Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1979. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / Ph.D.

Des résultats récents ont montré que les podzols équatoriaux stockent d’importantes quantités de carbone dans leurs horizons Bh profonds. Cette constatation amène deux questions principales : (1) comment et à quel rythme se sont formés ces sols (2) dans quelle mesure le changement climatique pourrait induire une production par ces sols de carbone atmosphérique susceptible d’impacter le système climatique mondial.Dans ce contexte, nous avons réalisé un modèle qui permet de contraindre les flux de carbone à la fois par les stocks observés et leur âge 14C. En situation suffisamment simplifiée, nous avons établi une relation formelle entre l’évolution des stocks et l’âge 14C de celui-ci. Appliqué aux podzols amazoniens, notre modèle a apporté des résultats nouveaux et inattendus. Il a permis de montrer que ce sont les horizons de surface des aires podzolisées les plus hydromorphes qui sont les plus gros contributeurs de MOD transférée vers le réseau hydrographique et la mer. On observe que la formation des Bh n’est possible qu’en envisageant deux compartiments, rapide et lent. Une estimation basse de leur temps de formation permet de différencier des podzols relativement jeunes (temps de formation de l’ordre de 15 103 - 25 103 ans), développés sur des sédiments Holocènes relativement récents, et des podzols âgés (temps de formation de l’ordre de 180 103 - 290 103 ans), développés sur des sédiments plus anciens. Le taux d’accumulation du carbone dans les podzols étudiés varie de 0,54 à 3,17 gC m-2 an-1, ce qui correspond à une séquestration de carbone de l’ordre de 3 1011 gC an-1, faibles à l’échelle annuelle, mais significative aux échelles géologiques.Les expérimentations de percolation en colonne nous ont permis de montrer la réactivité du Bh et la présence, malgré des rapports C/N très élevés (63 en moyenne), d’une activité bactérienne significative qui modifie la nature de la MOD qui le traverse. Cette dernière a la capacité de transporter Al et Fe sous forme de complexes organo-métalliques, complexes susceptibles de migrer à travers des matériaux très kaolinitiques. Ces résultats participent à la compréhension des transferts de MOD d’origine pédologique dans les nappes profondes.Dans l’hypothèse de l’apparition d’un climat à saisons contrastées, nous avons pu montrer qu’une durée sans pluie de 90 jours après disparition de la nappe perchée ne permettrait pas d’atteindre le point d’entrée d’air par assèchement des horizons superficiels. Néanmoins, dans l’hypothèse d’une entrée d’air, l’extrapolation des taux de minéralisation mesurés expérimentalement en conditions oxiques aboutit à une production de C atmosphérique de l’ordre de 2,0 1014 g de CO2 par an, ce qui peut impliquer une rétroaction positive du système climatique mondial. / Recent results have shown that equatorial podzols store large amounts of carbon in their deep Bh horizons. This leads to two main questions: (1) how and at what kinetics these soils were formed, (2) how climate change could induce atmospheric carbon production that could impact the global climate system.In this context, we have developed a model that allows to constrain carbon fluxes both by the observed C stocks and their 14C age. In a sufficiently simplified situation, we have established a formal relationship between the C stock evolution and its 14C age. Applied to Amazonian podzols, our model has brought new and unexpected results. It has been shown that the surface horizons of the most hydromorphic podzolized areas are the largest contributors of MOD transferred to the hydrographic network then to the sea. It is observed that the formation of Bh is only possible by considering two compartments, fast and slow. The estimate of their formation time (low estimate) allowed to differentiate between relatively young podzols (formation time 15 – 25 ky) developed on relatively recent Holocene sediments and old podzols (formation 180 – 290 ky) developed on older sediments. The carbon accumulation rate in the studied podzols ranges from 0.54 to 3.17 gC m-2 y-1, which corresponds to a carbon sequestration around 3 1011 gC an-1, which is significant at the geological scales.Column percolation experiments allowed us to show the reactivity of the Bh material and the presence, despite very high C/N ratios (63 on average), of a significant bacterial activity which modifies the nature of the MOD which percolates through it. This MOD has the capacity to transport Al and Fe in the form of complex organometallic complexes capable of migrating through very kaolinitic materials. These results contribute to the understanding of the transfers of pedologically formed MOD in the deep aquifers.Under the hypothesis of the appearance of a climate with contrasting seasons, we have been able to show that a 90-day period without rain after the disappearance of the perched water-table would not allow to reach the point of entry of air by drying of superficial horizons. Nevertheless, assuming an air entry, the extrapolation of the experimentally measured mineralization rates under oxic conditions results in a production of atmospheric C around 2.0 1014 g of CO2 per year, which may involve a positive feedback from the global climate system.

Cette thèse s'intéresse à la turbulence dans l'hélium (4He) superfluide, à des températures comprises entre 1.15 K et la température de transition superfluide, Tlambda = 2.17 K, ce qui correspond à une fraction de superfluide rho_s/rho comprise entre 97.6% et 0%. Il s'agit d'un travail essentiellement expérimental dont le but est de comparer la turbulence classique et la turbulence quantique, à l'aide de mesures locales de fluctuations de vitesse et de vorticité. Ces mesures sont complétées par l'analyse de champs de vitesse issus de simulations numériques.Nous avons développé une instrumentation spécifique, adaptée aux écoulements cryogéniques: des tubes de Pitot miniatures, dont les dimensions effectives ont pu être rendues sub-millimétriques, et un capteur original, basé sur la déflection d'une micro-poutrelle (300 microns x 100 microns x 1 microns) mesurée à l'aide d'un micro-résonateur supraconducteur dans la gamme de fréquence du GHz. Un premier prototype de ce capteur, micro-fabriqué à partir d'une galette de silicium, a été réalisé en salle blanche puis validé dans une conduite cryogénique. La résolution spatiale obtenue est du même ordre que celle des meilleurs anémomètres en He II, et il devrait être possible de l'améliorer d'une décade.Les tubes de Pitot ont été placés dans les souffleries superfluides TSF et TOUPIE. La première, fruit d'une collaboration nationale, a fourni un écoulement stationnaire de grille (Rlambda= 250, 1.65 K < T < 2.6 K, rho_s/rho < 80%) et un sillage proche. La seconde soufflerie, refroidie pour la première fois dans le cadre de cette thèse, a fourni un écoulement de sillage lointain (Rlambda = 1100, 1.55 K < T < 4.2 K, rho_s/rho < 86%).Les mesures ont mis en évidence dans ces écoulements des similarités fortes avec les écoulements classiques, aux échelles inertielles: spectre de vitesse en k^{-5/3}, constante de Kolmogorov et taux de turbulence identiques aux écoulements classiques, loi des 4/5, exposants anormaux pour les fonctions de structures des incréments de vitesse (intermittence). À plus petite échelle, les simulations numériques (1.15 K < T < 2.1565 K) mettent en évidence un comportement exotique : l'énergie s'accumule et tend vers l'équipartition, ce qui se traduit par un spectre de vitesse simulé en k^2. Ce phénomène s'accentue à basse température. Enfin, des mesures locales de fluctuation de vorticité ont été réalisées à l'aide de pinces à second son sur une gamme de température comprise entre 1.69 K (rho_s/rho=77%) et 2.01 K (rho_s/rho = 42%). Nous avons observé un raidissement de la pente des spectres de vorticité lorsque la température diminue. Ce résultat peut être interprété comme une conséquence du phénomène d'équipartition mis en évidence dans les simulations numériques à petite échelle. / The focus of this thesis is the turbulence of superfluid 4He at temperatures between 1.15K and the superfluid transition temperature Tlambda = 2.17K, corresponding to a superfluid fraction rho_s/rho between 97.6% and 0%. This work is mostly experimental. We aim to compare classical and quantum turbulence, using local velocity and vorticity fluctuations measurements. These measurements are backed up by numerical simulations.We developed dedicated probes, designed for cryogenic flows: Pitot tubes with sub-millimeter effective size and a new cantilever-based probe (300 microns x 100 microns x 1 microns) whose deflection is measured with a superconducting micro-resonator in the GHz frequency range. A first prototype was micro-machined from a silicon wafer in cleanroom and validated in a superfluid wind tunnel. The resolution was found similar to the one of the best anemometers operating in He~II and will be further improved.The Pitot tubes have been inserted inside two superfluid wind tunnels, TSF and TOUPIE. The former, designed and operated within a national collaboration, provided a stationary grid flow (Rlambda = 250, 1.65 K < T < 2.6 K, rho_s/rho < 80%) and a near-wake flow. The latter (Rlambda = 1100, 1.55 K < T < 4.2 K, rho_s/rho < 86%), which was cooled down for the first time during this thesis, provided a far-wake flow.The measurements have highlighted strong similarities with classical flows at inertial scales: k^{-5/3} velocity spectra, Kolmogorov constant and turbulence intensity indistinguishable above and below the superfluid transition, 4/5-law, anomalous velocity structure functions exponents (intermittency). At smaller scales, the numerical simulations (1.15 K < T < 2.1565 K) exhibit exotic behavior: kinetic energy piles up and tends to equipartition, which makes the simulated velocity spectrum scale like k^2. This phenomenon is enhanced at low temperature. Finally, local vorticity fluctuations measurements have been achieved using second sound tweezers over a temperature range, between 1.69 K (rhos/rho=77%) and 2.01 K (rhos/rho = 42%). We observed that the spectrum scaling steepens as the temperature decreases. This can be interpreted as a consequence of the equipartitioned reservoir evidenced by numerical simulations at small scale.

Utility companies and regulatory agencies are interested in understanding juvenile salmon swimming patterns as they approach hydropower dams because it can allow them to assess fish bypass efficiency and conduct fish survival studies. A model capable of predicting juvenile salmon swim paths can assist in the design of fish bypasses and diversion structures. This thesis presents a mechanistic model tailored to simulate swimming patterns of juvenile salmon swimming in forebays, tailraces, and free-flowing rivers. The model integrates information on juvenile salmon behavior at both field and laboratory scale and literature on juvenile salmon swimming capabilities. Simulated fish swim paths are determined by solving Newton's Second Law. Most of the model parameters are represented by probability distributions. Behavioral responses are triggered for the most part by the flow acceleration and pressure. The model uses conditional probability distributions of thrust magnitude and direction, given flow acceleration. Simulated fish select a swimming direction referenced to the flow velocity vector. To consider juvenile salmon's tendency to coast with the flow, the model intersperses periods of active swimming and gliding. Chinook salmon measured swim paths were analyzed. The flow variables at the fish locations were obtained from CFD simulations. Juvenile salmon mean thrust was determined from solving Newton's Second Law at every measured location. Results show that as flow acceleration increases, the juvenile salmon average thrust increases and the probability of gliding decreases. Chinook salmon tend to migrate tail-first as flow acceleration increases. For the flow accelerations of 5x10-4 m/s2 and 1x10-2 m/s2, approximately 85% and 95% of the analyzed fish migrated tail-first, respectively. The model capacity to predict fish migration route selection, fish-like trajectories, and residence times was tested at two hydropower dams. On average, migration routes were predicted with 17 percent of relative error. Model predictions for fish average residence times were within 10 percent of measured values.

Numerical modeling has become a major tool for managing water quality in surface waterbodies such as rivers, lakes, reservoirs, and estuaries. Since the two-dimensional longitudinal/vertical model CE-QUAL-W2 is a well-known model and it has been applied to thousands of waterbodies around the world successfully, its numerical scheme was adapted to develop a new three-dimensional numerical model for simulating hydrodynamics, temperature, and water quality in surface waterbodies. Finite difference approximations were used to solve the fluid dynamic governing equations of continuity, free water surface, momentums, and mass transport. No coordinate transformations were performed and the z-coordinate system has been used. Higher-order schemes (QUICK, QUICKEST, and ULTIMATE QUICKEST) in addition to the UPWIND scheme were used for the advective temperature and mass transport. A novel numerical approach was used for the numerical formulation of the three-dimensional scheme. This approach forced the numerical solution of the free surface equation to be a tri-diagonal matrix form rather than a more computationally intensive penta-diagonal matrix solution. This new approach was done by linking a method called line-by-line with the free water surface numerical solution. Another new approach was that the three-dimensional numerical scheme involved a simultaneous solution of hydrodynamics, temperature, and water quality at every model time level instead of saving the hydrodynamic results to be used later for water quality simulation. Hence, this scheme allowed feedback between the hydrodynamics and water quality every time step. In addition, various unique numerical algorithms were employed from CE-QUAL-W2 such as the W2 turbulence model, selective withdrawal theory, surface heat fluxes, and water quality sources and sinks, making the three-dimensional model built on well-tested algorithms. To test the model structure and assumptions, an analytical verification was performed by comparing model predictions to known analytical exact solutions test cases. Good agreement was showed by the model for all of these tests. A computation of the volume balance over the simulation period was also incorporated within the model to assess how well the code performed. Sensitivity tests were also made varying bed and wind shear. The model was also applied to three reservoirs in the USA as field case studies: Lake Chaplain in WA, Laurance Lake in OR, and Cooper Creek Reservoir in OR. The model was validated by comparing the model predictions of water levels, velocities, vertical temperature profiles, and dissolved oxygen with field data. Through these real applications, the numerical predictions of the 3D model showed good agreement with field data based on error statistics. The model results of each field case study were discussed separately. In the Lake Chaplain model application, the study was focused on the importance of the higher-order schemes compared to the first-order UPWIND scheme. The model predictions of temperature were determined by using the UPWIND, QUICK, and QUICKEST scheme and compared with field data. The Error statistics of the model predictions compared to field data were an absolute mean error (AME) of 0.065 m for the water level predictions and an overall AME of 1.62 °C, 1.09 °C, and 1.23 °C for the temperature predictions by using the UPWIND, QUICK, and QUICKEST scheme, respectively. In the Laurance Lake model application, a comparison was performed between the present 3D model and the 2D CE-QUAL-W2. Since the 3D model was build based on CE-QUAL-W2, differences between the two models were evaluated. Error statistics between the model predictions of water level and temperature compared to field data showed that both models were in good agreement with field data. However, the 3D model AME (0.30 m for the water level predictions and 0.48 °C for the temperature predictions) was higher than the 2D model (0.03 m for the water level predictions and 0.42 °C for the temperature predictions). Finally, the Cooper Creek Reservoir case study was done to show the model predictions of temperature and dissolved oxygen. In this application, vertical temperature profiles were covered the entire simulation period in order to show how the model transfer heat between stratification and non- stratification conditions. The model showed good agreement with field data (0.12 m AME for the water level predictions, 1.00 °C overall AME for the temperature predictions, and 1.32 g/m3 overall AME for the dissolved oxygen predictions). Finally, comparisons were made between CE-QUAL-W2 and the 3D model. The 2D model generally performed better in the tests cases if the model user is unconcerned about lateral impacts. The 3D model is important to use when lateral currents and variation in the lateral dimension are important.

Blind Pass, a heavily structured wave-dominated tidal inlet on the west central coast of Florida, has undergone substantial morphologic changes in the past 150 years. Initially Blind Pass was a mixed-energy inlet. In 1848 a hurricane opened a new inlet to the north called Johns Pass, which captured a large portion of the tidal prism of Blind Pass. Since then Blind Pass migrated southward until it was structurally stabilized in 1937. The decreasing tidal prism resulted in significant inlet channel filling. The channel has been dredged 12 times since 1937. The present inlet is stabilized by two jetties and a series of seawalls. Detailed time-series field measurements of bathymetry and tidal flows were conducted between 2001 and 2004, after the last channel dredging in the summer of 2000. The measured depositional rate in the inlet channel approximately equals the net southward longshore transport rate. This suggests that the inlet has served as a trap for the southward longshore transport allowing negligible bypassing to the eroding downdrift beach. Most of the active sedimentation occurs on the northern side of the inlet. The sediment in the thalweg is largely coarse shell lag, indicating adequate sediment flushing by the ebbing tide. The cross-channel flow measurements revealed that ebb flow was approximately twice as high in the channel thalweg as compared with the rest of the channel. The flood flow was largely uniform across the entire inlet and dominated over the northern portion of the inlet due to the weak ebb flow there. This cross-channel flow pattern is crucial to the understanding of the sedimentation patterns in the Blind Pass channel. Two years after the last dredging the mouth has become shallow enough to induce wave breaking across the shoal area. Distinctive seasonal patterns of sedimentation were measured thereafter in the inlet channel, influenced by seasonal wave climate. The sedimentation is event driven from passage of cold fronts bringing elevated wave energy that accelerates the southward longshore transport. During normal conditions the sediment deposited in the mouth area is redistributed further into the inlet by the flood current combined with wave-driven current.

Bunces Pass is an unstructured tide-dominated inlet just north of the main entrance to Tampa Bay, Florida. The inlet has been stable for at least 130 years, as the size, shape, and orientation have remained unchanged. The morphological evolution of the Bunces Pass ebb-tidal delta is influenced by adjacent inlets. Historically, the ebb tidal delta was extremely large, due to the presence of the south channel of Pass-A-Grille Pass. As the tidal prism decreased through the south channel, the sheltering effect produced by the large ebb tidal delta diminished, and large volumes of sand began migrating shoreward. Sediment from the ebb tidal delta accreted along "the Reefs", formed both North Bunces Key and South Bunces Key, and accreted on Mullet Key south of the inlet. Tidal currents at Bunces Pass are primarily ebb-dominant during both summer and winter seasons, though there is flood dominance for several days during neap tides. The ebb dominance is primarily due to the large back-barrier embayment of Tampa Bay, which results in a spring ebb tidal prism of 2.02 x 107 m³. This tidal prism is more than 400 times the corresponding littoral drift. It is primarily responsible for maintaining the inlet's stability, as well as the development of its large ebb-tidal delta. Sediments from the ebb tidal delta at Bunces Pass reflect different degrees of wave versus tidal energy. The strongest tidal currents present throughout the entire ebb tidal delta complex mechanically weather shell gravel in the main channel, producing a shelly, fine quartz sand with relatively high amounts of shell gravel and carbonate sand. This sub-facies is also present on the north channel margin linear bar, due to the interaction of waves, tidal currents, and a southerly littoral drift along this coastal reach. Fine, quartz sand dominates the off shore and swash platform environments. The present situation at Bunces Pass shows a stabilized, tide-dominated inlet with a large, elongate ebb delta that is unlikely to change significantly in the future if present conditions are maintained. The prevalent ebb-dominance suggests that the inlet is hydraulically connected to the adjacent and much larger Egmont Channel inlet system, which also serves Tampa Bay. Strong ebb-tidal currents have kept Bunces Pass in dynamic equilibrium with its surrounding environment. The large ebb tidal prism is responsible for explaining how a tide-dominated inlet is maintained in a microtidal environment.

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