GPS-determination of along-strike variation in Cascadia margin kinematics: Implications for relative plate motion, Subduction zone coupling, and permanent deformation

High‐precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along‐strike variation in Cascadia margin kinematics. These results constrain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of magnitude longer than the decades‐long history of seismic monitoring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station velocities along the coast. After correction for a simple elastic dislocation model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3–7 mm/yr) to northern and central Oregon (∼9 mm/yr), consistent with oblique convergence and geologic constraints on permanent deformation. The margin‐parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north‐westward velocities reflect the influence of Pacific‐North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin

Real-time Bayesian estimation of stage-discharge rating curves during floods

International audienceStreamflow estimation in real time is required for data assimilation by flood forecasting systems. Streamflow time series are established using 'rating curves' that approximate the stage-discharge relationship. A major problem is that the river bed at many hydrometric stations can evolve due to morphogenic floods, leading to rating changes, or 'shifts'. The methods proposed in the literature for updating the rating curve are based on a retrospective statistical analysis of calibration data (or gaugings). The aim of the proposed method is the automatic detection of shifts and the estimation of their magnitude in real time, using both observational data and hydraulic knowledge of the site. It is built on an existing Bayesian framework for estimating probabilistic stage-discharge rating curves with quantitative uncertainties. Before starting a real-time application, a retrospective analysis is performed to detect past changes typically based on available gaugings. The retrospective results are then used to calibrate the methods for detecting and estimating shifts and their magnitudes in real-time. To this aim, the method uses different sources of information available in real time, e.g. stage records, estimates of the sediment transport that affects the bed evolution, changes in the shape of recession curves, changes in correlations with neighbouring hydrometric stations, etc. The proposed method was applied to the Ardèche River at Meyras, France, a river with a gravel-bed degrading during each flood. Both retrospective analysis and real-time estimation of the stage-discharge rating curves yielded encouraging results, with consistent detection of shifts and estimation of their magnitudes. Further research includes testing the method at more challenging sites and the use of other useful information available in real-time. The final goal is to provide real-time streamflow estimates with uncertainties that can be assimilated in models and decision making

Estimation de l'évolution du lit des rivières en correspondance des stations hydrométriques à l'aide du limnigramme

International audienceWe propose an original method for estimating riverbed evolution at hydrometric stations through the analysis of stage recessions. The main goal is the detection and automatic estimation of variations in the mean bed elevation using stage record only, in the absence of discharge measurements (aka gaugings), rating curves and bathymetry measurements. The method assumes the stage recession physically tends towards the riverbed elevation as streamflow tends towards zero. Thus, a change in the shape of the recession may indicate a potential riverbed evolution. This work focuses on rivers subjected to sudden morphological changes. The proposed method has been applied to the Ardèche River at Meyras (Barutel) in France, a river with a gravel bed degrading during each flood. The method yielded encouraging results with the detection of net riverbed change. Perspectives include the validation of the method by considering more challenging case studies.Nous proposons une méthode originale d'estimation du lit d'une rivière en correspondance des stations hydrométriques par l'analyse des récessions de la hauteur d'eau. L'objectif principal c'est la détection et l'automatisation estimation des variations de la cote moyenne du fond du lit à l'aide uniquement du limnigramme, sans mesures de débit (jaugeages), ni des courbes de tarage, ni des mesures bathymétrique. La méthode suppose que la récession tend physiquement vers la cote du fond du lit de la rivière comme le débit tend vers zéro. Donc, un changement dans la forme de la récession peut indiquer une évolution potentielle du lit de la rivière. Ce travail se concentre sur les rivières soumises à des changements morphologiques soudains. La méthode proposée a été appliquée à l'Ardèche à Meyras (Barutel) en France, une rivière avec un lit en gravier qui se dégrade lors de chaque crue importante. La méthode a donné des résultats encourageants avec la détection de changements nets du lit de la rivière. Les perspectives comprennent la validation de la méthode en considérant des cas d'étude plus complexes

Detéction des détarages en correspondance des stations hydrométriques

International audienceRiver discharge time series are usually established using "rating curves" (hereafter called RCs) that approximate the stage-discharge relation. These models are fitted using occasional stage-discharge measurements, also known as gaugings. However, two major problems affect the RC estimation: firstly, gaugings and RCs are both uncertain; secondly, the stage-discharge relation can be unstable and subjected to sudden or transient changes (also known as "rating shifts"). For the first problem many authors have proposed methods for the uncertainty quantification. As regards the second issue transient changes require a dynamic approach while sudden changes require detecting the times of shift and estimating an RC for each stable period. We propose some tools for the detection of effective or potential sudden shift times in a retrospective analysis. To do this we use two main sources of information: the gaugings and the water level (or "stage") record. The proposed retrospective analysis is based on the segmentation of residuals between gaugings and a reference RC; the segmentation of the recessions of the stage record; a sediment transport proxy model. The proposed method has been applied to two case studies mainly affected by morphological changes yielding quite encouraging if compared to the official shift dates.Les séries chronologiques de débit de la rivière sont généralement établies à l'aide de "courbes de tarage" (ci-après appelées CdT) qui approximent la relation hauteur-débit. Ces modèles sont calibrés à l'aide de mesures occasionnelles de hauteur-débit, les jaugeages. Cependant, deux problèmes majeurs affectent l'estimation des CdT: premièrement, les jaugeages et les CdT sont incertaines; deuxièmement, la relation hateur-débit peut être instable et soumise à des changements soudains ou transitoires ("détarages"). Pour le premier problème, de nombreux auteurs ont proposé des méthodes de quantification de l'incertitude. En ce qui concerne le deuxième problème, les détarages transitoires nécessitent une approche dynamique tandis que les détarages soudains nécessitent de détecter les temps de détarage et d'estimer une CdT pour chaque période stable. Nous proposons quelques outils pour la détection de temps de détarage soudain effectifs ou potentiels à travers une analyse rétrospective. Nous utilisons deux sources principales d'information: les jaugeages et la hauteur d'eau. L'analyse rétrospective proposée est basée sur: la segmentation des résidus entre les jaugeages et une CdT de référence; la segmentation des récessions du limnigramme; un modèle proxy de transport sédimentaire. La méthode proposée a été appliquée à deux cas d'études principalement affectées par des changements morphologiques, ce qui donne des résultats assez encourageants par rapport aux dates officielles de changement

Shift happens! Ajustement des courbes de tarage hauteur-débit aux évolutions morphologiques à des moments connus

International audienceEstablishing streamflow time series in unstable rivers is challenged by changes in the stage-discharge relation after floods. Then, the field hydrologist must develop a new stage-discharge rating curve using new calibration data but also using some information from previous calibration data and previous rating curves. The process includes a large amount of informal expert knowledge and hydraulic assumptions seldom made explicit. This paper develops a stage-period-discharge (SPD) model based on the physical interpretation of changes in the stage-discharge relation across a series of stability periods defined by known dates and times. Using simple hydraulic equations, the user provides prior knowledge on the controls, their static and varying parameters, and their possible changes. As a single model is used for all the periods, the estimation of all rating curves can be performed in one go: All gaugings hence provide information to estimate the static parameters and the varying parameters for the relevant periods. Bayesian inference is used, providing a natural way to include prior knowledge and to quantify uncertainty. The generality and some key properties of the method are demonstrated through application to two hydrometric stations, differing in hydraulic configuration and in number and type of changes. Specific experiments demonstrate the ability of the SPD model to transfer information across periods. Consequently, rating curves are more precisely estimated than by separately estimating them for each period. The SPD model provides a hydraulically based, transparent, and user-friendly approach to replace manual shift corrections traditionally applied in operational practice, with a quantification of uncertainties