclusion is that it is necessary to study the same physical forces in different aquatic environments, since responses appear to differ, for example, between freshwater, brackish, and marine environments. Factors explaining these differences have not yet been expressed, making small-scale studies and modelling a challenge for future research.</p>

Numerous scenarios exist whereby radioactive particulates are transported between spatially separated points of interest. An example of this phenomenon is, in the aftermath of a Radiological Dispersal Device (RDD) detonation, the resuspension of radioactive particulates from the resultant fallout field. Quantifying the spatial distribution of radioactive particulates allow for the calculation of potential radiation doses that can be incurred from exposure to such particulates. Presently, there are no simulation techniques that link radioactive particulate transport with subsequent radiation field determination and so this thesis develops a coupled Computational Fluid Dynamics (CFD) and Monte Carlo (MC) Radiation Transport approach to this problem. Via particulate injections, the CFD simulation defines the spatial distribution of radioactive particulates and this distribution is then employed by the MC Radiation Transport simulation to characterize the resultant radiation field. GAMBIT/FLUENT are employed for the CFD simulations while MCNPX is used for the MC Radiation Transport simulations. / UOIT

Abstract The South China Sea (SCS) is the largest marginal sea in the world and connects with the East China Sea (ECS) through the Taiwan Strait (TS). This study investigates the distribution and biogeochemical behavior of both particulate and dissolved organic matter in the SCS and the TS based on samples collected on several cruises of the R/V Ocean Researchers I and III. Dissolved inorganic nitrogen and phosphorus (DIN and DIP), particulate organic carbon and nitrogen (POC and PON) as well as dissolved organic nitrogen and phosphorus (DON and DOP) concentrations were determined. Concentrations of DON and DOP in the SCS are in the range of 1.2-9.9 mMN and 0.04-0.21 mMP, respectively. The surface DON concentration is the highest in the northern SCS, whereas it is the lowest in the southern part. The DOP does not show a similar trend. DON and DOP concentrations all decrease with depth but increase slightly near the bottom, perhaps on account of sediment resuspension. Because of the preferential degradation of DOP over DON, the maximum concentration of DOP appears at a shallower depth than that of DON. Approximately 11 % and 2 % of DIN and DIP respectively are attributed to the degradation of DON and DOP above 500 m in the SCS. Concentrations of POC and PON in the SCS are in the range of 1.06-2.84 mMC and 0.07-0.36 mMN, respectively. The distributions of POC and PON show similar patterns with a correlation coefficient of 0.97. The concentrations of these are the highest at the surface layer, decrease with depth, but then increase slightly near the bottom, perhaps again because of resuspension of the bottom sediments. The ratio of PON/POC is 0.138 in the euphotic zone, a value close to the Redfield ratio of 0.15. In the TS and the adjacent coastal zones, the effect of terrestrial input is obvious and results in higher POC, PON, DON and DOP nearshore. Ranges of these concentrations are 0.06-59.6 mMN, 0.01-1.29 mMP, 3.80-57.1 mMC and 0.19-3.4 mMN, respectively. There was an attempt to use the one-dimensional diffusion-advection model to estimate the DIN and DIP production rates and the DON and DOP consumption rates over the depth range of 900-2500 m. These values are, respectively, 0.036, 0.006, 0.021 and 0.002 mmol/kg/yr.

This thesis presents a two-phase liquid-solid numerical model using Smoothed Particle Hydrodynamics (SPH). The scheme is developed for multi-phase flows in industrial tanks containing sediment used in the nuclear industry for decommissioning. These two-phase liquid-sediments flows feature a changing interfacial profile, large deformations and fragmentation of the interface with internal jets generating resuspension of the solid phase. SPH is a meshless Lagrangian discretization scheme whose major advantage is the absence of a mesh making the method ideal for interfacial and highly non-linear flows with fragmentation and resuspension. Emphasis has been given to the yield profile and rheological characteristics of the sediment solid phase using a yielding, shear and suspension layer which is needed to predict accurately the erosion phenomena. The numerical SPH scheme is based on the explicit treatment of both phases using Newtonian and non-Newtonian Bingham-type constitutive models. This is supplemented by a yield criterion to predict the onset of yielding of the sediment surface and a suspension model at low volumetric concentrations of sediment solid. The multi-phase model has been compared with experimental and 2-D reference numerical models for scour following a dry-bed dam break yielding satisfactory results and improvements over well-known SPH multi-phase models. A 3-D case using more than 4 million particles, that is to the author’s best knowledge one of the largest liquid-sediment SPH simulations, is presented for the first time. The numerical model is accelerated with the use of Graphic Processing Units (GPUs), with massively parallel capabilities. With the adoption of a multi-phase model the computational requirements increase due to extra arithmetic operations required to resolve both phases and the additional memory requirements for storing a second phase in the device memory. The open source weakly compressible SPH solver DualSPHysics was chosen as the platform for both CPU and GPU implementations. The implementation and optimisation of the multi-phase GPU code achieved a speed up of over 50 compared to a single thread serial code. Prior to this thesis, large resolution liquid-solid simulations were prohibitive and 3-D simulations with millions of particles were unfeasible unless variable particle resolution was employed. Finally, the thesis addresses the challenging problem of enforcing wall boundary conditions in SPH with a novel extension of an existing Modified Virtual Boundary Particle (MVBP) technique. In contrast to the MVBP method, the extended MVBP (eMVBP) boundary condition guarantees that arbitrarily complex domains can be readily discretized ensuring approximate zeroth and first order consistency for all particles whose smoothing kernel support overlaps the boundary. The 2-D eMVBP method has also been extended to 3-D using boundary surfaces discretized into sets of triangular planes to represent the solid wall. Boundary particles are then obtained by translating a full uniform stencil according to the fluid particle position and applying an efficient ray casting algorithm to select particles inside the fluid domain. No special treatment for corners and low computational cost make the method ideal for GPU parallelization. The models are validated for a number of 2-D and 3-D cases, where significantly improved behaviour is obtained in comparison with the conventional boundary techniques. Finally the capability of the numerical scheme to simulate a dam break simulation is also shown in 2-D and 3-D.

Nedbrytning av organiskt material är en nyckelfaktor som påverkar omvandlingar av de många grundämnen som utgör eller är associerade till just organiskt material. En stor del av nedbrytningen av akvatiskt organiskt material (OM) sker i gränsskiktet mellan sediment och vatten. Eftersom så många biogeokemiska cykler styrs av nedbrytningen av OM är det viktigt att ha kunskap om processer och påverkansfaktorer både på mikro- och makronivå. Mineraliseringshastigheten av OM är en vanligt förekommande mätparameter, men vanligtvis inkluderar mätningarna inte de naturliga processer som kan påverka nedbrytnings-hastigheterna, t.ex. fysiska krafter. Syftet med den här studien är att studera mineraliseringshastigheten av det OM som finns naturligt i ytsediment i söt- och brackvatten när det utsätts för fysiska krafter som orsakar förändringar i redox-förhållanden, resuspension eller advektivt porvattenflöde. Fem laborativa experiment har utförts för att belysa syftet: Åldrat ytsediment från en sötvattens å utsattes för olika redox förhållanden där oxisk respiration, sulfatreduktion respektive metanogenes gynnades. Resultaten visade ingen skillnad i mineraliseringshastighet beroende på behandling. Detta motsäger studier utförda i marina miljöer, där anoxiska förhållanden ger en lägre mineraliseringshastighet än oxiska. Vidare gjordes två studier på brackvattensediment där effekten av resuspension var i centrum. Den ena studien fokuserade på frekvens och varaktighet av resuspensionstiderna, den andra på olika typer av sediment. Studierna visade att väldigt korta resuspensionstider med upp till 48 timmars stillhet mellan varje resuspension ökade mineraliseringstakten med fem gånger jämfört med diffusivt utbyte, och mer än dubblerades i jämförelse med kontinuerlig resuspension eller resuspension i långa perioder. Resuspensionen under kort tid var troligen gynnande då resuspension fysiskt stör bildningen av stabila bakteriesamhällen. Mineraliseringshastigheterna i sediment som domineras av väldigt fin, fin eller medium sand visade lika hastigheter, medan grov sand visade en signifikant lägre mineraliseringshastighet. Likheterna mellan de tre första sedimenttyperna kan dock ha påverkats av tillgång på lättnedbrytbart OM då sediment och vatten hämtades in under en algblomning. Till sist studerades effekten på mineraliseringshastigheten av advektivt porvattenflöde. Detta gjordes på åldrat sediment dels från en sötvattensbäck dels från en brackvattenstrand. Inget av de två sedimenttyperna visade någon skillnad i mineraliseringshastighet i jämförelse med diffusivt styrda system. Det är i motsats till tidigare marina studier, men är i linje med den första studien, där mineraliseringshastigheten var oberoende av redox-förhållande. Den generella slutsatsen från den här studien är nödvändigheten att studera samma aspekter i olika typer av akvatiska system, eftersom responsen verkar vara annorlunda beroende på system, t.ex. söt- brack- och saltvatten. Faktorer som kan förklara de här skillnaderna finns ännu inte, vilket gör att småskaliga studier och modeller blir viktiga verktyg för att utreda detta. / Organic matter mineralisation is a key parameter that affects most other element transformations associated with organic matter. A substantial part of aquatic organic matter (OM) mineralisation takes place at the interface between sediment and water. Understanding OM mineralisation is important at both the micro and macro scales, since it drives many biogeochemical cycles. OM mineralisation rates are widely measured, but generally not all the natural factors possibly affecting the rates, such as physical forcing, are considered. This thesis examines the mineralisation rates of indigenous OM in fresh and brackish surface sediments, subjected to different physical forces inducing changed redox conditions, resuspension, and advective pore water flow. Five experiments were performed to this end. Aged surface sediment from a freshwater river was subjected to different redox conditions favouring oxic respiration, sulphate reduction, and methanogenesis, respectively. Results indicated no difference in mineralisation rate irrespective of treatment. This contradicts what has been found in marine environments, where anoxic mineralisation rates are lower than oxic ones. Further, two studies of resuspension of brackish sediments were performed, one addressing the impact of the frequency and duration of the resuspension events, and the other addressing the impact of resuspension on different types of sediments. The studies found that very brief resuspension events followed by calm periods of up to 48 h increased mineralisation rates by five times compared to diffusion, and more than doubled the rate compared to continuous or long-term resuspension. The short-term events were possibly favoured because resuspension physically disturbs the arrangement of a stable bacteria community. Mineralisation rates on sediments dominated by very fine, fine, or medium-grained sand were the same, while coarse sand displayed a significantly lower rate. The similar rates of the three first sediment types could stem from access to labile OM, due to an ongoing algae bloom when the sediment and water samples were collected. Finally, the effect of advective pore water flow on aged sediment from one fresh and one brackish sediment was studied. Neither of the sediments displayed a mineralisation rate different from those occurring in incubations in which only diffusive exchange occurred. This contradicts the findings of previous marine studies, but is in line with the first study, which did not detect different mineralisation rates irrespective of redox conditions. The general conclusion is that it is necessary to study the same physical forces in different aquatic environments, since responses appear to differ, for example, between freshwater, brackish, and marine environments. Factors explaining these differences have not yet been expressed, making small-scale studies and modelling a challenge for future research.