AcknowledgementsI would like to thank Benoît and Paolo for their very competent supervision of this master thesis. AbstractAlthough riparian vegetation is present in or along many water courses of the world, its active role resulting from the interaction with flow and sediment processes has only recently become an active field of research. Especially, the role of vegetation in the process of river pattern formation has been explored and demonstrated mostly experimentally and numerically until now. In the present work, we shed light on this subject by performing a linear stability analysis on a simple model for riverbed vegetation dynamics coupled with the set of classical river morphodynamic equations. The vegetation model only accounts for logistic growth, local positive feedback through seeding and resprouting, and mortality by means of uprooting through flow shear stress. Due to the simplicity of the model, we can transform the set of equations into an eigenvalue problem and assess the stability of the linearized equations when slightly perturbated away from a spatially homogeneous solution. If we couple vegetation dynamics with a 1D morphodynamic framework, we observe that instability towards long sediment waves is possible due to competitive interaction between vegetation growth and mortality. Moreover, the domain in the parameter space where perturbations are amplified was found to be simply connected. Subsequently, we proceed to the analysis of vegetation dynamics coupled with a 2D morphodynamic framework, which can be used to evaluate instability towards alternate and multiple bars. It is found that two kinds of instabilities, which are discriminated mainly by the Froude number, occur in a connected domain in the parameter space. At lower Froude number, instability is mainly governed by sediment dynamics and leads to the formation of alternate and multiple bars while at higher Froude number instability is driven by vegetation dynamics, which only allows for alternate bars.

Abstract. One-dimensional hydrodynamic models are nowadays widely recognized as key tools for lake studies. They offer the possibility to analyze processes at high frequency, here referring to hourly timescales, to investigate scenarios and test hypotheses. Yet, simulation outputs are mainly used by the modellers themselves and often not easily reachable for the outside community. We have developed an open-access web-based platform for visualization and promotion of easy access to lake model output data updated in near-real time (http://simstrat.eawag.ch, last access: 29 August 2019). This platform was developed for 54 lakes in Switzerland with potential for adaptation to other regions or at global scale using appropriate forcing input data. The benefit of this data platform is practically illustrated with two examples. First, we show that the output data allows for assessing the long-term effects of past climate change on the thermal structure of a lake. The study confirms the need to not only evaluate changes in all atmospheric forcing but also changes in the watershed or throughflow heat energy and changes in light penetration to assess the lake thermal structure. Then, we show how the data platform can be used to study and compare the role of episodic strong wind events for different lakes on a regional scale and especially how their thermal structure is temporarily destabilized. With this open-access data platform, we demonstrate a new path forward for scientists and practitioners promoting a cross exchange of expertise through openly sharing in situ and model data.

a b s t r a c tWe investigate the influence of vegetation on river morphological instabilities using an analytical framework. We first discuss the important role of the hydrological (flooding frequency) and biological (vegetation development rate) timescales. As long as the changes in riverbed morphology and vegetation over an interval comprising one flood and one low-flow period are small, we show that it is possible to simplify the description of a vegetated river with non-constant discharge. We propose physically-based and effective (neural) models for the feedback between vegetation and morphodynamics. Physically-based approaches use equations of morphodynamics extended to account for the interplay between flow, sediment and vegetation dynamics. While their foundation is solid, a physically-based description is only feasible for simple vegetation cover (grass to shrubs). For complex vegetated obstacles we present as an alternative effective approaches, explicitly including interactions (local and non-local) between obstacles. We focus on the role of vegetation in the emergence of ridge patterns observed in the presence of an ephemeral flow and correspondingly derive a set of conditions for patterns.

Lake Kivu, East Africa, is well known for its huge reservoir of dissolved methane (CH 4) and carbon dioxide (CO 2) in the stratified deep waters (below 250 m). The methane concentrations of up to~20 mmol/l are sufficiently high for commercial gas extraction and power production. In view of the projected extraction capacity of up to several hundred MW in the next decades, reliable and accurate gas measurement techniques are required to closely monitor the evolution of gas concentrations. For this purpose, an intercomparison campaign for dissolved gas measurements was planned and conducted in March 2018. The applied measurement techniques included on-site mass spectrometry of continuously pumped sample water, gas chromatography of in-situ filled gas bags, an in-situ membrane inlet laser spectrometer sensor and a prototype sensor for total dissolved gas pressure (TDGP). We present the results of three datasets for CH 4 , two for CO 2 and one for TDGP. The resulting methane profiles show a good agreement within a range of around 5-10% in the deep water. We also observe that TDGP measurements in the deep waters are systematically around 5 to 10% lower than TDGP computed from gas concentrations. Part of this difference may be attributed to the non-trivial conversion of concentration to partial pressure in gasrich Lake Kivu. When comparing our data to past measurements, we cannot verify the previously suggested increase in methane concentrations since 1974. We therefore conclude that the methane and carbon dioxide concentrations in Lake Kivu are currently close to a steady state.

Abstract. One-dimensional hydrodynamic lake models are nowadays widely recognized as key tools. They offer the possibility to study processes at high frequency, here referring to hourly time scale, to analyse scenarios and test hypothesizes. Yet, simulation outputs are mainly used by the modellers themselves and often not easily reachable for the outside community. We have developed an openly accessible web-based platform for visualization and promotion of easy access to lake model output data updated in near real time (https://simstrat.eawag.ch/). This platform was developed for 54 lakes in Switzerland with potential for adaptation to other regional areas or even at global worldwide scale using appropriate forcing input data. The benefit of this data platform is here practically illustrated with two examples. First we show that the output data allows for assessing the long term effects of past climate change on the thermal structure of a lake. In the second case, we demonstrate how the data platform can be used to study and compare the role of episodic strong wind events for different lakes on a regional scale and especially how they temporary destabilize their thermal structure. With this open access data platform we demonstrate the path forward for scientists and practitioners promoting a cross-exchange of expertise through openly sharing of in-situ and model data.