LeprojetCRUEX++

Le projet CRUEX++ est la suite du projet CRUEX, initié une vigntaine d’année en arrière. Le but du projet est la détermination des crues extrêmes. Le projet CRUEX visait surtout une méthodologie PMP-PMF. De nombreux outils ont été développés et différents modèles et méthodes ont été élaborés. Le projet CRUEX++ vise la combinaison et l’enrichissement, voire le complément, des résultats des dernières années dans une méthodologie d’estimation de crues extrêmes et de finaliser ce grand projet. La méthodologie est développée dans l’optique de vérifier la sécurité des barrages en Suisse. Une thèse a été initiée en 2012 afin de mener le projet CRUEX++ et de d’élaborer une méthodolgie d’estimation de crues extrêmes

Development of a methodology for extreme flood estimations in alpine catchments for the verification of dam safety

In the context of dam safety, reliable safety flood estimation methods are necessary to guarantee a sufficiently designed spillway capacity. Todayâs state of the art allows to approach this topic under different angles. Statistical extrapolations are common practice, but do not grant to address all needs of dam safety verifications. Due to the evolution of informatics in the last years, computational power has been pushed to a level where hydrological modelling can be performed under optimal conditions. Practitioners are, however, not yet very familiar with hydrological modelling and its potential is not completely exploited. The present research work develops an approach combining statistics and hydrological modelling techniques to propose an innovative ready-to-use methodology to address the complex issue of extreme flood estimations. In order for this to be successful, some lacks of scientific knowledge had to be addressed before dwelling on the combination between statistics and simulations. Concerning the temporal rainfall distribution, it could be shown that a unique rainfall mass curve is admissible for the entire territory of Switzerland. Regarding a coherent combination of temperature and extreme precipitations, linear relations between the duration of the precipitation event and the zero degree isothermal altitude could be determined. In the context of hydrological modelling, the influence of initial conditions for extreme flood simulations have been assessed and methodological recommendations for the choice of initial conditions for extreme flood simulations could be formulated. Furthermore, the maximum admissible spatial expansion of the PMP events derived from the Swiss PMP maps, elaborated during the CRUEX project, could be estimated to be 230 km2. Finally, the combination of the simulation results with the approach of upper bounded statistical extrapolations could be shown to be advantageous: the sample sensitivity is reduced and the plausibility of the extrapolations is enhanced compared to conventional statistical distributions. Besides these scientific challenges, the methodology has to be pragmatic and ready-to-use as it is destined to engineers. The methodology was developed to be easily communicable. Therefore, its development has been undertaken under respect of common practices. Ultimately, a holistic methodology for extreme flood estimations could be formulated. The main advantage of the methodology is that it allows to estimate extreme flood hydrographs using hydrological simulation and to plausibly attribute a return period to the simulated peak discharge taking a deterministically determined upper discharge limit into account by referring to the PMP-PMF approach. The methodology combines thus the possibility of flood attenuation estimations with the knowledge of the occurrence probability of the peak discharge of the event, that is normally a reference quantity in flood safety guidelines. The application of the developed methodology to three catchments with different characteristics could prove its ease of utilization and, more importantly, its advantages compared to conventional approaches. A computer tool could be developed to make the CRUEX++ methodology accessible and readily applicable

Innertkirchen compensation basin outlets – Flap gate combined with small stilling basin

In the context of an update of their hydropower plants, the Hydroelectric Company KWO plans an adaption of their powerhouses Innertkirchen 1 and 2. One aim of the project is, among many others, an optimized restitution regime of the turbined water to the Aare River. Therefore, a compensation basin is planned to reduce the currently pronounced hydro-peaking. The basin is situated downstream of the Hasliaare and the Gadmerwasser confluence, next to the outlets of the two powerhouses. The spatial situation and the geology implicate strong restrictions regarding the volume of the compensation basin, allowing for a storage capacity of approximately 20’000 m3. This volume is more than doubled by a voluminous tailrace tunnel between the turbines and the basin. The regulation of the basin is assured by a flap gate and a radial gate. The basin intents to limit hydro peaking, and thereby ameliorates the ecology of the Aare River between the basin and Lake of Brienz. It will facilitate to access the Gadmerwasser for the fishes, which is known as excellent spawning ground. The efficiency and reliability of the basin regulation structures is a key item for the operation of the two powerhouses. The basin outlet gates were thus model-tested at the Laboratory of Hydraulic Constructions of EPFL, Switzerland, to assure an sufficient discharge capacity even for elevated water levels in the Aare River, to avoid unwanted erosion on the area, to avoid sedimentation of the structure by the bed load of the Aare River, and to generated acceptable conditions for fish passage along the Aare River. The modeled hydraulic perimeter covered the basin outlet structure, a length of some 250 m of the Aare River, as well as a part of the compensation basin. The model is set-up with a geometrical scale factor of 1:40 and operated under the similitude of Froude. In the experiments, the efficiency of the basin regulation gates is tested. Additionally, water levels, flow velocities and the erosion of the river bed are measured. The paper describes this particular case study with some optimization steps; finally leading to a solution with satisfies the principle requests. Given the importance of the ecological aspects and the Swiss legislation, the present compensation basin can provide ideas and concepts for similar structures to foresee in the near future

Analysis of the variation of the 0°C isothermal altitude during intense rainfall events

The analysis aims at analysing the evolution of the 0°C isothermal altitude during intense rainfall events. The following steps are followed: • The rainfall events of around 100 meteorological stations measuring at hourly time step are considered. • Two meteorological sounding stations launching two weather balloons per day are considered. • The variation of the 0°C is analysed depending on the rainfall event duration. • The analysis is carried out distinguishing between winter (December-February) and summer (June-August)

Impacts du changement climatique sur la production hydroélectrique alpine : comment une nouvelle retenue à Oberaletsch pourrait garantir la gestion durable des installations existantes

Suite à décision du Conseil Fédéral de sortir du nucléaire, l’hydroélectricité suisse jouera un rôle central pour réussir la transition énergétique. Cependant, le changement climatique semble avoir un impact négatif sur la production hydraulique à long terme. En effet, avec l’accélération de la fonte des glaciers, les apports hydrauliques vont en général augmenter jusqu’en 2050, puis diminuer avec la disparition d’un grand nombre de glaciers. Ce phénomène va laisser place à des sols très érodibles, les moraines, ce qui se traduira par une augmentation globale de l’apport en sédiments aux retenues existantes. La diminution globale des apports en eau va donc impacter la gestion des barrages, comme celui de Gebidem, dans le canton du Valais en Suisse, déjà très concerné par la gestion des sédiments. Ce barrage est situé en aval du grand glacier d’Aletsch et exploite les eaux de la Massa à la centrale de Bitsch. Le volume de la retenue ne représentant que 2% de l’ensemble des apports de son bassin versant. Le retrait du glacier, et notamment de son bras d’Oberaletsch laisse apparaître un site idéal pour l’implantation d’une nouvelle retenue en amont de Gebidem, ce qui permettrait d’une part d’exploiter une nouvelle hauteur de chute et d’autre part de pouvoir augmenter la capacité de stockage saisonnier permettant une meilleure valorisation de la ressource en eau et une plus grande flexibilité d’exploitation. L’article présente une étude du potentiel du site d’Oberaletsch focalisé sur la possibilité de convertir les futurs impacts du changement climatique en opportunités. La construction d’un nouvel aménagement à Oberaletsch à caractère modulable permettrait de capter l’abondance des apports dans les prochaines décennies et de se préparer pour leur réduction attendue après 2050. Les effets combinés d’une nouvelle retenue et d’un moindre transit des apports en eau en rivière, réduiraient la mobilisation des dépôts de moraine et les apports en sédiments à Gebidem. Une exploitation coordonnée des deux aménagements permettrait des gains d’efficience dans l’usage de l’ensemble des ressources en eau, du territoire et des infrastructures, toute en réduisant et modulant les investissements en nouvelles installations

Relevance of the correlation between precipitation and the 0 C isothermal altitude for extreme flood estimation

Extreme floods are commonly estimated with the help of design storms and hydrological models. In this paper, we propose a new method to take into account the relationship between precipitation intensity (P) and air temperature (T) to account for potential snow accumulation and melt processes during the elaboration of design storms. The proposed method is based on a detailed analysis of this P-T relationship in the Swiss Alps. The region, no upper precipitation intensity limit is detectable for increasing temperature. However, a relationship between the highest measured temperature before a precipitation event and the duration of the subsequent event could be identified. An explanation for this relationship is proposed here based on the temperature gradient measured before the precipitation events. The relevance of these results is discussed for an example of Probable Maximum Precipitation-Probable Maximum Flood (PMPPMF) estimation for the high mountainous Mattmark dam catchment in the Swiss Alps. The proposed method to associate a critical air temperature to a PMP is easily transposable to similar alpine settings where meteorological soundings as well as ground temperature and precipitation measurements are available. In the future, the analyses presented here might be further refined by distinguishing between precipitation event types (frontal versus orographic)

New Approach to Identifying Critical Initial Conditions for Extreme Flood Simulations in a Semicontinuous Simulation Framework

Extreme flood simulation with synthetic extreme precipitation events raises unavoidable questions about the choice of initial conditions. State-of-the-art extreme flood estimation frameworks propose to address these questions with the help of semicontinuous modeling and reanalysis of simulated state variables. In this context, the present work proposes a new method for the selection of initial conditions for extreme flood simulation. The method is based on generating sets of initial conditions from the matrix of state variables corresponding to a long simulation run of the selected hydrological model. Two sets of initial conditions are obtained: a deterministic set composed of selected state variable quantiles and a stochastic set composed of state variable vectors randomly drawn from the complete state variable matrix. The extreme flood simulations corresponding to both sets are compared in detail, and the stochastic simulations are used in a sensitivity analysis to identify the dominant state variables and possible interactions. The aim hereby is to provide a tool to analyze the role of initial conditions and the importance to account for state variable interactions in extreme flood estimation. The proposed method is applied to probable maximum flood estimation for the Swiss Mattmark Dam catchment with a semilumped hydrological model. The obtained results for this case study show that for high flood peak quantiles, the initial soil saturation is dominating other state variables, and deterministic initial conditions are sufficient to generate extreme floods