527 research outputs found
Simulation of proglacial forefield morphodynamics and their implications for the filtering of subglacial sediment export following glacier retreat
No full textAbstract Proglacial forefields are geomorphologically active zones with rivers that may buffer sediment connectivity between glaciers and downstream regions. To date, there are few studies of the magnitude of this process and the role of system‐internal perturbations, such as meltwater discharge rates, subglacial sediment supply and changing valley topography (e.g. valley slope, lateral accommodation space) following glacier retreat. Here, we use a physically‐based morphodynamic model, calibrated using continuous field‐collected data, to investigate the geomorphic response of these fluvial systems to varying topographical, sedimentological and hydrological boundary conditions. Results confirm the central role of the supply to transport capacity ratio in determining both proglacial stream geomorphic response and downstream sediment delivery rates, particularly for bedload particles; while suspended sediment transport is less affected by morphodynamic processes. Simulations also showed that the filtering effect is influenced by the lateral accommodation space and valley bottom slope. These observations not only suggest that the filtering varies between geomorphological settings, but also that the form of a valley (width, slope), as it is revealed following glacier retreat, will also impact filtering. These findings are important for understanding the future evolution of the longitudinal sediment connectivity in deglaciating catchments, in light of likely peak water and peak sediment. In the near‐future, the evolution of the proglacial margin filtering will have repercussion for the supply of sediment towards downstream regions, for the management of hydropower plants and for aquatic ecosystems.Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung https://doi.org/10.13039/50110000171
The effects of river restoration on catchment scale flood risk and flood hydrology
No full textA rising exposure to flood risk is a predicted consequence of increased development in vulnerable areas and an increase in the frequency of extreme weather events due to climate change. In the face of this challenge, a continued reliance on engineered at-a-point flood defences is seen as both unrealistic and undesirable. The contribution of ‘soft engineering’ solutions (e.g. riparian forests, wood in rivers) to integrated, catchment scale flood risk management has been demonstrated at small scales but not larger ones. In this study we use reduced complexity hydrological modelling to analyse the effects of land use and channel changes resulting from river restoration upon flood flows at the catchment scale. Results show short sections of river-floodplain restoration using engineered logjams, typical of many current restoration schemes, have highly variable impacts on catchment-scale flood peak magnitude and so need to be used with caution as a flood management solution. Forested floodplains have a more general impact upon flood hydrology, with areas in the middle and upper catchment tending to show reductions in peak magnitude at the catchment outflow. The most promising restoration scenarios for flood risk management are for riparian forest restoration at the sub-catchment scale, representing 20–40% of the total catchment area, where reductions in peak magnitude of up to 19% are observed through de-synchronization of the timings of sub-catchment flood waves. Sub-catchment floodplain forest restoration over 10–15% of total catchment area can lead to reductions in peak magnitude of 6% at 25 years post-restoration
Tudor and early Stuart vessel glass: an archaeological study of forms and patterns of consumption in England, 1500 to 1640
No full textThe aims of this thesis are twofold. The first is concerned with the establishment of a typology for vessel glass in England between 1500 and 1640. There has been no morphological classification for glass of this period and one is constructed in this thesis from museum collections, published and unpublished material derived from archaeological excavations. The second aim of this thesis is to explore the way that glass vessels were used in Tudor and early Stuart society. The rise of consumerism and role of consumption in early modern Europe has been explored by a number of scholars, but there has been little attempt to link these ideas with excavated material culture. To achieve these aims twelve groups of glass from a variety of well contexted sites have been examined. The glass from these, in conjunction with seventy-four published excavation reports, forms the basis for the vessel classification. Although classified primarily by their form the typology considers questions concerning the manufacturing provenance and the decorative techniques used on the vessels. Likewise the twelve study sites are used as the basis for a more contextualised material culture study. Differences between assemblages from urban and elite sites are considered, as are their relative forms of disposal. Further questions concerning the role of glass during dining and the importance of vessel decoration as a means of conveying social messages are addressed. Finally contrasting patterns of repair and conspicuous consumption are considered. Whilst providing a framework for future research into the glass used in Tudor and early Stuart England, this thesis advocates a new methodological approach for material culture studies. It has demonstrated that through a more contextualised study of artefacts, a greater understanding of material culture use can be achieved
The role of proglacial forefields in filtering the signal of subglacial sediment export
No full textThe rapid recession of glaciers due to climate warming is increasing the total surface of proglacial margins, landscapes relatively poor at first in terms of organic matter but shaped by high geomorphic activity, including hillslope processes and river morphodynamics. Under suitable conditions, proglacial forefields may develop and contain morphologically active braid plains. These are zones of accumulation, transport and erosion of glacially-outwashed sediment characterized by a network of shallow, unstable channels which continually shift their position in response to discharge and sediment load variation associated with glacial meltwater and sediment supply.
Field measurements, numerical simulations and scaled laboratory experiments have all been used to study the formation, maintenance and evolution of these fluvial systems. However, knowledge on how forefield morphodynamics interact with upstream boundary conditions (i.e. subglacial sediment export and meltwater runoff variation), and the effects of these interactions on downstream sediment transport, is still lacking. This gap arises from practical limitations related to the inability to collect continuous (subglacial) sediment transport rates, notably for bedload sized sediment, concurrent with spatially extensive, high resolution and high precision information on river morphodynamics.
Given the above, the core aim of the thesis is to undertake the first, coupled study of the relationship between continuous records of subglacial sediment export, proglacial forefield morphodynamics and downstream sediment fluxes for a retreating Alpine glacier. The focus is on the Glacier d’Otemma proglacial floodplain (south-western Swiss Alps), a region particularly suitable for this study as characterized by a high disconnectivity between the valley-sidewalls and the valley-bottom minimizing hillslope influences on the river. A set of methodological approaches for continuously quantifying sediment transport for both suspended sediment and bedload particles through the proglacial forefield were deployed. The comparison of sediment transport data collected at the glacier terminus with that at the forefield outlet using established signal post-processing techniques revealed that morphodynamic processes rapidly filter (i.e. dampen and delay) the subglacial sediment export signal related to bedload, while that of suspended sediment passes almost unimpeded through the proglacial margin.
To understand the operation of this filter, an intensive survey of fluvial morphodynamics and surface sedimentology was undertaken. Daily drone surveys of the floodplain were used to quantify spatial patterns of erosion and deposition. Central to the latter was the development of a heuristic model for predicting water depth distribution in highly-turbid braided rivers to generate elevation models for both dry and inundated areas. These were then combined with the subglacial sediment export rates to quantify the influence of proglacial morphodynamics for downstream sediment transport. Changes in the balance between glacial sediment supply and transport capacity resulted in rapid changes in river morphodynamics. During periods when subglacial bedload supply exceeded transport capacity there was aggradation in the forefield, accompanied by more intense bar construction, increased channel instability and rising braiding intensity and surficial coarsening of floodplain deposits. The forefield acted as a sink for both bedload and suspended load sediment. In contrast, when the subglacial bedload export rates were lower than transport capacity, the forefield continued to act as a sediment sink for fine sediment, but the system was net degradational for bedload sediment.
To generalize these findings to situations with different topographical, sedimentological and hydrological settings, it was used an hydromorphological calibrated with field-collected data. Results confirmed the central role of the supply to transport capacity ratio in determining both proglacial stream geomorphic response and downstream sediment delivery rates; with lower ratios leading to more hydraulically efficient scenarios and more ready transfer of the subglacial sediment export signal to downstream. However, this effect was also influenced by larger-scale forcing of valley topography including accommodation space, suggesting that the filtering effect varies between geomorphological settings.
The results of this thesis demonstrate that proglacial forefields play a significant role for the sediment connectivity between glacier termini and downstream regions. These findings have important implications for natural hazards mitigation, ecosystem development, hydropower plants management and glaciological studies in the actual context of rapid glacier recession.
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La rapide récession des glaciers due au réchauffement climatique accroît la surface totale des marges proglaciaires, paysages relativement pauvres en matière organique mais façonnés par une activité géomorphologique intense, comprenant les processus de versant et la morphodynamique fluviale. Dans des conditions géomorphologiques appropriées, des marges proglaciaires peuvent se former et contenir des cours d’eau tressés morphologiquement actifs. Ces derniers sont des zones d'accumulation, de transport et d'érosion des sédiments glaciaires caractérisées par un réseau de chenaux peu profonds et instables qui changent continuellement leur position en réponse aux variations de débit et de charge sédimentaire associées à la fonte glaciaire et à l'approvisionnement en sédiments.
Les mesures sur le terrain, les simulations numériques et les expériences de laboratoire à l'échelle réduite sont des outlis qui ont été utilisés pour étudier la formation, la maintenance et l'évolution de ces systèmes fluviaux. Cependant, les connaissances dont la morphodynamique des avant-pays interagit avec les conditions limites en amont (c'est-à-dire l'exportation de sédiments sous-glaciaires et la variation du ruissellement de fonte), et les effets de ces interactions sur le transport sédimentaire en aval, font encore défaut. Cette lacune découle de limitations pratiques liées à l'incapacité à collecter en continu des taux de transport de sédiments (sous-)glaciaires, notamment pour les sédiments transportés en contact avec le lit du cours d’eau, simultanément avec des informations spatialement étendues, à haute résolution et précision sur la morphodynamique fluviale.
L'objectif principal de la thèse est d'entreprendre la première étude couplée de la relation entre les enregistrements continus de l'exportation de sédiments subglaciaires, la morphodynamique des marges proglaciaires actives et les flux sédimentaires aval pour un glacier alpin en retrait. L'accent est mis sur la plaine d'inondation proglaciaire du Glacier d'Otemma (Alpes suisses du sud-ouest), une région particulièrement propice à cette étude car caractérisée par une forte déconnexion sédimentaire entre les versants et le fond de la vallée. Un ensemble d'approches méthodologiques pour quantifier en continu le transport de sédiments en suspension et du charriage de fond à travers la marge proglaciaire ont été déployé. La comparaison des données de transport de sédiments collectées au terminus du glacier avec celles à la sortie du système proglaciaire à l'aide de techniques de post-traitement du signal ont révélé que les processus morphodynamiques filtrent rapidement (atténuent et retardent) le signal d'exportation de sédiments sous-glaciaires lié au charriage, tandis que celui des sédiments en suspension passe presque sans entrave à travers la marge proglaciaire.
Pour comprendre le fonctionnement de ce filtre, une enquête intensive sur la morphodynamique fluviale et la sédimentologie de surface a été menée. Des relevés quotidiens par drone de la plaine d'inondation ont été utilisés pour quantifier la distribution spatiale d'érosion et de dépôt. Au cœur de cette analyse, il y avait le développement d'un modèle heuristique pour prédire la distribution de la profondeur de l'eau dans les rivières tressées très turbides afin de générer des modèles numériques de terrain pour les zones sèches et inondées. Ces derniers ont ensuite été combinés avec les taux d'exportation de sédiments sous-glaciaires pour quantifier l'influence de la morphodynamique proglaciaire sur le transport sédimentaire aval. Les changements dans l'équilibre entre l'approvisionnement en sédiments glaciaires et la capacité de transport ont entraîné des changements rapides dans la morphodynamique fluviale. Pendant les périodes où l'exportation en charriage sous- glaciaires dépassait la capacité de transport, il y avait une aggradation de la plaine alluviale, accompagnée d'une construction de barres plus intense, d'une instabilité accrue des chenaux et d'un grossissement des dépôts superficiels. La marge proglaciaire a agi comme un puits pour les sédiments fins et grossiers. En revanche, lorsque les taux d'exportation de sédiments de charriage sous-glaciaires étaient inférieurs à la capacité de transport, la plaine alluviale a continué à agir comme un puits pour les particules fines, mais le système était en nette dégradation pour les sédiments grossiers.
Pour généraliser ces résultats à des situations avec des configurations topographiques, sédimentologiques et hydrologiques différentes, un modèle hydromorphologique a été calibré avec les données collectées directement sur le terrain. Les résultats ont confirmé le rôle central du rapport entre l'approvisionnement et la capacité de transport pour la détermination de la réponse géomorphologique des rivières proglaciaires et des débits sédimentaires vers l’aval; avec des ratios plus faibles conduisant à des scénarios plus hydrauliquement efficaces et un transfert plus facile du signal d'exportation de sédiments sous-glaciaires à travers la marge proglaciaire. Cependant, cet effet était également influencé par les forçages à plus grande échelle de la topographie de la vallée, notamment l'espace d'accommodation et la pente, suggérant que l'effet de filtrage varie beaucoup en fonction des configurations géomorphologiques.
Les résultats de cette thèse démontrent que les cours d’eau proglaciaires tréssés jouent un rôle significatif pour la connectivité sédimentaire entre les terminus glaciaires et les régions aval. Ces résultats ont d'importantes implications pour l'atténuation des risques naturels, le développement des écosystèmes, la gestion des centrales hydroélectriques et les études glaciologiques dans le contexte actuel de rapide récession des glaciers.
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Il riscaldamento climatico sta modificando profondamente il paesaggio alpino, causando una rapida recessione dei ghiacciai e aumentando la superficie totale dei loro margini, detti margini proglaciali. I margini proglaciali sono zone relativamente povere in materia organica ma modellate da un'attività geomorfologica intensa, compresi i processi di versante e la morfodinamica fluviale. In condizioni geomorgologiche adeguate, possono formarsi margini proglaciali ospitanti corsi d’acqua intrecciati e morfologicamente attivi. Questi corsi d’acqua sono zone di accumulo, trasporto ed erosione dei sedimenti glaciali e sono caratterizzati da una fitta rete di canali poco profondi e instabili che cambiano continuamente la loro posizione in risposta alle variazioni di portata e di carico sedimentario che, a loro volta, sono associati alla fusione glaciale e all'approvvigionamento in sedimenti.
Gli strumenti utilizzati in passato per studiare la formazione, il mantenimento e l'evoluzione di questi sistemi fluviali sono le misure sul campo, le simulazioni numeriche e gli esperimenti di laboratorio in scala ridotta. Tuttavia, si conosce ancora poco di come l'esportazione di sedimenti subglaciali e la variazione del deflusso di fusione a monte interagisca con la morfodinamica dei margini proglaciali attivi, e gli effetti di queste interazioni sul trasporto sedimentario a valle. Questa lacuna deriva da limiti pratici legati all'incapacità di raccogliere in modo continuo e simultaneo sia i tassi di trasporto di sedimenti (sotto-)glaciali (in particolare per i sedimenti trasportati in contatto con il letto fluviale), sia le informazioni sulla morfodinamica fluviale su ampie superfici, ad alta risoluzione e di alta precisione.
Considerando quanto sopra, l'obiettivo principale della tesi è di intraprendere il primo studio congiunto della relazione tra registrazioni continue dell'esportazione di sedimenti subglaciali, morfodinamica dei corsi d’acqua proglaciali intrecciati e flussi sedimentari a valle per un ghiacciaio alpino in ritirata. La zona di studi scelta per questa tesi è la pianura alluvionale proglaciale del Ghiacciaio di Otemma (Alpi svizzere sud-occidentali), una regione particolarmente adatta per questo progetto poiché caratterizzata da una forte disconnessione sedimentaria tra i versanti e il fondo valle attenuante l’apporto di sediment verso il fiume. Il margine proglaciale è stato analizzato usando molteplici approcci metodologici per quantificare in modo continuo e simultaneo sia il trasporto di sedimenti in sospensione, sia il trasporto di sedimenti di fondo. I dati di trasporto dei sedimenti raccolti al portale glaciale sono stati confrontati con quelli all'uscita del margine proglaciale utilizzando tecniche di post- elaborazione del segnale; tali dati hanno rivelato che i processi morfodinamici filtrano rapidamente, ovvero attenuano e ritardano, il segnale di esportazione di sedimenti subglaciali legato al trasporto di fondo, mentre quello dei sedimenti in sospensione passa quasi senza ostacoli attraverso il margine proglaciale.
Per comprendere il funzionamento di questo filtro, è stata condotta un'indagine intensiva sulla morfodinamica fluviale e sulla sedimentologia superficiale. Sopralluoghi giornalieri con drone della pianura alluvionale sono stati utilizzati per quantificare la spazialità dell’erosione e della deposizione. Al centro di quest'ultimo vi era lo sviluppo di un modello euristico per predire la distribuzione della profondità dell'acqua nei fiumi intrecciati torbidi al fine di generare modelli numerici del terreno sia per le aree asciutte che per le aree inondate. Questi ultimi sono stati poi combinati con i tassi di esportazione di sedimenti subglaciali per quantificare l'influenza della morfodinamica proglaciale sul trasporto sedimentario a valle. I cambiamenti nell'equilibrio tra l'approvvigionamento di sedimenti sotto-glaciali e la capacità di trasporto hanno comportato cambiamenti rapidi nella morfodinamica fluviale. Durante i periodi in cui l'approvvigionamento sotto-glaciale di ghiaia superava la capacità di trasporto, c'era una aggradazione della pianura alluvionale, accompagnata da una costruzione di isole più intensa, una maggiore instabilità dei canali, un'intensificazione dell’’intreccio, e un ingrossamento dei depositi superficiali. Il margine proglaciale ha agito come un pozzo sia per i sedimenti grossieri che per quelli fini. Al contrario, quando i tassi di esportazione sotto-glaciale di ghiaia erano inferiori alla capacità di trasporto, la pianura alluvionale ha continuato ad agire come un pozzo per i sedimenti fini, ma il sistema era netto degradativo per i sedimenti grossieri.
Per generalizzare questi risultati a situazioni con diverse configurazioni topografiche, sedimentologiche e idrologiche, è stato utilizzato un modello idromorfologico calibrato e validato utilizzando dati raccolti direttamente sul campo. I risultati hanno confermato il ruolo centrale del rapporto tra approvvigionamento sedimentario e capacità di trasporto nella determinazione sia della risposta geomorfologica dei corsi d'acqua proglaciali sia dei tassi d’esportazione sedimentaria a valle; con rapporti più bassi che portano a scenari più idraulicamente efficienti e un trasferimento più facile del segnale di esportazione di sedimenti subglaciali attraverso il fiume proglaciale. Tuttavia, questo effetto è anche influenzato da fattori a più grande scala come la topografia della valle, in particolare lo spazio di alloggiamento laterale e la pendenza, suggerendo che l'effetto di filtraggio varia tra le configurazioni geomorfologiche.
I risultati di questa tesi dimostrano che i margini proglaciali attivi svolgono un ruolo significativo per la connettività sedimentaria tra i termini glaciali e le regioni a valle. Questi risultati hanno importanti implicazioni per la mitigazione dei rischi naturali, lo sviluppo degli ecosistemi, la gestione delle centrali idroelettriche e gli studi glaciologici nel contesto attuale di rapida recessione dei ghiacciai.
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Die rasche Gletscherschmelze infolge der Klimaerwärmung vergrößert die Gesamtfläche der proglazialen Ränder, Landschaften, die anfangs relativ arm an organischem Material sind, aber durch hohe geomorphologische Aktivität, einschließlich Hangprozesse und Flussmorphodynamik, geformt werden. Unter geeigneten Bedingungen können sich proglaziale Vorfelder entwickeln und morphologisch aktive verzweigte Flussebenen enthalten. Diese Zonen der Akkumulation, des Transports und der Erosion von glazial abgelagerten Sedimenten sind durch ein Netzwerk von flachen, instabilen Kanälen gekennzeichnet, die ihre Position kontinuierlich in Reaktion auf Abfluss- und Sedimentlaständerungen ändern, die mit Gletscherschmelzwasser und Sedimentzufuhr verbunden sind.
Feldmessungen, numerische Simulationen und maßstabsgetreue Laborexperimente wurden verwendet, um die Entstehung, Aufrechterhaltung und Entwicklung dieser Flusssysteme zu untersuchen. Es mangelt jedoch noch an Wissen darüber, wie die Morphodynamik der Vorfelder mit den Bedingungen am Oberlauf (d. h. dem subglazialen Sedimentexport und der Variation des Schmelzwasserabflusses) interagiert und welche Auswirkungen diese Interaktionen auf den Sedimenttransport flussabwärts haben. Diese Lücke entsteht aus praktischen Einschränkungen, die es unmöglich machen, kontinuierliche (subglaziale) Sedimenttransportraten zu erfassen, insbesondere für Sediment in Geschiebekornfraktion, zusammen mit räumlich ausgedehnten, hochauflösenden und präzisen Informationen über die Flussmorphodynamik.
Vor diesem Hintergrund ist das Hauptziel der Dissertation, die erste gekoppelte Studie zur Beziehung zwischen kontinuierlichen Aufzeichnungen des subglazialen Sedimentexports, der Morphodynamik proglazialer Vorfelder und den Sedimentflüssen flussabwärts für einen sich zurückziehenden alpinen Gletscher durchzuführen. Der Fokus liegt auf der proglazialen Überflutungsebene des Glacier d’Otemma (südwestliche Schweizer Alpen), einer Region, die sich besonders gut für diese Studie eignet, da sie durch eine hohe Dissoziation zwischen den Talwänden und dem Talboden gekennzeichnet ist, was den Einfluss von Hangprozessen auf den Fluss minimiert. Eine Reihe methodischer Ansätze zur kontinuierlichen Quantifizierung des Sedimenttransports für sowohl Schwebstoffe als auch Geschiebesedimente durch das proglaziale Vorfeld wurden eingesetzt. Der Vergleich der am Gletschertor gesammelten Sedimenttransportdaten mit denen am Ausgang des Vorfelds unter Verwendung etablierter Signalverarbeitungstechniken zeigte, dass morphodynamische Prozesse das Signal des subglazialen Sedimentexports, das mit Geschiebe verbunden ist, schnell filtern (d. h. dämpfen und verzögern), während das der Schwebstoffe fast ungehindert durch das proglaziale Vorfeld gelangt.
Um das Funktionieren dieses Filters zu verstehen, wurde eine intensive Untersuchung der Flussmorphodynamik und der Oberflächensedimentologie durchgeführt. Tägliche Drohnenvermessungen der Überflutungsebene wurden verwendet, um räumliche Muster von Erosion und Ablagerung zu quantifizieren. Im Mittelpunkt dieser Analyse stand die Entwicklung eines heuristischen Modells zur Vorhersage der Wassertiefenverteilung in stark trüben, verzweigten Flüssen, um Höhenmodelle für sowohl trockene als auch überflutete Bereiche zu erstellen. Diese wurden dann mit den subglazialen Sedimentexport-Raten kombiniert, um den Einfluss der proglazialen Morphodynamik auf den Sedimenttransport flussabwärts zu quantifizieren. Veränderungen im Gleichgewicht zwischen der glazialen Sedimentzufuhr und der Transportkapazität führten zu schnellen Veränderungen in der Flussmorphodynamik. Während Perioden, in denen die subglaziale Geschiebezufuhr die Transportkapazität überstieg, kam es zu einer Aufhöhung im Vorfeld, begleitet von intensiverer Barrenbildung, erhöhter Kanalinstabilität, gesteigerter Verzweigungsintensität und Oberflächenvergröberung der Überflutungsebenenablagerungen. Das Vorfeld fungierte als Senke sowohl für Geschiebe als auch für Schwebstoffe. Im Gegensatz dazu, wenn die subglazialen Geschiebe-Export- Raten unter der Transportkapazität lagen, fungierte das Vorfeld weiterhin als Senke für feine Sedimente, aber das System war netto degradiert für Geschiebesedimente.
Um diese Erkenntnisse auf Situationen mit unterschiedlichen topographischen, sedimentologischen und hydrologischen Gegebenheiten zu verallgemeinern, wurde ein hydromorphologisches Modell verwendet, das mit Felddaten kalibriert wurde. Die Ergebnisse bestätigten die zentrale Rolle des Verhältnisses von Zuführung zu Transportkapazität bei der Bestimmung der geomorphologischen Reaktion proglazialer Flüsse und der Sedimentlieferungsraten flussabwärts. Niedrigere Verhältnisse führten zu hydraulisch effizienteren Szenarien und einem leichteren Transfer des subglazialen Sedimentexport- Signals nach flussabwärts. Dieser Effekt wurde jedoch auch durch großmaßstäbliche Einflüsse der Taltopographie wie den Akkommodationsraum beeinflusst, was darauf hindeutet, dass der Filtereffekt zwischen geomorphologischen Einstellungen variiert.
Die Ergebnisse dieser Dissertation zeigen, dass pro
Ajuster la planification spatiale aux enjeux de l'eau : des pratiques à l'horizon des possibles
No full textÀ la croisée de la planification spatiale et des sciences de l’eau, cette recherche examine les enjeux rassemblant ces différents champs disciplinaires. Si la planification spatiale s’envisage en effet comme un facteur central de la gestion intégrée des eaux, leur réunion n’a été imposée que récemment par de nouvelles dispositions légales et par l’adoption du paradigme de durabilité. L’un des principaux motifs de ce travail a été de comprendre pourquoi la réalisation d’objectifs partagés pouvait s’avérer délicate. Les écueils et les articulations, entravant ou facilitant leur poursuite, ont été analysés à travers deux études de cas, le Canton de Vaud et la Région métropolitaine de Barcelone, soit deux entités territoriales soumises à des contraintes d’intensité variable, dès lors qu’elles se caractérisent par une répartition bien distincte des ressources en eau et des établissements humains.ESPRICEA
ICE-MARGINAL SUBGLACIAL CHANNELS AND THEIR RELATIONSHIP TO THE RAPID RETREAT OF TEMPERATE ALPINE GLACIERS
No full textCarbon fluxes and productivity regimes in Alpine streams
No full textHigh-altitude catchments have a major role in the transport of organic matter to streams due to the storage of dissolved organic carbon (DOC) in soils and glacier ice and the subsequent mobilization during the melting processes. Yet, stream function goes beyond the conveying downstream of the terrestrial and glacier derived DOC since they are also highly active in the mineralization, retention and production of organic matter. Stream ecosystem metabolism integrates the processes that regulate the conversion between the organic and inorganic forms of carbon in streams and is a fundamental measure to determine whether carbon accumulates or it is lost within the ecosystem. In a global warming scenario, high-mountain stream ecosystems are faced to modifications in their structure and function as a result of climate-driven hydrological changes. Thus far, despite the active role of alpine streams in the global carbon cycle, studies on the impacts from changes in snowmelt, glacier ice melt and groundwater contribution to streamflow have been largely focusing on alterations in hydrological regimes, water availability, geomorphology and biodiversity. To fill this gap, the aim of this thesis is to examine DOC fluxes dynamics and productivity regimes across a range of glacierized and non-glacierized alpine catchments to anticipate the possible consequences of projected hydrological changes on alpine stream biogeochemistry and ecosystem functioning.
This thesis is supported by the collection and analysis of high-frequency time series of physicochemical parameters, geomorphological data and streamwater samples from different Alpine streams with contrasting glacier coverage. The first part of the thesis investigates the response across all the streams of the annual DOC export to runoff primarily driven by snowmelt and glacier melt. In the second part, with the use of dissolved oxygen time series, the ecosystem energetics regimes in a glacier-, groundwater- and snowmelt-fed stream are explored during two years and related to their physical template. The third and last part is focused on providing estimates of gross primary production (GPP) rates for the energetic regimes established in the three Alpine stream types.
The obtained results show a varied response of DOC export to runoff across the catchments which was related to the degree of glaciation and vegetation cover. Our findings also reveal different stream ecosystem energetic regimes among the streams and highlight discharge as the major modulator of drivers, such as light, gas exchange rate or stability, on the seasonal and daily dissolved oxygen dynamics. Lastly, the magnitude and the temporal patterns of ecosystem GPP are the result of the light and streambed disturbance conditions that are largely determined by the hydrological and turbidity regime.
I argue that glacier shrinkage together with changes in snowmelt hydrology will alter the response of DOC yield to discharge, with consequent impact on the timing and magnitude of the lateral DOC fluxes from terrestrial to stream ecosystems. Also, an eventual reduction in glacier runoff and snowpack duration and content will alter the physical template for primary producers, which may lead to a greater production of autochthonous organic matter across high-mountain streams. Overall, I assume a shift in the magnitude and temporal patterns of energetic inputs, with consequences for the stream biogeochemistry and function.RIVE
Infiltration through clogged streambeds: theory and assessment of microbial mat clogging
No full textStreams and lakes are losing water when their surface is higher than the regional water table. The seepage flux depends on the surface water level, the geometry and hydraulic properties of the streambed and the underlying aquifer, as well as on the position of the regional water table. In particular, when naturally or artificially lowering the latter, surface water eventually disconnects from groundwater, resulting in an unsaturated zone in-between both saturated components.
Here we first study the one-dimensional configuration, for which disconnection requires the presence of a clogging layer, i.e., a layer having a lower permeability than the underlying sediments. We provide asymptotic solutions to the steady-state Richards equation for generic unsaturated hydraulic conductivity functions. The solutions are readily applicable to common parametrizations, such as van Genuchten or Brooks and Corey. The identification of three regimes, one dominated by the clogging layer, one by the underlying sediments and one balanced by both layers, motivates a refined classification of clogging. We also argue that infiltration rate roughly grows linearly with ponding depth. Some applications based on these findings are proposed, such as novel formulas for seepage modeling, a method to identify disconnection and a capillary zone height estimation formula. For multidimensional seepage, disconnection is also possible without clogging layer, but this eventuality is often discarded. Considering a generic seepage problem, we introduce a framework based on constitutive relations to qualitatively define all possible seepage states. Recognizing the need to quantify these relations to improve our ability to identify seepage states, we advocate for simple models to compute them.
Despite their proximity with glaciers, terraces of glacial forefield are often water scarce due to high sediment permeability and groundwater disconnection. Microbial mats are thought to contribute to primary succession in these nascent ecosystems, notably by impermeabilizing tributary streams and, thus, increasing water retention. Here, this biogeomorphic potential is explored in two steps. First, observations of very similar microbial mat thickness and permeability dynamics from a series of outdoor flume experiments mimicking tributary conditions, suggest relative clogging homogeneity within the considered streams. These experiments also highlight daily cycles of permeability caused by photosynthetic activity. Furthermore, we show that the permeability is not strongly affected by desiccation and identify a potential evaporative pumping mechanism driven by microbial mats. Second, an idealized terrace model, based on one-dimensional seepage solutions, is introduced to assess under what conditions surface water extent is expected to be significantly affected by clogging. The ratio between flooded distances within clogged and unclogged channels is shown to be almost independent on the inflow for sufficiently shallow streams, to scale sublinearly with the clogging resistance for intermediate clogging and to converge to a maximum value for larger clogging. The overall effect of microbial clogging on water retention on glacial forefield terraces remains poorly constrained, because strongly sensitive to the permeability of the underlying sediments. Field and experimental measurements are suggested to fill this gap.RIVE
Periphyton development in recently-deglaciated floodplains: from physical habitat constraints to ecosystem engineering
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